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A Practical Approach to Design for Efficient Fabrication and Construction By Alan Hayward, Neil Sadler and Derek Tordoff Steel Bridges Publication Number 34/02

Steel Bridges

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Page 1: Steel Bridges

A Practical Approach to Design for Efficient Fabrication and Construction

By Alan Hayward, Neil Sadler and Derek Tordoff

Steel Bridges

Publication Number 34/02

Page 2: Steel Bridges

Apart from any fair dealing for the purposes of research orprivate study or criticism or review, as permitted under theCopyright Design and Patents Act 1988, this publication maynot be reproduced, stored or transmitted in any form by anymeans without the prior permission of the publishers or in thecase of reprographic reproduction only in accordance with theterms of the licences issued by the UK Copyright LicensingAgency, or in accordance with the terms of licences issued bythe appropriate Reproduction Rights Organisation outside theUK.

Enquiries concerning reproduction outside the terms statedhere should be sent to the publishers, The BritishConstructional Steelwork Association Ltd at the address givenbelow.

Although care has been taken to ensure, to the best of ourknowledge, that all data and information contained herein areaccurate to the extent that they relate to either matters of factor accepted practice or matters of opinion at the time ofpublication, The British Constructional Steelwork AssociationLtd, the authors and the reviewers assume no responsibilityfor any errors in or misinterpretation of such data and/orinformation or any loss or damage arising from or related totheir use.

The British Constructional Steelwork Association Limited(BCSA) is the national organisation for the steel constructionindustry: its Member companies undertake the design,fabrication and erection of steelwork for all forms ofconstruction in building and civil engineering. AssociateMembers are those principal companies involved in thepurchase, design or supply of components, materials,services related to the industry. Corporate Members areclients, professional offices, educational establishments whichsupport the development of national specifications, quality,fabrication and erection techniques, overall industry efficiencyand good practice.

The principal objectives of the Association are to promote theuse of structural steelwork; to assist specifiers and clients; toensure that the capabilities and activities of the industry arewidely understood and to provide members with professionalservices in technical, commercial, contractual and qualityassurance matters. The Association’s aim is to influence thetrading environment in which member companies operate inorder to improve their profitability.

A current list of members and a list of current publications andfurther membership details can be obtained from:

The British Constructional Steelwork Association Ltd4 Whitehall Court, Westminster, London SW1A 2ESTelephone: +44 (0) 20 7839 8566Fax: +44 (0) 20 7976 1634Email: [email protected]

BCSA’s website, www.SteelConstruction.org, can beused both to find information about steel constructioncompanies and suppliers, and also to search for adviceand information about steel construction related topics,view publications, etc.

Publication Number 34/02

First Edition March 1985Second Impression March 1990Third Impression April 1991

Second Edition December 2002

ISBN 0 85073 040 6

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

© The British Constructional Steelwork Association Ltd

Designed and Printed by Box of Tricks www.bot.uk.com

Page 3: Steel Bridges

PREFACE TO THE SECOND EDITION

Design of steel bridges is achieved most effectivelywhen it is based on a sound understanding of both thematerial and the methods adopted in processingsteelwork through to the final bridge form. The aim ofthis publication is to provide a basis for thisunderstanding by reference to the factors whichinfluence safe, practical and economic fabrication anderection of bridge steelwork.

The first edition of this publication was prompted bythe need for the steel construction industry to givegeneral guidance and information to designers of smalland medium span steel bridges in the UK and was toprovide an insight into the practical aspects offabrication and erection that would help designers usesteel more efficiently and economically to achieve theirclients' requirements. That need is just as vital today,but since publication of the first edition in 1985 manychanges in working practice have occurred that makea second edition essential.

The text has been amended and updated to reflectchanges in procurement practice, the continuingEuropean harmonisation of codes and standards,technological advances in manufacturing andconstruction, and modernisation of fabricationworkshops. The original structure of the content hasbeen retained: Chapters 1, 2 and 3 provide guidanceon conceptual design, steel quality, and design ofmembers; Chapters 4 and 6 give practical informationon bolting and welding for connections, and onprocedures for managing them in implementing thedesign; Chapter 5 discusses the accuracy offabrication and its significance for the designer; someguidance on cost is given in Chapter 7 in qualitativeterms. The guidance is supported with a new set ofcase studies illustrating 'good' deck layouts and cross-section arrangements, fabrication details andeconomic design solutions for some recent specificbridge requirements. A substantial reference list isincluded so that the reader may follow up the subjectin greater depth to meet his particular needs.

Throughout the text there are many references toCodes of Practice, British Standards and other designdocuments – particularly the standards and advicenotes contained in the Design Manual for Roads andBridges issued on behalf of the highways overseeingorganisation in England, Scotland, Wales and NorthernIreland. In a period of considerable change, thereferences relate to documents current in 2002, andtheir antecedents where relevant. Change will go onand in the life of this edition it is probable that theEurocodes will be implemented. Given that most of theguidance and information is practical andtechnological, the reader should be confident inmaking continued use of the publication provided hehas an awareness of changes in requirements whichaffect responsibilities, procedural arrangements, orquantitative values eg fabrication tolerances.

This second edition has been prepared by CassHayward & Partners for the BCSA, with substantialinput from Alan C G Hayward and Neil Sadler; AlanHayward also made significant contribution to theoriginal text. Thanks are given to Ian E Hunter,Consultant - formerly Cleveland Bridge Ltd, for draftingparts of the text and an overall editorial review, and toRichard Thomas of Rowecord Engineering Ltd fordrafting Chapter 6. The valued assistance of thefollowing is also acknowledged:

C V Castledine formerly Butterley Engineering Ltd

J Dale Rowecord Engineering Ltd

N Iannetta Fairfield-Mabey Ltd

A Jandu Highways Agency

P Lloyd Fairfield-Mabey Ltd

G M Mitchell MBE BCSA

S Waters Fairfield-Mabey Ltd

P J Williams BCSA

STEEL BRIDGESA Practical Approach to Design for Efficient Fabrication and Construction

Page 4: Steel Bridges

PREFACE TO THE FIRST EDITION (Reproduced)

The aim of this publication is to provide guidance uponthe practical aspects of fabrication that influence theefficient design of steel bridges. Extensive consultationhas taken place with experts in the design, fabricationand erection of steel bridges and therefore thispublication represents a compilation of many years’knowledge and experience.

The proof of the effectiveness of this approach is thata number of steel bridge designs based on theseconcepts have proved to be less expensive than themore traditional designs in concrete and steel.References are made to BS 5400 Part 3 (Code ofpractice for the design of steel bridges) and Part 6(Specification for materials and workmanship, steel)and their application in practice is demonstrated.

Examples are given of ‘good’ deck layouts and crosssectional arrangements, fabrication details andeconomic design solutions to specific bridgerequirements.

The author, Derek Tordoff, has worked on the design,construction and inspection of steel and concretebridges both with consultants and with a contractor.He undertook research work at the University of Leedson computer aided techniques for the efficient designof steel bridges. He joined BCSA in 1976 where he isnow Director General with wide ranging responsibilitiesin the steel construction industry

ACKNOWLEDGEMENTS (First Edition - reproduced)

The author is indebted to JS Allen (NEI Thompson Ltd)and ACG Hayward (Cass Hayward & Partners) for theircomments, advice and contributions to the text.

Acknowledgements for their assistance are also due to:

P H Allen BCSA

J G Booth John Booth & Sons (Bolton) Ltd

Prof F M Burdekin University of Manchester Institute of Science & Technology

M M Chrimes Institution of Civil Engineers

W R Cox Sir William Arrol NEI Cranes Ltd

K Dixon Cleveland Bridge and Engineering

J E Evans ex Fairfield Mabey Ltd

H J Gettins Robert Watson & Co (Constructional Engineers) Ltd

A N Hakin BCSA

J Kinsella Dorman Long Bridge & Engineering

F I Lees Butterley Engineering Ltd

J E Leeson NEI Thompson Ltd Horseley Bridge

J W Pask ex Redpath Dorman Long Ltd

J L Pratt Braithwaite & Co (Structural) Ltd

W R Ramsay BSC Sections and Commercial Steels

D C Shenton BSC Sections and Commercial Steels

T J Upstone ex Redpath Dorman Long Ltd

F G Ward ex Constrado

A Waterhouse NEI Thompson Ltd Horseley Bridge

The diagrams, charts and figures have been preparedwith the assistance of Cass Hayward & Partners.

Page 5: Steel Bridges

1. Design Conception1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Initial Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Cross-Sections for Highway Bridges . . . . . . . . . . . . . . . . . . 101.4 Cross-Sections for Railway Bridges . . . . . . . . . . . . . . . . . . . 111.5 Cross-Sections for Footbridges . . . . . . . . . . . . . . . . . . . . . . 121.6 Camber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.7 Dimensional Limitations. . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.8 Erection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.9 Repairs and Upgrading . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2. Steel Qualities2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2 Brittle Fracture and Notch Toughness Requirements. . . . . . . 172.3 Internal Discontinuities in Rolled Steel Products . . . . . . . . . . 172.4 Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.5 Weathering Steels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.5.1 Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.5.2 Materials and Weldability . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.5.3 Bolting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.5.4 Availability of Weathering Steel . . . . . . . . . . . . . . . . . . . . . . 222.5.5 Suitability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3. Design of Members3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.2 Plate Girders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.3 Box Girders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.4 Bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4. Connections4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.2 Bolted Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.3 High Strength Friction Grip Bolts . . . . . . . . . . . . . . . . . . . . . 334.4 The Friction Grip Joint. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.5 Installation of HSFG Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . 344.5.1 Torque Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.5.2 Part-Turn Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.5.3 Direct Tension Indication . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.5.4 Sequence of Tightening . . . . . . . . . . . . . . . . . . . . . . . . . . . 354.6 Inspection of HSFG Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . 354.7 Welded Connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5. Fabrication5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375.2 Fabrication Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385.3 Checking of Deviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385.4 Causes of Fabrication Distortion . . . . . . . . . . . . . . . . . . . . . 425.5 Methods of Control of Distortion . . . . . . . . . . . . . . . . . . . . . 425.6 Effects of Design on Distortion. . . . . . . . . . . . . . . . . . . . . . . 425.7 Distortion Effects and Control of

Various Forms of Construction. . . . . . . . . . . . . . . . . . . . . . . 435.8 Correction of Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.9 Trial Erection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

CONTENTS

Page 6: Steel Bridges

6. Welding6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.2 Principal Welding Standards . . . . . . . . . . . . . . . . . . . . . . . . 456.3 Types of Joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.4 Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466.5 Preparation of Welding Procedure Specifications . . . . . . . . . 476.6 Procedure Trials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476.7 Avoidance of Hydrogen Cracking. . . . . . . . . . . . . . . . . . . . . 486.8 Welder Approval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486.9 Inspection and Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496.10 Weld Quality Levels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

7. Costs7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537.2 Project Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537.2.1 Everything has to be paid for. . . . . . . . . . . . . . . . . . . . . . . . 537.2.2 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537.2.3 The Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537.2.4 Specification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547.2.5 Programme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547.2.6 Conditions of Contract . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557.2.7 Commercial Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557.3 The Fabrication Process . . . . . . . . . . . . . . . . . . . . . . . . . . . 557.3.1 A Manufactured Product . . . . . . . . . . . . . . . . . . . . . . . . . . . 557.3.2 Starting the Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557.3.3 Planning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557.3.4 Drawings and Production Data . . . . . . . . . . . . . . . . . . . . . . 567.3.5 Material Procurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567.3.6 Receipt of Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567.3.7 Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567.3.8 Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567.3.9 Cutting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577.3.10 Preparation of Edges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577.3.11 Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577.3.12 Pressing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577.3.13 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577.3.14 Managing Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587.4 Protective Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587.5 Erection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597.6 Guidance on Costs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597.6.1 What is a useful measure of cost . . . . . . . . . . . . . . . . . . . . . 597.6.2 Cost per Square Metre . . . . . . . . . . . . . . . . . . . . . . . . . . . . 607.6.3 Relative Cost per Tonne . . . . . . . . . . . . . . . . . . . . . . . . . . . 607.7 Achieving Best Value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

8. Case Studies8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 618.2 Tonna Bridge Replacement . . . . . . . . . . . . . . . . . . . . . . . . . 618.3 Road Bridge A34 Trunk Road – Newbury Bypass. . . . . . . . . 658.4 Westgate Bridges Deck Replacement, Gloucester . . . . . . . . 678.5 Churn Valley Viaduct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 698.6 A500 Basford Hough Shavington Bypass,

London – Crewe Railway Bridge . . . . . . . . . . . . . . . . . . . . . 718.7 Doncaster North Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . 738.8 A69 Haltwhistle Bypass Railway Viaduct . . . . . . . . . . . . . . . 758.9 River Exe Viaduct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778.10 Newark Dyke Rail Bridge Reconstruction

over the River Trent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Page 7: Steel Bridges

9. Competence in Steel Construction9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839.2 The Register of Qualified Steelwork Contractors . . . . . . . . . . 839.3 Bridgeworks Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839.4 Use of the Bridgeworks Register . . . . . . . . . . . . . . . . . . . . . 84

10. References10.1 Codes and Standards referred to in this edition . . . . . . . . . . 8510.2 Other documents, references,

standards referred to in this edition . . . . . . . . . . . . . . . . . . . 8610.3 Suggested further reading and sources of information . . . . . 8610.4 References used within the text of the first edition . . . . . . . . 8710.5 Articles about early steel bridges . . . . . . . . . . . . . . . . . . . . . 87

LIST OF ILLUSTRATIONS AND TABLES

Figures Title

1 Composite I Girders – Guide to Bracing . . . . . . . . . . . . . . . . . . . . . . . 52 Composite I Girders – Bracing Types . . . . . . . . . . . . . . . . . . . . . . . . . 63 Composite I Girders – Guide to Make-Up . . . . . . . . . . . . . . . . . . . . . . 74 2-Lane Composite Cross Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Footbridge Cross Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Half-Through and Rail Bridge Cross Sections . . . . . . . . . . . . . . . . . . 127 Available Plate Lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 Economy of Flange/Web Thickness Changes . . . . . . . . . . . . . . . . . . 289 Typical Single Span Girder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2910 Typical Bolted & Welded Splices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3011 Plate Girder Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3112 Detailing Do’s & Don’ts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3913 Orthotropic Plate Stiffeners. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4014 Weld Distortion and Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4115 Tonna Bridge Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6216 A34 Newbury Bypass – Donnington Link . . . . . . . . . . . . . . . . . . . . . 6617 Westgate Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6818 Churn Valley Viaduct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7019 A500 Basford Hough, Bridge over Railway . . . . . . . . . . . . . . . . . . . . 7220 Doncaster North Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7421 A69 Haltwhistle Bypass Railway Viaduct . . . . . . . . . . . . . . . . . . . . . . 7622 River Exe Viaduct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7823 Newark Dyke Rail Bridge Reconstruction . . . . . . . . . . . . . . . . . . . . . 81

Tables Title

1 Guide to Minimum Cambers in Universal Beams . . . . . . . . . . . . . . . . 132 Road Transport Length/Width Restrictions . . . . . . . . . . . . . . . . . . . . 133 Relative Efficiency and Limiting Thickness of Steel Grades . . . . . . . . . 184 Weathering Steel Corrosion Allowance to BD7/01

Related to 120 Year Design Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 Compression Outstand Limit to Flanges . . . . . . . . . . . . . . . . . . . . . . 23

Page 8: Steel Bridges
Page 9: Steel Bridges

1.1 Introduction

Steel is usually the material best-suited to meet therequirements in the UK for highways and railwaybridges, footbridges and moving bridges. Steelconstruction is, though, a peculiar part of civilengineering construction as all of the planning andpreparation is done off site, most elements critical tothe bridge are made in a remote factory, and indeedthe work at site may last but a few days. Production ofthe bridgeworks in a factory has many advantages interms of precision, quality, economy and safety but thedifferences from other civil engineering activities needto be understood if the potential of steel is to beexploited by the designer. The objective of thispublication is to facilitate an understanding of thoseaspects of fabrication and erection that influence thequality and economics of steel bridgeworks so that thedesigner is able to achieve an optimum solution for his client.

Fabrication of the steelwork represents a significantpart of the overall cost of a bridge. Each fabricationshop has a layout which has been developed withequipment appropriate to the types and scale ofstructures in which the company specialises. There isnot always a single preferred method for fabricating aparticular component or detail. Some details may suitautomated fabrication processes but may be lessappropriate for those companies which use moretraditional practices - good quality work can beproduced in highly automated shops, almost withoutthe use of fabrication drawings, and also by skilledtradesmen in basic shops with modest equipment.However, there are certain limitations for most of thedetails and impractical arrangements to be avoidedwhich, if anticipated knowledgeably at the designstage, will enable economical and easy fabrication.

Most of the advice given in this publication relates tohighway, railway and pedestrian bridges in the UK ofshort and medium span of conventional types. Steel iseminently suitable for long span bridges, includingsuspension and cable-stayed spans, and formechanical bridges which fall outside the scope of thisbook, though much of the content is relevant to them.Similarly the design of bridges for other countries, andof bridgeworks for export, involves considerationswhich are outside the scope of this book.

The production of a new bridge for its sponsor involvesa team of organisations and individuals workingtogether. Traditionally there was the Engineer and theContractor, but with new forms of procurement theseroles do not necessarily exist within a contract: so it is

convenient for the purpose of this book to use theterms 'designer' and 'steelwork contractor' to identifytwo of the key roles in building a steel bridge. However,removal of the term 'Engineer' from the text has notbeen possible since current British Standards,including BS 5400: Part 6, retain it. The term 'designer'is used to identify the organisation responsible for thedesign of the permanent works, whether employed bythe client, main contractor or steelwork contractor, andwhose responsibilities include all the technical mattersinvolving design, decision-making and approval for theimplementation of the design during the contract. Therole of 'steelwork contractor' covers responsibility forfabrication and erection (and certainly these are bestundertaken by one organisation even though it maychoose to subcontract part): it includes responsibilityfor choosing fabrication methods, fabricationdrawings, material procurement, erection method,construction engineering and temporary works design,protective treatment and, when required andappropriate, bearing supply.

The steelwork contractor and the designer for a steelproject must understand each other's roles andresponsibilities to obtain the best outcomeeconomically and technically. The designer shouldhave close contact with the steelwork contractor andbe able to discuss ideas with him where they have amutual bearing. Equally it is important that thesteelwork contractor is free to raise points with thedesigner where the end product and the projectoutcome for the client will be improved. This is madeeasier when a design and construct contract is used,as has been demonstrated on many successful bridgeprojects over the last 20 years.

Design of bridges for the UK is currently carried out toBS 5400, with Part 3 covering design of steel bridges,Part 5 the design of composite bridges, and Part 6 theworkmanship of steel bridges. For highway bridgesreference has to be made to Departmental Standardsand for railway bridges to Railtrack Group Standards;these standards are referred to in the text, as are manyof the associated British Standards. Through the life ofthis edition it is certain that British Standards will bereplaced by Euronorms and Eurocodes, but in mostinstances they will not invalidate the practical adviceand technical information provided by the book. It isnot clear how the implementation of the Eurocodesmay affect practices, for example, in the definition ofworkmanship tolerances compatible with designassumptions, but the codes cannot change theessential characteristics of working with steel.

1

CHAPTER 1DESIGN CONCEPTION

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Sustainability is a crucial issue for today's constructionindustry. Construction has to be socially andenvironmentally responsible as well as economicallyviable. Steel is a sustainable construction material touse for bridgework and good design will enhance itssustainability characteristics which are very evident inthe material itself, the manufacture of the structuralcomponents, and the relatively straightforward andswift construction of most steel bridges today:

a) New steel plate and sections contain a significantproportion of recycled material, and they are fullyrecyclable at the end of the life of the structure.

b) Innovation and technological advances - infabrication, in equipment and infrastructure fordelivery, and in construction plant and techniques -have enabled the move to more complete off-sitemanufacture so that typical bridge members aretruly manufactured products, with all theadvantages that brings. These include higherproductivity, less waste, energy efficiency, betterworking conditions, and a more easily controlledprocess, with less adverse impact on theenvironment.

c) Steel bridge construction is typically a brief andhighly organised activity in the overall constructionprogramme, utilising highly mobile equipment and asmall workforce of skilled people. Delivery can betimed to minimise inconvenience; relatively littlespace is required to operate; the steelwork is clean,with no dust, little waste, and noise is not a majorproblem. Erection schemes can be designedreadily to meet environmental constraints andconcerns. The speed of the erection processmeans that any inconvenience or environmentalimpact is reduced to a minimum period.

Sustainability in the bridge project is an issue for boththe steelwork contractor and the designer: it is anotherimportant quality aspect which benefits from thequality of the care and skill with which they fulfil theirrespective roles.

Since the publication of the first edition of this book,the regulatory environment for health and safety inconstruction has been much enhanced and health andsafety considerations have become an integral part ofdesign as well as planning and carrying outconstruction works - the Construction and Design(Management) Regulations have been a major factor inthis development. Although this edition has not beenextended specifically to cover such considerations, itsunderlying purpose is to improve the designer’sunderstanding of steelwork and his competence inusing it for bridge construction. So, for example, thedesigner must also consider issues such as designingfor the work to be done in the workshop rather than onsite, elastic instability of slender girders and access forsite tasks. Similarly the client, with professional advice,

has to satisfy himself that the chosen steelworkcontractor is competent to undertake the challenge ofhis design. The establishment of the independentRegister of Qualified Steelwork Contractors (RQSC)addresses this by including those steelworkcontractors who can demonstrate their relevant set ofcommercial and technical qualifications to undertakesteel bridgeworks. The Register, which is described inChapter 9, categorises the competent firms by size(turnover) and capability for different types of bridge: itis open to any company which can meet the objectivecriteria for competence assessed by independentengineers of standing. The Highways Agency nowrequires that only registered steelwork contractors areengaged for UK highway bridgeworks.

1.2 Initial Conception

For new build highways or railways the requirementsfor bridges are determined by the alignment, localterrain and the obstacles which are to be crossed.Ideally the designer should be involved at an earlystage to optimise overall geometry for the bridge spansand the interfaces with earthworks taking account ofthe cost influences of sighting distances, skew,curvature and construction depth. At the time ofconstruction of the early motorways from the 1950’s tothe 1980’s greenfield conditions allowed considerablefreedom of choice in alignments and methods ofconstruction. However, with the growing urbanisationand traffic density in the UK and elsewhere from the1980’s considerable restrictions arose in the provisionof new transport systems and the extension of existingsystems. Thus the number of requirements for bridgesacross obstacles is increased and designers are oftenconstrained to adopt curved, tapered or skewedstructures with severe limitations on constructiondepth. Moreover, the public objections to trafficdisruption mean that the methods and speed ofconstruction heavily influence bridge design now.

These trends have encouraged a move towardsprefabrication of elements favouring the use ofstructural steel as the primary medium for bridgespans, whilst capitalising on the merits of concrete andother materials in substructures, for formwork and inbridge deck slabs.

For replacement spans, or new bridges beneathexisting highways or rail tracks, the form ofconstruction will be dictated by the needs of a livehighway or railway in keeping disruption of traffic to anabsolute minimum. This favours prefabricated formsof construction which can be erected rapidly duringpossession, or which can be assembled adjacent tothe highway or rail track for speedy installation byreliable sliding, rolling-in or wheeled transportationmethods. Steel is ideal as the main structural materialin these situations.

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Where there is some freedom in the choice of spanlengths it should be borne in mind that the optimumsolutions for steel and concrete bridges are not alwaysthe same. For single span steel bridges a span lengthof 25 to 45m is economic, but this can extend to about60m. It is far more economic in steel to use continuousmultiple spans because fully rigid site joints, eitherwelded or bolted, are easily achieved in steel and leadto savings in amounts of material, bracings, bearingsand expansion joints. Cantilever and suspended spanscan exceptionally be used where differential settlementof the foundations is predicted as being substantial.The optimum for multiple spans ranges from 30m to80m. Continuous spans require much lessmaintenance of bearings and expansion joints,compared with simple spans, and offer improvedappearance of the piers. Typically for continuous spansthe end span should be about 80% of the penultimatespan for economy, but may be decided by otherfactors.

Span lengths can be designated as:Short up to 30m

Medium 30 to 80m

Long greater than 80m

A long span is demanded where a significantobstruction is to be crossed, such as a navigablewaterway or deep valley. Multiple medium to longspans become necessary across river estuaries orwhere ground conditions demand very expensivefoundations. For high level viaducts (with soffit morethan say 8m above ground level) the costs of piers,erection and concreting increase and so economicspan lengths tend to increase to balance superstructureand substructure costs.

For highway bridges Universal Beams with compositeconcrete slabs can be used for continuous spans of upto approximately 30m; however, because themaximum readily available length of UBs is 24m, plategirders are favoured for greater spans to avoid buttwelds. For spans up to 100m, plate girders are usuallycheaper than box girders; but box girders maysometimes be preferred for their cleaner aestheticappearance or when curvature demands torsionalrigidity. For long spans it is necessary to use box girdersto avoid excessive flange thicknesses and to providetorsional resistance against aerodynamic effects.Cable-stayed bridges are suitable for (and in multi-stayform prove economical for) spans ranging from 200m toabove 400m; recent international projects have usedcable-stayed spans of over 800m; suspension bridgesare used for all very long spans (up to 1990m to date).Through or half-through bridges are appropriate whereconstruction depth is critical as in water-way crossingsin flat terrain and for railway bridges: arch and trusstypes are suitable for medium spans, and half-throughgirders for short spans.

Steel is able to deal with skewed or curved alignmentsefficiently although, for single spans, it is oftenconvenient to provide a straight bridge and to increasethe width appropriately. This is likely to be economicwhere the width increase does not increase the grossplan area by more than say 5%. In other cases andespecially for multiple spans the bridge should becurved. Where the radius of curvature is greater thansay 800m, the girders are conveniently fabricated asstraight chords between the locations of site splices.For continuous spans such splices are set at areas ofminimum bending moment to minimise connection size,to facilitate erection, and to suit practicable length foreconomic delivery to site. If straight chords are chosen,curvature of the deck edge is achieved by variation ofthe concrete deck cantilevers. Lateral bracing needs tobe provided adjacent to the splices where curvature isaccommodated to cater for torsional effects. Typicalbracing layouts are shown diagrammatically in Figure 1;where substantially curved girders are used more lateralbracings are generally required, but to some extent theextra cost is offset by reduction of cantilever slab costs.

In plate girder deck type bridges lateral bracing isusually necessary for erection stability of thecompression flanges and during concreting of the deckslab. In the service condition lateral bracing may alsobe necessary to stabilise the bottom flange adjacent tointermediate supports of continuous spans. Some formof lateral bracing is usually required at the supports, andat the abutments this can be combined with a steel endtrimmer supporting the deck slab. Often solutions forconstruction and in-service requirements can becombined especially where the design utilises bracing inwide decks (>20m) to assist in transverse distribution ofconcentrated live loadings. Lateral bracings are also anecessity where accidental impact forces on the bridgesoffit must be designed for (bridges over highwayswhere headroom is less than 5.7m).

Only in exceptional cases should temporary bracingsbeneath a completed deck slab be specified forremoval on completion of construction because that isa potentially hazardous, as well as costly activity: indeedit should be utilised permanently to minimise girdersizes. In general, except for spans exceeding 60m, useof full length plan bracing systems should not benecessary, reliance being made upon the lateralstrength of the connected girders to resist wind effects.During construction the girders can often be erected,and delivered, as braced pairs to achieve mutualstability: intermediate lateral bracings are used whichserve to provide mutual torsional restraint. Temporarystabilisation of pairs of girders may be necessary duringerection. Once erected the braced girders need topossess sufficient lateral strength against wind loadinguntil the deck slab is sufficiently cured to provide therestraint, otherwise temporary plan bracing systemsmay be needed.

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Figures 1 and 2 provide a guide to bracing ofcomposite I-girders: Figure 1 shows layouts for simplysupported and continuous spans with recommendedspacing to suit flange sizes, and Figure 2 illustratesvarious types of bracing for use at supports and inspan. The width of the top flange of plate girders iscritical for stability during handling and constructionand it is recommended that this should not be lessthan 400mm. Figure 3 provides a guide to the make-up of composite I-girders for various ranges of span upto 70m to optimise site splice locations and availablematerial lengths consistent with the cost of makingshop butt welds.

Use of steel allows a variety of pier shapes to be usedto suit functional or aesthetic requirements. For widedecks spanning highways it is often preferable to avoidsolid leaf type piers which give a "tunnel" effect tousers of the highway below. Piers can be formed fromsteel or concrete prismatic or tapered columns, orportal frames. The number of columns within a pier canbe reduced by the use of integral steel crossheads (seeFigure 4), an early example being the M27 RiverHamble Bridge built in 1974. More recent examplesinclude the Thelwall Viaduct widening and viaducts onthe approaches to the M4 Second Severn Crossing.Integral steel crossheads are expensive to fabricateand erect, particularly for curved and superelevateddecks, so the benefits must be balanced against theextra cost and time required for them.

Movable bridges fall outside the scope of thispublication, but are nearly always constructed in steelso as to minimise dead weight and thereby thesubstantial costs of the operating machinery, bridgeoperation and maintenance. Modern examplesinclude bascule, swing, vertical lift, retractable bridgesand roll-on/roll-off ramps as used in port areas and onhighways where it is impracticable or uneconomic toprovide a fixed bridge with enough navigationheadroom. The type depends upon the requirednavigation width, height, deck width, frequency ofopening and aesthetic requirements.

1.3 Cross Sections for Highway Bridges

For short and medium spans cross sections aregenerally of composite deck-type (as shown in Figure4 for two-lane decks but applicable to multi-lanespans) but half-through type (as Figure 6) or throughtype are used where construction depth is critical. Fordeck type construction an economic constructionexcluding surfacing depth is generally about 1/20th ofthe span, but this can be reduced to about 1/30th ofthe span where necessary. It is usual to cantilever thedeck slab beyond the outer girder, which has anumber of advantages as it

– is visually attractive, giving a shadow line whichreduces the apparent depth of the girder;

– protects the steelwork from rain washing andsubsequent staining;

– reduces the width of piers; and

– optimises the deck slab design.

However, the deck cantilever should normally berestricted to 1.5m for minimising cost and be no longerthan 2.5m to avoid high falsework costs. Wherenecessary the cantilever length can be increased byuse of steel cantilever brackets in conjunction withcross girders between the main girders. Where it isnecessary to use high containment (P6) parapets, thenthis will affect the overall design demanding athickening of the deck slab locally and limitation of thelength of edge cantilevers.

Figure 4 shows typical two-lane highway bridgecomposite cross sections, each of which may beeconomic in given situations. The simplest form ismultiple Universal Beams for spans up to 24m. Thenumber and spacing of the beams will depend uponthe width of the deck, available construction depth andwhether service troughs have to be provided.Universal Beams have relatively thick webs; thereforeshear is not usually a problem rendering intermediatestiffeners unnecessary. Plate girders are generallymore suitable and economic for spans greater than20m. Generally an even number of girders should beused to facilitate optimisation on material ordering andto allow erection in braced pairs where appropriate.The most cost effective multigirder solution will havegirders at 3.0 to 3.5m spacing.

An important aspect to be considered in the designand construction of plate girders is stability duringerection and the possible need for temporary bracing.Figure 1 gives a guide to the location of bracings withconsistent flange widths. The assumptions made inthe design in relation to the sequence of erection,concreting, use of temporary bracing and method ofsupport of falsework should be clearly stated on thedrawings or otherwise specified. Generally the designassumption made is that the steelwork is selfsupporting and remains unpropped during concretingof the slab so that composite behaviour is utilised forsuperimposed loads and live loading only.

For medium spans, twin plate girders with crossgirders ("ladder-deck" bridges) offer certainadvantages for bridges up to about 24m in width, asshown in Figure 4 for a two-lane bridge - there arefewer main girders to fabricate and erect and there iseconomy of web material. Steel cantilevers are afeasible option with this cross section to achieve adeck cantilever greater than 2.5m. Ladder-decks aresuited to erection of the complete deck structure bylaunching. For longer spans twin box girders withcross girders and cantilevers become a viable option.

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Multiple box girders are suitable for medium spanswhere appearance of the bridge soffit is very importantand the presence of bracings is deemed unacceptableas for example in a prominent urban area wherepedestrians view the soffit. Open topped box girdersare used extensively in North America and there aresome examples in the UK, including the River NeneBridge at Northampton and the A43 Towcester Bridge.Temporary lateral and plan bracing systems are,however, necessary to maintain stability duringconstruction. Closed boxes, which are more stableduring fabrication and erection, are also used, butcurrent health and safety regulations for confinedspace working make work inside them very expensive.

For long spans where the weight of the deck becomesdominant, it is more appropriate to use an all steelorthotropic stiffened deck plate instead of a concreteslab. For the primary members either single or twinbox girders are used. Such boxes and deck areassembled from separate stiffened plate elements at aconstruction yard convenient for the site, andtransported to site for erection as large units. Such aprocedure was used for example on the SevernSuspension Bridge and Humber Bridge.

To reduce site assembly and erection costs, wherethere is access from the sea, complete spans may beassembled in a shipyard or port and taken by bargetransport for erection in place. Examples are the FoyleBridge built in Belfast for erection in Londonderry, andScalpay Bridge, which was taken complete to theOuter Hebrides.

1.4 Cross Sections for Railway Bridges

For replacements of existing spans cross sections areinfluenced by construction depth limitations becauseexisting decks often have substandard ballasted trackdepth, or the rails are mounted directly via longitudinaltimbers. Modern standards typically demand 300mmballast depth beneath sleepers giving a total trackdepth of approximately 690mm below rail level,allowing 25mm for floor waterproofing. Half throughconstruction is most usual for deck replacements andfor new bridges beneath existing railways to avoidtrack lifting which may be impracticable or very costly.Spans are generally simply supported to permitpiecemeal construction under traffic conditions.

Half through cross sections should, where possible,avoid centre girders which project in excess of 100mmabove rail level within the "six foot" space betweentracks. It is important to check that any part of thestructure or furniture does not infringe the structuregauge or lateral clearance for walkways taking accountof any curvature and cant, allowing for centre-throwand end-throw of rail vehicles. Modern standardsrecommend the provision of a robust kerb extending at

least 300mm above rail level to contain derailedvehicles, although many existing bridges do notpossess this.

For short spans up to about 17m the girders need notproject more than 110mm above rail level and eachtrack can be supported by a separate deck, whichfacilitates piecemeal replacements and site delivery.The Railtrack Standard ‘ZED’ type bridge uses shallowplate girders of zed configuration, so that maintenancespace is available between adjacent decks, with crossgirders and enveloping deck slab. A variant is theCass Hayward U-deck which integrates the maingirders with a deck of either all steel or composite formto achieve a single piece for fabrication and erection.Where practicable a robust kerb can be incorporatedin these designs by deepening the outermost girdersand locating them within the platform gauge.

For spans up to about 40m girders can be located sothat they fit within the platform gauge extending notmore than 915mm above rail level. The Railtrackstandard box girder type bridge which covers a spanrange from 12m to 39m uses trapezoidal box girderswith a transverse ribbed steel deck spanning betweennotionally pin-jointed shear plate connections: the boxgirders are stabilised by linear rocker bearings centredbeneath the inner web. This design is particularlysuited to piecemeal crane erection during trackpossession. For some recent projects, plate girderalternatives have proved economic where the site hassufficient width to accommodate them.

For spans exceeding about 40m the girder depthdictates that they are located outside the structuregauge, so increasing the span of the deck between themain girders. Half through plate girders or box girderscan be used, plate girders often being modelled on theformer type ‘E’ bridge with rolled section cross girdersspanning between rigid shear plate bolted connectionsin line with external stiffeners to give ‘U’ frame stability.The deck can be either in situ concrete partiallyencasing the cross girders or of stiffened steel plateconstruction, depending on the envisaged erectionmethod. For spans in excess of about 60m, throughor half through trusses or bowstring girders becomeappropriate. Examples are bridge 70A at Stockportacross the M63 and M64 motorways having a trussspan of 120m, and the Newark Dyke bridgereconstruction with a braced arch span of 77m.

For new build railways it is often possible to adopt adeck type solution with the benefit of an efficient crosssection of a concrete slab acting compositely withplate or box girders. Here the advice given in 1.3 fordeck type highway bridges would generally apply.Where train speeds in excess of 125mph areenvisaged special design criteria need to be appliedconcerning limits on vibration and deformation whichmay influence, in particular, the depth of constructionand form of bridge deck.

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1.5 Cross Sections for Footbridges

Footbridges are economically constructed in steel for

short and medium spans using all steel or concrete or

timber decks. Figure 5 shows a variety of footbridge

cross sections. Other information is given in the Corus

publication "The Design of Steel Footbridges". The

advantage in using a steel deck plate is that the whole

cross section including parapets can be fabricated at

the works for delivery and erection in complete spans

of minimal weight. Deck type spans with twin

Universal Beams, plate girders or a single box girder

may be suitable. Half-through cross sections are

popular to reduce the length of staircase or ramp

approaches and may use warren girders, Vierendeel

girders, universal beams or plate girders depending

upon span. Through-type cross sections are suitable

where the bridge is to be clad, such as when used to

inter-connect buildings in motorway service areas or

for foot passenger Ro-Ro linkspans. Cross sections

are influenced by the form of parapets which need to

be of solid form and increased height across railway

tracks. It is usually convenient for the supporting

columns of footbridges to be of steel using braced

trestles or single tubular members. It is customary to

use thinner material than in railway or highway bridges,

such as 6mm or 8mm for deck plates. Fillet welds

should be correspondingly lighter to reduce distortion

during fabrication, in particular to prevent water

ponding and unsightliness in parapets.

1.6 Camber

Changes take place in the shape of a girder from the

start of assembly of the prepared plate components in

the fabrication shop until the time it is in service in the

bridge. The pre-fabrication shape of the girder must

anticipate these changes if the required final profile is to

be achieved. Allowances need to be considered for:

(a) changes of shape during fabrication by shrinkage

and distortion due to flame cutting, welding and

assembly sequence;

(b) deflection from the fabricated shape of the

steelwork that takes place at site under its self-

weight, the weight of the concrete slab, and the

superimposed dead loads of surfacing and finishes.

(For composite bridges the sequence of concrete

pours will influence the deflection to some extent);

(c) long term effects such as concrete deck shrinkage

and creep; and

(d) the shape of the specified vertical geometry of the

road or railway carried.

A permanent pre-camber is also often specified for

appearance reasons or to achieve positive drainage

fall, especially on footbridges.

Fabrication precamber as in (a) needs to be allowed forby the steelwork contractor, often based onexperience as well as theoretical calculations. Thedesigner should define camber geometry to cover (b)(c) and (d) and supply the steelwork contractor with acamber or deflection diagram to the designer'sassumed erection and concreting sequence. It is goodpractice to define the girder shape after erection iscomplete and before concreting begins so that it maybe checked and verified at this handover stage. Themain contractor may vary the concreting sequence,with the designer’s approval, which would requirerecalculation of (b). For significantly skewed multipleplate girder beam bridges it may be necessary tospecify a pre-twist at end supports – see 5.3.

Variations in camber can arise in practice for severalreasons:

– fabrication pre-camber to allow for flame cutting andwelding effects is difficult to achieve with precisiondue to the number of imponderables which areinvolved, including the residual stresses which existin the material;

– "shake-out" of residual stresses may occur duringtransport and during erection, leading to changes inshape of components;

– for composite bridges the design assumptions madefor continuous spans relating to cracking of concretein tension areas may be inaccurate, leading todeflections not being as predicted;

– for composite bridges camber changes due toshrinkage and creep may be of a long term natureand are difficult to predict with accuracy; and

– temperature variations within the structure at thetime of checking cambers on site.

For these reasons, tolerance should be permitted inspecifying and approving cambers, although BS 5400Part 6 contains no specific tolerances for camber. Asa guide a tolerance of ± SPAN/1000 is reasonable, butnot less than 6mm or greater than 25mm in any onespan. In some cases it may be convenient to specifya final upward camber of this magnitude so that adownward sagging profile does not result if cambererrors occur up to the tolerance limit. Details should,where possible, permit some degree of camber variationbetween adjacent girders. For continuous spans, wherefabricated lengths are joined at site, as assembly anderection proceed splice details should permit rotationtolerance by use of suitable details (see Figure 12), suchas bolted joints with gaps and clearance holes. Forwelded joints some degree of trimming and fairing ofjoints will often be needed at site.

A typical camber diagram for a simply supported span

is shown in Figure 9.

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Universal Beams can be cambered at the steel mill.The operation is performed by bending the beams coldto approximate a circular curve, but the beam may losesmall amounts of this camber due to release ofstresses put into the beam during the camberingoperation. Alternatively, where only small pre-cambersare specified sufficient to counteract deflection (saySPAN/1000 or 25mm for a 25m long Universal Beam)then cambering by the steelwork contractor or otherspecialist will need to be carried out. For very shortspans where appearance is not critical the designershould consider whether any cambering is justified.

As a guide only, Table 1 gives the minimum cambers inmm which are likely to remain permanent aftercambering for beams cambered at the steel mill.

Greater accuracy can be achieved by cambering in aspecialist rolling process, in which case camberingtolerances are ± 3mm or + 5 – 0mm on mid ordinateheight, with no minimum camber as such.

1.7 Dimensional Limitations

For road transport of fabricated items within the UK therestrictions in Table 2 apply.

Maximum height depends upon vehicle height andshape of the load but generally items up to 3.0m highcan be transported. For rail transport advice must besought, but generally items up to 3.0m high, 2.9mwide and 30.0m long can be carried by arrangement:transport of steelwork by rail is currently extremely rare.

These restrictions and the requirements for erectionhave a considerable effect upon the member sizes andlocation of site splices. The Figures are for guidanceonly, eg 45m plate girders have been fabricated and

transported to site using a transporter with rearsteerable bogies. The steelwork contractor should beallowed flexibility in fabricating in longer lengthsprovided he can obtain the necessary permission totransport such loads. Constraints at site or on theroute to site may restrict delivery dimensions or weightso some flexibility to allow additional site splices maybe required.

Except for very large projects it is usually uneconomicfor large members (such as box girders) to be siteassembled from individual elements. Generally, thelongest possible members should be fabricated toachieve the minimum number of site joints. Erectioncosts are considerably influenced by unit weight socrane sizes should be optimised wherever possible:the crane size is determined by piece weight and theradius for lifting which is dictated by the site layout. Ageneral guide is a maximum unit weight of 50 tonnes,although with modern cranes larger lifts can beachieved. In general site splices and girder lengthsshould be chosen to facilitate erection and theappropriate erection method for the particular site.

When galvanizing is specified for corrosion protectionparticular care must be adopted in the design anddetailing of the steelwork. Means of access anddrainage of the molten zinc and venting of the gasesfrom internal compartments is essential for eachassembly. During the galvanizing process (immersionat about 450oC for approximately five minutes) stressrelief can sometimes cause distortion of light gaugesteelwork. The stresses can arise from welding,cutting or cold working. As far as possible, eachassembly should be symmetrical and the weldingstresses balanced. Plate and box girders will tend to

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TABLE 1. Guide to minimum cambers in Universal Beams

Beam Depth Beam Length (metres)(mm) 26 24 23 21 20 18 17 15 14 12 11 9

914 95 - 75 - 57 - 38 32 25 19 - -834 100 - 83 - 63 - 44 38 32 25 - -762 115 - 89 - 70 - 50 38 32 25 - -686 127 - 101 - 75 - 50 44 38 32 - -610 127 - 115 - 82 - 63 50 38 32 - -533 - 127 - 114 - 82 - 57 44 38 25 20

TABLE 2. Road Transport length/width restrictions

Method of Transport Max Width Max Rigid Length Max Laden Weight Max Axle Weight (metres) (metres) (tonnes) (tonnes)

Free movement 2.9 18.6 38 10.5Police to be informed 5.0 27.4 150 16.5– escort required

Special movement order More than 5.0 More than 27.4(at least 8 weeks notice)

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twist longitudinally due to the release of welding stress:this is rarely an issue in assembly of plate girderbridges but it does need consideration on box girderbridges.

Consideration should also be given to factors (such asdetailing, material grade, locked-in stresses, goodfabrication practice) that will overcome the minimal riskof cracking arising from the galvanizing process (liquidmetal assisted cracking or embrittlement). Advice isavailable from galvanizing companies and from theGalvanizers Association on these issues.

The maximum size and weight of assemblies that canbe galvanized depends upon the size of the bath andcraneage available. The largest baths, in 2001, are upto 21m long by 1.5 to 2.0m wide by 2.7 to 3.5m deep.By ‘double dipping’ (immersing part of the structureand then reversing it for length and depth) larger sizescan be galvanized subject to limitations on craneagecapacity.

1.8 Erection

Prior to the 1970's the erection of small and mediumspan bridges required the steelwork contractor toexercise ingenuity and expertise to devise schemesusing cranes of fairly limited capacity - a 50t derrickcrane mounted on travelling bogies was the largestavailable. Extensive temporary works were commonwith steel trestles in span to support short membersfor splicing, or rolling in, cantilevering or launchingschemes; and, on occasion using floating craft forcranes or girder movements. The steelwork contractorhad a significant amount of construction engineering todo which, for launching and cantilevering schemes,involved analysis and modification of the permanentworks design to suit. These schemes required muchmore activity at site to prepare and carry out, andwould take weeks rather than days.

With the advent of larger, and yet larger, capacitymobile cranes from the 1970’s and the ability to deliverlonger components via the motorway system, erectionof many major bridges could be carried out morerapidly and economically without resort to temporarysupports. The introduction of composite constructionin continuous multiple spans favoured delivery anderection in longer lengths too. These developmentsled to simpler quicker erection for many bridges butsignificant stresses could arise during construction,and elastic instability of steel members duringconstruction became much more of an issue. This,together with modern safety legislation, means thedesigner has to anticipate the erection scheme andcompletion of deck slab construction in his design ofthe bridge. The production of method statements,safety plans and risk assessments is required of thedesigner as well as the steelwork contractor; andtoday the checking of steelwork strength and stabilityat all stages of erection, as well as of any temporary

works, is required to be rigorous and documented. Thecontractor is responsible for the erection scheme aswell as its implementation: the designer has toanticipate it properly.

Erection of short and medium span steel bridges ismost commonly carried out using road mobile ortracked crawler cranes. Road mobile cranes requirefirm ground conditions to get on to site and at the workpositions where they use outriggers to develop fullcapacity. The largest cranes, and with derrickcounterweight installed, have the capacity to lift morethan 50 tonnes at 50m radius, or 100 tonnes at 28mradius: these cranes are suitable for erection of mostsmall and many medium span bridge girders where aunit weight not exceeding 50 tonnes is involved. Inpoor but level ground conditions crawler cranes haveflexibility in being able to travel with the load and,typically, can lift up to 15 tonnes at 50m radiustravelling. Hire costs and crane assembly periods formobile cranes increase substantially for the largercranes, which may be demanded for example where ‘I’girders are lifted in pairs, but it is advantageous iferection can be performed using the minimum numberof lifts and crane positions. Crawler cranes are noteconomic for short hire periods, but are more costeffective than road mobile cranes for long periods.Ground supported temporary works are avoidedwherever possible, and personnel access is assistedby use of mobile access platforms or cherry pickers.For heavy lifts, cranes may be used in tandem, subjectto more severe lifting conditions, which cansignificantly increase the erection costs. Mobile cranesare usually hired by the steelwork contractorspecifically for the erection, so it is most economic if allthe steelwork within a bridge can be erected in onecontinuous operation. Girders are generally lifted ontheir centre of gravity using double slings connected totemporary lifting lugs welded to the top flange. Slinglengths are usually selected commensurate withstability of the girder, whilst being lifted, so as to limitthe crane jib length needed and maximise on the cranecapacity.

For single spans, up to say 60m in length, any spliceswhether bolted or welded would usually be made withthe girders aligned on temporary stillages at groundlevel, before each girder is lifted complete. Forcontinuous spans, then ‘span’ and ‘pier’ girders wouldbe spliced similarly at ground level with erectionproceeding span by span and oversailing each pier toavoid the need for any ground supported temporaryworks. For plate girders erected singly of lengthgreater than about 40m then stability may demand useof bowstrings or other temporary works to reduce theeffective flange length against buckling once the craneis released. These erection methods are economicand of little hindrance to other site operations for theyallow construction to advance rapidly without need forsubstantial ground supported temporary works.

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Erection by mobile cranes is generally the mosteconomic method provided access and space isavailable for such cranes and for delivery of steelwork.Where temporary supports are required standard ‘L’trestling is often used with foundations using timbersleepers or concrete, depending on groundconditions. For long span bridges significanttemporary works will usually be necessary includingthe site pre-assembly of main members from separateflange and web elements. If welded splices need to becarried out in the final position rather than at groundlevel some form of temporary works and weldingshelters with access for inspection and testing arenecessary – that is rather more provision than forbolted connections.

Where erection has to be carried out during a limitedperiod such as in a railway possession or road closurethen lifting of complete bridge spans is preferable,even though this increases the size or number ofcranes. Rail mounted cranes were much used in thepast for rail bridge erection, but have limited capacity;they may be appropriate where no access is availablefor road mobile cranes, however they are of limitedavailability. Where crane access is not feasible oroverly costly, such as for a waterway crossing, thenother erection methods including launching or rolling-inmay be called for: such schemes are less commontoday but, in the hands of competent steelworkcontractors, are powerful ways of overcoming difficultobstacles or logistical constraints.

Launching is most suitable for new build highway orrailway multiple continuous span bridges with constantheight girders or trusses. Where possible thesteelwork is assembled full length off one end of thebridge and launched forward on rollers or slide unitsmounted on tops of the permanent piers. A taperedlaunching nose is used to minimise stressing of thegirders and remove the cantilever deflection of theleading end as it approaches each support.Propulsion may be by pulling with winches or strandjacks, or by incremental jacking, followed by jackingdown on to the permanent bearings. Generally thesteelwork alone is launched, followed by deck slabconcreting, but launching of concreted spans can beadvantageous. An important design check is thestability of the girder web or truss bottom chord abovethe roller or slide units; the camber shape of the girdersto be taken into account; and the interface of thegirders with the temporary works must suit the slidingor rolling action.

Lateral sliding in, rolling in or transporter units are usedfor bridge replacements. The whole structure isnormally constructed and completed on temporarysupports alongside the bridge before it is movedtransversely and then jacked down onto its permanentbearings. Railway bridges have for many years beenrolled in using 76mm (3") diameter steel balls running

on bullhead rails laid flat and surmounted by rollingcarriages beneath the steelwork: with the advent ofpolytetrafluoroethylene (PTFE) and other low frictionmaterials sliding in is increasingly favoured in thatheavier loads can be carried. Propulsion is generallyby strand jacks or incremental jacking, followed byjacking down onto permanent bearings. Acombination of longitudinal launching and lateralsliding may be appropriate.

The large crane is not the universal solution: modernbridges of modest scale can still present very realchallenges to the ingenuity of the steelwork contractorand the skill of the designer. For example, the NewarkDyke rail bridge, which required complete replacementin a three day closure of the East Coast main line at asite of very restricted access, was effected by firstlylaunching in turn the two braced arch girders acrossthe river, traversing them to receive cross girders anddeck concrete before the possession; then the lateralslide in of the complete bridge including tracks duringthe three days which included sliding out the existingstructures for removal by pontoon.

The erection of steel bridges requires detailedconsideration by the designer, for it has to be safe andpracticable, and significant construction engineeringon the part of the steelwork contractor who willdevelop and implement the scheme which is actuallyused. It should be noted that fabrication and erectionmay be carried out by different steelwork contractors,but it is generally desirable that one steelworkcontractor is responsible for both. Site appliedprotective treatment is generally carried out aftererection, and after deck concreting in the case ofcomposite bridges; this work is usually sub-let by thesteelwork contractor.

1.9 Repairs and Upgrading

The enormous increase of highway traffic since the1980’s and the influence of heavier commercialvehicles has led to revised bridge loading standards sothat many modern bridges are now regarded as sub-standard. Major bridges strengthened since the early1990’s include the Avonmouth, Severn, Wye, Forth,Tay, Friarton and Tamar bridges, as well as smallerbridges of all types. Although railway loadings havenot significantly changed, many rail bridges are wellover 100 years old and are becoming life expired. Theplanned introduction of higher train speeds above125mph has led to new design criteria againstexcessive vibration to maintain passenger comfortlevels and ballast stability, and consequently somebridges require additional stiffness. All in all asignificant volume of steel bridgework now involvesrepairs and upgrading.

Steel is particularly suitable for strengthening by addedmaterial or duplicate members using bolted or sitewelding where appropriate. In some cases such as the

15

Page 24: Steel Bridges

M6 Thelwall Viaduct a composite riveted plate girderstructure strengthening was achieved by bolting on ofadditional material and replacement of the existingdeck using stronger concrete and additional shearconnection. In other cases strengthening hasconsisted merely of replacement of bearings and minorworks. Some bridges originally designed noncompositely have been made composite byreplacement of rivets by shear connectors.

Repairs to existing bridges have also been increasinglyrequired due to accidental collision of vehicles withbridge soffits. Damage to steel members has generallybeen found to be fairly local without fracture or risk ofoverall collapse, due to the ductile properties of steel.Depending upon the severity of damage several girderbridges have, since 1999, been repaired by heatstraightening processes, as an alternative to costlygirder replacement: techniques as used in the USAhave been developed by UK steelwork contractors forthis work.

Designers and steelwork contractors undertakingrepair and strengthening works can face challengesand hazards not met in new works; these can includeunderstanding the structure and how it works,technical issues such as those presented by weldingto older materials; and safety hazards with access,confined space working and the presence ofpotentially toxic lead-based paints and cadmiumplating. Even the smallest of such projects should beundertaken only by engineers and organisations withthe relevant knowledge and expertise.

16

Page 25: Steel Bridges

2.1 Introduction

The performance requirements for materials for steelbridges are contained in BS 5400: Part 3 (Design)which requires the minimum UTS to be not less than1.2 x the nominal yield stress, a ductility correspondingto a minimum elongation of 15% and a notchtoughness to avoid brittle fracture. It is important toremember that the ductility is an important property ofsteel which allows it to be fabricated and shaped bynormal workshop practices. The requirements are metusing the appropriate strength grade and impactqualities given in material standards BS 7668, BS EN10025, BS EN 10113, BS EN 10137, BS EN 10155and BS EN 10210 and are specified by the designer.Grade S355 is the most common grade used in UKbridges. Strength grades higher than S460 are notcovered by the design rules. BS 5400: Part 6(Materials and Workmanship) contains detailedrequirements for materials including laminar defectlimits in critical areas, thickness tolerance, andperformance requirements for steels to standardsother than those above. The steel is required to besupplied with a manufacturer’s certificate and to havedetails of ladle analysis provided, so that the steelworkcontractor can check the details for weldingprocedures.

2.2 Brittle Fracture and Notch ToughnessRequirements

The possibility of brittle fracture in steel structures isnot confined to bridges for it has to be consideredwherever stressed elements are used at lowtemperatures, especially in thick material wherestresses are tensile, there are stress raising details andthe loading is applied rapidly. Steel having adequatenotch toughness properties at the design minimumtemperature should be specified so that when stressedin the presence of a stress raising detail the steel willhave a tendency to strain rather than fracture in a brittlemanner. In fact, structural steelwork has a goodhistory of very few brittle fracture failures. There are anumber of contributory factors which help to preventbrittle fracture from occurring, eg the risk of brittlefracture is highest when the first high tensile stresscoincides with a very low temperature – where the firsthigh tensile stress occurs at a higher temperature, thensome element of "proofing" takes place. The first UKlimitation of notch toughness for steel in weldedtension areas of steel bridges was introduced in BS153 in the mid 1960’s.

The standard Impact Test prescribed in EN 10045-1provides a measure of the notch toughness of a steel.

The test specimen is usually of square section 10 x10mm and 60mm long with a notch across the centreof one side: it is supported horizontally at each end inthe test machine and hit on the face behind the notchby a pendulum hammer with a long sharp strikingedge. A pointer moved by the pendulum over a scaleindicates the energy used in breaking the specimen,which is expressed as the Charpy energy in Joules atthe temperature of testing. The resistance of a steelreduces with temperature so the test is usually carriedout at room temperature, and a range of lowtemperatures to suit the specified requirement, say,0oC, -10oC, -15oC, -20oC.

In BS 5400: Part 3: 2000 the required impact quality isderived from an equation relating the maximumpermitted thickness of a steel part to:

- steel grade,

- design minimum temperature (see BS 5400: Part 2),

- construction detail,

- stress level, whether tension or compression, and

- rate of loading.

A key part of the equation is a k-factor which classifiessteel parts for fracture purposes.

For convenience table 3C in BS 5400: Part 3: 2000gives maximum thickness limits for steels for k=1which covers many situations where the designstresses exceed 0.5sy and are tensile. For typical UKminimum design temperatures the limiting thicknessesare shown in Table 3, which takes account of nominalyield strength variations with thickness.

Whilst stress relieving is generally allowed by thestandards, it is normally impractical for bridges due totheir large scale.

2.3 Internal Discontinuities in Rolled SteelProducts

Internal discontinuities are imperfections lying within thethickness of the steel product. These may be planar orlaminar imperfections, or inclusion bands or clusters.Typically such laminar imperfections run parallel to thesurface of a rolled steel product. They can veryoccasionally originate from two main sources in theingot or slab from which the plate or section is rolled:

(a) entrapped non-metallic matter, such as steelmakingslag, refractories or other foreign bodies (NB: Suchmaterial may not necessarily form a ‘lamination’, if thebody fragments into smaller pieces on rolling, they mayappear as discrete or clusters of ‘inclusions’.

17

CHAPTER 2STEEL QUALITIES

Page 26: Steel Bridges

S275

S355

S460

S355(WR)

-J0J2N,MNL,ML

-J0J2K2N,MNL,ML

-QN,MQLNL,MLQLI

J0WJ2WK2W

-0-20-30-50

-0-20-30-30-50

-20-30-40-50-60

0-20-30

43A43C43D43DD43EE

50A50C50D50DD50DD50EE

-55C-55EE-

WR50AWR50C

- -77124149229

-50749333147

50607796100

507493

-70114136224

-45688585134

4255648892

456885

-6098124191

-40597474123

3850587784

405974

-5589114174

-36546868112

3546537177

365468

-07798149

-050595993

3242466070

05059

-07089136

-045545485

2937425564

045554

0 -5 -10 -15 -20 -25 -30

Max thickness in mm for parts in tensionDesign min temp ºC

265

345

440

345

1.01.051.091.121.22

0.840.850.860.890.890.98

0.89

0.91

1.041.051.07

SecondarySteel

MainSteel

MainSteel

Not commonlyused

UnpaintedStructures

StrengthGrade

FormerGrade inBS4360

Yield N/mm2

(16-40mm plate)Material Cost /Yield(S275 or 43A is 1.0)

Main Use

Grade

Quality

T27JºC

TAB

LE 3

.Rel

ativ

e E

ffic

ienc

y an

d L

imit

ing

Thi

ckne

ss o

f S

teel

Gra

des

Page 27: Steel Bridges

The distinction between a lamination and an inclusionis purely arbitrary);

(b) a pipe

or from non-uniform distribution of alloying elements,impurities or phases.

When an ingot solidifies it may contain shrinkagecavities known as pipes. Providing they are notexposed to the atmosphere during reheating forsubsequent rolling, pipe cavities generally weld upduring the rolling process to give sound material. If apipe is exposed, say by trimming off the head of theingot, then its surface will oxidise during subsequentreheating and this will prevent the cavity from weldingup. The resulting ‘lamination’, consisting largely of ironoxide, is then a plane of weakness. Ingots are veryrarely used now and almost all steel produced inEurope today is by the continuous casting process,which eliminates this source of discontinuity.

Where a ‘lamination’ is wholly within the body of a plateor section and is not excessively large it will not impedethe load-bearing capacity for stresses which are whollyparallel to the main axes of the member. However,‘laminations’ can cause problems if they are at or inclose proximity to a welded joint: thus a ‘lamination’ willbe a plane of weakness at a cruciform joint where it issubjected to stresses through the thickness of theplate or section. A related phenomenon is lamellartearing, which may occasionally occur at joints inthicker sections subject to through-the-thicknessstresses under conditions of restraint during welding.This results from a distribution of micro-inclusions,which link up under high through-thickness stressesresulting in a distinctive stepped internal crack. It isgenerally less common with modern steel-makingpractice which produces lower average sulphurcontents than was the case up to the 1970’s. Inpractice it is unlikely that failure will occur in service,but full penetration welded cruciform joints shouldgenerally be avoided because the heat inputs frommulti-run welding and back gouging processes cancause a significant strain within the plate thickness,resulting in a tearing kind of failure. Cruciform weldsshould ideally have fillet welds or partial penetrationwelds reinforced if necessary by fillet welds. If jointdetails are such that lamellar tearing may be aproblem, then steel with improved through-thicknessductility (eg Z-grades to BS EN 10164) should bespecified. Such steels are produced with ultra lowsulphur levels.

To reduce the risk of lamellar tearing, BS 5400: Part 6:1999 specifies limits on laminar imperfections inaccordance with BS 5996: 1993 (superseded by BSEN 10160 in 1999) in critical areas such as girder websclose to welded diaphragms and stiffeners, or to otherareas which may be specified by the designer. BS EN10160: 1999 has a number of acceptance classes:

classes S0 to S3 refer to decreasing sizes andpopulation densities of discontinuities anywhere in themain area or body of a plate; and classes E0 to E4refer to decreasing sizes and numbers of imperfectionsnear the edges of a plate.

For bridge applications two classes of BS EN 10160:1999 are relevant:

Class S1 which defines a maximum permitted areafor an individual lamination of 1000mm2 asdetected with a scan over the whole bodyof a plate.

This is specified for the material in thevicinity of diaphragms, stiffeners orcruciform joints.

Class E1 which refers to a band 50mm, 75mm or100mm wide (depending on the platethickness) from the plate edge and nodiscontinuity over 50mm length, or1000mm2 area, is permitted in this zone. Italso stipulates a maximum number ofsmaller discontinuities (between 25mmand 50mm long) per 1m length.

This is required for plate edges to becorner-welded (NOT butt welded).

Whilst BS EN 10160: 1999 refers to steel plate, it canalso be applied to steel sections by specialarrangement with the steel manufacturer. However,the ultrasonic testing of sections should really bespecified to BS EN 10306: 2002.

2.4 Material Selection

Table 3 gives an indication of the relative efficiency (interms of a base cost/yield ratio) of the various gradesof steel. These are based on typical costs at the timeof publication and on yield values for plates 16mm –40mm thick. They may vary with market conditionsbut can be regarded as reasonably representative.

From Table 3 it can be seen that grade S355 steel ismore economically attractive than grade S275 and soits use by designers and its availability have greatlyincreased: with a 30% strength/mass advantage gradeS355 steel offers cost savings for transportation anderection too. Although grade S460 appears to beslightly more advantageous for NL, ML qualities, suchqualities are usually only required for thicker plates.Grade S460 steel is less readily available than S355steel, particularly for thick plates, and has yet to findwidespread use in the UK. Fabrication costs, and thecost of consumables, increase with use of such higherstrength grades as the requisite welding proceduresbecome increasingly more demanding and theadditional strength is of little benefit where fatigue orslenderness govern the design.

19

Page 28: Steel Bridges

The availability of plate lengths in different widths is a

function of the rolling process: Figure 7 shows the

availability from Corus. The generally available

maximum sizes of plate dimensions should be

adopted as a guide in design; however, it is often

possible to obtain larger sizes, subject to consultation

with the rolling mills, at a cost premium. Butt welding

shorter plates can be more cost effective than

specifying a very large single plate.

It should be noted that high premiums are paid on steel

orders (of one size and one quality for delivery from one

works at one time to one destination) of low quantities,

typically less than 5 tonnes. Therefore designers

should try to standardise on material sizes to avoid

cost penalties. Where small tonnages occur (say for

gussets and packs) it is often more convenient to

obtain material from a stockholder, even though costs

are greater; normally only grades S275 and S355 are

available so that other grades for such small items

should be avoided.

2.5 Weathering Steels

Weathering steels (or weather resistant steels) are high

strength low alloy steels originally developed by

American steel makers to resist corrosion and abrasion

in their own coke and ore wagons. They were first

used for structural purposes in 1961 when the John

Deere building was constructed in North America, with

an exposed steelwork and glass exterior. The first

weathering steel bridge in the United Kingdom was a

footbridge at York University, built in 1967, and many

more have been built since. The advantage of

weathering steel is that the structure requires virtually

no maintenance, apart from occasional inspection.

The whole life costs of the structure are reduced

because all the direct and consequential costs of initial

protective treatment and of periodic repainting are

eliminated. Examples of bridges constructed using

weathering steels are shown in the Corus publication

‘Weathering Steel Bridges’ and advice on details is

given in a ECCS publication ‘The Use of Weathering

Steel in Bridges’.

2.5.1 Performance

These steels owe their weathering resistance to the

formation of a tightly adherent protective oxide film or

‘patina’ which seals the surface against further

corrosion. The patina gradually darkens with time and

assumes a pleasing texture and colour which ranges

from dark brown to almost black, depending on

conditions of exposure. The type of oxide film which

forms on the steel is determined by the alloy content,

the degree of contamination of the atmosphere and

the frequency with which the surface is wet by dew

and rainfall and dried by the wind and sun.

Steels with alloy elements in the proportionsconsidered for weathering steels (up to about 3% total)generally corrode at much the same rates as mild steelif they are permanently immersed or buried. Theirimproved resistance to atmospheric corrosion isrelated to the nature of the rust layers formed. Theatmospheric corrosion rate of newly-exposedweathering steel is initially similar to that of mild steelbut it slows down as the patina builds up; significantlythe degree of slowing down is greatest where theexposure conditions most markedly follow a repeatedcycle of wetting and drying.

The formation of the protective oxide film or ‘patina’ isprogressive with exposure to the atmosphere duringand after construction. For the best appearanceexposed surfaces should be blast cleaned to removemill scale; and paint marks should be avoided toensure uniform patina formation. For most structures,only the very visible exposed faces may need thecleaning treatment as it would serve little purpose tospend money cleaning the inner surface of a plategirder when that surface is not prominently seen - butmill scale must be removed to allow the patina to form.Piece marks and erection marks, normally painted onmembers by the steelwork contractor, must be placedwhere they will not normally be visible. Measuresshould be taken in design and construction to controlrun-off staining, especially during the initial weatheringperiod to avoid staining of adjacent materials, forexample by protection of supporting concrete piersand using steelwork details which encourage waterrun-off clear of the supports.

As with normal good design practice, crevices orledges where water or debris can collect should beavoided, for these conditions may acceleratecorrosion. In detailing care should be taken to keepwater run-off from expansion joints clear of thesteelwork. A useful expedient on deck type bridges isto use a full depth concrete diaphragm encasing theends of girders there, or, alternatively protectivetreatment by painting can be applied to critical areas.For railway bridges measures should be taken toprevent continuously wet ballast from coming intocontact with the steel by use of concrete encasementup to the top of ballast level, together with use ofweather flats. Weathering steel should not be used inhighly corrosive atmospheres, including up to 2kmfrom coastal waters or where the headroom is lessthan 2.5m over waterways. Highways AgencyStandard BD7/01 authorises its use for highwaybridges subject to certain restrictions relating to thequality of the environment, where de-icing salts wouldlead to chloride being deposited on the steel, or wherethe steel would be continuously wet or damp. A longterm corrosion allowance should be made as shown inTable 4.

20

Page 29: Steel Bridges

A former restriction on use of weathering steel overhighways with less than 7.5m headroom has beenremoved following the outcome of research into thecorrosivity under bridges and in-service performance inthe UK and elsewhere. There should therefore be norestriction on the use of weathering steel in bridgesover highways or in rail underbridges subject to therequirements of BD7/01.

BD7/01 requires steel thickness measurements to becarried out over a series of principal inspections toconfirm that the required slowing-down of thecorrosion process is achieved. A useful developmentfor such measurements is the ‘EMA Probe’ which canbe used for measuring the thickness of sound steelbelow any rust layer using the generation of ultrasonicwaves by electro-magnetic induction.

2.5.2 Materials and Weldability

BS EN 10155 1993 (for plates and sections) and BS7668 (for hollow sections) specify the chemicalcomposition and mechanical properties of weatheringsteel grades. An early weathering steel developed bythe US Steel Corporation was called Corten; this is apatented name and the generic name is "weatheringsteel". Strength grade S355 is generally used forbridgework and is available in similar impact qualitiesto non weathering grades. Weathering steels havebeen proved amenable to the normal fabrication andwelding procedures appropriate to structural steels ofthe same strength.

In common with other high yield steels for generalstructural applications, procedures and precautions forwelding have to be adopted to avoid cracking and toobtain adequate joint properties. The total level of alloyadditions is generally higher than for most high yieldsteels giving carbon equivalent values (cev) sometimesranging as high as 0.53, although developments insteel making are generating significant improvementsin weldability. Corus is currently able to offer steels upto 65mm thick with cev typically of 0.44 (0.47 max) andthicker plates of typical 0.5 (0.52 max). The higher thecarbon and alloy content of a steel, the harder andmore brittle the heat affected zone near a weldbecomes and the more susceptible it is to cracking.‘Carbon equivalent’ formulae are widely used asempirical guides relating composition and crackingtendency.

The formula adopted is:

To determine the carbon equivalent of a steel thechemical symbols in the formula are replaced by thepercentages of the respective elements in the steel.

The ‘weldability’ of a material can be considered as thefacility with which it can be welded without theoccurrence of cracking or other defects which render itunfit for the intended application. Long practicalexperience in the welding of weather resistant steels inNorth America and the UK has shown that they arereadily weldable provided that the normal precautionsapplicable to steel of their strength level and carbonequivalent are taken.

For most applications of weathering steels theweathered appearance of the finished structure is oneof its prime characteristics. It is important, therefore,that any welds which are exposed to public viewshould also, over a reasonably short period of time,weather in the same manner as the adjacent parentmaterial. A wide range of electrodes is available withproperties which are compatible with the parent steel.

It is not normally necessary to use electrodes withcompatible weathering properties for small single passwelds, or for internal runs of multi-pass welds. In theformer case, sufficient absorption of alloying elementsfrom the base steel will give the weld metal a corrosionresistance and colouring similar to the base, whilst inthe latter there is no need for the submerged runs tohave such resistance. It is, of course, vital thatelectrodes with adequate mechanical properties arechosen.

2.5.3 Bolting

In a painted steel structure bolted connections areprotected from the ingress of water by the paintcoatings. This is not the case, however, with aweathering steel structure and as the connection isfully exposed it is inevitable that at least some ingressof water to the interior joint surfaces can occur. Thismay be minimised with suitable bolt spacings, edgedistances and sealing but to avoid any electrochemicalreaction it is important that the bolts, nuts and washershave a similar electro chemical potential to that of the

21

TABLE 4 – Weathering Steel Corrosion Allowances to BD7/01 related to 120 year design life

Atmospheric Condition under ISO 9223

Mild

1.0 1.5 0.5Corrosion allowance perexposed surface (mm)

* Including all bridges across highways subject to the use of de-icing salt.

Severe * Interior of Sealed Box Sections

CE = C + Mn + Cr + Mo + V + Ni + Cu6 5 15

Page 30: Steel Bridges

steel structural member. Appropriate specifications forthese would have chemical compositions complyingwith ASTM A325, Type 3, Grade A. Bolt spacingsshould not exceed 14t (maximum 180mm) and edgedistances should not be greater than 8t (maximum130mm). Such connections have performedsuccessfully over many years of service.

The designer should limit the connectors on aweathering steel structure to sizes M20, M24 or M30for availability reasons, and M24 is the preferred size.For a structure using 100 tonnes of weatheringstructural steel, typically only 1–1.5 tonnes ofconnectors are required. This is far too small to justifya special rolling of bar in the steel mill; the productionof nuts and bolts is dependent on the availability ofright sizes of bar kept in stock by the fastenermanufacturers or steel stockholders dealing inweathering steels. It is advantageous in design usingM24 bolts to adopt spacings appropriate to 1"(25.4mm) diameter bolts (ie minimum spacing say65mm) so as to permit procurement of bolts from theUSA if necessary.

2.5.4 Availability of weathering steel

Plates are readily available from the mill.

The availability of sections should be checked at anearly stage in the design. The minimum quantity forsections direct from the mill is typically 50t per size andweight. Smaller quantities may be available dependingon the availability of suitable feedstock and gaps in therolling programme, but this cannot be relied on at the

design stage. Hence, unless rationalisation of thedesign yields such quantities, it is often best to specifyfabricated sections rather than the UB's or UC's. Alimited range of weathering grade angles and channelssuitable for bracing elements is currently held in stockin the UK, and is available in small quantities. Forfurther details on availability contact Corus.

For hollow sections, the minimum quantity is 150t perorder, as a cast has to be made specially.

2.5.5 Suitability

The selection of weathering steel for bridge structuresis a matter of engineering judgement. Some of thefactors to be evaluated are:

– environment: consideration of overall bridge sitecondition. Any geographic or site conditions whichcreate continuous wetting and very highconcentration of chlorides must be avoided.

– economics: comparison of the cost of initial paintingin other steels versus the cost of weathering steels.Recent experience is that costs of weathering steeland painted steel bridges are similar taking intoaccount the added thickness for long term corrosionwith weathering steels. Overall, when commutedmaintenance costs are taken into account, theweathering steel alternative is likely to be cheaper.

– safety: elimination of maintenance painting overtraffic, and for box girders the elimination of muchhazardous confined space working throughout thewhole life of the bridge.

22

Notes:1. Intermediate gauges are available2. Plates may be available in longer lengths by arrangement3. For precise details of plate availability, please contact Corus

FIGURE 7. Available Plate Lengths

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

18.3

18.3

18.3

18.3

18.3

17.0

17.0

17.0

17.0

17.0

9.2

8.5

7.9

7.4

6.9

18.3

18.3

18.3

18.3

18.3

18.3

17.0

17.0

17.0

17.0

17.0

16.2

15.1

14.1

13.2

12.4

18.3

18.3

18.3

18.3

18.3

18.3

17.0

17.0

17.0

16.8

15.4

14.2

13.2

12.3

11.5

10.9

18.3

18.3

18.3

18.3

18.3

17.0

17.0

17.0

16.4

14.9

13.7

12.6

11.7

10.9

10.3

9.7

18.3

18.3

18.3

18.3

17.0

17.0

17.0

16.4

14.8

13.4

12.3

11.4

10.6

9.9

9.2

8.7

18.3

18.3

18.3

18.3

17.0

17.0

16.8

14.9

13.4

12.2

11.2

10.3

9.6

9.0

8.4

7.9

18.3

18.3

18.3

18.3

17.0

17.0

15.4

13.7

12.3

11.2

10.3

9.5

8.8

8.2

7.7

7.2

18.3

18.3

18.3

17.0

17.0

16.2

14.2

12.6

11.4

10.3

9.5

8.7

8.1

7.6

7.1

6.7

10.0

18.3

18.3

17.0

17.0

15.1

13.2

11.7

10.6

9.6

8.8

8.1

7.5

7.0

6.6

6.2

15.0

15.0

14.1

12.3

10.9

9.9

9.0

8.2

7.6

7.0

6.6

6.2

5.8

12.0

12.0

12.0

11.5

10.3

9.2

8.4

7.7

7.1

6.6

6.2

5.8

5.4

> 1250

≤ 1500

> 1500

≤ 1750

> 1750

≤ 2000

> 2000

≤ 2250

> 2250

≤ 2500

> 2500

≤ 2750

> 2750

≤ 3000

> 3000

≤ 3200

> 3200

≤ 3500

> 3500

≤ 3750

> 3750

≤ 4000

PlateGauge(mm)

Plate Width (mm)

Page 31: Steel Bridges

3.1 Introduction

This chapter deals with the various practical aspects ofthe design and selection of the appropriate form of theprincipal structural members in a steel or compositeconstruction bridge.

The simplest form uses rolled section universal beamswith little fabrication other than cutting to length,welding of bearing stiffeners (discussed in more detailbelow for plate girders), and welding of shearconnectors. As a characteristic of the rolling process,most deep rolled beams have web thicknesses greaterthan needed for structural purposes. Consequentlyweb stiffeners are rarely required; however, where theyare needed, web stiffeners should not be fullpenetration butt welded to the webs of rolled sectionsbecause of the risk of lamination.

3.2 Plate Girders

Flange thicknesses limited to 75mm are generallyadvisable to avoid weld procedures needing excessivepreheat; but flanges up to 100mm thickness or morecan be used subject to notch toughness requirementsand availability. Butt welds will be necessary forforming long flanges, allowing the use of smaller platesin regions of reduced moment; however, the number of

plate changes should be the minimum commensuratewith plate availability and economy, because of thehigh cost of butt welds. Figure 8 is a guide as to whereit is economic to change plate thicknesses withingirder lengths. The Figure is typical for flanges or websof medium size plate girders. The relative economymay vary in individual cases depending upon thewelding processes used and general fabricationmethods. It is usual to cut flange and web plates inmultiple widths from economic wide plates suppliedfrom the rolling mill.

Flanges should generally be as wide as possiblecommensurate with outstand limits imposed by BS5400: Part 3, so as to give maximum lateral stability.Table 5 gives outstand limits for a common range ofcompression flange thickness. Tension flanges arelimited to an outstand of twenty times thickness toensure suitably robust construction.

A typical plate girder for a single span bridge is shownin Figure 9; because the span of 32.25m exceeded27.4m an optional splice is shown towards one end.Suitable bolted or welded splices are shown in Figure10. In cases where the steelwork contractor candeliver the girders to site in one length then the sitesplice would be unnecessary.

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CHAPTER 3DESIGN OF MEMBERS

15

275

205

400

355

180

350

460

158

300

20

278

550

243

450

216

400

25

347

650

304

600

269

500

30

265

417

800

345

365

700

440

323

650

35

486

950

426

850

377

750

40

556

1100

487

950

431

850

45

637

1250

556

1100

491

1000

50

255

708

1400

335

618

1200

430

545

1100

55

779

1550

679

1350

654

1300

60

850

1700

741

1450

654

1300

65

939

1850

815

1600

726

1450

70

245

1011

2000

325

878

1750

410

782

1550

75

1083

2150

941

1850

837

1650

y (N/mm2)

Outstand (mm)

Typ Fig

y (N/mm2)

Outstand (mm)

Typ Fig

y (N/mm2)

Outstand (mm)

Typ Fig

S275

S355

S460

Strength Grade Flange Thickness (mm)

o

o

o

TABLE 5 - Compression Outstand limit to Flanges (BS 5400: Part 3: 2000 Clause 9.3.)

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The steelwork contractor usually butt welds the flangesand web plates to full length in the shop beforeassembly of the girder. This means that such shopjoints can be in different positions in the two flangesand do not have to line up with any shop joint in theweb.

The fabrication process for plate girders variesbetween steelwork contractors depending on howextensively their workshops are equipped, from basicfacilities up to substantial automation. The alternativesare described in the following common sequence (seeFigure 11) for plate girder preparation, assembly, andfinishing:

A. Butt weld plates for flanges and webs into longerlengths as required, with the plate at full suppliedwidth, if possible, to minimise butt weld testingrequirements.

B. Mark ends and machine flame cut flanges to widthand length. If the girder is curved in plan then theflange will need to be profiled to the correct cut shape.If it is possible with the equipment available, then drillbolt holes for flange connections and mark stiffenerpositions. If not, then these will need to be handmarked and drilled later.

C. Machine flame cut welds to profile and camber,including any fabrication precamber. If it is possiblewith the equipment available, then drill bolt holes forweb connections and mark any longitudinal stiffenerpositions. If not, then these will need to be handmarked and drilled later.

D. Machine flame cut stiffeners to shape. If it ispossible with the equipment available, then drill boltholes for bracing connections and mark any planbracing cleat positions. If not, then these will need tobe hand marked and drilled later.

E. Assemble the girder

Either

Assemble flanges to web using tack welds, then semi-automatically weld the flanges to the web, rotating thegirder as necessary to minimise distortion and obtainthe necessary welding position. (This is used for veryshort girders, such as diaphragms, and highly shapedgirders.)

or

If ‘T + I’ automatic girder assembly equipment isavailable, then automatically weld web to first flange toform a ‘T’ section before turning the ‘T’ section over toassemble and weld the second flange to the webforming the ‘I’ girder.

or

If automatic girder assembly equipment is available,then place the web horizontally between the twoflanges and automatically weld the two uppermost

web to flange welds before turning the girder to weldthe other side.

F. Tack in stiffeners (marking positions if necessary),then fillet weld in place using manual, semi-automaticor automatic (robotic) welding procedures as availableand appropriate.

G. Check tolerances to specification for the fabricatedgirder.

H. Trial erect steelwork if required. With modernfabrication techniques trial erections should only benecessary if the implications of a minor problem on asingle connection is so critical as to justify the expense.For example, any section of bridge being erected inpossession of a motorway or railway would normallybe trial erected and this should be specified by thedesigner. Trial erections should not normally bespecified for ‘green field’ erection, but with somedesigns it may be necessary to trial erect and carry outthe fitting and welding of some components beforedismantling. Seek advice from steelwork contractors ifnecessary.

I. Blast steelwork and apply protective treatment tospecification, if required.

J. Pre-assemble braced pairs or longer lengths prior todelivery to site, depending on erection method and siteaccess. This can sometimes be combined with trialerection to minimise cost.

When designing plate girders, there is a temptation tominimise the weight of the girder by minimising thethickness of the web and introducing stiffeners atregular intervals along the web. This does not lead tothe most cost-effective design, as the workmanshipcost of profiling, fitting and welding these intermediatestiffeners can outweigh the material costs saved inreducing the web thickness. As a general rule,intermediate stiffeners should only be provided wherepermanent and temporary bracing members arerequired, at say 8m to 10m centres longitudinally for amulti girder design, with the web plate designed tomeet loading requirements unstiffened between thebracing stiffeners - except at bearing or jackingpositions. Ladder beam designs are stiffened at crossgirder locations, generally 2.5m to 4.0m depending onformwork design.

In design and detailing of web stiffeners for plategirders, the welding should be carefully considered forboth bearing stiffeners and intermediate stiffeners.Where stiffeners are closely spaced access for weldingmust be considered: a reasonable rule is that thespace between two elements should be at least equalto their depth. Thus for example, two stiffeners150mm wide should be separated by at least 150mm.

Bearing (and jacking) stiffeners should be attached toboth flanges using fillet welds. These stiffeners shouldalso be ‘fitted’ to the loaded flange (usually the bottom

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flange) to ensure sufficient bearing contact is madebetween the stiffener and the flange. The stiffenersmust be forced against the bearing flange before theyare tack welded to the girder web. The weldingsequence should be worked out so that the tendencyis always to compress the fitted end of the stiffeneragainst the loaded flange, so the fillet welds to theunloaded flange should be left until last. ‘Fitting’stiffeners to both flanges should be avoided as it is veryexpensive and is rarely necessary in bridges. At thebottom flange it may be necessary to use heavy filletwelds or partial penetration welds because BS 5400:Part 10 does not allow direct bearing to be assumed infatigue checks. However, full penetration welds shouldbe avoided because distortion of the flange may besignificant giving problems in fit of bearings. Forstiffeners up to 10mm thick, say in a footbridge, thebearing load can be carried from the flange into thestiffener by the fillet welds; so this avoids fitting suchsmall stiffeners to the flange.

Generally, intermediate web stiffeners do not need toextend to the tension flanges and may be terminatedat a distance up to five times the web thickness fromthe flanges (BS 5400 Part 3). These stiffeners willusually be attached to the compression flange but, asthey do not act in bearing, they need not be ‘fitted’ toensure bearing contact with the flange, only filletwelded.

All stiffeners should be cut to clear web to flangewelds. For painted bridges this should be achievedusing a close fitting snipe, which is then welded overthe web to flange weld. This detail avoids the need toapply the protective treatment system to the backs ofcope holes, which are difficult to access properly, andmakes for easier long term maintenance. Forweathering steel bridges, the issues concerning theprotective treatment system do not apply, and it isbetter to provide cope holes to ensure adequatedrainage along the girder length and to prevent thecollection of water on the bottom flanges at stiffenerlocations. The size of cope hole will vary with stiffenerthickness, ranging from 30mm radius for stiffeners upto 12mm thick, to 50mm radius for stiffeners between35mm and 50mm thick. The size of cope hole mayfurther increase if stiffeners are skewed.

Welding of stiffeners to girder webs and flanges shouldbe with fillet welds to avoid excessive distortion of thegirder section. All stiffeners should be detailed toenable the weld connections to be continuous aroundthe edges of the stiffener and flange without the needfor preparation of the plates involved.

Changes in flange thickness should ideally occur at theouter faces allowing a constant web depth, but oftenthis is not possible at top flange level due to conflictwith slab details; and for girders which are to belaunched this may be a consideration for the bottomflange. Designers should aim to avoid longitudinal webstiffeners although they may be necessary in the

support region of continuous deep girders of variabledepth to satisfy the requirements of BS 5400: Part 3.

All details should be designed with a view to simplicityand minimum number of pieces to be welded orconnected together. For example, where stiffenersneed to be shaped so as to connect to bracings theyshould be cut from a single plate rather than beingformed from separate rectangular pieces. All re-entrant corners should be radiused at 20mm or 1.25tminimum radius. Site connection details shouldprovide angular and length tolerance, bearing in mindthe earlier comments on camber prediction andvariation between adjacent girders. Figure 12 showssome detailing DOs AND DON’Ts and the principlesillustrated should be borne in mind throughout thedesign and detailing process. Some cusp distortion offlanges may occur due to the web to flange welds.However, this normally does not matter from a strengthpoint of view, although care must be taken duringfabrication at the location of the bearings to ensureproper fit-up and for this reason a separate bearingplate welded to the girder is desirable to ensure a flatinterface with the bearing.

Bracings are normally of rolled angle sectionconnected by HSFG bolts via one leg to web stiffeners.In detailing clearance gaps, a 30mm minimum shouldbe allowed at ends of bracings for the stiffener welds,painting access and for camber variation betweengirders. Edge distance to bolts from member endsshould be increased by 5mm above the minimum toallow for reaming or other adjustments which maybecome necessary. For example, for M24 bolts theminimum edge distance at member ends should be aminimum of 1.5 x 26mm hole + 5mm = 44mm, or say45mm. This should be applied to all site boltedconnections of lapped or cover plate type. Weldedbracing frames should be avoided as they can easilycause problems in overcoming intolerance cambervariations between erected girders.

3.3 Box Girders

Box girders tend to be used for long spans where plategirder flange sizes become excessive, or wheretorsion, curvature or aerodynamic considerationsdemand torsional rigidity. Other than in these casesplate girders will be a cheaper solution becauseassembly and welding of plate girders have beenlargely automated – they take up less space and timein the workshop compared with box girders.

Box girders of width exceeding 1.5m are usually madeup of pre-welded stiffened plate panels. Web panelspresent situations very similar to webs in plate girdersand, therefore, the stiffening tends to be similar. Topflanges generally need to be stiffened longitudinally toresist wheel loading on the bridge deck; for steelorthotropic deck plates various forms of stiffener canbe used as shown in Figure 13. For normal highway

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decks the stiffener to deck plate weld is highly fatiguesensitive, and an 80% partial penetration butt weld isessential: fillet welds for closed stiffeners may besufficient for low utilisation decks on Ro-Ro ramps forexample.

The shape of box girders needs to be retained duringfabrication by diaphragms at intervals which act asformers, normally placed on one flange, followed byassembly of the webs and other flange to them. Suchdiaphragms should form bearing diaphragms andintermediate frames as part of the permanent design.A rough guide for maximum spacing is 3 x the boxdepth to ensure that distortion (or lozenging) of the boxshape does not occur during fabrication or underaction of deck traffic loading. If sufficient diaphragmsare used then transverse bending of the longitudinalcorner welds under such loading is negligible andpartial penetration or fillet welds can be used. Fullpenetration welds in such situations are feasible, butare costly and less easy to guarantee. A suitable detailis to use a 6mm fillet weld inside the box (often doneas part of the assembly process) with external weldsbeing partial penetration single vee welds typically size8mm. A further simplification is to oversail the topflange and to use fillet welds both internally andexternally. Details should be devised to permit themaximum amount of fabrication and protectivetreatment prior to assembly of the box girders. Thusinternal diaphragms should allow for longitudinalstiffeners to slot through. Diaphragms shouldincorporate access holes of suitable dimensions forfinal welding and to permit maintenance access withdue regard for safety and emergency situations. Inrecognition of the designer’s responsibilities underCDM regulations both during construction and in-service, reference to health and safety guidelines isnecessary to confirm acceptable sizes of openings,depending upon overall dimensions of the box andlength of escape routes. It is suggested that aminimum size should be 500mm wide and 600mmhigh. Wherever possible openings should be flush withthe bottom flange to permit movement of a stretcher inemergency. Where a step is necessary, for example ata bearing diaphragm, the height of this should beminimised. During construction it should berecognised that further temporary access holes forwelding may be necessary, often through webs; thesenormally need to be reinstated with full penetrationwelded infill plates on completion.

For small boxes, say less than 1.2m x 1.2m, the insideshould be permanently sealed by welding. For flangesless than 1.2m wide, longitudinal stiffeners may beavoided, simplifying the fabrication. Bridgesconstructed of several such compact boxes caneliminate the need for any exposed lateral bracings.

The fit up and fabrication of box girders requiresconsiderable skill and experience. For pre-fabricated

stiffened panels the longitudinal stiffeners are usuallywelded first giving long runs where fully automaticwelding can be used to advantage. The plate must beclamped down to avoid the inevitable weld shrinkageon its upper surface from distorting the plate. Theclamps are retained until the transverse stiffeners havebeen welded in. Some steelwork contractors useautomated equipment to produce orthotropic stiffenedplate panels in bulk economically to high quality.

For corner welds the box may have to be turned overin the shop, after making the first two welds, tocomplete the second pair.

The hollow towers of cable-stayed and suspensionbridges as well as the ribs of some arch bridgesundergo stresses predominantly in compression. Thesite joints can be made by machining the ends of thecomponents and the load is then transferred by directbearing, the bolts or site welds being nominal forlocation and to carry any shear forces. In these casesmachining needs to be done after fabrication toovercome distortion due to welding. Drawings shouldclearly state that the end plate thickness quoted is"after machining".

3.4 Bearings

For many steel bridges it is convenient to useproprietary bearings, which are economic wheresliding together with rotation about both axes is to beaccommodated. The PTFE and/or elastomericmaterials within these bearings are able to dealefficiently with low sliding friction (5% typical) andarticulation. In cases where uplift, excessive rotation orrestraint about one axis only has to beaccommodated, steel fabricated bearings may bemore economic and suitable. In cases of uplift, aseparate device or fabrication which resists upwardforces may be preferable, for example, in resistingvehicle impact on bridge soffits. Long span bridgesgenerally require special bearings, such as pendel linksor enlarged versions of proprietary disc or sphericaltype. All bearings must be capable of replacementduring the design life of the bridge and it is prudent toprovide for jacking at supports. Given the designer'sobligations for maintenance under CDM he shouldprovide sufficient space and support for jacking to becarried out, including jacking stiffeners on thesteelwork if necessary.

Proprietary pot or disc type bearings are suitable formost short and medium span bridges at fixed and freelocations. They occupy minimal space and arenormally bolted to the steelwork through a taperedsteel bearing plate, to take up longitudinal gradient ofthe girder due to camber and overall geometry, suchthat the bearing is truly horizontal in the completedbridge. The bearing plate accommodates anydepartures from flatness of the flange: it is machinedso as to form the taper and to achieve a flat surface for

26

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the bearing. They are usually welded to the girder, thebearing being attached by through bolts or tappedholes into the bearing plate. At fixed bearings theattachment bolts are likely to be significantly loadedsuch that tapped holes are not satisfactory withoutsufficient engagement length. In such cases aconservative assessment of the minimum engagementis, using grade 8.8 bolts and S355 bearing plate:

0.9d x = 1.62d or 39m for M24 bolts.

This means that the bearing plate needs to be, say,45mm thick at least. Through bolts are an alternativesolution, but the effect of holes through the flangeneeds to be taken into account and taper washersmay be required. It is good practice for a template ofthe bearing holes to be supplied to the steelworkcontractor by the bearing manufacturer. Design ofproprietary bearings is normally carried out by themanufacturer to BS 5400: Part 9.

At free bearings the effect of eccentricity of the reactionmust be considered, assuming that the slidinginterface is above the bearing interface as is the casewith proprietary bearings unless they are inverted andprovided with debris skirts. For all proprietary bearingsit is important that full bearing details are supplied tothe steelwork contractor at an early stage beforefabrication starts in the shop: often the choice ofbearing and manufacturer is entrusted to the maincontractor such that bearings are not ordered until latein the construction stage and late information disruptsfabrication.

For small spans, or in footbridges, elastomericbearings may be suitable. The bearings should beretained in place by keep strips top and bottom andmounted on a lower steel plate secured to thesubstructure, or as an alternative by the use of epoxyadhesives. For footbridges supported by steelcolumns bearings can often be dispensed with, thethermal movements and articulation beingaccommodated by column flexure. Footbridgesshould include holding down bolts into thesuperstructure to resist accidental impact or theft.

For fixed bearings, and where rotation occurslongitudinally only, steel fabricated knuckle bearingsare appropriate. They are also suitable for all thebearings in abutment supported short spans up toabout 20m where higher friction can be accepted, asin many railway bridges. They consist simply of a steelblock with radiused upper surface welded on to a steelspreader plate bolted to the substructure. In cases ofuplift a fabricated pin bearing is suitable. Where bothlongitudinal movement and uplift occurs a swing linkpinned bearing can be used. Fabricated bearingswhich can accommodate larger rotations are used inarticulating Ro-Ro linkspans and movable bridges.Design of fabricated bearings would normally becarried out by the bridge designer, possibly incollaboration with the steelwork contractor, or abearing specialist. Roller bearings of hardened steelare suitable where only longitudinal rotation andmovement occurs and are capable of achieving verylow friction values down to 1%, suitable on slenderpiers: to reduce bearing height as special bearings theupper and lower curved surfaces can be radiused fromdifferent axes. Such fabricated bearings are used inthe standard box girder rail bridges for spans greaterthan 20m.

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4.1 Introduction

In general shop connections for short and mediumspan steel bridges are made by welding and siteconnections by bolting. Bolting is generally to bepreferred for the site connections for it can be carriedout more quickly than welding, and with lessinterruption to the flow of erection. Where the job islarge, all the impediments and costs of site welding,with its attendant plant, extra tradespeople, testing,protection of joints, temporary restraints and qualitymanagement can be spread sufficiently to achieverelative economy. For small jobs the Designer musthave very good cause to specify site welding. Wherewelding is used for site connections then often this isfor principal connections in main girders, whilstsecondary members are more conveniently bolted.Typical site connections for a plate girder, either boltedor welded, are shown in Figure 10.

Much of the workmanship in steel bridge erection istaken up in assembling minor components and makingthe connections. It is important that the Designer, inmaking choices and detail design for connections,should visualise how all the components can beassembled and how the tradesmen can carry out thebolting or welding. In some arrangements it is easy todesign components which cannot be fitted, boltswhich cannot be tightened with standard equipment,and position welds which are very arduous for thewelder and costly for the project. Where the Designerhas difficulty in arriving at a practicable solution whichmeets his design criteria, he should consult anexperienced steel bridge contractor.

4.2 Bolted Connections

The use of high strength friction grip (HSFG) bolts ismandatory under BS 5400 for all traffic loadedconnections, to give full rigidity. Untensioned bolts ofmild steel (grade 4.6) and high tensile steel (grade 8.8)in clearance holes may be used in footbridges, inelements which do not receive highway or railwaytraffic loading such as parapets, and for temporaryworks.

Countersunk HSFG bolts should not be used, exceptwhere a flush finish is absolutely essential for functional(not aesthetic) reasons. Similarly, the use of set screwsor set bolts in tapped holes should be avoidedbecause difficulties can arise in achieving accuratealignment and damage free holes and threads:difficulties may also arise in matching the strength ofthe threads in the main material with that of the threadson the fastener, particularly when using a high tensile

steel fastener for its strength in tension with smallthread engagement.

All lapped or cover plate connections should havetolerance in length to allow for site adjustments: thusat member ends, including bracings and bolted girdersplices, the minimum edge distance prescribed in BS5400: Part 3 should be increased by at least 5mm asdescribed in 3.2. End plate type connections areconvenient for cross girders in half-through bridgeswhere they can automatically square up the maingirders during erection; but they are generally morecostly to fabricate as end plates need machining toachieve proper contact. End plates do not givefreedom in interconnection of multiple braced girderswhere camber differences may be critical to fit up.

4.3 High Strength Friction Grip Bolts

Two types of HSFG bolts (known as pre-loaded boltsin Eurocodes) are specified in BS 4395 :

– General Grade bolts to BS 4395: Part 1, whichaccount for most of those used, and have a goodcompromise of high strength and ductility.Mechanical properties are similar to 8.8 grade forsizes up to and including M24; but to continue in thisgrade for larger sizes would mean the use of an alloyrather than a carbon steel and so the strength isreduced for sizes over M24 for economy. The boltsare tightened to give a shank tension of at least thespecified proof load.

– Higher Grade bolts to BS 4395: Part 2 are made of10.9 grade material which means the ductility islower than for the general grade. The limitedextensibility could lead to breakage or strain crackingin the threads from the combined action of axialtension and applied torque during tightening if thebolt were overstressed. BS 5400: Part 3 limits theassumed bolt tension to only 0.85 times the proofload of the bolt and reduces the slip factors by 10%.BS 4604: Part 2 covering the use of higher gradebolts restricts them to joints subjected to shearalone.

4.4 The Friction Grip Joint

The friction grip joint depends for its performance onthe tightening of HSFG bolts to the specified shanktensions so that the adjoining plies are brought intoclose contact and the shear load is transferred byfriction at the interfaces.

Frictional resistance at the interface is highlydependent on the surface conditions. Certain types ofsurface treatment result in very low slip factors and

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CHAPTER 4CONNECTIONS

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should be avoided: for example, the slip factor for etchprimer of 0.25 is too low for the economic transfer ofshear loading by friction. It is also important that thefaying surfaces are free from oil, grease, dust, dirt,loose rust, mill scale, paint (except in the case of aplanned coating) or any other form of contamination,which is detrimental to the development of friction.

In weathering steel construction, mill scale must beremoved from all surfaces, including the fayingsurfaces of friction grip joints. Minor rusting of weatherresistant steel is not detrimental; and indeed that istrue for faying surfaces of other steels.

BS 5400: Part 3 lists the following slip factors forvarious surface conditions:

0.45 for weathered surfaces clear of all mill scaleand loose rust,

0.50 for surfaces blasted with shot or grit and withloose rust,

0.50 for surfaces sprayed with aluminium,

0.40 for surfaces sprayed with zinc,

0.35 for surfaces treated with zinc silicate paint,and

0.25 for surfaces treated with etch primer.

When BS 4395: Part 2 bolts are used these valuesmust be reduced by 10%. This allows for the higherpreload carried by Part 2 bolts and the fact that the slipfactor is not independent of bolt preload.

Slip factors for surface conditions not listed in theCode must be established by slip tests as described inBS 4604 or obtained from other reliable sources at thediscretion of the Supervising Authority.

On assembly of the joint at site, any contamination ofthe faying surfaces by oil or grease is best removed bysuitable chemical means, as flame cleaning usuallyleaves harmful residues. If the joint cannot beassembled as soon as the surfaces have been treated,or as soon as any protective masking has beenremoved, it is sufficient to remove any thin films of rustor other loose material with a wire brush. During thisprocess the surfaces must not be damaged or madesmooth.

4.5 Installation of HSFG Bolts

Bolts may be tightened by three methods. Each aims atachieving at least the minimum specified shank tension.

4.5.1 Torque Control

This requires the use of a manual torque wrench orpower tool fitted with a torque cut-out which must firstbe calibrated on a bolt from the job batch using a boltload meter or similar device for determining bolttension. The procedure in BS 4604 requires the testbolt to be tightened to a load 10% above minimum

shank tension, and the torque setting to obtain thistension is used for tightening the bolts in the structure.It will be appreciated that torque can vary veryconsiderably from bolt to bolt, depending on a numberof factors including the condition of the threads andnut/washer interface, and the amount of lubricantpresent on the threads. Consequently the Standardrequires the wrench to be recalibrated frequently - atleast once per shift, or more often if required by thecontract supervisor, for each change of diameter orbatch and as specified for changes in bolt lengths.

4.5.2 Part Turn Method

This method is for use with general grade bolts only.After assembly of the joint the bolts are given apreliminary tightening with an impact wrench to bringthe surfaces together and to a prescribed beddingtorque. This tightening is intended to impart the bolttension to the point at which a power wrenchcommences solid impacting. A matching mark is thenmade on the nut and shank end. The nut is thenturned relative to the shank until the specified rotationgiven by BS 4604 is achieved and this will give atension in excess of the minimum proof load.

4.5.3 Direct Tension Indication

Direct tension indication is available by using eitherload indicating washers with standard bolts, or tensioncontrol bolts.

Load indicating washers to BS 7644: Part 1(proprietary name ‘Coronet’) are specially hardenedwashers with nibs on one face. The nibs bear againstthe underside of the bolt head leaving a gap betweenthe head and the load indicating face: as the bolt istightened the nibs are compressed and the gapreduced. At a specified average gap measured byfeeler gauge, the induced shank tension is at leastequal to the minimum required by BS 4604. Provisioncan be made for the load indicating washer to be fittedunder the nut, when this is more convenient using anut faced washer to BS 7644: Part 2 and tighteningthe bolt with the head.

Load indicating washers are not suitable for use withweathering steel bridges since the remaining gaps areundesirable.

Tension control bolts, which are manufactured inaccordance with Japanese standard JSS II-09: 1981,are HSFG bolts with a domed head and with thethreaded shank extended to form a splined end. Theassembly comprises a bolt, a nut and a hardened flatwasher under the nut. Tightening is achieved using aspecial shear wrench with a socket which locates onthe nut and spline: when the correct shank tension isreached the spline is sheared off. Tension control boltsoriginated in the UK in the 1950’s as Torshear bolts butthey were not widely used. TC bolts are of highergrade corresponding to BS 4395: Part 2. No torque is

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applied to the bolt shank itself during tightening whichmitigates the concern about higher grade bolts intension. If the bolts are assumed to be general gradebolts it is satisfactory to use them to carry tension.

4.5.4 Sequence of Tightening

Whichever method of bolt tightening is chosen, boltsand nuts should always be tightened in a staggeredpattern and when a bolt group comprises more thanfour bolts, tightening should be from the middle of thejoint outwards and ensuring that all the plies areproperly pulled together in full contact.

If due to any cause a bolt or nut is slackened off afterfinal tightening, the bolt, nut and washer must bediscarded and not reused.

4.6 Inspection of HSFG Bolts

Identification markings on the bolt head and nut shouldbe checked and the use of an appropriate hardenedwasher under the rotated nut or bolt head verified.Where necessary, the use and correct positioning oftaper washers should be checked.

With the part turn method it is necessary to ensure thatthe nut has been rotated sufficiently relative to the bolt.

With the torque control method a random sample oftightened bolts should be selected for checking, whichis most easily carried out by re-calibrating the wrenchto give the required shank tension and then setting thetorque reading 5% above this figure. This should notmove the nuts further.

With the ‘Coronet’ load indicator method it isnecessary to ensure that the average specified gapbetween the load indicating washer and the bolt headhas been reached.

In large bolt groups there is always a danger that onbedding down of the connected plies, some of the firstbolts to be tightened may have lost some of theirpreload. This is not always apparent in the case ofnormal inspection procedures for Part Turn and DirectTension Indication, so it is of paramount importancethat tightening, inspection and any remedial measuresin such situations are carried out in strict compliancewith the contract supervisor’s instructions.

4.7 Welded Connections

In-line web to web and flange to flange connectionsgenerally need to develop the full capacity of theelements and should be full penetration butt welds. If a section change occurs the larger plate should, forfatigue reasons, be tapered in thickness and width at amaximum slope of 1 in 4 down to that of the smaller.Where double-vee preparations are used it isunnecessary to form a taper where the ‘step’ is 2mmor less because this can be incorporated within thewidth of the weld. At changes of flange thickness thetaper should be provided to one face only (see also 3.2

and Figure 10). Weld preparation form and detailsshould normally be determined by the steelworkcontractor, based on approved weld procedures.Unless absolutely vital for fatigue reasons or to removecorrosion traps on weatheriing steel, grinding flush atwelds should not be specified because of restrictionsin meeting safety legislation requirements: in mostcases fatigue performance will be governed by otheradjacent details such as web to flange and stiffenerwelds such that undressed welds are acceptable, -they also have the advantage of being visiblyidentifiable in-service.

At flange site welds a cope hole (see Figure 10) shouldbe provided to the web with minimum radius of 40mm(or 1.25 x web thickness if greater) to allow weldingaccess. Such cope holes should be left open oncompletion and not filled with a welded insert, whichmay cause excessive restraint effects.

Generally the designer should seek to use fillet weldsfor all other welded connections, rather than fullpenetration butt welds which usually are not justifiedexcept in exceptional cases for fatigue reasons. Fullpenetration welds tend to cause distortion, which isparticularly critical for end plate type connections; and,they are relatively costly due to the operations involvedin forming weld preparations, multiple weld runsinvolving back gouging, measures to reduce distortion,and requirements for testing. Where a design requiresfillet welds larger than 15mm x 15mm it is preferable touse partial penetration butt welds, reinforced wherenecessary by fillet welds - for example at the lowerends of bearing stiffeners where the welds may becritical for fatigue.

Generally fillet welds should be at least 5mm or 6mmleg length other than in footbridges or parapetconstruction where smaller welds of maximum 2/3 xplate thickness will be more suitable to minimisedistortion in the thinner sections. On long continuouswelds, such as web to flange fillet welds, up to 10mmfillet may be the optimum and suitable for laying in asingle pass using the submerged arc processes.Advantage should be taken of the penetration, whichis achievable on fillet welds using automatic weldprocesses. Where procedure trials are able to showthat a given penetration is consistently achieved thenthis can be included in the throat dimension assumedfor design. BS 5400 Part 3 allows a root penetrationof 0.2 x throat (but not more than 2mm) to be assumedwithout any need for trials if the submerged arcprocess is used.

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5.1 Introduction

The object of this chapter is to discuss the accuracy ofsteel fabrication and its significance for the designer aswell as for construction.

The dimensions of any artefact vary from those definedby the designer: such variations stem from the natureand behaviour of the material as much as from theprocess of making it. Modern steel fabrication involvesthe manufacture of large and often complex weldedassemblies of components from rolled steel products:high temperature processes are used to make thesteel products to form the components and join themtogether so dimensional variation from the design isinherent and unavoidable. This behaviour hasimplications for the designer, for the steelworkcontractor, and for the bridge builder and each has toanticipate the variations in carrying out their role. Theimportant questions are - which dimensional variationsare significant, what limits must be put on those whichare significant, and how should variations be managedto ensure that the design is implemented to meet itsperformance requirements without delay?

In steel bridge construction dimensional variation issignificant in a number of ways for it involves precisemechanical components, structural steelworkmanufactured remote from the site, and civilengineering works. These interface with each otherand yet their precision varies from the highly accurateto the inaccuracies inherent in placing concrete. It isconvenient to distinguish between:

• Mechanical fit which is vital for example for functionbetween nut and bolt, between bearing and girder,between machined faces of compression members.

• Fit up of fabricated members which is essential forefficient assembly for example of a bolted site splice,yet the dimensional accuracy of the bolt group isimmaterial to the strength.

• Deviation from flatness or straightness which affectsthe strength or function of components for examplein reduced buckling capacity.

• Accuracy of assembly at site where steel spansmust match the substructure positions and girderprofiles must correspond to maintain deck slabthicknesses.

• Interface with substructures where the designer hasto provide adjustment in construction, say by largeH.D. bolt pockets and variable grout layers toaccommodate relatively inaccurate concrete toprecise steel components.

The control of dimension by tolerancing is fundamentalto the mechanical engineering discipline without whichno mechanism could work, no parts would beinterchangeable: no mechanical drawing is completewithout tolerances on all dimensions, limits and fits onmating parts, and flatness tolerances on surfaces. Incontrast civil engineering construction has largelyignored the concept of tolerancing, depending on thecalibration of its metrology to build the productsatisfactorily in situ. Historically steel fabrication founda workable compromise making large manufacturedproducts using workshop techniques which assuredtheir efficient assembly at a remote site – tolerancinghas not been part of that process as a rule, it wasimplicit in much of the work and explicit only formechanical bridge parts. Indeed the level of accuracycommon to a mechanical engineering workshop isgenerally unnecessary for steel bridgework – for whichit has to be justified because it comes at a substantialcost and needs special facilities, including machining.For example the variation of flatness and thickness ofa steel plate from the rolling mill is perfectly satisfactoryfor a girder, but it would be unnacceptable for amachine part. With the widespread use of automatedprocesses from the 1980's for plate preparation, holedrilling, girder assembly and welding, the geometricalaccuracy to which steel fabrication can be made hasmuch improved: this has been driven by theeconomics of practicable manufacture and thereplacement of labour intensive traditional practice.

A demand for formal tolerances in steel bridge workwas first put forward in 1968 when a committee wasset up to revise the then bridge standard BS 153. Theneed was highlighted in the investigations following thecollapse of the Milford Haven and West Gate (Yarra,Australia) box girder bridges in 1970 when theMerrison Committee produced the Interim DesignWorkmanship Rules (IDWR) in February 1973 for boxgirder bridges. These rules contained limits forimperfections including the flatness of plate panels andstraightness of stiffeners which were shown tosignificantly affect the design capacity againstbuckling.

For each new job the steelwork contractor will assessthe design to determine how best to undertake thefabrication and control dimensions to ensure proper fitup and assembly at site. For box girders in particularand large bridges with steel decks, this may wellinclude a project-specific regime of dimensionaltolerances on sub-assemblies such as deck panels:these would be compatible with the tolerances set bythe designer for the finished bridge.

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CHAPTER 5FABRICATION

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The following sections concentrate on the fabricationprocess, the behaviour of steel in fabrication, andthose aspects of accuracy which bear particularly onthe strength of members and the shape ofcomponents, and are of primary concern to thedesigner.

5.2 Fabrication Tolerances

The fabrication tolerances and workmanship levelsdefined explicitly in BS 5400 Part 6 have beendeveloped from a considerable amount of theoreticaland practical investigations. It should be appreciatedthat most of the tolerances as stated apply only tosome elements of the structure and are required fromconsiderations of strength only: by themselves they donot address the purposes of appearance and fit up,and it is for the individual steelwork contractor tochoose his own additional standards for these. Thetolerances in Table 5 of Part 6 are compatible with thedesign rules given in BS 5400 Part 3, but are generallyenhanced by 20% to allow for any uncertainties inmeasurement and interpretation. The non dimensionalpresentation of the strength rules contained in Part 3enable some of the tolerances to be related to the steelgrade.

Plate flatness tolerances apply to the webs of plateand box girders and to the panels of stiffenedcompression flanges and box columns. The tolerancerefers to the flatness at right angles to the platesurface, measured parallel to the longer side and asimple expression is given relating the tolerance to thegauge length of measurement. The tolerances in BS5400 Part 6 are at least double the conservative valueswhich had been specified by the Merrison Rules.Straightness tolerances are applied to longitudinalcompression flange stiffeners in box girders, boxcolumns and orthotropic decks and to all webstiffeners in plate and box girders. Measurements atthe time of introduction of BS 5400: Part 6 on sixbridges showed that 97.5% of stiffeners and platepanels would satisfy the tolerances.

The tolerance on the bow of compression members islinked with the assumptions made in the design andwas developed from an extensive programme ofpractical tests and theoretical investigations based onan assumed sinusoidal bow of length/1000. Thetolerance on the straightness of individual flanges ofrolled and fabricated girders is related to the reductionin the strength of the girder due to lateral torsionalbuckling and the twisting moment at the support thatcan be caused by an overall bow of the girder. Thetolerance on the flatness of rolled webs is related to thestrength of the web acting as a strut under points ofconcentrated loading.

5.3 Checking of Deviations

The potential difficulty associated with working to

specified tolerances is the amount of checkingrequired in the fabrication shop. The specification ofreasonable tolerances should not increase fabricationcosts as a good steelwork contractor should be ableto comply with the values without special proceduresor rectification measures. However, as well as thedirect cost of checking, costs can be incurred whenchecking activities delay the work-piece from enteringthe next phase of production: checking adds time andcost to the overall fabrication process.

Normally all member components are visuallyexamined by the steelwork contractor qualitatively fordeviations in excess of the specified tolerances andany parts quantitatively checked where necessary.This is a safeguard to ensure that obvious excessivedeviations, not caught within the representative 5% or10% sampling specified by BS 5400: Part 6, do notescape. For those member components where onlyrepresentative checks are required, the Engineer isrequired to specify the areas where half of the checksare to be made. These will be in critical areas wherethe steelwork is fully or nearly fully stressed and thestrength is sensitive to imperfection. The remainder ofthe checks are to be made in areas selected at randomby the Engineer. 100% checking is required for theoverall straightness of columns, struts and girders, andthe webs of rolled sections at internal supports.

In making any checks the scanning device is to beplaced so that local surface irregularities do notinfluence the results. The checks are performed oncompletion of fabrication, and then at site oncompletion of each site joint; local checks have to bemade on flatness of plate panels and straightness ofstiffeners. It is clear that the verticality of webs atsupports can only be checked during any trial erectionof the steelwork or after erection at site. For the endsupports of significantly skewed multiple I-beambridges (skew angle exceeding say 30o) significanttwist of the girders is likely to occur when dead loadsare applied especially from concrete deck slabs. Thisarises from the incompatibility between the deflectionof adjacent interconnected girders. In such cases thedesigner should either supply values for the predictedtwist (along with the specified precamber information –see 1.6) for adding to the specified verticality tolerance,or alternatively specify that girders are to be pre-twisted at site so as to counteract the twist due todead loads. For further advice refer to Steel BridgeGroup Guidance Note 7.03.

Where the tolerances specified in BS 5400 Part 6 areexceeded remedial actions may be taken to removethe distortion such as by heat or other straighteningmeasures. In some cases where this is difficult orimpracticable the information is submitted to thedesigner for consideration. Often further analyticalchecks which consider the actual deviation may showthat this is acceptable. In the case of plate panels,

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stiffeners, cross girders, cross frames and cantilevers,and webs of rolled girders, if 10% or more of thechecks exceed the appropriate tolerances then theEngineer may call for additional checks.

5.4 Causes of Fabrication Distortion

Distortion is a general term used to describe thevarious movements and shrinkages that take placewhen heat is applied in cutting or welding processes.All welding causes a certain amount of shrinkage andin some situations will also cause deformation from theoriginal shape. Longitudinal and transverse shrinkagein many circumstances may only be a minor problembut angular distortion, bowing and twisting can presentconsiderable difficulties if the fabrication is not inexperienced hands.

A full awareness of distortion is vital to all concernedwith welding including the designer, detailer, shopforeman and the welders, as each in their actions cancause difficulties through lack of understanding andcare. Weld sizes should be kept to the minimumrequired for the design in order to reduce distortioneffects, eg in many cases partial penetration welds canbe used in preference to full penetration welds. Figure12 showing detailing DOs and DON’Ts illustrates howthe effects of distortion may be avoided or reduced.Some distortion effects can be corrected, but it ismuch more satisfactory to plan to avoid distortion andthereby avoid the difficulties and costs of straighteningto achieve final acceptability of the job. Consider a filletweld (see Figure 14) made on a ‘T’ section. On coolingthe weld metal will induce a longitudinal contraction, atransverse contraction and an angular distortion of theup-standing leg. A similar section with double weldruns will induce greater longitudinal and transversecontraction and the combined forces will produce anangular distortion or bowing of the table. Thelongitudinal shrinkage is likely to be about 1mm per 3mof weld and transverse contraction about 1mmprovided the leg length of the weld does not exceedthree quarters of the plate thickness.

The contractions produced by a single V butt weld (seeFigure 14) induce longitudinal and transverse shrinkageproducing angular distortion and possibly somebowing. The transverse contraction will be between1.5mm and 3mm and the longitudinal contractionabout 1mm in 3 metres. Angular distortion occursafter the first run of weld cools, contracts and drawsthe plates together. The second run has the sameshrinkage effect but its contraction is restricted by thesolidified first run, which acts as a fulcrum for angulardistortion. Subsequent runs increase the effect. Theangular distortion is a direct function of the number offiller runs and not the plate thickness, although ofcourse the two are related.

The use of a double V preparation to balance thevolume of weld about the centre of gravity of the

section will significantly reduce any angular distortion.To allow for the effect of back gouging, asymmetricpreparations are often used to advantage, but it mustbe remembered that longitudinal and transversecontractions will still be present. The contractions in astructure can be assessed, but a number of factors willaffect the result. The fit-up is most important as anyexcess gap will affect the weld volume and increaseshrinkage. The largest size of electrodes should beused and where possible semi-automatic andautomatic processes should be employed to reducethe total heat input and the shrinkage to a minimum.

In certain circumstances residual rolling stresses in theparent metal can have considerable effect and maycause otherwise similar sections to react differently.The extent of final distortion will be a combination ofthe inherent stresses and those introduced by welding.

5.5 Methods of Control of Distortion

All members which are welded will shrink in their lengthso each member should either be fabricated over-length and cut to length after welding, or an estimateof shrinkage should be added to anticipate the effectduring the fabrication of the member. For the controlof angular distortion and bowing there are twomethods of control which can be considered if thedistortion is likely to be of significance (see Figure 14):

Pre-setting. The section is bent in the oppositedirection to that in which it is expected to distort andwelding is then carried out under restraint. When cool,and the clamps are removed, the section should springstraight. Trials and experience can determine theextent of pre-bend for any particular member.

Clamping. The units are held straight by clamps whilstthe welding is carried out which reduces the distortionto tolerable amounts.

5.6 Effect of Design on Distortion

A good design will use the minimum amount of weldmetal consistent with the required strength. Atchanges in direction of flanges bending should beconsidered to avoid introduction of butt welds.Welded sections should, if possible, be designed withtheir welds balanced about the neutral axes of thesections; so welded asymmetric cross sections shouldbe avoided. In these ways, little distortion will occurand only allowances for the overall contraction need bemade (see Figure 14).

Over-welding is a serious risk and all details should beconsidered, even small cleats. The optimum size ofweld should be specified on the drawing. The amountof distortion is directly proportional to the amount ofweld metal and it is bad practice to specify ‘weld on’or ‘weld all round’ without specifying the weld size, asthe minimum necessary size – 6mm leg fillet-weld ispractical for bridgework.

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5.7 Distortion Effects and Control on Various

Forms of Construction

Fabrication of girders. Butt joints in flanges or webs of

girders should be completed before the girders are

assembled wherever practical. Run-on/run-off pieces

should be clamped at each end of these joints: they

should be of the same thickness as the plate material

and have the same weld preparation. Extension

pieces are removed after the completion of the welding

and the flange edges carefully dressed by grinding.

The direction of weld runs should be alternated to

avoid the tendency for the joint to distort in plan. It

may be necessary to balance the welding of the butt

joints by making a number of runs in one side of the V

preparation and then turning the flange over to make

runs in the second side and so on. Back chipping or

gouging must be carried out before commencing

welding on the second side. The use of suitable

rotating fixtures should be considered by the steelwork

contractor to enable long flanges to be turned over

without risk of cracking the weld when snatch lifted by

cranes.

On completion of all web and flange butt joints the

girder is assembled and, if automatic welding is to be

used for the web to flange welds, the stiffeners are

added after these welds are complete. If, to suit the

available equipment the web is horizontal, it may be

advisable to assemble flanges slightly out of square to

allow for the greater effect of the welding of the first

side fillet welds (see Figure 14). Where manual welding

is used on girders it is normal practice to fit the

stiffeners before the welding and these are usually

sufficient to maintain the squareness of the flanges.

Distortion can come to the steelwork contractor’s aid

where bearing stiffeners need to be fitted. Local flange

heating can be used to bow the flanges locally allowing

insertion of the stiffener, the subsequent cooling

causing the flanges to come into tight contact with the

stiffener end. Such controlled heat input operations

are part of the fabrication art and are generally not

detrimental.

Box columns. Light sections, such as boxed channels

or joists with welds balanced about the neutral axes,

should give no difficulties provided suitable allowances

have been made for overall shrinkage. Heavier boxes

will have diaphragms and it is important that these are

square before assembly: also the side plates must be

free from twist. Such sections lend themselves to

automatic welding. Provided that no stresses are

introduced into the sections due to out of straight

material or unsquared diaphragms, no difficulty should

arise from welding but allowances must be made for

overall shrinkage.

Site Welded Girder Splices. It is usual to weld theflange joints before the web, for the flange beingthicker and requiring a greater number of runs of weld,will shrink more than the thinner web joint. Otherwisethe web may buckle as a result of flange shrinkage. Inthe fabrication of such joints it is necessary toanticipate this procedure by fabricating the web jointwith a root gap larger than that specified by the weldprocedure by an amount equal to the expected weldshrinkage of the flange joint (see Figure 10). In heavygirder joints a variation of the procedure should beadopted whereby the flange joints are completed inbalance to about two thirds of their weld volumes; atthis stage the web joints may be welded and finally theflange welds. This method helps to minimise tensilestresses remaining in the web.

5.8 Correction of Distortion

Sections can be straightened with the aid of hydraulicpresses or special bar bending or straighteningmachines. Some sections are too large for this type ofstraightening and it is necessary to adopt techniquesinvolving the application of further heat: heat has to beapplied to the side opposite to that carrying the weldswhich caused the distortion. The techniques arebased on the fact that if heat is applied locally to amember, the heated area will tend to expand and beconstricted by the surrounding area of cold metalwhich is stronger than the heated area: upon cooling,the metal in the heated area will become compressedplastically to a lesser volume than before heating thuscausing the member to curve in the required direction.

The application of heat has to be carefully controlled toprescribed temperatures and considerable experienceis required before it can be successfully applied –overheating will cause metallurgical problems. Themethod of heat application can also be used tostraighten long strips of plate that have beenoxyacetylene flame cut along one edge, where releaseof the internal residual rolling stresses and the effect ofthe heat of the cut have caused curving during cutting.The heat should be applied in triangular areas on theedge opposite to the flame cut edge (see Figure 14).Out-of-tolerance distortions in plate panels can bereduced by suitable local heating of the panel (seeFigure 14), sometimes combined with jacking toprovide restraints.

5.9 Trial Erection

Trial erection of bridge steelwork at the fabricationworks is a traditional way of ensuring that fit-up andgeometry can be achieved at site so reducing the riskof delays in erection or damage to protectivetreatment. With the much improved accuracyachieved by automated fabrication procedures theneed for trial erection has been reduced in recentyears. Today trial erection of most bridges isunnecessary and indeed complete trial erection of a

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large structure may be impracticable. However, wheredelays in assembly at site are totally unacceptable orremedial measures would be extremely difficult trialerection is of considerable benefit, for example, for arailway or highway bridge to be erected during alimited possession. Partial trial erection involvingcomplex or close fitting connections, such as inskewed integral crossheads or the shear plateconnections of the standard rail underbridge boxgirders, is also justifiable. This may also enable thefabricator to position or adjust and weld somecomponents of the connection, such as end platesduring trial erection, as a practical way of achieving fitup. BS 5400: Part 6 Clause 5.9 requires trial erectionwhere this is specified by the Engineer - generally thedesigner. The extent of trial erection should beconsidered, bearing in mind that simultaneous trialerection of a large bridge off site may be totallyimpracticable: full trial erection is on the critical path so,apart from the substantial costs, it adds considerabletime to the fabrication programme. If a multiple spanbridge is to be trial erected, partial or staged trialerections are appropriate and will depend upon theamount of space which the steelwork contractor hasavailable. Depending upon the degree of repetitionand the fabrication methodology, trial erection of aparticular span only may be sufficient. Often the needsfor full trial erection can be reduced or dispensed withonce the early stages of erection have successfullybeen proved.

The designer, or specifier, in considering the need fortrial erection needs to evaluate the risks andconsequences of delay at site – who would be most atrisk, is it worth the client in effect paying a largepremium for assurance for the risk involved? For hispart, the experienced steelwork contractor will plan hisfabrication, fit-up and checking procedures tominimise the risk to himself and the project.

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6.1 Introduction

Today welding is the primary joining process in steelbridge construction: virtually all shop joints, andfrequently site joints, are welded. The output of thewelding process is dependent on many factors andvariables, so correct application and control areessential to assure weld integrity and achieveeconomic production levels.

Welding technology uses its own special terminologyand its application to steel bridge construction issubject to an extensive range of British, European andInternational Standards, which are in process ofchange and development. The design of a weldthrough to its final acceptance is a process, whichinvolves dialogue between designers, non-specialistengineers, and those directly executing weldingoperations. The aim of this chapter is to give someinsight into this process, the terminology, theapplicable standards, the common ways in whichwelds are made and the techniques for quality control.

The profusion of standards is confusing, particularly inthe progressive change to European Standards, so aflowchart illustrates the respective functions andrelationships between the standards in regular use.

6.2 Principal welding standards

Previous chapters have discussed the design of steelbridges and the development of the specification andperformance requirements. The current UKspecification for workmanship, BS 5400: Part 6:1999,states that metal-arc welding should comply with therequirements of BS 5135, unless otherwise specifiedby the Engineer. BS 5135 has been superseded by BSEN 1011: Welding - Recommendations for welding ofmetallic materials - Part 1: General guidance for arcwelding, and Part 2: Arc welding of ferritic steels. Theguidance and information in this specification isupdated and substantially similar to the previousstandard and to all intents and purposes can bedirectly applied; there are some exceptions andattention is drawn to these in this chapter. Theassumption made in each standard is that execution ofits provisions is entrusted to appropriately qualified,trained and experienced personnel.

The formulation and approval testing of weldingprocedures is necessary to establish methods and toanticipate and overcome any difficulties likely to beencountered in the fabrication process. Approvaltesting of welding procedures and welders is carriedout in accordance with BS EN 288-3 and BS EN 287-1 respectively. Inspection and testing requirementsand acceptance criteria, developed specifically forbridgework, are detailed in BS 5400: Part 6.

Additions and amendments to standards are ofteninvoked through contract specifications. Highway andrail bridges in the UK invariably have additionalrequirements detailed in appendices andsupplementary specifications.

6.3 Types of joint

Structural welded joints are described as either buttwelds or fillet welds. Butt welds for bridgework arenormally in-line plate joints in webs and flanges, eitherto accommodate a change of thickness or to make upavailable material to girder length. The positions ofthese joints are allowed for in the design, althoughmaterial availability constraints or the erection schememay require the Engineer and contractor to agree onfinal positions. Tee butt weld joints are only requiredwhere there are substantial loading or fatigueconsiderations in bearing stiffening or transverseconnections.

Butt welds for bridges are full or partial penetrationjoints made between bevelled or chamfered materials.Full penetration joints are designed to transmit the fullstrength of the section. It is possible to weld thesejoints from one side but material thicknesses in bridgesare such that they are usually welded from both sidesto balance distortion effects, with an in-processbackgouging and/or backgrinding operation to ensurethe integrity of the root area. Single sided butt weldswith backing strips, ceramic or permanent steel, arecommon for joining steel deck plates and where thereare closed box sections or stiffeners, which can onlybe accessed for welding from one side. Fatigueconsiderations limit the use of partial penetrationwelds.

Every effort should be made to design out butt weldingas far as possible due to the costs associated withpreparation, welding time, higher welder skill levels andmore stringent and time consuming testingrequirements. In addition, butt welds tend to havelarger volumes of deposited weld metal; this increasesweld shrinkage effects and results in higher residualstress levels in the joint. Careful sequencing of weldingoperations is essential to balance shrinkage and todistribute residual stress thus minimizing distortion.

Most other joints on bridgework use fillet welds in a teeconfiguration. They include the web to flange joints andstiffener, bearing and bracing connections.

Weld sizes must be detailed on the project designdrawings together with any fatigue classification designrequirements. British practice uses leg length to definefillet weld size, but this is not universal as throatthickness is used in some foreign practice.

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6.4 Processes

The four main processes in regular use in UK bridgemanufacturing are described below; variations of theseprocesses have been developed to suit individualmanufacturer’s practices and facilities. Otherprocesses also have a place for specific applications.

The important factors for the steelwork contractor toconsider when selecting a welding process are theability to fulfil the design requirement and, from aproductivity point of view, the deposition rate that canbe achieved and the duty cycle or efficiency of theprocess. The efficiency is a ratio of actual welding orarcing time to the overall time a welder or operator isengaged in performing the welding task. The overalltime includes setting up equipment, cleaning andchecking of the completed weld.

Process numbers are defined in BS EN ISO 4063.

Submerged arc welding with electrode (SAW) -process 121

This is probably the most widely used process forwelding bridge web to flange fillet welds and in-linebutts in thick plate to make up flange and web lengths.The process feeds a continuous wire into a contact tipwhere it makes electrical contact with the power fromthe rectifier. The wire feeds into the weld area, where itarcs and forms a molten pool. The weld pool issubmerged by flux fed from a hopper. The flux,immediately covering the molten weld pool, meltsforming a slag protecting the weld during solidification;surplus flux is re-cycled. As the weld cools the slagfreezes and peels away leaving high quality, goodprofile welds.

Solid wires of diameters from 1.6 to 4.0 mm arecommonly used with granular fluxes. Mechanicalproperties of the joint and the chemistry of the weld areinfluenced by careful selection of the wire/fluxcombination.

The process is inherently safer than other processes asthe arc is completely covered during welding, hencethe term submerged; this also means that personalprotection requirements are limited. High depositionrates are a feature of the process because it is normallymechanized on gantries, tractors or other purposebuilt equipment. This maintains control of parametersand provides guidance for accurate placement ofwelds.

The ability to exercise precise procedure controlenables contractors to take advantage of the deeppenetration characteristic of the process. The crosssectional profile of fillet welds deposited is such that toachieve a design throat thickness a smaller leg lengthweld is required. BS 5400: Part 3 provides appropriatedesign guidance.

Process variants include twin and tandem wire feedsand metal powder additions. These all increasedeposition potential but the equipment requirements

become more complex. The process is better suited toshop production but site use can be justified whereapplications include long runs and/or thick plate jointsand the area can be weather-proofed.

Metal-active gas welding (MAG) - process 135

This is the most widely used manually controlledprocess for shop fabrication work; it is sometimesknown as semi-automatic or CO2 welding. Continuoussolid wire electrode is passed through a wire feed unitto the gun usually held and manipulated by theoperator. Power is supplied from a rectifier/invertersource along interconnecting cables to the wire feedunit and gun cable; electrical connection to the wire ismade in a contact tip at the end of the gun. The arc isprotected by a shielding gas, which is directed to theweld area by a shroud or nozzle surrounding thecontact tip. Shielding gases are normally a mixture ofargon, carbon dioxide and possibly oxygen or helium.

Good deposition rates and duty cycles can beexpected with the process, which can also bemechanised with simple motorised carriages. The gasshield is susceptible to being blown away by draughts,which can cause porosity and possible detrimentalmetallurgical changes in the weld metal. The processis therefore better suited to shop manufacture,although it is used on site where effective shelters canbe provided. It is also more efficient in the flat andhorizontal positions; welds in other positions aredeposited with lower voltage and amperageparameters and are more prone to fusion defects.

Flux-cored wire metal–arc welding with activegas shield (FCAW) - process 136

This process utilizes the same equipment as MAGwelding, however the consumable wire electrode is inthe form of a small diameter tube filled with a flux. Theadvantage of using these wires is that higherdeposition rates can be used particularly when welding"in position", ie vertical or overhead. The presence ofthin slag assists in overcoming gravity and enableswelds to be deposited in position with relatively highcurrent and voltage thus reducing the possibility offusion type defects. Flux additions also influence theweld chemistry and thus enhance the mechanicalproperties of the joint.

Metal-arc welding with covered electrode (MMAor stick welding) – process 111

This process remains the most versatile of all weldingprocesses however its use in the modern workshop islimited. Alternating current transformers or DC rectifierssupply electrical power along a cable to an electrodeholder or tongs. Electrical earthing is required tocomplete the circuit. A flux coated wire electrode (or"stick") is inserted in the holder and a welding arc isestablished at the tip of the electrode when it is struckagainst the work piece. The electrode melts at the tipinto a molten pool, which fuses with the parent materialforming the weld. The flux also melts forming a

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protective slag and generating a gas shield to preventcontamination of the weld pool as it solidifies. Fluxadditions and the electrode core are used to influencethe chemistry and the mechanical properties of theweld.

Hydrogen controlled basic coated electrodes aregenerally used for steel bridge welding. It is essential tostore and handle these electrodes in accordance withthe consumable manufacturer’s recommendations inorder to preserve their low hydrogen characteristics.This is achieved either by using drying ovens andheated quivers to store and handle the product, or topurchase electrodes in sealed packages specificallydesigned to maintain low hydrogen levels.

The disadvantages of the process are the relatively lowdeposition rate and the high levels of waste associatedwith the unusable end stubs of electrodes.Nevertheless it remains the main process for sitewelding and for difficult access areas where bulkyequipment is unsuitable.

Shear stud connector welding

Composite bridges require the welding of shear studconnectors to the top flange of plate or box girders andother locations where steel to concrete compositeaction is required. The method of welding is known asthe drawn-arc process and specialist equipment isrequired in the form of a heavy-duty rectifier and apurpose made gun. Studs are loaded into the gun andon making electrical contact with the work, the tippedend arcs and melts. The arc is timed to establish amolten state between the end of the stud and theparent material. At the appropriate moment the gunplunges the stud into the weld pool. A ceramic ferrulesurrounds the stud to retain the molten metal in placeand to allow gas generated by the process to escape.The ferrules are chipped off when the weld solidifies.Satisfactory welds have a clean "upset" completelysurrounding the stud.

The equipment for stud welding is not particularlyportable, so if only a few studs are to be installed orreplaced at site, it is more economic to use a manualprocess.

6.5 Preparation of welding procedurespecifications

The drawings detail the structural form, materialselection and indicate welded joint connections. Thesteelwork contractor proposes methods of weldingeach joint configuration to achieve the performancerequired. Strength, notch toughness, ductility andfatigue are the significant metallurgical and mechanicalproperties to consider. The type of joint, the weldingposition and productivity and resource demandsinfluence the selection of a suitable welding process.

The proposed method is presented on a weldingprocedure specification (WPS), which details theinformation necessary to instruct and guide welders toassure repeatable performance for each joint

configuration. An example format for a WPS is shownin BS EN 288-2.

Welding procedure specifications for shop and sitewelds are submitted to the Engineer for his approvalbefore commencement of fabrication. It is necessary tosupport the submissions with evidence of satisfactoryprocedure trials in the form of a welding procedureapproval record (WPAR). The guidance clauses of BS5400: Part 6 confirm that consideration should begiven to the results obtained from procedure trialsundertaken on previous contracts to avoid thenecessity of repeating trials for every project. The majorUK steelwork contractors have pre-approved weldingprocedures capable of producing satisfactory welds inmost joint configurations likely to be encountered inconventional bridgeworks.

For circumstances where previous trial data is notrelevant it is necessary to conduct a welding proceduretest or trial to establish and to confirm suitability of theproposed WPS. Note the conflict of terminology, BS5400 uses the term trial whereas BS EN standards usetest. The next sections use the terms within thecontext of the standard being discussed.

6.6 Procedure trials

When it becomes necessary to conduct a weldingprocedure trial, BS 5400: Part 6 refers to weldingprocedures complying with the requirements of BS5135, BS EN 288-1, -2 and -3 and BS 4570, asappropriate. BS EN 1011 substitutes for BS 5135 withthe various parts of BS EN 288 supporting detailedwelding procedure testing. BS 4570 refers to thewelding of steel castings.

Procedure trials on welded stud shear connectors areundertaken, when specified by the Engineer. BS 5400:Part 6 describes the metallographic and destructivetest requirements to prove the integrity of stud welds.

There is an additional general requirement concerningprocedure trials that where paint primers are to beapplied to the work prior to fabrication, they areapplied to the sample material used for the trials.

BS EN 288-3 defines the conditions for the executionof welding procedure tests and the limits of validitywithin the ranges of approval stated in thespecification. The test commences with thepreparation of a preliminary welding procedurespecification (pWPS). For each joint configuration,either butt or fillet weld, consideration is given to thematerial thickness and anticipated fit up toleranceslikely to be achieved in practice. Process selection isdetermined by the method of assembly, the weldingposition and whether mechanization is a viableproposition to improve productivity.

Joint preparation dimensions are dependent upon thechoice of process, any access restrictions and thematerial thickness. Consumables are selected formaterial grade compatibility and to achieve themechanical properties specified, primarily in terms of

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strength and toughness. For S355 and higher gradesof steel, hydrogen controlled products are used.

The risk of hydrogen cracking, lamellar tearing,solidification cracking or any other potential problem isassessed not only for the purpose of conducting thetrial but also for the intended application of the weldingprocedure on the project. Appropriate measures suchas the introduction of preheat or post heat are includedin the pWPS.

Distortion control is maintained by correct sequencingof welding. Backgouging and/or backgrinding toachieve root weld integrity are introduced asnecessary.

Welding voltage, current and speed ranges are notedto provide a guide to the optimum welding conditions.

The ranges of approval for material groups, thicknessand type of joint within the specification are carefullyconsidered to maximize the application of the pWPS.Test plates are prepared of sufficient size to extract themechanical test specimens including any additionaltests specified or necessary to enhance theapplicability of the procedure. The plates and thepWPS are presented to the welder; the test isconducted in the presence of the examiner and arecord maintained of the welding parameters and anymodifications to the procedure needed.

Completed tests are submitted to the examiner forvisual examination and non-destructive testing inaccordance with BS EN 288-3: Table 1. Non-destructive testing techniques are normally ultrasonictesting for volumetric examination and magneticparticle inspection (MPI) for surface examination andcrack detection. Satisfactory test plates are thensubmitted for destructive testing, again in accordancewith Table 1 of BS EN 288-3.

There is a series of further standards detailing thepreparation, machining and testing of all types ofdestructive test specimen. Normally specialistlaboratories arrange for the preparation of testspecimens and undertake the actual mechanicaltesting and reporting.

The completed test results are compiled into a WPARendorsed by the examiner. Project specific weldingprocedures based upon the ranges of approval arethen prepared for submission to the Engineer.

6.7 Avoidance of hydrogen cracking

Cracking can lead to brittle failure of the joint withpotentially catastrophic results. Hydrogen (or cold)cracking can occur in the region of the parent metaladjacent to the fusion boundary of the weld, known asthe heat affected zone (HAZ). Weld metal failure canalso be triggered under certain conditions. Themechanisms that cause failure are complex anddescribed in detail in specialist texts.

Recommended methods for avoiding hydrogencracking are described in BS EN 1011: Part 2, AnnexC. These methods determine a level of preheating tomodify cooling rates and therefore to reduce the risk of

forming crack-susceptible microstructures in the HAZ.Preheating also lessens thermal shock andencourages the evolution of hydrogen from the weld,particularly if maintained as a post heat on completionof the joint.

One of the parameters required to calculate preheat isheat input. A notable change in the standard is todiscontinue use of the term arc energy in favour of heatinput to describe the energy introduced into the weldper unit run length. The calculation is based upon thewelding voltage, current and travel speed and includesa thermal efficiency factor; the formula is detailed inPart 1 of the Standard.

High restraint and increased carbon equivalent valuesassociated with thicker plates and higher steel gradesmay demand more stringent procedures. Low heatinputs associated with small welds may alsonecessitate preheating. Experienced steelworkcontractors can accommodate this extra operationand allow for it accordingly.

BS EN 1011 confirms that the most effectiveassurance of avoiding hydrogen cracking is to reducethe hydrogen input to the weld metal from the weldingconsumables. Processes with inherently low hydrogenpotential are effective as part of the strategy, as well asthe adoption of strict storage and handling proceduresof hydrogen controlled electrodes. Consumablesuppliers’ data and recommendations provideguidance to ensure the lowest possible hydrogenlevels are achieved for the type of product selected inthe procedure.

Further informative Annexes in BS EN 1011-2 describethe influence of welding conditions on HAZ toughnessand hardness and give useful advice on avoidingsolidification cracking and lamellar tearing.

6.8 Welder approval

There are no specific clauses in BS 5400: Part 6concerning approval of welders. BS 5135 madespecific reference to approval and testing of welders,however the new standard, BS EN 1011-1 Annex A,requires this information to be supplied by thepurchaser. Clause 20 indicates the appropriatestandard is the relevant part of BS EN 287; for steelbridges this is Part 1. The standard prescribes tests toapprove welders based upon process, type of joint,position and material.

Welders undertaking successful procedure trials gainautomatic approval within the ranges of approval in thestandard.

Welder approvals are time limited and need re-validating depending on continuity of employment,engagement on work of a relevant technical nature andsatisfactory performance. The success of all weldingoperations relies on the workforce having appropriatetraining and regular monitoring of competence byinspection and testing.

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6.9 Inspection and testing

Procedure trials

Specimens for destructive testing are cut andmachined from the test plate. Typical specimens for anin-line plate butt weld include transverse tensile tests,transverse bend tests, impact tests and a macro-examination piece on which hardness testing isperformed.

Tensile tests are required to cover the full sectionthickness of the joint and under most specifications theresult has to at least match the correspondingminimum strength of the parent material.

Root and face bend tests are to have the weld root andface respectively in tension, although for materialsequal to or greater than 12mm side bend tests arepreferred. Flaws greater than 3mm are not permitted,although those appearing at the corners of thespecimen are to be ignored.

Hardness and impact testing criteria for bridgework arevaried by BS 5400: Part 6, however it is wise to test allwelding procedures to the limit of potential applicationto avoid repeating similar tests in the future.

The notch toughness of weld metal and HAZ isimportant as initiation of cracks could occur fromdefects located in these areas. The toughnessrequirements in BS 5400: Part 6 are specifically forCharpy V-notch impact tests for tension areas and areapplied to butt welds including corner or T-butt weldsparallel or transverse to the main tension stress. Thenotch toughness of weld metal and HAZ is importantas initiation of cracks could occur from defects locatedin these areas. The toughness requirements in BS5400: Part 6 are specifically Charpy V-notch impacttests for tension areas and are applied to butt weldsincluding corner or T-butt welds parallel or transverseto the main tension stress. The minimum energyabsorption requirements and the testing temperatureare the same as those required for the parent materialin the joint.

Other specifications may vary the Part 6 requirements.Care is needed to ensure the project requirements arefully understood.

The guidance clauses in BS 5400: Part 6 providefurther information and discuss the acceptance criteriafor hardness and impact testing in tension areas.Notch positions in impact specimens in the weld metaland heat-affected zones are shown for various jointconfigurations.

Examination of stud shear connector weld proceduretests is based on a sample of six studs. Three studsare bent sideways for a distance approximately half thelength of the stud, and are required to be bent straightagain without failure of the weld, the other three studsare used to prepare macro sections which must befree from macroscopic defects visible to the nakedeye. Hardness values for the weld metal have to be in

the range 150-350 HV30 and the HAZ must notexceed 350 HV30.

Production tests

Production testing is specified as destructive or non-destructive. Destructive tests are mechanical tests onspecimens taken from "run-off" plates generallyattached to in-line butt welds. The "run-off" plates haveto be larger than normal to accommodate the testspecimens and any re-tests which may be required.

(a) Destructive testing:

Unless otherwise specified by the Engineer, productiontests required in BS 5400 are approximately 1 in 5transverse butt welds in tension flanges and 1 in 10other butt welds.

Specimens for testing include transverse tensile andtransverse bend tests. Charpy V-notch impact testsare required on the weld metal of butt welds transverseto and carrying the main tension stress and wherespecified by the Engineer on the fusion boundaryregion of the HAZ.

Transverse tensile tests are required to achieve atensile strength of not less than the correspondingspecified minimum value for the parent metal. Failurecan occur in the parent or weld metal part of thespecimen.

The transverse bend tests in thicknesses of 10mm andover are side bend tests; otherwise face and root bendtests are carried out as described under proceduretrials. The former diameter and angle of bend are asrequired by BS EN 288-3. Defects revealed are to beinvestigated to establish cause prior to acceptance orrejection: this is to avoid costly repair of relativelyinsignificant discontinuities such as minor porosity.Slight tearing at the edges of the specimen is also nota cause for rejection.

Charpy V-notch impact tests are required to achievethe acceptance criteria specified for the proceduretests.

Further testing is permitted under the standard,however in the event of failure to meet test criteria theEngineer will determine the next course of action on allwelds represented by the production test plate.Actions to consider may include additional non-destructive testing or a stress relieving post weld heattreatment Rejected joints should be repaired inaccordance with an approved procedure.

(b) Non-destructive testing:

BS 5400: Part 6 describes the methods and frequencyof inspection and acceptance criteria for all joints on aproject where the fatigue classification is other than Bor C. Critical areas of the structure requiring aminimum fatigue classification are defined by thedesigner on the drawings. This influences the testprocedures, the extent of inspection and theacceptance criteria. All testing of welds should take

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place not less than 48 hours after welding.

The methods and acceptance criteria in Part 6 havebeen prepared especially for bridgework and aredesigned to achieve a level of performance basedupon the fitness for purpose of production welds.

Surface inspection methods are visual and magneticparticle inspection; sub-surface inspection methodsare radiography and ultrasonic inspection.Radiography demands stringent health and safetycontrols; it is relatively slow and needs specialistequipment and use of the process has declined onbridgework compared with the safe and more portableequipment associated with ultrasonic inspection.Safety exclusion zones are required, in works and onsite, when radiography is in progress. The standardnotes that radiography may be used in cases ofdispute to clarify the nature, sizes or extent of multipleinternal flaws detected ultrasonically. Specialisttechnicians with recognised training are required for allnon-destructive testing methods.

All welds require 100% visual inspection. From apractical point of view welds should be visuallyinspected immediately after welding to ensure obvioussurface defects are dealt with promptly.

Partial non-destructive inspections should be at least300mm in length or the total length for shorter joints.The standard directs partial inspections to includeareas where visual inspection has indicated thatinternal quality is suspect. Where unacceptablediscontinuities are found the examination area isincreased accordingly.

The extent of magnetic particle and ultrasonicinspection is defined and increases particularly where aminimum fatigue class requirement is shown on thedrawings. Acceptance criteria are given for all methodsof inspection but quality levels vary according towhether a minimum fatigue class is specified, thelocation of the joint and the type of discontinuity.

Production stud shear connector welds are ring testedby using a 2kg hammer to strike each stud. A selectionof studs, nominally chosen by the Engineer andnormally around 1 in 50, are tested by using a 6kghammer to displace the stud sideways to a distance0.25 x the height of the stud. The bent studs are notto be straightened. Failed studs are to be replaced inaccordance with an approved procedure.

6.10 Weld quality levels

The effect of defects on the performance of weldedjoints depends upon the loading applied and uponmaterial properties. It may also depend on the preciselocation and orientation of the defect, and upon suchfactors as service environment and temperature. Themajor effect of weld defects on the serviceperformance of steel structures is to increase the riskof failure by fatigue or by brittle fracture. Types ofwelding defect can be classified under one of thegeneral headings:

(a) Cracks.

(b) Planar defects other than cracks, e.g. lack of penetration, lack of fusion.

(c) Slag inclusions.

(d) Porosity, pores.

(e) Undercut or profile defects.

Crack or planar defects penetrating the surface arepotentially the most serious form of defect. Embeddedslag inclusions and porosity are unlikely to initiatefailure unless very excessive. Undercut is not normallya serious defect unless significant tensile stressesoccur transverse to the joint.

In order to prevent brittle fracture the Engineer selectsmaterial with adequate notch toughness in accordancewith the requirements of BS 5400: Part 3 from thestandards listed in Part 6. This is of particularimportance for thick joints and low temperatureapplications.

When a joint is subject to fatigue loading, and thecorrect precautions are taken for the selection ofmaterials, the effect of any weld defects is primarily onthe development of fatigue cracks.

In selecting the appropriate weld quality for a particulartype of joint the following aspects are considered:

(a) Type of joint (fillet, butt or tee-butt).

(b) Direction of principal stresses at the joint in the plateand in the weld, relative to the length of the weld.

(c) Magnitude of the stress range under fatigue loadingand the specified load spectrum for the service life.

Where defects are detected as a result of inspectionand testing, then repair may be necessary.Alternatively in many cases the particular defects maybe assessed on the concept of ‘fitness for purpose’.This is dependent upon the stress levels and thesignificance of fatigue at the location. The size, formand position of the defect are considered taking intoaccount static strength, fatigue and brittle fracturecriteria for the service life of the structure. This is amatter for speedy consultation between the contractorand designer for if acceptable, costly repairs can beavoided as well as other problems such as increasedrisk of distortion, the potential for introducing otherdefects in the repair and programme delay.

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BS 5400: Part 6

BS EN 288 - 3

Welding procedure trials Welding recommendations,guidance and application

Flowchart showing the relationship between the principal workmanship and welding standards for bridgework

Welder approvaltesting

Inspection andtesting

BS 5135(withdrawn)

BS EN 1011 BS EN 287 - 1

Submerged-arc welding process

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MAG or flux cored arc welding process

Manual metal arc welding process

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7.1 Introduction

The likely cost of steelwork is an importantconsideration in feasibility studies for a bridge project,as well as in planning and budgeting for theconstruction. Equally important for the client are theeffects on the outturn price he will pay, of decisionstaken in design and procurement of steelwork as partof the overall project.

Very little guidance is published on the cost of bridgesteelwork because it is difficult to generalise for thewide range of bridge types and configurations found inthe UK; and of course at any particular time pricesreflect the state of the market. The price a client paysfor the steelwork in a new bridge covers the cost ofmany activities and services as well as the basic costof materials used and the direct workmanship infabrication and erection. In this chapter the aim is toprovide guidance on relative costs and, moreimportantly, to show how the decisions and actions ofclients and designers can increase or reduce the costof bridge steelwork irrespective of the value and qualityof the finished product.

Understanding and anticipating the whole process ofproduction from the outset of the project is essentialfor success; so in the following sections cost isdiscussed in relation to

– the defined project requirements,

– the production process for a typical bridge, and

– cooperation for efficiency and economy.

7.2 Project Requirements

7.2.1 Everything has to be paid for

The client's aim is to secure the new bridge to thedesired standard as economically and efficiently aspossible: to that end, whatever the contractualarrangements or procurement method, the steelworkcontractor will start work with a defined set of projectrequirements. These will include a range of technicaland commercial documents – drawings,specifications, bills of quantities, programme andconditions of contract – all of which bear on the cost ofthe work to be done and the price to be paid.

The requirements should be all that are appropriate,necessary and sufficient to achieve the client's aim andno more. To go beyond that may require services oractivities which give no added value, or set limitationswhich inhibit enterprise or economy, and increase costand price. It is worth reviewing each category of therequirements.

7.2.2 Scope of Work

The typical sub-contract scope of works for steelbridgework covers supply, fabrication, protectivetreatment and erection. Each of the UK steel bridgecontractors, in its own particular way, is able toundertake additional activities beyond that basicscope. This can offer benefits for cost and riskmanagement; for example if the provision of an in situconcrete deck on a small rail bridge is taken on by thesteelwork contractor at the factory, or if the steelworkcontractor were to supply and install the bearings.

The opportunity for synergy between main contractorand steelwork contractor depends on their respectiveskills and experience and requires thoughtful definitionof contractual interfaces, for example, in the provisionof groundworks and access for delivery and erection.Similarly if the steelwork contractor is required toundertake work outside its normal experience, risks forthe project could increase, leading to higher costs - intime and money.

As a general rule, in defining the scope of work of eachparty, it is best to place control of costs and risk withthe party which has most relevant experience andopportunity to deal with them. The scope of work willbe defined in detail by the drawings, the specifications,the bill of quantities and the method of measurementfor the project.

7.2.3 The Design

A bridge designer is challenged to produce a solutionwhich meets all of a wide range of criteria, including theminimum cost consistent with all other criteria. Heportrays the chosen solution primarily on the contractdrawings with the support of the specification. Incoming to an overall view of the relative cost of aparticular design, the key issues are its efficiency,complexity and buildability.

Efficient design of steelwork aims at using the steelcomponents to their safe design capacity within anoverall economy of resources for fabrication anderection. Minimum cost design is very unlikely to be thelightest design and certainly it will not be the heaviest.Weight saving is not an end in itself; for example flangeweights can be reduced at the expense of additionalbutt welds, or webs can be thinner if more stiffenersare added. Achieving the most efficient design requirescareful selection of concept and layout, and judgementin drawing the balance between weight andworkmanship in detail. Guidance on efficient design ofcomposite plate girder highway bridges, including likelyweights, has been published by Corus (CompositeSteel Highway Bridges). The efficiency of a design has

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to be judged against the particular projectrequirements of course; thus severe restriction ofconstruction depth will increase the weight of themembers, however efficient the design is.

In general more complex fabrication is more expensive,especially if measured on a cost per tonne basis.Fabrication is generally more economic if connectionsare simple, geometry is straightforward, and theamount of welding is minimised. The complexity of adesign is the outcome of the designer's response tothe imposed constraints and the choices he makes:thus for example, a viaduct may have to conform tocomplex highway geometry but a decision to useintegral crossheads would compound the complexityand cost of fabrication and erection substantially.Modern CAD systems and CNC fabrication equipmentcan well accommodate such complex geometry toease the drawing and production processes, but theycannot eliminate all the extra costs in workmanship,organisation and management. In detailing, everyadditional component, every extra weld adds to thecost so it pays the designer to consider the optionsand anticipate relative costs in making his choices.

The buildability of structural steelwork is a cost issueas well as a safety issue for the designer; indeedfulfilment of his obligations under CDM to reduceexposure to hazards should contribute to overalleconomy. Design for function and purpose has toanticipate the fabrication and erection processes.Good detailing is the first key to ensuring buildability –say in the fabrication of box girders to minimisehazardous and expensive internal activities, or inensuring that bolts can be entered and readilytightened in site connections. And, secondly, forconstruction work on site the structural layout,member sizing, and connections need to be consistentwith a practicable economic erection method: siteconstraints on time, say for rail or road closures, andphysical restrictions on access, crane position,temporary supports and the like will determine whatthat method will be. Connections should be detailed tofacilitate site fitting with some rotational anddimensional tolerance, bearing in mind the factors ofcamber prediction and fabrication tolerances: this isimportant for safety and cost, and can be met readilywith bolted joints. In some circumstances extrastiffening or strength will need to be designed in to themembers, say for launching or a big lift, and that will beless costly if done at the outset. The level of erectioncosts is determined by the potential difficulty of the siteand the project constraints; the designer has realscope to increase or reduce that cost.

To achieve good quality design which satisfies all thecriteria is not easy; it requires skill in the use of steel atthe concept and detail stages of design. Somedesigners are more experienced in steel bridge designthan others: greater experience should lead to simplerand more economic design which will reduce total costs.

7.2.4 Specification

The project specification is an essential part of thedesign, and it is germane to what has to be paid for.The project specification should express clearly whatthe designer requires, be up to date and refer tocurrent national and international standards.Conformance with well recognised specifications forfabrication and erection (eg based on BS 5400: Part 6)will reduce uncertainty and help to minimise cost;specifiers should avoid introducing personalisedclauses demanding, say, extra fabrication tolerances,testing procedures, or architectural finishes as thesewill increase fabrication costs. Some projects willrequire special clauses to meet particular needs, butthese should reflect recognised good practice and notover-anticipate problems. The SCI publication "ModelAppendix 18/1" (Specification of Structural Steelworkfor Bridges) contains a series of model clauses whichmay be inserted into a project specification and whichis compatible with the Specification for HighwayWorks, BS 5400 Part 6, and associated standards. Inthe limit, any tendency towards "preferentialengineering" adds to the cost and price of a steelbridge.

7.2.5 Programme

The project programme should reflect an informedview of achieving the client's objective consistent withallowing sufficient time to complete all the necessarytasks – administrative, intellectual, and practical, at siteand elsewhere – within the logical constraints of thedesign and other project requirements. Within thatprogramme, the steelwork contractor needs sufficienttime to do all that he has to do – many activities are inhis direct control, others such as supply of steel arenot. For most bridge steelwork the time required tocomplete the work most economically can be reducedby special measures, or taking risks, but suchmeasures come at a price. Therefore it is important notto compress the steelwork programme below theallowance of sufficient time for economy, withoutrecognising the enhanced cost and risk to the project.This is discussed in more detail in section 7.3. Typicalfabrication periods range from four weeks from receiptof steel for small simple structures through to manymonths for large complex structures: the necessaryperiod depends on the capacity of the factory and itsworkload too.

Within the detail of the project programme featureswhich tend to increase the site cost of the steelwork are

- time constraints on deliveries and hours of working

- phasing of erection requiring more that one visit tosite

- possession work requiring multi-shift and night work

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- timing of work such as welding and painting inunseasonable periods of the year, requiring extrameasures and risks of excessive down time.

7.2.6 Conditions of Contract

The conditions of contract particular to a project andthe related subcontract conditions, introduce elementsof cost for the steelwork contractor which are reflectedin his price. Comparatively aggressive terms andconditions will result in higher prices.

Factors which increase the cost of structural steelworkinclude the financing of materials and fabricatedsteelwork at the factory until it is paid for, and the riskprovision for unreasonable damage clauses which aredisproportionate to the size of the subcontract. Theprovision of interim payments for steel and steelworkshould be included where it is required that work bedone well in advance of erection dates. As with allother aspects of project requirements the applicationof the contract conditions to a project needs to bethought through to ensure that they are in the bestinterests of achieving a successful project even downto minor considerations such as times for approvals.Price is related to risk, and offloading risk on to thecontractor or subcontractor increases the price to bepaid for the work.

7.2.7 Commercial Factors

The price to be paid for steelwork will reflect not onlythe costs and risks inherent in the explicit requirementsof the project documents, but also less tangible factorssuch as the relationships between the parties andmarket conditions.

As with any specialist work, the perceived degree ofrisk will depend upon the trust that each of the partieshas with the others. When the parties (client, designer,main contractor and steelwork contractor) haveworked together successfully in the past by keeping tocontractual agreements such as payment terms andby maintaining good working relationships, greatercertainty of cost and fewer post-contract claims canbe expected. The prices tendered will reflect that.

Market conditions will affect the steelwork contractor'scosts, for example in the price and availability ofmaterial or labour and plant in remote locations. Thecontractor's margins will reflect the state of his forwardwork load and the overall demand and supply balance.However, no steelwork contractor can afford to beinfluenced by the immediate situation whenconsidering projects, which may become orders in themedium or long term.

7.3 The Fabrication Process

7.3.1 A Manufactured Product

In any project using fabricated steelwork it is surprisinghow few people outside the factory doors understandwhat goes on within them - even when their roles givethem an active interest or part in the process.

Steelwork is a manufactured product with a keyfunction in a civil engineering project on site: most of itsadded value accrues over many weeks before deliveryto site where it is often erected in a few days. Thefundamental differences between production in afactory and production of civil engineering works onsite, and in particular in their cost structures, lead tomisunderstandings and commonly to unintendedcommercial surprises. Mutual recognition andunderstanding of the differences help relationships,control of costs, and ensure better value for the client;and this is particularly so in managing any change ofrequirements, and dealing with problems.

Between receipt of the final design input and thedelivery of complete steel members to site is asequence of dependent activities – through planning,preparation of data and drawings, procurement ofmaterial, receipt and preparation of steel, assemblyand welding, possibly trial erection, and protectivetreatment. The factory, which represents a large fixedoverhead, is laid out with covered space, cranes,machinery and equipment to suit the steelworkcontractor's product range with efficiency andeconomy – the facility to manufacture buildingstructures is quite different from one for plate girders orfor steel decks. Steel bridge members are large, heavyand bulky, so the factory layout is designed to lift, moveand manipulate the steelwork economically and safelybetween production activities. This non-productivework and the occupation of floor space represent asubstantial proportion of bridge fabrication costs.Unlike a site, the factory works on a number of projectsin parallel to achieve profitable utilisation of the factoryand its permanent workforce, and thereby to be able tooffer steelwork at competitive prices and maintain thecompany's expertise.

The sequence of activities for bridgework is describedmore fully in the following paragraphs to dispel some ofthe mystery about what does happen in the factory.

7.3.2 Starting the Process

Before fabrication can begin, the work has to beplanned and programmed, drawings and productiondata produced, material ordered and first steeldelivered. The target is to provide precise data and allthe appropriate material for the first components to theshop floor by the planned date; and then to maintainthe flow of data and material to meet the programmewithout delay or disruption.

7.3.3 Planning

The steelwork contractor is committed to deliver anderect the bridgework to meet the overall projectprogramme, given receipt of the requisite designinformation by agreed key dates. Fabrication isplanned to complete the members in sequence to suitthe agreed erection sequence - to minimise thecritical path for each member and to avoid stockpilingof finished work. Delivery dates for members to site are

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determined by an erection programme with sufficientduration to do the work safely and to specificationwithin all site constraints. Within the factory, efficientuse of the space, equipment, and the substantiallyfixed level of resource requires the programme to fitinto the overall works programme to balance demandand avoid bottlenecks. Work may be planned for sub-letting for specialist operations, say bending ormachining, or to deal with overload.

Specialist planning is done, to meet the programme, inpreparing method statements for more complexactivities, inspection and test plans, risk assessmentsfor site work, erection method statements, and healthand safety plans - all within a company specific qualitymanagement system.

Preparation of all this information, which is prerequisiteto doing the job properly, depends on timely andsufficient accurate input from the other parties to theproject, and timely response to submissions requiringapproval.

Erection planning will include a measure ofconstruction engineering, dependent on the scale andcomplexity of the bridgeworks, including structuralanalysis, temporary works design and independentchecks. This in turn requires full information from thesite and agreement between the parties on method,timing and responsibilities before it can be completed.

7.3.4 Drawings and Production Data

The design input has to be processed to schedulematerial for procurement, to generate work instructionsfor tradesmen or machines on the factory floor, and forconstruction engineering at site. With progressiveautomation of fabrication, the industry is movingtowards making fabrication drawings obsolescent –the design data are converted directly into digitalinstructions for machines. Each steelwork contractorwill format the data to suit his particular factory facilityand such fabrication drawings as are made, are for hispurposes in fabrication and erection of the bridgework.

To implement the design of the steelwork efficientlyand in a timely manner, it is vital that the design inputexpresses the designer's intent clearly and completelyat the outset and certainly before the commencementof CAD detailing or modelling: typically this informationis required between four and six weeks beforefabrication starts. Where members are required to becambered the designer should provide the data on thedesign drawings, which can come from his computermodel; it is not cost effective for the steelworkcontractor to set up a new model and make hiscalculations on the critical path of his work - addingtime, cost, and risk of delay.

7.3.5 Material Procurement

Steelwork contractors do not carry stocks of material,except a basic minimum for projects in progress, somaterial is bought specifically for each new project. For

most bridges, costs can be minimised by orderingplates and sections directly from the steel mills, incontrast to the UK market for building steelwork, whichdepends much more on steel stockholders. Somebridge materials, including most special grades suchas S460 or WR steels and larger sizes of section, arenot available from stockholders at any price. Time hasto be allowed in any bridge procurement programmefor supply from the mills with typical lead times of six totwelve weeks for plate and between six and sixteenweeks for larger sections and tubes. Full designinformation is not necessary to enable orders for steelto be placed but sufficient detail must be available todefine all components in advance of rolling dates tominimise waste and costs.

For members fabricated from plate, most platecomponents including flanges, webs and stiffeners,are cut from plates of economic size and width. Hencesteel listing for ordering includes a computerisednesting process to achieve best utilisation andminimise waste to no more than a few percent. To thesame end, the designer should avoid mixing of gradeswhere possible and rationalise the range of platethicknesses and section sizes. Advice is given inChapters 1 and 2 on plate sizes and the availability ofsteel grades.

7.3.6 Receipt of Material

As material is received at the factory it is marshalled fortransfer into the preparation area in the programmedsequence. Usually as the steel is picked up it receivespre-fabrication cleaning by grit-blasting to remove allmill-scale, rust and dirt. This provides clean surfacesfor marking and welding and reveals any superficialdefects in the material, should there be any, beforework starts.

7.3.7 Preparation

This term is used to cover the set of operations whichconverts the plain material into finished plate or sectioncomponents ready for assembly and welding. Therange of processes and equipment used depends onthe requirement for each component: it includesmarking, cutting, edge preparation, drilling, pressingand machining: sections may be processed throughautomated saw and drill lines for cutting and drilling.

7.3.8 Marking

The day of the large template loft with string line andplywood is long gone, to be replaced by desktop PC.Some steelwork contractors have automated theprocess, but it still has a major cost which can beminimised by keeping the structure design simple. Forexample, complex arrangements of shear studs areinevitably more expensive than simple arrangementswhich can utilise templates for marking and are easy tocheck – ensuring that there are no fouls withreinforcing bars at site.

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7.3.9 Cutting

Components for box and plate girders will usually becut from plate by plasma or oxy-propane equipmentmounted on CNC machines. These closely controlledtechniques are generally accurate and efficiency isdependent largely on the process. It is possible toincrease cutting costs by adding complexity; forexample, creating too many types of stiffener preventsmulti-head profiling. As plates are cut up, each piece ismarked for identification and material traceability.

Other components such as bracings should bedetailed for simplicity with the aim of minimising thenumber of pieces to cut. Ends of bracings should becut square rather than mitred and single membersshould be used in preference to back-to-backmembers.

7.3.10 Preparation of Edges

Unnecessary preparation of edges should be avoided.The hardness of flame cut edges can be controlledquite simply through qualified cutting procedures sothat grinding of edges is not necessary. Fillet weldsrather than penetration welds should be used wherepossible, for the same reason. The costly practice ofgrinding arrises to a prescribed radius, of 3mm say, toavoid edge defects in protective treatment is no longernecessary with the high solids chemical cure paintsystems used today.

Machining of edges should not be specified unlessproven to be absolutely essential to achieve thewelded connection – for example in achieving partialpenetration welds for deck stiffeners with highpenetration without blowthrough.

7.3.11 Drilling

Usually all holes for site-bolted connections are drilledin the factory, and for welded members most holes aredrilled in preparation before assembly using CNCmachines. Drilling holes in large assembled membersis more costly because less efficient portable machineshave to be mounted on the member. For complexassemblies final holes may have to be drilled in trialassembly with the joint material to ensure a goodmatch at site. Small components, particularly spliceplates, are drilled in packs so simple detail andrepetition contribute to cost reduction.

7.3.12 Pressing

Bridge steelwork contractors and specialist workshopsare equipped with a variety of presses which may beused to form members from plate, or for remedialworking of deformed components. Designers areadvised to seek specialist advice before using coldformed details in design. The details need to suit theavailable equipment and the specification for therelevant material may need modifying. The introductionof plate bending adds an activity, which the steelworkcontractor may have to sublet, so time and cost effects

need to be considered.

7.3.13 Assembly

The fabrication process is arranged so that work oneach principal member is a continuous process withthe minimum standing time. Thus the set ofcomponents for a member is prepared as a batch andmoved on to the assembly area or machines withoutdelay to be put together and welded, for such non-productive time has a cost.

The assembly of plate girders and box girders isdescribed in some detail in Chapter 3. Methods varybetween steelwork contractors for similar products:each one chooses the most efficient and economicway of using the factory's equipment and space -some with large capital investment and fewermanhours, some with modest facilities and muchgreater skilled manhours. The same volume of weldingis undertaken, the same issues of accuracy anddistortion have to be controlled.

Welding procedure and processes are described indetail in Chapter 6. There are two main factors thataffect welding costs, the choice of process and theefficiency of application of that process. Higherdeposition processes such as submerged arc welding(SAW) can reduce welding costs, but they are limitedby gravity and are intolerant of damp steel orconsumables, so they are not practical for most sitewelding. High deposition processes, mounted onautomatic or semi-automatic machines are usedwhenever possible in shop assembly, normally utilisingmetal active gas (MAG) techniques which are mostcommon in UK steelwork contractors. Manual metalarc welding (MMA), commonly called 'stick' welding, isfar less efficient, but can be the only choice forcomplex joints and details - a choice which isimposed by the complexity of design and where detailshave not taken account of how welds can be made.Generally the route to minimum cost of welding in theUK is to maximise the use of automated equipment inthe fabrication works and minimise welding on site.

Grinding of butt welds in members requires anotheroperation on the critical path of fabrication which ishazardous, time consuming and relatively costly for theperceived benefit. The vibration effects of the tools arehazardous and so their use is now strictly regulated toprotect the operator. Consideration should be given tothe need to grind flush particularly for longitudinal buttwelds, where welds can be completed with a neatprofile, and especially on internal surfaces.

As described earlier the cost of lifting and turning largeassemblies to facilitate welding, particularly of boxgirders, can be reduced by careful design of theconnections of webs and flanges. Similarly, hazardousand high cost work in the interior of box girders is verydependent on the thought given to internal detail.

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Some types of bridge require members to bemachined after completion of assembly and welding,for example the members of an arch or at connectionsfor suspension systems on major bridges. Steelworkcontractors have limited capability for in housemachining, so subletting to specialists at high cost orusing portable machines with limited cutting capacityare options. Designers should seek to avoid machineryat this stage of production unless there is very goodcause.

Trial erection has been discussed elsewhere, but it isworth mentioning in a discussion of cost as it has amajor impact on the length of the fabricationprogramme: a member required in trial erection cannotbe released for painting and delivery to site until the lastmember is fabricated and the trial assembly is checkedand released. Generally the considerable cost ofmanhours, craneage, and space in the works is non-productive save for the assurance of minimising risk ofdelay at site. Given the accuracy of modern fabricationmachinery and the variety of techniques available tothe steelwork contractor to demonstrate that the fit willbe achieved, trial erection is to be avoided.

As assembly of members progresses inspection, NDTand mechanical testing are carried out to minimisedelay to release of the completed members.

7.3.14 Managing Change

Fabrication is a continuous programme of activitiesfrom receipt of first information to completion anddelivery of the finished member. The specification andother contract requirements require reference atvarious stages to the designer, the Engineer, and otherparties - they have a part to play in the process, andhow they fulfil their roles bears on the steelworkcontractor's performance.

Few projects reach their conclusion without the needfor change, stemming from any of the parties andimpacting on the others. The cost effects of changewhich impact on fabrication become more severe thelater they occur in the fabrication programme: evenapparently quite modest changes can have substantialtime and cost consequences, for they not only put theproject fabrication at risk, but they can disrupt work onother projects in the factory at the same time.

Experienced steelwork contractors make prudentassessments of the risks of delays in information,approvals and consents, and develop contingencyplans to deal with problems. To the extent thattechnical factors are involved, it is important thatdesigners and supervising engineers develop goodworking relationships and effective channels ofcommunication with the steelwork contractors; mutualunderstanding can make a great difference to the costof the project and its success.

7.4 Protective Treatment

Conventional practice requires the completed steelmembers to be blast cleaned, perhaps metal sprayed,and part painted before delivery to site; finish coats areapplied after the bridge is built. The steelworkcontractor carries out the first stage at the fabricationworks, given the capital investment in proper blastingand painting facilities, or en route to the site at a sub-contractor's facility which incurs extra handling andtransport costs. Very bulky bridge members take upconsiderable space in the facility so each one is blast-cleaned and painted as quickly as possible, whilstmaintaining intercoat times, to maximise throughput ofthe plant.

Although protective treatment of bridge steelwork islargely outside the scope of this book, the way it isorganised within the project programme - when,where and how - does affect the overall cost of thebridge. Commonly, project specifications will constrainthe application of the protective treatment withoutregard to the construction of the bridge; andsometimes the work can thereby be more expensiveand protection be impaired. It is well worth reviewingthe application requirements for each project to ensurebest value.

The primary function of protective treatment is just that– to protect the steel from corrosion for as long as canbe achieved with the chosen technology: theappearance of the finished bridge is not unimportant,but it should take second place to protection in thespecifier's mind. Performance of the treatmentdepends on four fundamental factors

– design to avoid vulnerable details,

– very effective surface preparation,

– selection of an appropriate system, and

– application of the system within the manufacturer'scriteria

Of these, the surface preparation and the applicationrequire suitable ambient conditions, which are bestachieved in an indoor and preferably controlledenvironment. Dust and dirt, high humidity, lowtemperature and poor access all aggravate the risks ofpremature failure and reduced life before firstmaintenance.

Site conditions generally militate against goodapplication standards, and the processes can havesevere environmental impact. For example:

- the essential ambient temperature and humiditycriteria are very difficult to meet in wet or coldweather; so the winter months in the UK are virtuallya close season for site painting;

- exclusion of dust and dirt from the process, andbetween coats on a civil construction site ischallenging;

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- access to erected bridgework for safe treatmenthas a high cost, as does shielding of the processand the environment;

- site welded joints require blast-cleaning which is avery dirty process and a threat to the siteenvironment;

- painting bridges over live highways or rail track inpossessions, and at night, puts quality at risk.

Thus it is beneficial to the performance of the finishedsystem and value to minimise protective treatment atsite, and to eliminate it if practicable. The example ofthe Foyle Bridge, where less than 1% of a 508m longbox girder bridge was painted at site, may be difficultto emulate but the potential benefits are substantial –directly and indirectly too in simplifying work overall onsite. The practice of fully painting girders, sometimes incomplete braced pairs, is increasing: certainly thereseems to be a good case for leaving just theconnections to be painted at site, and applying finishcoat on the few surfaces that impact on the overallappearance of the bridge.

Damage to painted surfaces in transit, and duringerection and construction of deck slabs is a very realproblem, but with properly planned and managedsystems of work should not be a major issue. Evenwhen it does occur, the remedial measures shouldcost far less than full painting on site. Risk of damageduring construction is not just a consideration for thesteelwork contractor, but also for the designer and forthe main contractor in detailing and construction of thesubstructures and composite decks.

In brief, as with most aspects of the bridge, theapplication of the protective treatment needs to bethought through by the designer in designing andspecifying the work as part of seeking best value forthe client.

7.5 Erection

The painted fabricated steelwork is delivered to site insequence to meet the planned erection method andprogramme. For major roadworks projects steelerection is phased to suit the construction ofroadworks; generally for railway bridges it will beorganised around the key track possession dates.

The discussion of erection in Chapter 1 illustrates thevariety of methods used with today's plant andequipment. Each new bridge presents a uniqueerection problem. The solution has to match thedetermining factors for that bridge on that site; sonotionally identical structures on two different sitescould well require different erection methods with quiteunrelated levels of cost. Erection costs are determinedfundamentally by the method to be used; so toestimate the cost, the steelwork contractor works up amethod and then calculates the requirements for time,resources, temporary works, and engineering.

The erection of the bridge is an opportunity for thebridge-builder to use his ingenuity and experience tocontribute to the best value of the project: he will dothat to help the steelwork contractor to produce themost competitive tender and successful job. Bridgelayout, configuration and details can help or hinderefficient safe erection so in an ideal world the designerwould anticipate the optimum erection method: aprincipal benefit of design and construct projects isthat design for function and service can go hand-in-hand with design for construction.

As discussed in Chapter 1 the designer has to makeassumptions about sequence to complete his analysisand design, and to satisfy himself that the bridge canbe built in a safe and practicable manner to fulfil hisCDM obligations. It is quite possible that thecontractor, who is responsible for the method which isused, will propose a different method: this may requirethe designer to revisit the design, but any cost to theclient in doing that should be measured against thebenefits of the more competitive offer. Clearly thesteelwork contractor has to satisfy himself and thedesigner that the proposed method is structurallyviable and safe.

Whatever the method, the steelwork contractor willsubmit method statements, temporary works designsand, when required, independent check certificates forapprovals. It is not uncommon for the vagaries ofconstruction, particularly of major roadworks, toprevent final details and documentation of schemes tobe complete until shortly before the work is to be done.Again, good channels for technical communicationbetween the parties are essential for safe efficientoperations.

7.6 Guidance on Cost

7.6.1 What is a useful measure of cost?

Some believe that they can use a simple measure suchas 'rate per tonne' to evaluate schemes; perhaps thatview is encouraged by such in the common methodsof measurement and bills of quantities. This is noteffective and can be misleading because only a smallpercentage of the overall cost of steelwork can berelated simply to tonnage - in fabrication or erection.Even for the basic steel material cost there is not asimple 'rate per tonne', with many factors such as thegrade, source, section size, plate length, plate width,test requirements and quantity all having a significanteffect on the price of the material from the mills - andbefore such factors as wastage are taken into account.Many erection costs, say for temporary works or hire ofa crane, are virtually independent of the tonnage. Thisin itself raises commercial difficulties in evaluatingchanges for bridge steelwork through a bill ofquantities at contract stage.

Almost all of the cost of bridge steelwork is influenceddirectly by the detail of the design and theconfiguration of the structure on the site which are of

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great variety. Therefore a designer needing realisticestimates would be well advised to discuss his optionswith appropriately experienced steelwork contractors,even if only at an early stage of design, to assessbudget figures. Budget rates per square metre offinished bridge deck will give a far more accurateestimate of cost than any 'guessed' rate per tonneapplied to a budget estimate of weight.

7.6.2 Cost per square metre

Cost per square metre of finished bridge deck willdepend on structure type, span configuration, designloading, erection method, and protective treatmentsystem, amongst other factors. A steelwork contractorwith a comprehensive track record of carrying out thetype of work involved should be able to provide adesigner with a global range of budget rates. Advicemay vary between steelwork contractors due todiffering expertise and fabrication facilities; so choosinga steelwork contractor with relevant experience andcapability for the type and size of bridge envisaged isimportant. Any guidance on cost for fabrication orerection must be treated with care.

7.6.3 Relative costs per tonne

For designers contemplating alternatives to plategirders where relevant, the following is a breakdown ofcosts per tonne relative to plate girders at a datum of100:

Clearly these figures are only indicative andconsiderable variations are to be anticipated for eachparticular bridge.

Universal beam bridges are uncommon now as theytend to be heavier than plate girder structures andoffering no benefit on a rate per tonne basis, they areuncompetitive – except possibly for very small spans.

Box girders are inherently more expensive to fabricate,especially with greater recognition of the hazards ofworking in confined spaces and the measuresnecessary to deal with them. Protective treatment anderection costs for box girders can be reduced throughthe use of weather resistant steels and careful choiceof site connections.

7.7 Achieving best value

The aim of this publication is to provide information andguidance to designers to achieve best value for theirclients in the design and construction of steel bridges.

The emphasis in this chapter on cost should not beconstrued as advocating that the lowest cost design isthe optimum, but rather to advise how money shouldbe well spent in giving the client the bridge he wants.

The services of the technical advisory staff and therelevant publications of the BCSA, the SCI and Corusare commended to designers engaged in steel bridgedesign. The Steel Bridge Group Guidance Notes,published by the SCI, are recommended for theircoverage of many design topics beyond the scope ofthis book.

The steelwork contractors specialising in bridgeworkwould encourage designers to familiarise themselveswith fabrication and erection, by visiting their worksand construction sites. Designers are not expected tobe experts in steel construction but it is surprising howmuch can be learnt from relatively short visits. There isnothing like walking over the first steel bridge you havedesigned.

There is much to be gained from involvement ofappropriate steelwork contractors in the design stagesof a project. It is self evident that the prospect of bestvalue will be the greater if the design anticipatesfabrication and erection processes effectively.Throughout the 1990’s, the UK construction industrywas charged with improving performance - to seek todeliver the product to clients through cooperation andteamwork. The universal experience of the steel bridge

contractors over many years is that through such anapproach everybody wins, and best value has beenachieved. Partnering with a steelwork contractor in theteam at tender stage can lead to reduced costs,especially in design & build projects and valueengineering, to minimise outturn costs and ensurequality. Maximum benefit will come from earlyinvolvement.

The number of UK bridge steelwork contractors isrelatively small: they vary in size, product range, andexperience. The independent Register of QualifiedSteelwork Contractors for Bridgework includes allthose with audited technical capability and commercialstanding: it provides information on their size andrange of capabilities. Further details of the Register aregiven in Chapter 9.

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Bridge Type Universal Beam Plate Girder Box Girder

Steel Materials 45 30 30

Fabrication 30 40 60

Protective Treatment 15 15 25

Erection 15 15 25

Total 105 100 140

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8.2 Tonna Bridge Replacement

Client: Neath Port Talbot County Borough Council

Engineer: Director of Technical Services,Neath Port Talbot County Borough Council

Design: Director of Technical Services,Neath Port Talbot County Borough Council

Main Contractor: Balfour Beatty Construction

Steelwork Subcontractor: Rowecord Engineering Ltd

Completion date: December 2001

Approximate steelwork weight: 122 tonnes

Tonna Road Bridge carries the B4434 over the RiverNeath, providing a key link from the A465 Trunk Roadto villages in the Neath Valley. The load carryingcapacity of the old riveted plate girder bridge wasassessed as only suitable for vehicles up to 3 tonnesgross vehicle weight. Enforcement of this weight limitwould have severely disrupted the operation of theCalor Gas Distribution Depot located on the northernside of the river and impacted on local bus services,businesses and residents. Council funding wassecured to replace the 30m long, two span, single-lanebridge built in 1911. The new bridge provides a 7.3mwide carriageway with footways on both sides,spanning 35m over the River Neath. The steel andconcrete composite deck consists of seven 1075 x500 profiled welded plate girders, carrying a 175mmthick reinforced concrete deck slab. The depth of thebeams was constrained by the vertical highwayalignment, and the 1 in 100 year flood profile of theriver. So ‘compact’ beams were required to supportthe deck, cambered to match the vertical curve of thehighway. The mass concrete substructures wereconstructed directly behind the existing masonryabutments, supporting the superstructure onFreyssinet bearings.

To maintain vehicular access, the new bridge had to beconstructed in two halves. In the first phase of the

project the existing bridge was kept in service duringthe construction of the downstream half of the bridge.The old bridge carried a multitude of statutoryundertakers' services including a high pressure watermain and gas and electricity services, so a great dealof work was required to transfer each of these mainsonto the new bridge whilst keeping the road open.

The first half of the new bridge was opened to traffic inSeptember 2001, allowing the demolition of the oldbridge, the construction of the second half of the newbridge, and subsequent removal of the adjacentfootbridge. The second phase included most of theroadworks construction and tie-ins, and wascompleted for the opening of the completed bridge on10th December 2001. The footbridge was refurbishedand recycled to reinstate a breach in the publicfootpath network, some five miles north of the bridgesite.

The project was let under the I.C.E. Conditions ofContract (5th Edition), but managed as an informalpartnering agreement. This allowed the design andconstruction teams to value engineer a number ofoptions, including the use of permanent formwork,working around service diversions and designing outthe need for a mid-span prop during deckconstruction.

8.1 Introduction

This chapter describes eight recently constructed short to medium span steel highway and rail bridges. All utilisecomposite construction and the majority were procured under design and construct contracts with steel providing themost competitive solution. The highway bridges have been selected to demonstrate the use of simple andcontinuous spans, multiple girder cross sections and ‘ladder’ bridges. The rail bridge case studies, together withsome of the road bridges, provide excellent examples of the use of steel construction to overcome severe siteconstraints.

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CHAPTER 8CASE STUDIES

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8.3 Road Bridge A34 Trunk Road – Newbury Bypass

Client: Neath Port Talbot County Borough Council

Client: The Highways Agency

Engineer: Mott MacDonald

Design: Mott MacDonald

Main Contractor: Costain Civil Engineering

Steelwork Subcontractor: Kvaerner Cleveland Bridge Ltd

Completion date: 1998

Approximate steelwork weight: 460 tonnes

This 11.3m wide Donnington Link Bridge carries theprevious northbound carriageway of the A34 over theNewbury Bypass at a skew of 66 degrees. Thecurvature of the A34 at this point required significantwidening of the central reserve and inside verge forvisibility, affecting the positioning and size of the bridgesupports. To minimise these increases single circularcolumns were selected for the intermediate supportsresulting in a four span bridge with an overall length of157m. The two centre spans are 40m in length andtwo end spans 33.5m.

A deck with twin steel box girders was selected toprovide the torsional rigidity required by the singlecolumns. Twin boxes were chosen as opposed to awider single box to ease transport to site, and inverted"top hat" girders with an insitu concrete top slabselected to simplify fabrication. All site connectionswere detailed for bolting. The use of single columnpiers avoided the visual conflict that multi-column piershave on high skew bridges.

The girders were delivered to site singly in sections.They were lifted into position by crane on tointermediate temporary supports for splicing andconnection at the permanent supports by soliddiaphragms. Additional temporary supports were

provided under outer webs at the piers to providestability until the deck slab had been cast and theboxes had achieved their torsional strength: these tookthe form of reinforced concrete columns supported onthe pier bases using formwork for the pier of anotherbridge to cast them. The intermediate temporarysupports were removed once the girders had beenbolted together longitudinally, joined at the piers andabutments by the transverse diaphragms, and thepermanent bearings grouted.

The box girders and the transverse diaphragms actcompositely with the insitu deck slab, which was castusing permanent formwork within the boxes. GKN"Paraslim" formwork was used for the cantilevers.

Ease of maintenance was given a high priority in thedetailing of the bridge. Access is available fromchambers at the abutments along the whole length ofthe inside of the boxes, and through the diaphragms,with crossings provided between the girders atintermediate positions. Positive drainage is providedfrom the carriageway over the bridge and from theexpansion joints should they leak. The girders areprotected with a four-coat external paint systemfinished with a polyurethane gloss to provide a longmaintenance-free life.

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8.4 Westgate Bridges Deck Replacement

Client: Gloucestershire County Council

Engineer: Halcrow Group Ltd

Contractor: John Mowlem & Company plc

Steelwork Subcontractor: Fairfield-Mabey Ltd

Completion date: June 2000

Approximate steelwork weight: 590 tonnes

The Westgate Bridges provide a strategic access toGloucester City from the West. The original bridgeswere constructed in 1972 and comprised two identical92m post tensioned concrete bridges with precastpre-tensioned concrete suspended spans. Eachbridge carries one carriageway of the dual A417 overthe Eastern Arm of the River Severn at Gloucester.

Deterioration of the prestressed concrete was sosevere that a 3 tonne weight limit was imposed andprevented gridlock of the roads in Gloucester. Bridgedeck replacement was compared with repair optionsincluding works to the halving joints and replacementof the concrete span with a lightweight structure.‘Whole Life Costing’ techniques confirmed that repairswere not financially viable and the bridge deck wouldneed to be replaced.

Replacement of the superstructure was governedentirely by the site constraints and sensitive traffic flowson roads and river. The site constraints included:

– extensive services throughout the site,

– site of archaeological interest,

– maintenance of waterway headroom,

– essential maintenance of peak hour traffic flows toavoid gridlock,

– poor ground conditions,

– works located within flood plain,

– vehicular and pedestrian access to be maintainedthrough the site,

– residential and commercial properties adjacent tosite, and

– existing footbridge located between road bridges.

The superstructure type was chosen for ease ofconstruction and to minimise maintenancerequirements over the River Severn. The chosensuperstructure has two continuous three spanoverbridges with fabricated girders in weathering steelacting compositely with an in situ reinforced concretedeck slab.

Close co-operation between the Designer andContractor produced an innovative method ofconstruction using a bridge slide to maintain two lanesof traffic at peak hours in both directions throughoutthe works and prevent gridlock of Gloucester. Valueengineering was encouraged throughout the projectand a number of significant changes were made to thedesign. These included the retention of the existingpile caps and the use of polystyrene backfill behind theabutments.

To prevent gridlock and maintain two lanes of traffic thefirst new superstructure was constructed between theexisting bridges. This with the existing South Bridgewould carry in and outbound traffic enabling demolitionand replacement of the North Bridge. Traffic wouldthen use the two new superstructures duringdemolition of the South Bridge. During a weekendroad closure the new South Bridge was slid into its finalposition. The bridge weighed approximately 2000tonnes and took only four and a half hours to slide the12m along PTFE stainless steel tracks into position.The bridge was re-opened to traffic some 45 hoursafter the road closure.

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8.5 Churn Valley Viaduct

Client: RMS (Gloucester)

Designer: KBR (Brown & Root)

Main Contractor: Road Management Group –comprising Amec, AlfredMcAlpine, Dragados and Brown & Root

Steelwork Sub-contractor: Fairfield-Mabey Ltd

Completion date: 1998

Approximate steelwork weight: 890 tonnes

The A419/A417 forms a strategic link between the M4and the M5 near Gloucester. In 1994, a contract toupgrade and manage the road was let as a DBFOcontract to the Road Management Group consortium(RMG). Speed of construction was a key driver behindthe design.

Churn Valley Viaduct is the major structure on theA417/A419 scheme. It comprises a 250m long, sevenspan continuous steel plate girder composite deck,crossing the River Churn and floodplain, the A435 andan access track. The project was approved by theRoyal Fine Arts Commission and Joint AdvisoryCommittee for the Cotswold Area of OutstandingNatural Beauty.

Each carriageway comprises two lanes with 1m hardstrips giving an overall deck width of 22.5m. Thespans were chosen to provide a similar aspect ratiobetween the span and the height above the valley floor.Each girder is constructed from separate pier and spansections with bolted splices at approximate points ofcontraflex. The eight plate girders are cranked at thesplices to maintain a near 2.5m constant spacingaround the curved horizontal profile. The steelwork topflanges were designed with a constant width tostandardise the design of the EMJ permanentformwork, deck slab and girders. The bottom flangewidth and thickness were varied to provide efficiency indesign

Churn Valley Viaduct is at the bottom of a large sagcurve for the road alignment, so the bridge drainagecovers a significant catchment area with the low pointapproximately midway along the bridge. Bridge deckkerb drainage units along the inner channels feed intoa collector pipe located within the central reserve. Thisis hidden beneath the deck within the girder depth forappearance.

Multiple fixed and guided bearings were incorporatedto keep these elements within the standard rangesupplied by the manufacturer. These were designed toaccommodate the thermal movements and flexuralresponse of the structure.

Driven piled foundations were designed for economy inconstruction and programme efficiency; they alsosatisfy the Environment Agency requirements for theaquifer below bearing strata in the Churn Valley. Thepiles support flared leaf piers with two full heightreinforced concrete abutments on each side of thevalley. The substructure elements provide directsupport to each of the main girders with sufficientspace on the bearing shelf for flat jacks. Jackingpoints are provided beneath deep U-frame supportbracing to keep the piers slender.

The design team included representatives from theconstruction partners to ensure the delivered productsuited the particular skills of the Contractor. Steelworkmember sizes and details were standardised with inputfrom subcontractors to ensure efficiency and speed ofconstruction issues were embodied within the design:the additional cost of material was insignificantcompared to the savings in programme and easierconstruction.

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8.6 A500 Basford Hough Shavington Bypass,London to Crewe Railway Bridge

Client: Highways Agency

Design: Babtie Group

Main Contractor: John Mowlem & Company plc

Steelwork Subcontractor: Fairfield–Mabey Ltd

Completion date: 2002

Approximate steelwork weight: 1270 tonnes

The bridge is located just south of Crewe and carriesthe A500 over the four tracks of the west coast mainrailway line and ten other tracks. All fourteen tracks areelectrified with overhead catenaries supported fromhigh gantries.

The bridge is 194m long, 22.6m wide with a skew of23 degrees and consists of five continuous spans witha maximum length of 54.5m. Each half of the deckcarries one carriageway and is supported on two mainlongitudinal weathering steel girders 2.5m deep,interconnected with skew transverse cross girders andbracing frames. The skew of the deck elements wasdictated by the layout of the piers and for ease oflaunching. The deck slab of the bridge consists ofinsitu lightweight concrete on permanent ribbed grpformwork with precast concrete P6 parapets.

Each pier consists of a column under each bearingposition, linked at low level by reinforced concretewalls. Pier size was dictated by the need toaccommodate permanent bearings, temporary launchskates and jacks. Provision was also made for lateralguide rollers and a "fail safe" concrete upstand toprevent the girders falling sideways off the piers. Thesubstructures are supported on 600mm diameter CFApiles to a maximum depth of 23m.

The deck was constructed in two halves on thepartially constructed east approach embankment andlaunched longitudinally into position on the previouslyconstructed substructure. This avoided the need forintricate staged erection between overhead linesrequiring multiple possessions. The procedure isbelieved to be a world first for a composite bridge deckof this scale, in that the most of the concrete deck slabwas cast prior to the launch, including precastconcrete P6 parapets. The parapets and deck of the

leading and trailing spans were left off as they are notover railway lines. Each half of the bridge was launchedseparately, and both halves were completed in a 30hour period during a 53 hour possession at Christmas2001.

Temporary reinforced concrete foundations wereconstructed on the approach embankment to supportthe bridge before and during the launch. These wereset to follow an extrapolation of the 10,000m radiusvertical curve of the road over the railway. Temporaryskates were located at each of the temporaryfoundations and permanent piers, and a winch systemwas constructed at the eastern end of the decks.

A steel launching nose with a tapered soffit wasattached to the leading end of the deck to cater forcantilever deflections of up to 750mm. The soffit taperallowed the nose to land on the pier skates and guidethe girders back to the correct level. During launchingthe bridges deflected in cantilever by up to 380mm attheir maximum extent.

Fabrication of the deck steelwork began off-site in July2001. Construction of the substructure began inAugust, and construction of the embankment,temporary supports, deck and winch system tookplace between the beginning of October andChristmas.

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8.7 Doncaster North Bridge

Client: Doncaster Metropolitan Borough Council

Viaduct designer: Robert Benaim & Associates

Highway/abutment designer: Mott Macdonald

Main Contractor: AMEC

Steelwork Subcontractor: Watson Steel Ltd

Completion date: 2001

Approximate steelwork weight: 2000 tonnes

The viaduct is the main structure on the DoncasterNorth Bridge project, a design and construct schemecarrying a new dual carriageway road across the riverDon, the Sheffield and South Yorkshire Navigation, theEast Coast Mainline (ECML) and a number of local raillines and sidings. The viaduct has 15 spans and anoverall length of 620m. The maximum span is 47.3mover the railways. The superstructure consists of asingle continuous structure with two main steel girdersof 2.45m depth and cross girders at 3.8m centresforming a ladder arrangement. Permanent formworkspans longitudinally between the cross girders tosupport the 225mm concrete deck slab. The maingirders are curved in plan and use welded splices.Precast P6 parapets and P2 precast concrete copingswith a steel parapet form the edge of the bridge.

Intermediate elliptical shaped piers are located squareto the girders. Pot bearings are used throughout thestructure. The abutment consists of a reinforced earthwall directly behind the first pier. Single large diameterbored pile foundations are used throughout, except forpiers adjacent to the railway where multiple 600mmdiameter pile foundations were used.

The steel design was submitted as an alternative to theconcrete reference design to ease construction overthe railway and optimise programme and costrequirements. The main span steelwork over the ECMLwas erected in one 150 tonne lift. A 60m ladder ofmain girders and cross beams was constructed ontemporary supports adjacent to the railway and lifted induring an overnight possession of the railway,permanent formwork, edge cantilevers and concretingof the deck were completed in subsequentpossessions. A similar technique was used for theconstruction of the nearby spans over the river Don.

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8.8 A69 Haltwhistle Bypass Railway Viaduct

Client: Road Link (A69) Ltd for Highways Agency

Design – Bridge Deck: Cass Hayward & Partners

Design – Substructure: Pell Frischmann

Main Contractor: A69 Haltwhistle BypassJoint Venture

Steelwork Subcontractor: Kvaerner Cleveland Bridge Ltd

Completion date: 1997

Approximate steelwork weight: 770 tonnes

This new viaduct carries the A69 Haltwhistle Bypassimprovement over the Newcastle to Carlisle railwayand Haltwhistle Burn in the north of England. The A69was the first project under DBFO concession to becompleted in the UK and the railway viaduct was themost prominent new structure on the route.

The bridge follows a tight 540m radius curve andsupports a single 7.3m wide carriageway with 1m widehard strips and a variable width extended verge on thesouth side to ensure sight lines are maintained. Thebridge width varies as a consequence andintermediate support locations were constrained bythe skewed alignment of the railway and river below. Awide "ladder" bridge provided the logical choice toaccommodate these geometrical criteria using a pair oftruly curved steel main plate girders, 3.2m deep, withcross girders at approximately 3.5m spacing betweenthem. Concrete deck cantilevers 2.0m long weremaintained throughout the length of the structure foraesthetic reasons and were designed toaccommodate the loading specification for the P6parapet required over the railway spans. Threeintermediate piers comprising twin reinforced concretecolumns within the 214m length provided supportunder each main girder leading to variable spansincluding a maximum span of 68 metres over therailway. All site joints were made using HSFG boltedconnections and RSA knee braces provide restraint tobottom flanges in compression at and near to supportsand adjacent to main girder splice positions. Thelonger span girders required two site splices within thelength between piers and these were arranged to suitan erection sequence using cranes progressing fromone abutment and phased installation over the railwayduring track possessions.

A special feature of the design was the use of part-depth precast concrete units for the deck slabcantilever construction encouraged by the restrictionsof time available for working over the railway. The deckwas cast in three phases, the first phase was for thecomplete slab width between the main girders usingOmnia-style permanent formwork spanning betweencross girders. The second phase involved theinstallation of precast edge units of two separate forms– as suited to the P2 and P6 parapet requirements –and incorporating a temporary over-deck restraintsystem which obviated any need to work below topflange level; separate units provided the wall elementfor the P6 parapets. The final phase involved placingreinforcement and concrete to achieve structuralcontinuity of the deck and parapet concrete. Theadoption of this method achieved significantprogramme benefits and helped to eliminate risks ofdelays from winter working in the exposed location.

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8.9 River Exe Viaduct

Client: Railtrack Great Western Zone

Design: Cass Hayward & Partners

Main Contractor: Hochtief Construction Ltd

Steelwork Subcontractor: Butterley Engineering Ltd

Completion date: October 1997

Approximate steelwork weight: 500 tonnes

This strategic rail bridge over a flood-prone river hadcome to the end of its life. Adoption of lightweightcontinuous steel construction for its replacement madeit possible to retain the existing sub-structures safely.Steel’s advantages of flexibility, shallow depth and lowweight were fully exploited to effect the replacementrapidly in two track possessions by rolling-in, includinga controlled jacking-down procedure to redistribute thereactions onto the substructures. New steelcrossheads used to transfer deck loading to theexisting columns were also designed as rolling-inbeams so minimising temporary works in the river anddemonstrating the advantages of the design andconstruct method of implementation.

The two-track bridge has spans of 24m, 27.3m and24m with severe skew and a plan taper towards oneend. The original was built in 1846 by Brunel as asingle track timber viaduct with a second track addedin 1861 of iron construction. In 1896 this was replacedby a three girder arrangement of three lattice girderssupported by wrought iron concrete filled columns inthe river. By 1997 a replacement for this structureproved essential and this was required to carryballasted tracks and to maintain the existing headroomover the river.

The two rail tracks are now carried by separate deckscontinuous over the three spans allowing each to beconstructed alongside the existing structure prior torolling-in during two track possessions. Each deckcomprises two steel plate girders with a speciallyshallow composite floor to accommodate extra ballastdepth without reducing existing headroom over theriver. The design was arranged so that lateralclearance is provided from the existing centre girderwhilst the new downline structure was rolled in. Thisenabled both tracks to be maintained in use at alltimes, except during the two possessions when onetrack was still available. At the piers the girders sit onsteel trimmers on bearings which are located inboardof the main girders so that they would be clear of theexisting column heads during construction. The

trimmers incorporate cantilever spreaders whichsupport the deck whilst rolling-in and later in serviceact as jacking points for maintenance.

The new deck has been designed so that the totalloads borne by the existing foundations arecomparable with those existing previously despite theprovision of extra ballast depth. However, theadoption of continuous spans with its technicaladvantages implied an increase of loads on theintermediate piers: this has been eliminated in designand construction by precambering the steelworkduring manufacture by approximately 300mm. Aftersteelwork preassembly, but before slab concreting androlling-in the girder ends were raised by controlledjacking having the effect of transferring sufficientreaction from piers to abutments.

At each river pier the wrought iron columns have beenretained and are now enveloped by a new steelcrosshead located above river level which has enabledthe existing substructure to remain intact. Thecrosshead consists of a pair of fabricated steel girders1.2m deep with concrete infill which was prestressedby Macalloy bars so as to grip the existing columns.The steel girders also acted as runways for rolling-inthe new superstructure and were specifically designedfor these dual functions. The existing abutments havebeen retained but with new precast sill beams todistribute loads from the new deck.

During each of two 57 hour weekend possessionsdemolition of the existing bridge floors was carried outby mobile impact plant working on the deck intofloating craft below. Mobile cranes removed theexisting metalwork. The rolling-in carriages,incorporating jackdown facility as already mounted onthe crossheads were used for controlled jacking of thesteelwork at pre-assembly, had 76mm diameter steelballs running within pairs of bullhead rails. Controlledpropulsion of each deck, complete with ballast andtrack weighing approximately 800 tonnes, wasachieved with hydraulic rams.

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The Newark Dyke rail bridge reconstruction demandeda high profile solution at a strategic river crossing onthe East Coast Main Line. Railtrack demanded anaesthetically pleasing solution whilst specifying newhigh speed design criteria and demanding safetyrequirements so as not to disturb the live railway. Steelwas chosen by the design and construction team asthe ideal structural material on which to base theirproposals to secure the contract. Steel was used forthe primary feature of the new main span, for specialsubstructures and the extensive temporary works,including piling, needed for launching and slide-inoperations. Steel’s high strength/weight ratio, shallowconstruction depth, flexibility, durability and robustqualities were essential ingredients in the success ofthis project.

Two previous bridges had carried the railway on askewed alignment over the River Trent – the originalwrought iron and cast iron trusses constructed in 1852being replaced by the all steel Whipple Murphy trussesin 1890, one for each track, which survived until now.After a series of short-term strengthenings, Railtrackdecided to replace the structure and, at the same time,take the opportunity to seek a solution that met theirfuture aspirations for higher speed trains, by increaseof the line speed from 100mph to 140mph. Theexisting double truss bridge involved reverse trackcurves and hence limited speeds at the site.

The 77m span bowstring half through bridge is carriedon new outboard foundations to avoid uncertaintiesassociated with re-use of the existing abutments. Thenecessarily heavier new superstructure, with itsballasted track and greater dynamic effects fromhigher speed trains, was considered likely to give longterm safety risks if the existing abutments on timberpiles were retained. The bridge itself is squarespanning, unlike the original, so as to eliminatepotential problems with track maintenance anddynamic behaviour. Main bowstring trusses are

diagonally braced to minimise deformations and arespaced at 11.25m centres to allow the tracks to be re-spaced for higher speed running. The top chord is ofopen ‘H’ steel plated section offering the maximumlateral inertia for stability whilst eliminating the need foroverhead bracings between the trusses and is 1.5mwide and 1.0m deep. Flanges and web are up to60mm plate thickness and the chord is straightbetween node points coinciding with a circular arc inelevation giving the best aesthetic appearance. Waterrun-off from the top chord is ensured by elimination ofstiffening on the top surface of the web. Diagonalsconsist of fabricated ‘I’ sections measuring 500mmtransverse to the bridge with flanges typically 325mmwide. Members of this form facilitate practicable andfatigue resistant welded end connections by elegantlyshaped integral gussets to the chords and offerrobustness against damage. Screwed rods, wireropes, strand or hollow sections have been used inbowstring bridges, but were, in this case, rejected dueto potential difficulties with compressive capability,durability, fatigue, creep or excessive maintenance ofpinned connections. Spacing of node joints isgenerally 8.46m, but is decreased at the ends foraesthetic reasons and to facilitate rigid U-frameconnection to the end three cross girders where thediagonals are of deeper section.

The bottom chord consists of a fabricated plate girder1.5m deep with its top flange level with the top of thedeck slab upstand robust kerb. Shear connectors areprovided along the full length of the bottom chord toachieve composite behaviour. At the bridge ends thetop and bottom chords converge to form a combinedstiffened fabrication with a downstand to bearing level.The main bearings are of fabricated steelwork andeliminated the necessity for limited life low frictionmaterials. Fixed end bearings are of linear rocker typewith roller type bearings at the free end; these bearingtypes also assist in stability of the bowstring topchords.

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8.10 Newark Dyke Rail Bridge Reconstructionover the River Trent

Client: Railtrack London North East Zone

Design – Superstructure: Cass Hayward & Partners

Design – Substructure: Corus Rail

Main Contractor: Skanska UK Ltd

Steelwork Subcontractor: Cleveland Bridge UK Ltd

Completion date: August 2000

Approximate steelwork weight: 670 tonnes

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The deck is of minimum depth to maintain headroomover the river and is supported by composite crossgirders 550mm deep at 2.82m spacing, cranked at theends to form rigid HSFG bolted end plate connectionswith the bowstring bottom chord. The slab is 250mmthick cast onto GRP permanent formwork andincorporates edge upstands containing the ballast andforming a robust kerb at least 400mm above rail levelto meet Railtrack standards.

Two continuous longitudinal steel stringers at 4.0mspacing interconnect the cross girders and serve todistribute concentrated live loadings effectivelybetween cross girders, forming virtually an isotropicfloor which fully participates as part of the bowstringbottom chord. This floor configuration enables thecalculated acceleration levels to be controlled to thespecified limits as demanded by new criteria fordynamic response under higher speed trains.

The solution for the substructures completelyeliminated risks involved in excavation beneath the liverailway. During a railway possession at Christmas1999 mined openings were formed through each ofthe existing abutments which otherwise remainedintact to support the existing superstructures. Newindependent foundations were constructed outside thehazard zone of the railway to receive the ends ofprefabricated steel box girder needle beams whichwere installed through each opening. The needlebeam supports the permanent bearings and alsofacilitated installation of the superstructure by sliding-inon paths on its top surface fully prepared in advance.

Main bowstring girders manufactured and trial-assembled in Darlington were welded up to full lengthstanding upright at site before launching outindividually over the river immediately adjacent to theexisting live bridge. Transverse slides enabled thegirders to be positioned at the correct spacing toreceive cross girders which were erected using apurpose-made steel gantry employing the permanentrunway beams suspended beneath. Followingcompletion of the deck slab and waterproofing thebridge was ready to be slid into place during theAugust 2000 Bank Holiday railway possession, whichwas successfully achieved along with re-alignment ofthe tracks and erection of new steel overheadelectrification masts well within the 72 hours allocated.

The new bridge is the first to be completed in the UKwhich is designed to counteract the dynamic effects ofhigh speed 140mph trains under European basedcriteria.

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9.1 Introduction

The objective of this book is to help designers of steelbridges to achieve optimum solutions for their clients,but it is appropriate in conclusion to reflect on howdesign solutions are to be constructed successfully.The client requires a robust supply chain forprocurement of his project: each party has to becompetent and have the resources to fulfil its role.

Successful construction is also safe construction andcompetence for the task is absolutely essential for asafe outcome as well as a technically and commerciallysuccessful project. Evolving UK health and safetylegislation was rationalised by the Health & Safety Actin 1974 but it was the Construction (Design andManagement) Regulations of 1994 which made explicitthe indisputable requirement for competence in designand construction: the Regulations placed obligationson the parties down the supply chain to assesscompetence and satisfy themselves of thecompetence of parties before placing work with them.In brief, responsibility for ensuring safe practice cannotbe escaped by subcontracting. How do Clients, orDesigners or Principal Contractors, ensure thatsubcontracted work is done safely?

The problem of assessing competence in steelconstruction, let alone for bridgework, may often beaggravated because the Client, the Engineer and theMain Contractor are not expert in the intricacies ofworking with steel. This is a problem for the industrytoo, because this can increase the risk of a clientaccepting an unsustainably low price from aninexperienced steel firm obviating fair competition.How can this risk be reduced?

9.2 The Register of Qualified SteelworkContractors

The government sponsored research into procurementin the construction industry included the issues ofprocuring competence and quality performance. Thisresearch, undertaken by the BCSA, led to theformation of the Register of Qualified SteelworkContractors Scheme to provide procurement agencieswith a reliable listing of steelwork contractors whichidentifies their capabilities for types of steelconstruction, and suggested maximum contract value.

A fundamental principle of the Register is thatcompanies may apply to join it and are subject toindependent expert audit before admission: theRegister is administered by the Association but it isopen to any capable steelwork contractor to join,member or non-member, British or foreign.

On receipt of an application to join the Register, theexperienced professional auditors visit the company at

its premises to assess its capabilities in elevencategories of building steelwork and/or six sub-categories of bridge construction. The auditors alsotake up relevant project references before theapplication is accepted. Registered companies makeannual returns and the auditors re-assess eachcompany at the works triennially and when there aresignificant changes. Entry in the Bridgeworks sectionof the Register involves a more extensive audit with awider range of acceptance criteria.

There are over 65 companies currently on the Registerwith several listed for Bridgeworks, including all theprincipal UK steel bridgework contractors (seewww.steelconstruction.org).

9.3 Bridgeworks Scheme

All companies on the Register have to satisfy theauditor of their financial standing and resources: to beregistered in the Bridgework Category, a companymust have a minimum turnover in steelwork forbridges** of £1 million in the most recent year oralternatively per annum if averaged over the last threeyears.

The company must present references for completedsupply and erect contracts that include at least sixbridgework** contracts undertaken over the last fiveyears, of which two must each exceed £100,000contract value completed within the last three years.

The company's track record and the company'ssystems, existing facilities and employed personnel willbe used to establish its capability.

– The track record will be based principally on the two£100,000 contracts. If necessary in addition othercontract references of comparable complexity (butnot necessarily of £100,000 value or as recent) canbe used.

– The contracts can have been undertaken withsublet erection, but must have been either forbridges** exceeding 20m span, or for mechanicallyoperated moving bridges. One contract mustinvolve the application of multi-coat treatment,which may have been sublet.

– The end-user clients, who must be different for thetwo contracts, will be contacted to establish theirsatisfaction with the work on the contracts.

– The company must have manufactured in-house atleast 75% of the steelwork for each of the twocontracts. Both contracts must have requiredmaterials and workmanship to BS 5400-6**. Oneof the contracts must have required thick platewelding such as the butt welding of S355J2 plate ina thickness of at least 40mm.

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– The company will need to demonstrate that it has

erected a bridge** of at least 30m span over water,

and a bridge** that involved the use of a railway

possession.

– The company's quality system must be

independently certified to meet the requirements of

BS EN ISO 9001 with a scope of registration that

includes steel bridges**.

– The company must have the ability undercover in its

own works to lift a single piece of 20 tonnes using

EOT cranes singly or in tandem. The company must

be able to demonstrate that it has the ability to

undertake trial assembly of large pieces post-

fabrication and prior to despatch.

– The company must employ at least one suitably

competent person with clearly designated tasks

and responsibility in each of three key management

disciplines: technical/design, welding and erection

methods.

• The technical/design manager should have

appropriate specialised technical knowledge

relevant to the assigned tasks, and at least five

year's steel bridge construction engineering

experience.

Note: In terms of knowledge, an individual with

Chartered/Incorporated membership of one of

the ICE, IStructE or IMechE would be

appropriate.

• The manager of welding coordination should

have appropriate specialised technical

knowledge relevant to the assigned tasks, and

at least five year's experience in the execution of

steelwork.

Note: In terms of knowledge, a welding

specialist with Specific knowledge to BS EN

719, or with the qualification of European

Welding Technologist, or with individual

Chartered/Incorporated membership of the

Welding Institute would be appropriate.

• The manager in charge of erection methods

should have a knowledge of the CDM and

CHSW Regulations, and be able to produce a

copy of the erection method statement that

he/she has authored for used on a complex

contract.

– The company must employ welders with suitable

approvals.

Based on evidence from the company's resources and

portfolio of experience, the Subcategories that can be

awarded are as follows:

FG Footbridges and Sign Gantries

PT Plate girders [>900mm deep],

trusswork [>20m long]

BA Stiffened complex platework in decks, box

girders, arch boxes

CM Cable-stayed bridges, suspension bridges,

other major structures [>100m]

MB Moving bridges

RF Bridge refurbishment

X Unclassified

Companies wishing to be registered in the Bridgework

Category but which do not possess suitably complete

bridgework experience may be registered as

unclassified companies. For acceptance in this sub-

category such companies need to fulfil all the

requirements set out above, but where the rules are

marked ** they may use contracts of comparable

complexity for steelwork other than bridgework.

Unclassified companies cannot be awarded other sub-

categories in the Bridgeworks Category. This enables

companies with the relevant technical and managerial

competence to enter the Bridgeworks Category and

reflects the auditor’s professional opinion of that

competence.

9.4 Use of the Bridgeworks Register

The Bridgeworks Register provides an effective pre-

qualification mechanism to match steelwork

contractors to the needs of particular bridge tenders.

Although the Register itself is not a quality assurance

scheme, listing in the Bridgeworks Category verifies

that the company's quality management systems are

third party accredited by an appropriate body.

Particular projects may present requirements or

challenges to prospective tenderers which go beyond

the range of criteria of the Bridgework Category: that is

for the procurement organisation to identify but it can

be assured that a Registered Bridgework Contractor

meets the essential basic requirements for

bridgeworks. The use of a Registered company

matched to the demands of the project is a prima facie

defence to any allegation that insufficient care was

taken in selecting a competent steelwork contractor.

The Highways Agency has given a lead to prospective

bridge owners by requiring that only firms listed on the

Register of Qualified Steelwork Contractors for the

type and value of work to be undertaken will be

employed for the fabrication and erection of

bridgeworks. This places an obligation on tendering

main contractors to anticipate this requirement in

pricing their bids; and indeed, it assists them in fulfilling

their duties under the C(D&M) Regulations.

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10.1 Codes and Standards referred to in this edition

• BS 4-1:1993 Structural steel sections. Specificationfor hot-rolled sections

• BS 153: Specification for steel girder bridges.

• BS 153-1 & 2:1972 (withdrawn) Materials andworkmanship. Weighing, shipping and erection

• BS 153-3A:1972 (withdrawn) Loads

• BS 153-3B & 4:1972 (withdrawn) Stresses. Designand construction

• BS 3692:2001 ISO metric precision hexagon bolts,screws and nuts. Specification

• BS 4190:2001 ISO metric black hexagon bolts,screws and nuts. Specification

• BS 4360:1990 (withdrawn) Specification forweldable structural steels

• BS 4395: Specification for high strength friction gripbolts and associated nuts and washers forstructural engineering.

BS 4395-1:1969 General grade

BS 4395-2:1969 Higher grade bolts andnuts and general grade washers

• BS 4570:1985 Specification for fusion welding ofsteel castings.

• BS 4604: Specification for the use of high strengthfriction grip bolts in structural steelwork.

BS 4604-1:1970 Metric series. Generalgrade

BS 4604-2:1970 Metric series. Highergrade (parallel shank)

• BS 4870 (withdrawn): Specification for approvaltesting of welding procedures.

BS 4870-1:1981 (withdrawn) Fusionwelding of steel

BS 4870-4:1988 (withdrawn) Specificationfor automatic fusion welding of metallicmaterials, including welding operatorapproval

• BS 5135:1984 (withdrawn) Specification for arcwelding of carbon and carbon manganese steels

• BS 5400: Steel, concrete and composite bridges.

• BS 5400-1:1988 General statement

• BS 5400-2:1978 (partially replaced) Specificationfor loads

• BS 5400-3:2000 Code of practice for design ofsteel bridges

• BS 5400-4:1990 Code of practice for design ofconcrete bridges

• BS 5400-5:1979 Code of practice for design ofcomposite bridges

• BS 5400-6:1999 Specification for materials andworkmanship, steel

• BS 5400-7:1978 Specification for materials andworkmanship, concrete, reinforcement andprestressing tendons

• BS 5400-8:1978 Recommendations for materialsand workmanship, concrete, reinforcement andprestressing tendons

• BS 5400-9.1:1983 Bridge bearings. Code ofpractice for design of bridge bearings

• BS 5400-9.2:1983 Bridge bearings. Specificationfor materials, manufacture and installation of bridgebearings

• BS 5400-10:1980 Code of practice for fatigue

• BS 5400-10C:1999 Charts for classification ofdetails for fatigue

• BS 5996:1993 (withdrawn) Specification foracceptance levels for internal imperfections in steelplate, strip and wide flats, based on ultrasonictesting

• BS 7644: Direct tension indicators.

BS 7644-1:1993 Specification forcompressible washers

BS 7644-2:1993 Specification for nut faceand bolt face washers

• BS 7668:1994 Specification for weldable structuralsteels. Hot finished structural hollow sections inweather resistant steels

• BS EN 287: Approval testing of welders for fusionwelding.

BS EN 287-1:1992 Steels

• BS EN 288: Specification and approval of weldingprocedures for metallic materials.

BS EN 288-1:1992 General rules for fusionwelding

BS EN 288-2:1992 Welding proceduresspecification for arc welding

BS EN 288-3:1992 Welding proceduretests for the arc welding of steels

• BS EN 1011: Welding. Recommendations forwelding of metallic materials.

BS EN 1011-1:1998 General guidance forarc welding

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CHAPTER 10REFERENCES

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BS EN 1011-2:2001 Arc welding of ferriticsteels

• BS EN 10025:1993 Hot rolled products of non-alloy structural steels. Technical delivery conditions

• BS EN 10045: Charpy impact test on metallicmaterials.

BS EN 10045-1:1990 Test method (V- andU-notches)

BS EN 10045-2:1993 Method for theverification of impact testing machines

• BS EN 10113: Hot-rolled products in weldable finegrain structural steels.

BS EN 10113-1:1993 General deliveryconditions

BS EN 10113-2:1993 Delivery conditionsfor normalized/normalized rolled steels

BS EN 10113-3:1993 Delivery conditionsfor thermomechanical rolled steels

• BS EN 10137: Plates and wide flats made of highyield strength structural steels in the quenched andtempered or precipitation hardened conditions.

BS EN 10137-1:1996 General deliveryconditions

BS EN 10137-2:1996 Delivery conditionsfor quenched and tempered steels

BS EN 10137-3:1996 Delivery conditionsfor precipitation hardened steels

• BS EN 10155:1993 Structural steels with improvedatmospheric corrosion resistance. Technicaldelivery conditions

• BS EN 10160:1999 Ultrasonic testing of steel flatproduct of thickness equal or greater than 6 mm(reflection method)

• BS EN 10164:1993 Steel products with improveddeformation properties perpendicular to the surfaceof the product. Technical delivery conditions

• BS EN 10210: Hot finished structural hollowsections of non-alloy and fine grain structural steels.

o BS EN 10210-1:1994 Technical deliveryrequirements

o BS EN 10210-2:1997 Tolerances, dimensions andsectional properties

• BS EN 10306:2002 Iron and steel. Ultrasonictesting of H beams with parallel flanges and IPEbeams

• BS EN ISO 4063:2000 Welding and alliedprocesses. Nomenclature of processes andreference numbers

10.2 Other documents, references, standardsreferred to in this edition

• Corus

Publications:

The Design of Steel Footbridges (10/2000)

Weathering Steel Bridges (01/2002)

Composite Steel Highway Bridges(02/2002)

Plate/Section Availability:

Plate Products: Range of Sizes

Structural Sections - To BS4: Part 1: 1993and BS EN10056: 1999

• European Convention for Constructional Steelwork

The Use of Weathering Steel in Bridges (Bridges inSteel no. 81 – 2001)

• Health and Safety Executive

Construction (Design and Management) (Amendment)Regulations 2000

• Highways Agency

Specification for Highways Works

Design Manual for Roads and Bridges

1.3 - BD13/90 - Design of Steel Bridges.Use of BS5400: Part 3: 1982

1.3 - BD24/92 - Design of ConcreteHighway Bridges and Structures. Use ofBS5400: Part 4: 1990

1.3 - BD16/82 - Design of CompositeBridges. Use of BS5400: Part 5: 1979

2.3 - BD20/92 - Bridge Bearings. Use ofBS5400: Part 9: 1983

1.3 - BD9/81 – Implementation of BS5400:Part 10: 1980 – Code of Practice forFatigue

2.3 - BD7/01 - Weathering Steel forHighway Structures

• Network Rail (Railtrack)

Railtrack Group Standards

• Steel Construction Institute / Steel Bridge Group.

Guidance Notes on Best Practice in Steel BridgeConstruction:

GN 5.04 – Plate Bending

GN 7.03 – Verticality of webs at supports

Specification of Structural Steelwork for Bridges: AModel Appendix 18/1

10.3 Suggested further reading and sources ofinformation

• CIRIA. Bridges – design for improved buildability(R155).

• Institution of Civil Engineers. Proceedings.Published papers (periodic) on the design andconstruction of steel bridges (see also section 10.X)

• nstitution of Structural Engineers. Journal.Published papers (periodic) on the design andconstruction of steel bridges.

• New Steel Construction. Bi-monthly publicationfrom BCSA and SCI, containing periodic articles onsteel bridge construction and specific projects.

• Steel Construction Institute. Steel Designers’Manual – 5th Edition

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• Steel Construction Institute / Steel Bridge Group.Guidance Notes on Best Practice in Steel BridgeConstruction – 3rd issue. 2002.

The Steel Bridge group is a technical forumestablished to consider matters of high priority andinterest to the steel bridge construction industry andto suggest strategies for improving the use of steel inbridgework. This publication presents a collection ofseparate Guidance Notes on a wide range of topicsconcerning the design and construction of steelbridges: it covers design, materials, contractdocumentation, fabrication, inspection and testing,erection and protective treatment. The representationof diverse interests in the Group means that theGuidance Notes can be considered to be guides togood, accepted practice.

• Steel Construction Institute. Series of publicationrelating to bridge design, including design guidesand worked examples, commentaries on relevantStandards and Specification guidance.

• The Welding Institute. Published papers on weldingaspects of the fabrication of steel bridges.

10.4 References used within the text of the first edition

• BS4: Part 1: 1980. Specification for hot rolledsections.

• BS153: Parts 3B and 4: 1972. Steel GirderBridges.

Part 3B: Stresses Part 4: Design and construction.

• BS4360: 1979. Specification for weldablestructural steels.

• BS5135: 1984. Process of arc welding of carbonand carbon manganese steels

• DD21: 1972. Quality grading of steel plate from12mm to 150mm thick by means of ultrasonictesting.

• PD6493: 1980. Guidance on some methods forthe derivation of acceptance levels for defects infusion welded joints.

• American Iron and Steel Institute - Task group onweathering steel bridges. 1982.

Performances of weathering steel in highway bridges- A first phase report.

• BCSA. New developments in steel constructionpart 11 – bridges.

• BCSA. Structural steelwork – fabrication.

• Branco, FA and Green R. 1982. Constructionbracing for composite box girder bridges.International conference on short and medium spanbridges, Toronto.

• British Bridge Builders. 1981. Bridging theHumber. Compiled by G Wilkinson.

• Constrado. 1983. Weather resistant steel forbridgework.

• Constrado. 1984. Steel bridge design guide –composite UB simply supported span (by GFJNash).

• Blodgett OW. 1972. "Distortion …. How MetalProperties Affect It". Welding Engineer, February.

• Burdekin, Prof. FM. 1981. "To control distortion,keep it flat, keep it straight". Metal Construction,October.

• Freeman RA. 1977. Erection of AvonmouthBridge. Acier: Stahl: Steel 5. 1977.

• Hansen, B. 1973. "Formulas for residual weldingstresses and distortions". The Danish WeldingInstitute. Publication P73044.

• Hayward ACGH. 1984. Alternative designs ofshorter span bridges. National Structural SteelConference. New developments in steelconstruction. London.

• HMSO. 1973. Inquiry into the basis of design andmethod of erection of steel box girder bridges.Report of the Committee – Appendix 1 Interimdesign and workmanship rules (known as MerrisonRules or IDWR)

• Hunter IE & McKeown ME, 1984 "Foyle Bridge:Fabrication and Construction of the Main Spans".Proceedings ICE, vol 76, May.

• Hyatt KE. 1968 "Severn Bridge: Fabrication &Erection". Proceedings ICE vol 41, September.

• Kerensky OA and Dallard NJ. 1968. The four levelinterchange between M4 and M5 motorways atAlmondsbury. Proceedings ICE vol 40 July. p295-321.

• Leggatt RH & White JD. "Predicted shrinkage &distortion in a welded plate". Proceedings of 1977Conference on Residual Stresses in WeldedConstruction and Their Effects. TWI.

• Paton J, Fraser DD, Davidson JB. 1968. Specialfeatures of Hamilton bypass motorway (M74).Proceedings, ICE vol 41, October.

• Roberts Sir Gilbert, 1968 "Severn Bridge: Designand Construct Arrangements". Proceedings, ICEvol 41, September.

• Wex BP, Gillespie NM and Kinsella J, 1984 "FoyleBridge: Design & Tender in a Design & BuildingCompetition". Proceedings, ICE vol 76, May.

• White JD, Leggatt RH and Dwight JB. 1979."Weld shrinkage prediction". Second InternationalConference on Offshore Structures. Paper 19,Imperial College, London.

10.5 Articles about early steel bridges

This list was provided by the Library of the Institutionof Civil Engineers to whom reference can be made toexamine or obtain copies of the articles given below:

• Baker, Sir B. Steel in railway structures. ICE MinProcs vol 85, 1886. pp140.

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Page 96: Steel Bridges

• Bernhard, C. Steel arched road bridge over theNeckar at Mannheim. ICE Min Procs vol 147, 1902.pp445-6; vol 150, 1902. p478.

• Bhvr, WH. The Ohio bridge of the Cincinnatti andConington Elevated Railway. ICE Min Procs vol 103,1890. pp411-415.

• Bischoff, F. On the use of steel in bridgeconstruction. ICE Min Procs vol 107, 1891.pp452-455.

• Brady, AB. Herbert River bridge, Gairloch,Queensland. ICE Min Procs vol 124, 1896. pp324-334.

• Bridge over the Mississippi at Memphis. ICE MinProcs vol 109, 1892. pp445-446.

• Burge, CO. The Hawkesbury bridge, New SouthWales. ICE Min Procs vol 101, 1890. pp2-12.

• Clarke, TC. The design of iron bridges of very largespan for railway traffic. ICE Min Procs vol 84, 1886.pp179-250, esp p192.

• Cooper, T. The use of steel for bridges. ASCETransactions, vol 8. pp283-.

• Crofts, HA. New bridge, Portland, USA. ICE MinProcs vol 165, 1906. pp392.

• Crultwell, GEW and Hornfray, SG. Tower Bridge.ICE Min Procs vol 127, 1897. pp35-82.

• Dumas, A. Alexander III bridge over the Seine. ICEMin Procs vol 130, 1897. pp335-6.

• Frere, FH. Reconstruction of Midland RailwayBridge across the River Trent. ICE Min Procs vol154, 1903. pp267-276.

• Goldsmith, AJ. Burnett and Kennedy Bridges,Bundaberg, Queensland. ICE Min Procs vol 153,1903. pp267-279.

• Guadard, J. Correspondence on the erection ofiron bridges. ICE Min Procs vol 157, 1904. pp248-253.

• Hackney, W. The manufacture of steel. ICE MinProcs vol 42, 1875 pp2-68

• Hallopeau, PFA. Employment of mild steel inrailway bridges. ICE Min Procs vol 96, 1889.pp371-372.

• Hanson, CR. Malay States Railways. ICE Min Procsvol 150, 1902. pp330-333.

• Hobler, GA. Cairns Railway. ICE Min Procs vol 153,1903. p28 + plate 5.

• Hobson, GA. Victoria Falls Bridge. ICE Min Procsvol 170, 1907. pp1-49.

• Jackson, CFV. Design and construction of steelbridge work with particulars of a recent example inQueensland. ICE Min Procs vol 142, 1900. pp253-271.

• Klette, H. Queen Carola Bridge, Dresden. ICE MinProcs vol 130, 1987. pp336-7.

• Krolm, R. Use of steel in bridge construction. ICEMin Procs vol 107, 1891. pp450-452.

• Krossen road bridge over the Oder. ICE Min Procsvol 170, 1907. pp409.

• Laurie, KA. Jubilee bridge over the River Hooghly.ICE Min Procs vol 158, 1904. pp374-379.

• Matheson, E. Steel for Structures. ICE Min Procsvol 69, 1882. pp1-78, esp pp63-64.

• Nansouty, M de. Road bridge over the Rhone atLyons. ICE Min Procs vol 108, 1892. pp430-432.

• Nansouty, M de. The new bridge over the Rhone atLyons. ICE Min Procs vol 106, 1891. pp360-2.

• New Westminster Bridge over the Fraser River,British Colombia. ICE Min Procs vol 164, 1906.pp447-448.

• Ridder, FW and Fox, FD. Great Central railwayextension. ICE Min Procs vol 142, 1900. pp5-75.

• Roberts-Austen, Sir W. Use of cast steel in theAlexander III bridge. ICE Min Procs vol 150, 1902.pp164-66.

• Sharpe, W. On some methods of steel bridgeerection. ICE Min Procs vol 150, 1902. pp352-360.

• Shaw, WR. Teesta bridge. ICE Min Procs vol 150,1902. pp361-375.

• Smith, JT. On Bessemer steel rails. ICE Min Procsvol 42, 1875. pp69-75. Discussion esp pp95-97.

• Smith, M. Steel and iron railway bridges in Canada.ICE Min Procs vol 117, 1893. pp315-318.

• Taylor, WO., Damodar Coal Line Bridge. ICE MinProcs vol 160, 1904. pp315-325.

• Walton, FTG. Construction of the Dufferin Bridgeover the Granges at Benares. ICE Min Procs vol101, 1890. pp13-24 and discussion pp38-72.

• White, Sir WH. Victoria Jubilee bridge, Montreal.ICE Min Procs vol 160, 1905. pp82.

• Whited, W. Oakland Bridge. ICE Min Procs vol 170,1907. pp408.

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