Composite Steel and Concrete

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Steel and Steel and ConcreteComposite Buildings

Companion Document toEN 1993 and EN 1994


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On 5th May 2006 the responsibilities of the Office of the Deputy Prime Minister (ODPM)transferred to the Department for Communities and Local Government (DCLG)

Department for Communities and Local GovernmentEland HouseBressenden PlaceLondonSW1E 5DU

Telephone: 020 7944 4400 Website: www.communities.gov.uk

Companion Document to EN 1993 and EN 1994 Steel and Steel and ConcreteComposite Buildings 2005Whilst this document provides practical guidance on the use of Eurocode BS EN 1993 and 1994

Steel and Steel and Concrete Composite Structures. It shall not be used for the design of actual projects until both the Eurocode and its National Annex are published by the British Standards Institution and approved for use by the First Secretary of State for England and Wales.

It should be noted that the guidance has been based on the latest draft Eurocode BS EN 1993 and 1994 available at the time of writing.

Crown Copyright, 2007

Copyright in the typographical arrangement rests with the Crown.

This publication, excluding logos, may be reproduced free of charge in any format or medium for research, private study or for internal circulation within an organisation. This is subject toit being reproduced accurately and not used in a misleading context. The material must be

acknowledged as Crown copyright and the title of the publication specified.

Any other use of the contents of this publication would require a copyright licence. Please apply for a Click-Use Licence for core material at www.opsi.gov.uk/click-use/system/online/pLogin.asp,or by writing to the Office of Public Sector Information, Information Policy Team, St ClementsHouse, 2-16 Colegate, Norwich, NR3 1BQ. Fax: 01603 723000 or email:[email protected]

If you require this publication in an alternative format please [email protected]

DCLG PublicationsPO Box 236

Wetherby West YorkshireLS23 7NBTel: 08701 226 236Fax: 08701 226 237Textphone: 08701 207 405Email: [email protected] online via the DCLG website: www.communities.gov.uk

January 2007

Product Code: 06 BD 04021 (f)


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CONTENTS

EXECUTIVE SUMMARY 7

CHAPTER 1 INTRODUCTION 8

1.1 Steel Structures 8

1.2 Composite steel and concrete structures 9

1.3 Aim and scope of this publication 9

CHAPTER 2 EUROCODES SYSTEM 10

2.1 Eurocodes Terminology 12

2.1.1 Types of clause used in the Eurocodes 12

CHAPTER 3 GENERAL DESIGN ISSUES 14

3.1 Convention for member axes 14

3.2 The explicit use of factors 14

3.2.1 Symbols used in the Eurocodes 15

3.3 Documents required when designing with the Eurocodes 16

CHAPTER 4 EN1993 STEEL STRUCTURES 17

4.1 Part 1-1: General Rules and Rules for Buildings 17

4.1.1 Material Properties 17

4.1.2 Ductility requirements for structural steel 17

4.1.3 Fracture toughness 18

4.1.4 Structural stability of frames 18

4.1.5 Structural imperfections 19

4.1.6 Buckling members in compression 194.1.7 Buckling uniform members in bending 20

4.1.8 Buckling uniform members in bendingand axial compression 21

4.2 Part 1-2: General rules Structural fire design 21

4.2.1 Material properties 21

4.2.2 Structural fire design 23

4.2.3 Members in compression 23

4.2.4 Combined bending and axial compression 23

4.2.5 Structural connections 23


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4.3 Part 1-8: Design of joints 23

4.3.1 Definitions 24

4.3.2 Material properties 24

4.3.3 Groups of fasteners 24

4.3.4 Analysis, classification and modelling 24

4.3.5 Structural joints connecting H or I sections 26

4.4 Part 1-10: Material toughness andthrough-thickness properties 27

4.4.1 Fracture toughness 27

4.4.2 Through-thickness properties 28

4.5 Part 5: Steel Piling 29

CHAPTER 5 EN1994 STEEL AND CONCRETECOMPOSITE STRUCTURES 31

5.1 Part 1-1: General rules and rules for buildings 31

5.1.1 Material Properties 31

5.1.2 Structural stability 32

5.1.3 Structural imperfections 32

5.1.4 Calculation of action (load) effects 32

5.1.5 Beams Ultimate Limit State 33

5.1.6 Beam serviceability limit state 345.1.7 Lateral torsional buckling 35

5.1.8 Members in compression 35

5.1.9 Composite joints in frames for buildings 36

5.1.10 Composite slabs with profiled metal sheeting 36

5.2 Part 1-2: Structural fire design 37

5.2.1 Fire exposure 37

5.2.2 Material Partial Factors 37

5.2.3 Structural analysis 37

5.2.4 Design procedures 37

5.2.5 Unprotected Composite Slabs 38

CHAPTER 6 EFFECTS ON UK STRUCTURALDESIGN PROCEDURES 40


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CHAPTER 7 DESIGN ROUTE MAPS 41

CHAPTER 8 REFERENCES 54

APPENDICES 56

Appendix A Eurocode clause reference tables 56

WORKED EXAMPLES 61

Anchored Sheet Pile Wall 62

Cantilever 83

Cantilever Sheet Pile Wall 90

Simply Supported Beam with Full Lateral Restraint 104

Simply Supported Beam with Lateral Restraint at the Load Points 111

Steel Driven Pile in Stiff Clay 119

Base Plate without Bending Moment 128

Simply Supported Beam with Full Lateral Restraint Fire Limit State 134

Simply Supported Composite Beam Fire Limit State 148

Partial Depth (flexible) End Plate Connection 157

Connections in Fire 176

Column in Simple Construction Fire Limit State 186

Column with Axial and Bi-Axial Moments (Due to simple connection) 195

Simply Supported Steel and Concrete Composite Beam 210

Concrete Filled CHS Composite Column 223

Continuous Steel and Concrete Composite Beam 235


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Companion Document to EN 1993 and EN 1994 Steel and Steel and Concrete Composite Buildings

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1 Introduction

The objectives of the Eurocodes are

To establish a common set of design rules for buildings and civil engineering works to beused across Europe.

To remove the barriers to free movement of products and engineering services betweenEuropean countries, by removing the obstacles caused by different nationally codifiedpractices for the assessment of structural reliability.

The emerging Eurocodes (ENs) have been developed following work undertaken to modifythe European Prestandards (ENVs). The ENVs were published with National ApplicationDocuments in the early 1990s to allow Designers to undertake provisional designs and make

comments on their content. Unlike the Eurocodes the ENVs did not have the status of European Standards.

Following a period of co-existence the current British Standards will be superseded by theEurocodes. These Eurocodes will be denoted as BS EN in the UK.

The Eurocodes can be considered to be divided into codes that provide fundamentalguidance for structural design (Basis of Structural design), guidance that may apply to alldesigns (loads, geotechnics and seismic) and detailed guidance for structural materials (steelconcrete etc.).

1.1 Steel Structures

EN 1993 (Eurocode 3) gives structural design rules for steel structures. It is divided into sixmain design areas, which are sub-divided into the following parts:

Part 1 General rules and rules for buildingsPart 1-1 General rules and rules for buildingsPart 1-2 Structural fire designPart 1-3 General rules Supplementary rules for cold-formed members and sheetingPart 1-4 Stainless steelsPart 1-5 Plated structural elementsPart 1-6 Strength and stability of shell elementsPart 1-7 Strength and stability of planar plated structures transversely loaded 1 Part 1-8 Design of joints

Part 1-9 Fatigue strength of steel structuresPart 1-10 Material toughness and through-thickness propertiesPart 1-11 Design of structures with tension components made of steelPart 1-12 Additional rules for the extension of EN 1993 to steel grades up to S700

Part 2 Steel Bridges

Part 3 Towers, masts and chimneysPart 3-1 Towers and mastsPart 3-2 Chimneys

Part 4 Silos, tanks and pipelines

1

It should be noted that while there is an ENV version of part 1-7 there may not be an ENversion of this part of Eurocode 3.


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Introduction

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Part 4-1 SilosPart 4-2 TanksPart 4-3 Pipelines

Part 5 Piling

Part 6 Crane supporting structures

1.2 Composite steel and concrete structures

EN1994 (Eurocode 4) gives structural design rules for composite steel and concretestructures. It is divided into two main design areas, which are sub-divided into the followingparts:

Part 1 General rules and rules for buildingsPart 1-1 General rules and rules for buildingsPart 1-2 Structural fire design

Part 2 Bridges

1.3 Aim and scope of this publication

The aim of this Companion Document is to provide UK designers with an overview of theEurocodes system, and with more detail given for parts of Eurocode 3 and Eurocode 4.

This Companion Document focuses on the guidance given for buildings. Design guidancepresented in the Eurocodes relating to bridges and other civil engineering works is notconsidered.

The main differences between the current British Standards (2004) and the Eurocodes 3 and4 are discussed. Where the design guidance is the same or there is little change between theCodes no discussion has been included.

To keep this document concise detailed design guidance is not presented.

The parts of Eurocode 3 and 4 that are covered by this companion document are:

Eurocode 3Part 1-1 General rules and rules for buildingsPart 1-2 Structural fire designPart 1-8 Design of jointsPart 1-10 Material toughness and through-thickness propertiesPart 5 Piling

Eurocode 4Part 1-1 General rules and rules for buildingsPart 1-2 Structural fire design


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Eurocodes System

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The individual material Eurocodes are divided into parts. Part 1 gives general rules and rulesfor buildings, Parts 2, 3 etc. give rules for other applications (bridges etc.). These high level parts are divided into sub-parts.

In addition to the inter-action between the materials codes and Basis of Structural Design theparts of each material code may cross-reference each other. This is due to the Eurocodespresenting guidance in only one place (i.e. rules are not repeated in several parts) andsubsequently referring to that clause in other parts of the Eurocode. In some cases parts of different material Eurocodes may be referenced e.g. a part of EN 1994 (Composite Steel andConcrete Structures) may reference a part of EN 1992 (Concrete Structures) or EN 1993(Steel Structures).

Each part of a Eurocode published by a National Standards Authority will be divided intodistinct sections, these are:

National title page National forward EN title page EN main text EN Annex(es)

o Normative Annexes contain design rules / methods / values to be used when designingto the Eurocode.

o Informative Annexes contain recommended design rules / methods or informativevalues, e.g. snow densities.

National Annex

The technical content of the EN main text and EN Annex(es) is the same across the whole of Europe. Those sections and the EN title page make up the 'EN' document published byCEN 3. The National Standards Authority (BSI in the UK) is responsible for developing andpublishing the National title page, National forward and National Annex. The addition of these

National sections in the UK makes the 'EN' document in to a 'BS EN' document.

Each part of a Eurocode will have an accompanying National Annex. These annexes willcontain information that should be referred to when designing a structure to be constructed inthat country. Therefore if a UK designer was designing a building to be constructed in Francethey would need to refer to the French National Annexes for all the Eurocodes used duringdesign and not the UK National Annexes.

The National Annex will contain information on the values / methods that should be used,where a national choice is allowed in the main text of the Eurocode. The national choices arecollectively referred to as Nationally Determined Parameters (NDPs). NDPs may be given for coefficient values, loads (both applied and self-weight) and where a choice in design

approach is given. The EN main text specifies recommended values / approaches, theNational Annex can either accept the recommendations given or specify different values /approaches to be used.

The National Annex will state how / if the content of an Informative EN Annex may be used for the design of structures to be constructed in that country. Information given in a Normative EN

Annex may only be altered by the National Annex if the EN text allows different rules / valuesto be given in the National Annex. References may be given to separate documents that giveguidance to help with the design of a structure. Such guidance is known as Non-ConflictingComplementary Information (NCCI) and may not be presented in the National Annex itself.

3 European committee for standardization


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Eurocodes System

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requirements and analytical methods for which no alternative is permitted unless specificallystated.

Application rules are generally denoted by a clause number without the letter P, e.g. 1.3(2). Application rules are generally recognised rules which comply with the Principles and satisfytheir requirements. It is permitted to use alternative design rules in place of those given in

Application rules. However, it must be shown that the alternative design rules meet therequirements of any relevant Principles. It must also be shown that the alternative rulesprovide equivalent structural safety, serviceability and durability to that expected from theEurocodes. If a design is carried out using an alternative rule to that given in an Applicationrule the design cannot be said to be wholly in accordance with the Eurocode. However, it canbe said that the design is in accordance with the Principles of the Eurocode. This may haveimplications for CE marking.

The Eurocodes also use different terms to identify when a rule must be used or when analternative to that given can be used. When the term shall is used in a clause the rule mustbe used (as for a Principle). If a clause contains the word should an alternative to that rulecan be used (as for an Application rule).

The majority of Eurocodes make the distinction between Principle and Application rules usingthe notation discussed earlier. However of the Eurocodes considered by this CompanionDocument, EN1993-1-1 (General rules), EN1993-1-2 (Fire design) and EN1993-1-10(material toughness and through-thickness properties), do not currently (November 2004) usethe letter P to denote a Principle, instead only the term shall identifies a rule as a Principle.EN1993-1-1 does present supplementary guidance for the design of steel buildings, denotedby the letter B after the clause number e.g. 5.1.1(4)B.


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3 General Design Issues

3.1 Convention for member axes

The Eurocodes define the member axes differently to BS 5950. The Eurocodes system is inkeeping with the system generally used in computer software for global structural analysis. Itdefines the longitudinal axis of the member as x-x , with the major axis of the cross-section asy-y and the minor axis as z-z . The convention used in BS 5950 defines the major axis of thecross-section as x-x , the minor axis as y-y and the longitudinal axis of the member as z-z .The same convention is used for the u-u and v-v axes for angle sections in both Eurocode 3and BS 5950.

Designers unfamiliar with using the Eurocodes should pay particular attention to thedifference in axes convention. This is particularly important when using section tables thatuse the BS 5950 convention. Figure 2 shows the axes convention and notation used for auniversal beam section.

Figure 2. Member axes convention and dimension symbols used in the Eurocodes

3.2 The explicit use of factors

In contrast to the current British Standards the Eurocodes do not hide the material partial

factors ( Mi). This results in expressions appearing more complex, or different property valuescompared with those currently used in the UK.

An example of expressions with an increase in the number of terms from the British Standardto the Eurocodes is the resistance of a cross-section for uniform compression:

0M

y Rd ,c

Af N

= For Class 1, 2 or 3 cross-sections

Where: N cRd is the resistance of the cross-section for uniform compression (N) A is the cross-sectional area (mm 2)f y is the yield strength (N/mm

2) M0 is the partial material factor for the resistance of the cross-section

d

t f

r

tw

b

h

z

z

y y

M0

( Mi

)


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General Design Issues

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3.2.1 Symbols used in the Eurocodes

The Eurocodes use different symbols for section properties compared with those used in BS5950. The section properties with different symbols used in the Eurocode and BS 5950 aregiven in Table 1 and shown in Figure 2. Section properties not included in Table 1 have thesame symbols in both codes.

Table 1. Section properties with different symbols used in the Eurocodes and BS 5950

Symbol used in design codeSection property BS 5950 EurocodeDepth of cross-section D hEffective section modulus Z eff Weff

Elastic section modulus Z W elFlange thickness T t f Net area of cross-section A n Aeff Outer diameter of circular sections

D d

Plastic section modulus S W pl

Radius of gyration r iRadius of root fillet -channel sections*

r r 1

Torsional constant J I TWarping constant H I wWeb thickness t t wWidth of cross-section B b* Symbol used for radius of root fillet for other sections does not differ between codes

In addition to the section property symbols given in Table 1, symbols for other coefficients andvalues differ between the Eurocodes and British Standards. Table 2 presents some Latinupper case letters used in the Eurocodes to define actions and forces. The letters given inTable 2 define a number of different terms within the British Standards therefore a directcomparison can not be given.

Table 2. Examples of Latin upper case letters used within the Eurocodes to define actionsand forces

Terms Latin upper case letter usedwithin the Eurocodes system

Actions (General) FPermanent action G

Variable action Q

Moment M

Axial force N

Shear force VResistance of element (used as themain symbol or as a subscript)

R

Effect of an action (used as a subscriptto one of the above)

E


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method used for global analysis. However when plastic global analysis is used the steelgrades must satisfy the following additional criteria:

f u /f y 1.20 Elongation at failure not less than 15% u 20 y

A comparison of the above limits shows that the EC3-1-1 limits are less onerous that thosegiven in BS 5950: Part 1.

The reason for the differences in the two sets of recommendations has been difficult toestablish but the following comments on the development of the limits used in both BS 5950and EC3-1-1 might be helpful in understanding the code writers' thinking.

The origin of the BS 5950: Part 1 rules was the old BCSA black book 23 or 29 whichextended plastic design from BS15 steels (later grade 43 and now called S275) to BS968steels (later grade 50 and now called S355). The 1969 amendment extended BS 449 tograde 50 for elastic analysis. For the early draft of BS 5950 the issue of allowing plasticdesign of grade 50 steel in the UK was considered. On the basis of specific tests itseemed plastic design could be allowed with smaller b/t and d/t limits i.e. for morecompact sections. The use of a general rule to avoid having to test every new grade of steel was investigated. Professor Horne was consulted and his view was that the onlyway to be sure a steel was NOT alright would be if it failed specific tests, but that it waspossible to make an informed judgement about parameters that would help decide if a testwas even necessary. As a result of these discussions a set of rules specific to plasticglobal analysis were developed which meant than any steel that satisfied them wassatisfactory. A steel that did not meet these criteria might also be satisfactory but specifictests were needed to be certain it could be used for plastic global analysis.

The EC3-1-1 drafting panel had a wider definition of plastic analysis than that used in theUK. Their understanding was that plastic analysis or even plastic design means not onlyplastic global analysis but that using the plastic modulus of a class 1 or class 2 cross-section is also plastic analysis. The wider definition may have contributed to thedifference in values given in EC3-1-1 and BS5950: Part 1 for the plastic analysis limits.

4.1.3 Fracture toughness

EC 3 and BS 5950 use different terminology and different approaches to establish the fracturetoughness of a material to avoid brittle fracture. BS 5950: Part 1 uses the minimum servicetemperature, T min , to determine fracture toughness. In the UK T min is usually taken as -5 Cfor internal steelwork and -15 C for external steelwork. The method used in EC3 is based on

a reference temperature of T Ed which is determined from equation 2.2 of EC3-1-10 (seesection 4.4.1 for further details).

4.1.4 Structural stability of frames

In both standards the designer is required to determine if the effects of the deformedgeometry of the structure will significantly affect or modify structural behaviour, for example byintroducing additional (secondary) moments. In EC3-1-1 this is achieved by checking that thecritical load factor, cr , for the structure under consideration satisfies the following limits:

cr 10 for elastic analysis

cr 15 for plastic analysis

f u/f y 1.20

u 20 y

cr1

cr

cr


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If cr is above these limits then the effects of deformed geometry (second order effects) canbe neglected and a first order analysis may be used. If cr is less than 10, or 15, then theeffects of the deformed geometry should be considered. This defines the boundaries, butunlike BS5950: Part 1 EC3-1-1 does not use the terms non-sway and sway sensitive todescribe the frames.

The limit used for elastic analysis in BS 5950: Part 1 is identical to that used in EC3-1-1. Theonly difference is that the limit in BS 5950: Part 1 is for clad structures where the stiffeningeffect of the cladding is not explicitly taken into account when calculating the elastic criticalload factor. No such limitation is placed on the method given in EC3. Consequently, baresteel frames designed using EC3-1-1 may be less stiff than those designed to BS 5950.

Unlike EC3-1-1, BS 5950: Part 1 includes two simplified methods for taking account of secondary effects for the plastic design of multi-storey rigid frames and a separate method for the plastic design of portal frames.

4.1.5 Structural imperfections

A feature of EC3-1-1 is its explicit allowance in the calculation procedures for practicalimperfections that have an influence on the resistance of members or structures. A number of alternative procedures are given in Section 5.3, some with limited scope. Generally theyconsider:

System imperfections An initial-bow imperfection is introduced in the design of braced bays and built upcompression members. In the case of bracing systems any additional deflections dueto the action of the bracing system in resisting externally applied forces also have tobe taken into account.

Frame imperfectionsThese are introduced into the analysis of all frames in the form of an equivalent initialsway. For convenience this can be replaced by a closed system of equivalent forces,except when determining reactions onto foundations. The frame imperfections areintended to account for the possible effects of other forms of imperfection which mayaffect the stability of frames such as lack-of-fit.

Member imperfectionsThese are introduced in the design of compression members through a series of imperfection factors which represent an equivalent lack of straightness. The values of the imperfection factors also account for the effects of typical residual stress patterns.Local bow imperfections of members, in addition to global sway imperfections, should

be included in the global analysis of frames that are sensitive to second order effects.

While BS 5950: Part 1 does not disallow this method of analysis system, frame and member imperfections are not explicitly included in the standard. An allowance is made for themwithin the buckling curves given in BS 5950: Part 1.

4.1.6 Buckling members in compression

BS 5950: Part 1 uses a modified Perry formula to determine member buckling resistance.This method is described in Annex C of BS 5950: Part 1. In EC3-1-1 the member bucklingresistance is derived from the resistance of the cross-section by applying a reduction factor, . Different values of are determined for flexural buckling ( y (y-y axis) or z (z-z axis)),

lateral torsional buckling ( LT ), torsional ( T ) and torsional-flexural buckling ( TF ). Thereduction factor is a function of an imperfection factor ( ) and the non-dimensional

BS 5950: Part 1 uses a modified Perry formula to determine member buckling resistance.This method is described in Annex C of BS 5950: Part 1. In EC3-1-1 the member bucklingresistance is derived from the resistance of the cross-section by applying a reduction factor, . Different values of are determined for flexural buckling ( y (y-y axis) or z (z-z axis)),lateral torsional buckling (

LT ), torsional (

T ) and torsional-flexural buckling (

TF ). The

reduction factor is a function of an imperfection factor ( ) and the non-dimensional

cr

cr


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slenderness ratio of the compression member. is a function of the slenderness ratio of the member Lcr /i , where Lcr is the buckling length in the plane of buckling. The buckling lengthis similar to the effective length used in BS 5950: Part 1. Unfortunately, unlike BS 5950: Part1, EC3-1-1 does not give guidance on the buckling lengths to be used. Consequently,guidance on the buckling lengths (or effective length) must be obtained from either BS 5950:

Part 1, design guides or appropriate textbooks.

Another feature of EC3-1-1 is the introduction of two additional checks for members with opencross-sections subject to compression. These checks are for the torsional and torsional-flexural buckling of members in compression. The methods use the same base equationsused for flexural buckling but with the non-dimensional slenderness replaced by either the

non-dimensional slenderness for torsional ( T ) or torsional-flexural buckling ( TF ). Theseparameters can be used to determined either T or TF and either the elastic torsional flexuralbuckling force or the elastic torsional buckling force of the member. EC3-1-1 does not includeguidance on how to calculate these two parameters and the designer must rely on anappropriate textbook.

4.1.7 Buckling uniform members in bending

In EC3-1-1 the lateral torsional buckling of a laterally unrestrained beam is determined fromthe resistance of the cross-section by applying a reduction factor, . The reduction factor, LT ,is a function of both the imperfection factor , LT , and the non-dimensional slenderness ratio,

LT , of the beam. This approach is similar to the method used for calculating the bucklingresistance of a column.

The method used in BS 5950: Part 1 is different and is based on a modified Perry-Robertsonexpression. A full description of this method is given in Annex B of BS 5950: Part 1.

The main difference between these two methods is that while BS 5950 is based on thecalculation of the equivalent slenderness , LT , EC3-1-1 requires the designer to evaluate theelastic critical moment for lateral-torsional buckling ( M cr ) as an intermediate step beforecalculating the non-dimensional slenderness ratio , LT . This is the traditional way of evaluating LT but unfortunately EC3-1-1 does not include data for the evaluation of M cr .Designers must therefore rely on an appropriate textbook.

Furthermore, EC3-1-1 contains two methods for calculating the lateral torsional buckling of amember. These are:

The general case, and

A method specifically for rolled sections or equivalent welded sections.The second method has been calibrated against test data and has been shown to givereasonable results for rolled sections. The calibration also showed the method to beunsatisfactory for welded sections. It is therefore suggested that designers use the generalcase for welded sections and the specific method for rolled sections. However, the UKNational Annex (once published) should be referred to for guidance on which method to use.

The second method includes a correction factor to allow for the shape of the bending momentdiagram. This correction factor is in addition to the equivalent uniform moment factors usedto allow for the differences between a uniform moment and the actual moment distributionalong the beam.


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4.1.8 Buckling uniform members in bending and axial compression

EC3-1-1 introduces two alternative methods for calculating the buckling resistance of amember subject to combined bending and axial compression. Both approaches useinteraction equations which have a similar general form to those used in BS 5950: Part 1.However, this is where the similarity ends. The methods in EC3-1-1 include interaction

factors, k , which account for the shape of the bending moment diagram and the class of thecross-section. The interaction factors have been derived from two alternative approachesand expressions for each interaction factor are included in Annex A for Method 1 and AnnexB for Method 2. Both methods require the evaluation of complex expressions in order todetermine the interaction factors. However, Method 2 is a little easier. A comparisonbetween Methods 1 and 2 and BS 5950 has shown that Method 2 is in better agreement withBS 5950: Part 1 than Method 1. Furthermore, there is some doubt over the applicability of Method 1 to asymmetric sections. For these reasons the National Annex may allow bothmethods to be used but restrict the scope of Method 1 to bi-symmetrical sections.

4.2 Part 1-2: General rules Structural fire design

The fire part of Eurocode 3 (EN1993-1-2 [4], hereafter referred to as EC3-1-2) is not radicallydifferent from the UK standard for the fire resistant design of steel structures. BS 5950 Part 8[5] is a performance based code that allows for calculation of fire resistance in addition to theuse of fire test data. The principal difference between the two codes is that the calculationprocedures in BS 5950: Part 8 are limited to a thermal exposure based on the standard firecurve while EC3-1-2 allows for alternative thermal exposures based on the factors influencingfire growth and development. The design procedure for EC3-1-2 is illustrated in Figure 3.Effectively the scope of BS 5950: Part 8 is restricted to the left hand branch of the diagram.

All the fire parts of the structural Eurocodes are designed to be used with the fire part of theEurocode for Actions (EN1991-1-2 [6] hereafter referred to as EC1-1-2). The thermal actions

(either nominal or parametric) are taken from this document and the resulting thermal andmechanical analysis undertaken using the principles and design methods detailed in EC3-1-2.

4.2.1 Material properties

For fire resistant design by calculation the most common method in the Eurocodes is to use amodified form of the equations for resistance at ambient temperature using reduced materialproperties corresponding to the appropriate temperature. For this reason EC3-1-2 containsdetailed guidance on the material properties of carbon and stainless steels. These arepresented as stress-strain relationships and as reduction factors relative to the ambienttemperature strength and elastic modulus. It is important to note that the variation of Young smodulus with temperature is different to the variation in steel strength . The information ispresented in the form of strength reduction factors ( k y, ) in EC3-1-2 and strength retentionfactors in BS 5950: Part 8. The strength reduction factors given in EC3-1-2 correspond to the2% strain values in Table 1 of BS 5950: Part 8. Elevated temperature properties are alsopresented for thermal elongation, specific heat and thermal conductivity. The relationshipsgiven in EC3-1-2 are identical to those in BS 5950: Part 8. The corresponding properties for stainless steel may be found in Annex C of EC3-1-2. Annex A of EC3-1-2 presents analternative stress-strain relationship for carbon steels allowing for strain hardening.

(k y )


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4.2.2 Structural fire design

In EC3-1-2 fire resistance may be determined either by simple calculation models, advancedcalculation models or testing. The current British Standard is based on fire resistance derivedfrom standard fire tests and fire resistance derived from calculations. The main difference inapproach is that BS 5950: Part 8 includes tabulated data for limiting temperatures and design

temperatures based on the results from standard tests while EC3-1-2 does not includetabulated data for design temperatures.

4.2.3 Members in compression

For compression members with Class 1, Class 2 or Class 3 cross-sections a non-dimensionalslenderness is calculated based on the buckling length in the fire situation. In general thebuckling length should be determined as for ambient temperature design. However, in abraced frame the buckling length may be determined based on continuity at the connectionsprovided that the fire resistance of the building components that separate the firecompartments is not less than the fire resistance of the column. Thus in a braced framewhere each storey comprises a separate fire compartment, intermediate columns areassumed to be fixed in direction at either end and the effective length is half of the systemlength. In the top storey the buckling length may be taken as 0.7 x the system length. This isdifferent to the approach used in BS 5950: Part 8 where the buckling length is determinedfollowing the guidance given for ambient temperature design i.e. current UK practice is moreconservative. It is anticipated that this issue will be addressed in the UK National Annex for EC3-1-2.

4.2.4 Combined bending and axial compression

For members subject to combined bending and axial compression the calculation method inEC3-1-2 is more complex than the corresponding calculation in BS 5950: Part 8 and differs

from the method in EC3-1-1. The interaction formula for the combination of axial load andminor and major axis bending is based on the procedure in the original draft for thedevelopment of the Eurocode, ENV 1993-1-1 as the new method in EC3-1-1 has not beenverified for the fire situation at the time of writing.

4.2.5 Structural connections

The latest version of BS 5950: Part 8 contains guidance on the calculation of the thickness of protection required for structural connections and takes into account the relative load ratio of the connection compared to that of the connected members. EC3-1-2 in addition to similar guidance includes a more detailed approach in Annex D where the design resistance of boltsin shear and tension, and the design resistance of welds can be calculated using a

temperature profile based on the temperature of the bottom flange of the beam at mid-span.This method is mainly applicable for simple connections although potentially could be appliedto all components of the connection using the approach in EN1993-1-8.

4.3 Part 1-8: Design of joints

EN1993-1-8 [7] (hereafter referred to as EC3-1-8) gives guidance for the design of steel jointssubject to predominantly static loads. Steel grades S235, S275, S355 and S460 are coveredby the guidance given.


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4.3.1 Definitions

EC3-1-8 starts by defining the different components that constitute a steel joint and makes aclear distinction between a connection and a joint. This can be confusing for UK designerswho generally use the words joint and connection interchangeably to describe the junctionbetween two steel members. In EC3-1-8 the word connection is used to define the location

at which two or more elements meet, while the word joint is used to define the zone wheretwo or more members are interconnected. Therefore a beam-to-column connection is theinterface between the flange (or web) of the column and the end of the beam, and includes allthe components (bolts, welds, end-plate, column flange etc) required to transfer the internalforces from the beam to the column. The joint however is the assembly of all the basiccomponents which play a part in the behaviour of the configuration. For example, a single-sided beam-to-column joint consists of a connection and a column web panel. It is importantthat UK designers recognise this distinction as it is used throughout the standard.

4.3.2 Material properties

In EC3-1-8 the nominal values of the yield strength, f yb , and the ultimate tensile strength, f ub ,for grade 8.8 and 10.9 bolts are considerably greater than the equivalent values used in BS5950: Part 1. This is due to the standards taking account of different effects within the quotedmaterial property values. EC3-1-1 gives ultimate values and BS5950: Part 1 givespermissible values. The partial material factors are included in the properties given in BS5950: Part 1 but are defined separately in EC3-1-8. BS5950 Part 1 material properties maybe used to account for prying actions without the direct calculation of the prying force byapplying a factor to the material properties, EC3 gives a separate check for prying action.

4.3.3 Groups of fasteners

The approach used in EC3-1-8 is different to that used in the BCSA/SCI publications on

Joints in Steel Construction [8, 9 & 10]. In EC3-1-8 the design resistance of a group of fasteners may be taken as the sum of the design bearing resistances of the individualfasteners provided the design shear resistance of each individual fastener is greater than or equal to the design bearing resistance. If this condition is not satisfied then the designresistance of a group of fasteners should be taken as the number of fasteners multiplied bythe smallest design resistance of any of the individual fasteners.

In the BCSA/SCI publications the design resistance of a group of fasteners is taken as thesum of the design resistances of the individual fasteners.

This difference in approach may cause problems for flexible end-plates. The currentapproach in the UK often means that the top bolts are designed for bearing failure and the

remaining bolts for shear. Because the EC3-1-8 rules do not allow mixed modes of failure thecapacity of the bolt group according to the Eurocode philosophy would often be based on thenumber of fasters multiplied by the design bearing resistance of the top bolts. Clearly thismay significantly reduce the apparent shear capacity of flexible end-plate connections and insome cases may result in an increase in the number of bolts needed.

4.3.4 Analysis, classification and modelling

Joint design depends very much on the designers decision regarding the method by whichthe structure is to be analysed. Both EC3-1-8 and BS 5950: Part 1 recognise that either elastic or plastic global analysis may be used, for frames that are simple, semi-continuous or continuous. When elastic analysis is adopted joint stiffness is relevant, when the analysis is

plastic then strength of the joint is relevant. EC3-1-8 goes a step further than the BritishStandard and includes a table that relates the type of framing, method of global analysis and


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the joint classification. Table 4 gives details (note that some of the terminology used in theEurocode has been slightly modified for clarity).

Table 4. Type of framing, analysis used and joint classification/requirements

Method of globalanalysis

Classification/requirements of joint

Elastic Nominally pinned Rigid Semi-rigid Rigid-plastic Nominally pinned Full-strength Partial-strengthElastic-plastic Nominally pinned Rigid and

full-strengthSemi-rigid and partial strength or Semi-rigid and full-strength or Rigid and partial-strength

Type of framing Simple Continuous Semi-continuous

Although the relationship between type of framing, method of global analysis and jointrequirements (represented by their classification) has been known for some time, its inclusionin a major structural code is new and some explanation of its use is required.

Simple frame design is based on the assumption that the beams are simply supported andthat the beam-to-column joints are sufficiently flexible and weak to restrict the development of significant beam end-moments. In continuous framing the type of joint used will depend onthe method of global analysis. When elastic analysis is used the joints are classifiedaccording to their stiffness and rigid joints must be used. When plastic analysis is used the

joints are classified according to their strength and full-strength joints must be used. Whenelastic-plastic analysis is adopted then the joints are classified according to both their stiffnessand strength and rigid, full-strength joints must be used.

Semi-continuous frame design recognises the fact that most practical joints possess somedegree of both stiffness and moment resistance. When elastic analysis is used the joints are

classified according to their stiffness and semi-rigid joints should be used. If plastic globalanalysis is used the joints are classified according to their strength and partial-strength jointsshould be used. When elastic-plastic analysis is used the joints are classified according totheir stiffness and strength, and semi-continuity could be achieved in a number of ways (seeTable 4).

The traditional UK approach of classifying a joint only recognises two types (pinned and rigid)and it is relatively straightforward to use engineering judgement to choose between these.For an extended system, such as the one used in EC3-1-8, the structural properties of a jointmay need to be quantified in order to classify it. EC3-1-8 includes methods for doing this, andit is the inclusion of these methods that constitutes the biggest difference between the designof joints to the Eurocode and the traditional methods used in the UK.

By comparing the quantified stiffness of a joint against the limits given in EC3-1-8 it can beclassified as pinned, rigid or semi-rigid. Similarly a joint can be classified by comparing itsquantified moment resistance with limits for pinned, full-strength or partial strength joints. Afuller description of a joint s behaviour can also be obtained by classifying it using bothstiffness and strength. Such a classification leads to joints which are pinned, rigid/full-strength, rigid/partial strength and semi-rigid/partial-strength.

One problem that this may cause is that joints which have traditionally been taken as pinnedor rigid may not be pinned or rigid under the new classification system. This situation iscomplicated by the fact that the Eurocode not only gives guidance on calculating stiffness andstrength (for some joint types), but clause 5.2.2.1 also allows classification on the basis of

experimental evidence or experience of previous performance . Clearly the results of thesethree approaches for a given joint may not always agree. This could prove problematical if


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checking authorities require designers to demonstrate that a joint is pinned or rigid, and couldlead to increased design time and/or changes to the UK s commonly used joints.

To establish the stiffness boundary between rigid and semi-rigid joints the relationshipbetween joint stiffness and the Euler buckling load for a single-bay, single-storey frame wasinvestigated [11]. It was decided that a semi-rigid joint can be considered as rigid providedthe difference between the Euler buckling load for a single-bay, single-storey frame with semi-rigid joints and the Euler buckling load of a similar frame with rigid joints was less than 5%.By adopting this approach a classification method based on the rigidity of the connectedbeam was developed. While such a system is easy to use it has attracted criticism, some of which is detailed below:

When compared to the stiffness limits given in some national standards the limits in EC3-1-8 appear to be conservative.

The classification system given in EC3-1-8 can be applied to any steel structure but asthe limits have been determined on the basis of a single-bay, single-storey frame theaccuracy of its application to multi-bay, multi-storey frames is questionable.

The stiffness boundaries between joint types have been determined on the basis of theultimate limit state and on the assumption that a difference of 5% between theperformance of a frame with rigid and semi-rigid joints is small and can be neglected.However, this does not necessarily mean that the differences at serviceability limit states,where displacements of the structure are more important, are equally small and can beneglected. Clearly, when deriving classification criteria both serviceability and ultimatelimit states should be considered.

4.3.5 Structural joints connecting H or I sections

The method described in Chapter 6 of EC3-1-8 for the design of joints between H or Isections is based on the component approach. Explicit guidance is only given for flush and

extended end plate joints, although a designer might need to calculate stiffness and/or strength for other types of joint in order to classify them. As well as this limitation it is worthnoting that the procedures for calculating the design moment resistance and rotationalstiffness of a joint are complex and time-consuming and are not suitable for hand calculation.Computer software is recommended for these complex calculations.

4.3.5.1 Design resistance

In the given method for calculating moment resistance the potential resistance of eachcomponent is calculated. These potential resistances are then converted to actual forces bybalancing the forces in the tension components with those in compression. The momentcapacity of the joint is then calculated by summing the product of the forces in the tensioncomponents and their distances from the centre of compression. This approach is verysimilar to the method described in the BCSA/SCI publication Joints in Steel Construction:Moment connections [9] (which was in fact heavily based on the Eurocode). However, theEurocode also includes methods for calculating a joint s rotational stiffness and rotationcapacity. Both of these methods will be new to UK designers and are therefore brieflydescribed below.

4.3.5.2 Rotational stiffness

Calculating the stiffness of any joint can be a difficult process. For this reason Reference 9takes a pragmatic approach and gives simple rule-of-thumb detailing guidelines which, if

followed, will in most circumstances ensure an appropriate joint stiffness, so that frame designassumptions are not invalidated.


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EC3-1-8 incorporates a method for calculating the stiffness of a bare steel joint based on workinitially carried out by Zoetemeijer [12] and more recently by Jaspart [13 & 14]. This methoduses the component approach in which the rotational response of the joint is determined fromthe mechanical properties of the different components (end-plate, cleat, column flange, boltsetc.). The advantage of this approach is that the behaviour of any joint can be calculated by

decomposing it into its components.

The stiffness of each joint component is represented by a linear spring with a force-displacement relationship. Tables are included in EC3-1-8 which give expressions for evaluating the stiffness of the different components. The combined effect of the componentsis determined by considering each spring, with an appropriate lever arm, to give a rotationalstiffness.

4.3.5.3 Rotation capacity

The rotation capacity of a joint is very important (a pin or plastic hinge must be able torotate sufficiently) but this is difficult to calculate accurately. However, numerous researchers

have investigated rotation capacity and have identified many sources of ductility in joints,some of which are listed below:

Column web panel in shear Column flange in bending End plate in bending Tension flange cleat in bending

EC3-1-8 gives a number of practical rules for checking the rotation capacity of a joint. Theserules are based on the above sources of ductility for bolted joints and entail checking that thecritical mode of failure is based on one of the above components.

4.4 Part 1-10: Material toughness and through-thickness properties

EN1993-1-10 [15] (here after referred to as EC3-1-10), gives design guidance for theselection of steel for fracture toughness and through-thickness properties.

4.4.1 Fracture toughness

To determine the maximum permissible thickness of a steel element using EC3-1-10 thereference temperature, steel grade and stress at the reference temperature are required. Thefollowing expression is used to determine the reference temperature:

T Ed = T md + T r + T + T R + T + T cf

WhereT md is the minimum service temperature with a specific return period, given in EN 1991-1-5 T r is an adjustment for radiation loss, obtained from EN 1991-1-5 T is the adjustment for stress and yield strength of material, crack imperfections and

member shape and dimensions, given in EN 1993-1-10 T R is a safety allowance, if required, to reflect different reliability levels for different

applications, obtained from EN 1993-1-10 T is the adjustment for a strain rate other than the reference strain rate, obtained from

EN 1993-1-10 T cf is the adjustment for the degree of cold forming, defined in EN 1993-1-10

Elastic analysis should be used to determine the stress at the reference temperature. Themaximum element thicknesses given in Table 2.1 of EC3-1-10 relate to three levels of stress,

T Ed = T md + T r + T + T R + T + T cf

WhereT md is the minimum service temperature with a specific return period, given in EN 1991-1-5T r is an adjustment for radiation loss, obtained from EN 1991-1-5T is the adjustment for stress and yield strength of material, crack imperfections and

member shape and dimensions, given in EN 1993-1-10TR is a safety allowance, if required, to reflect different reliability levels for different

applications, obtained from EN 1993-1-10T is the adjustment for a strain rate other than the reference strain rate, obtained from EN

1993-1-10Tcf is the adjustment for the degree of cold forming, defined in EN 1993-1-10


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0.25f y(t), 0.5f y(t) and 0.75f y(t). Where f y(t) is the nominal yield strength adjusted for thethickness of the element.

The current UK guidance gives maximum thickness values for minimum temperatures of -5C, -15 C, -25 C, -35 C and -45 C. The minimum temperature of -5 C for internalsteelwork given in BS 5950: Part 1 relates to the temperatures experienced duringconstruction, when it is vulnerable to brittle fracture. The values given in EC3-1-10 consider awider range of temperatures, +10 C to -50 C in 10 C intervals. Interpolation between thevalues is allowed, but extrapolation beyond the extreme values given in the table is notpermitted.

The minimum temperature used in BS 5950: Part 1 and the reference temperature ( T Ed ) of EC3-1-10 are not equivalent to each other. The minimum temperature used in BS 5950: Part1 is similar to the minimum service temperature with a specific return period ( T md ).

Maximum element thickness values are given for different steel grades in both codes,although more steel grades/types are considered in BS 5950: Part 1. Table 5 gives the steelgrades/types considered in both standards. Comparing the steel grades covered by BS 5950:Part 1 and EC3-1-10 it appears that no allowance has been made in Table 2.1 of EC3-1-10for the steel grades used for hollow sections. EC3-1-10 allows the use of fracture mechanicsfor a numerical evaluation. Therefore this method may be used for the steel grades used for hollow sections.

Table 5. Material property standards for which maximum element thicknesses are given inBS 5950: Part 1 and EN 1993-1-1

Material property standards for which maximum element thicknesses are given in standardsBS 5950: Part 1S275 to S460 steel grades

EN 1993-1-1S275 to S690 steel grades

BS EN 10025 BS EN 10025

BS EN 10113 BS EN 10113BS EN 10137 BS EN 10137BS EN 10166BS EN 10210BS EN 10219BS7668

A note to clause 2.2(5) of EC3-1-10 allows the National Annex to give maximum values of therange between T Ed and the test temperature and also the range of Ed , to which the validity of values for permissible thickness in Table 2.1 may be restricted. A further note to this clauseallows the National Annex to limit the use of Table 2.1 for steel grades up to S460. The UK

National Annex to EC3-1-10 is currently under development and no comment can be made atthis time on the values that may be included in it.

4.4.2 Through-thickness properties

Section 3 of EC3-1-10 gives a method for determining the susceptibility of steel to lamellar tearing. Lamellar tearing is a weld induced flaw and is usually detected during ultrasonicinspection of welds. The main risk of lamellar tearing is with cruciform joints, T-joints, corner

joints and when full penetration welds are used.

Ed ,


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To check if lamellar tearing may be ignored EC3-1-10 requires the available design andrequired design Z -values 4 to be compared. The available design Z -value is given in BS EN10164 [16]. The required design Z -value is obtained from coefficients given in EC3-1-10relating to weld depth, shape and position of welds, material thickness, restraint of shrinkageand influence of preheating. BS 5950: Part 2 [17] states that the material shall be tested for

through-thickness properties to the specified quality class in accordance with BS EN 10164.

The inclusion of the Z -value check in EC3-1-10 may result in designers having to perform thischeck for every welded joint in a structure. Currently in the UK only joints identified as beingat risk from lamellar tearing are checked. EC3-1-10 allows the National Annex to limit thescope of section 3 to certain steel products . This may be used in the UK National Annex tolimit the Z -value checks to specific types of welded joints.

4.5 Part 5: Steel Piling

EN1993-5 [18] (hereafter referred to as EC3-5) gives guidance for the design of all types of steel piles including hot and cold formed sheet piles, bearing piles and piling systems built upfrom component parts. It gives guidance for different shapes, sizes and arrangements of steel piles. Although some of these are not common in the UK at present they may find futureapplication.

The fields of application considered by the Eurocode are:

Steel piled foundations of civil engineering works on land and over water Temporary or permanent structures necessary for the execution of steel piling Temporary or permanent retaining structures composed of steel sheet piles, including all

kinds of combined walls.

Guidance for steel piles filled with concrete is also included in EC3-5.

EC3-5 contains an annex giving detailed rules for the design of cold formed pile sections andcombined walls. These areas have not previously been dealt with in UK guidance.

Current UK standards do not contain an equivalent code to EC3-5. BS 8002 [19] is basicallya geotechnical code that requires input from BS 5950: Part 1 to allow the design of steel piles.Current SCI documents cover some aspects of UK steel pile design. However, the guidancegiven in these documents does not give the detail required for a full design, and it onlyapplies to simple structures.

EC3-5 introduces some new concepts to the traditional UK design process, these include:

The use of plastic design for piling Four classes for sheet piling and the resultant different design approaches A more formal system for assessing the structural performance of piling structures

The checks on shear in a sheet pile wall, which are perhaps covered by inspection in currentpractice need to be formally assessed, as do shear buckling and combined moments, shear and axial loading. Many of these checks will require section data and it is likely that either data sheets giving member capacities or the basic geometric information will be provided bythe sheet pile manufacturers. The effects of water pressures on the structural design are alsoto be taken into account (which is a new concept for UK designers), and specific rules for thetransfer of shear in the interlocks of piles and its effect on the strength and stiffness of pile

4 Z -value is the transverse reduction of area in a tensile test of the through-thickness ductilityof a specimen, measured as a percentage


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sections are included. This issue is addressed qualitatively in BS 8002 but is covered in moredepth in EC3-5, as it has a much higher profile outside the UK. Compared with current UKpractice EC3-5 deals more formally with combined-walls and cellular structures, as well ashigh modulus walls.

Conflicting views have emerged within the UK industry on the implementation of EC3-5.These views have emerged because of the significant differences in scope and approachbetween current UK practice and the Eurocode system. One of the major areas for concern isthe effect that a move from lumped factor design to a partial factor approach will have ondesign requirements. This is compounded by changes in the specific calculations that arerequired to satisfy the new code. There are situations where formal calculations are nowrequired which would previously have been dealt with by inspection in the UK. There is alsoconcern that these design changes may make designs less efficient, or effective, comparedwith current UK practice.

One of the most difficult areas to assess is the effect that the plastic design rules will have onthe design process as there is little or no experience with these design rules within the UK.

The design calculations need to consider the situation at all stages in the life of the structureand if the proposed section has appropriate parameters, the wall can be designed on thebasis of plastic section properties and moment redistribution. This assumes that the pilesection is capable of sustaining a moment of resistance as the pile rotates plastically and thisability may change with the amount of corrosion that the section has sustained. This may beaccepted practice in structural designs but the response of soil when the system is at or approaching plastic conditions is not understood.

There is reference made to EN 12063, the standard covering site activities which goes intosignificantly more detail than current British Standards on some aspects of site work (i.e.welding).

One potential area of conflict with current UK methods is the fact that there is no overtdifference between the requirements for temporary and permanent construction. This waspreviously dealt with by allowing increased stress levels in temporary works piling (BS8002:1994 [19]) and not considering corrosion on the section properties. Under the new rules therewill be no change in stress, which may be a retrograde step in some minds.


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EN 1994 Steel and Concrete Composite Structures

31

5 EN1994 Steel and Concrete Composite Structures

The main differences between the design guidance given in Eurocode 4 and BS 5950 arediscussed in this section.

5.1 Part 1-1: General rules and rules for buildings

Eurocode 4 applies to the design of composite structures and members for building and civilengineering works. The Eurocode is concerned only with the requirements for the resistance,serviceability and durability of composite steel and concrete structures. EN1994-1-1 [20](hereafter referred to as EC4-1-1) gives a general basis for the design of composite structuresalong with specific rules for buildings.

EC4-1-1 provides design guidance for some types of element not common in the UK, such aspartially encased concrete beams, composite columns in buildings, high strength structuralsteels and composite joints together with various methods of continuous beam design and thedetailing of the continuous joints.

5.1.1 Material Properties

5.1.1.1 Concrete

Unless given in EC4-1-1, concrete material properties must be obtained from EN1992-1-1[21] (hereafter referred to as EC2-1-1) for both normal weight and lightweight concrete.

However, EC4-1-1 does not cover the design of composite structures with concrete gradeslower than C20/25 or higher than C60/75. EC4-1-1 therefore extends the range of concretestrengths compared to those available in BS 5950.

The classification for normal weight concrete used in the Eurocodes system (C x /y ) gives thecylinder strength ( x ) and the cube strength ( y ) in N/mm 2. The design strengths used in theEurocodes are based on the cylinder strengths and not the cube strengths, so care should betaken by designers to use the correct value.

5.1.1.2 Structural steel

Reference should be made to EC3-1-1, clauses 3.1 and 3.2 for the properties of structural

steel, however, EC4-1-1 does not cover steel grades with a characteristic strength greater than 460N/mm 2. This is in common with BS 5950: Part 3 where the design strength of structural steel is obtained by reference to BS 5950: Part 1. However, a comment is made inBS 5950 that due to a lack of test evidence, the design strength should not be taken asgreater than 355N/mm 2. This limit is lower than that given in EC4-1-1.

Research has shown that to prevent premature concrete crushing some design rules shouldbe modified for steels with strength greater than 355N/mm 2. Such modifications have beenincorporated into EC4-1-1 so that it can cover steels with strengths up to 460N/mm 2.


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5.1.2 Structural stability

The effects of the deformed geometry of the structure must be considered and an importantdesign Principle outlined states that second-order effects should be considered if theyincrease the action effects significantly or modify significantly the structural behaviour.

It is suggested that this increase in internal forces may be neglected if the increase in forcesdue to second-order effects is less than 10% of the forces determined in first-order analysis.

The Eurocode also states that if second-order effects in individual members and relevantmember imperfections are fully accounted for in the global analysis of the structure, individualstability checks on the members (such as lateral torsional buckling presumably) are notnecessary.

This is in contrast to BS5950 where there is no specific requirement to consider the increasein internal forces due to second-order effects but individual stability checks are required.

An additional Principle stated is that appropriate allowances must be made for creep andcracking of concrete and for the behaviour of joints when determining the stiffness of thestructure.

Part 3, BS5950, uses a slightly different approach where the specific effects of concrete creepdo not have to be considered provided that material values given are used when calculatingthe modular ratio.

The effects of concrete cracking are considered in BS5950, where the cracked sectionmethod is used to determine member stiffness for elastic analysis, although the un-crackedsection is used to calculate deflections.

5.1.3 Structural imperfections

When using EC4-1-1 appropriate allowances must be made to cover the effects of imperfections, including residual stresses and geometrical imperfections present even in theunloaded structure, such as lack of verticality, out of straightness and the unavoidable minor eccentricities present in joints. The assumed shape of any imperfections must take intoaccount the elastic buckling mode of the structure or member in the most unfavourabledirection and form, in the plane of buckling considered.

Equivalent geometric imperfections should be used unless the effects of local imperfectionsare included in the member resistance design formulae. EC4-1-1 gives values of initial bowimperfections for composite columns and whilst there are no specific imperfectionrequirements for beams, EC4-1-1 incorporates the effects of imperfections within the formulaefor the buckling resistance moment of laterally unrestrained composite beams. A similar approach is adopted in the current British Standard. The designer should refer to EC3-1-1 for the effects of global imperfections and for the formulae for buckling resistance of steelmembers, which also incorporate the effects of member imperfections.

No specific requirements for dealing with member or global imperfections are outlined withinBS 5950: Part 3.

5.1.4 Calculation of action (load) effects

Action effects are generally calculated by elastic global analysis even where the capacity of across-section is based on its plastic or non-linear resistance. Elastic global analysis shouldalso be used for serviceability limit states, with, where appropriate, corrections for non-linear effects.


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Allowance must also be made for shear lag. This is achieved for continuous beams by usingan effective width of slab. In much the same way as in BS 5950: Part 3, EC4-1-1 outlines anumber of provisions for determining the effective width of concrete slab, with the totaleffective width for the sagging portion of a beam (noted as b eff1 in EC4-1-1) being the familiar Le /4 but no greater than the geometric distance between the beam centres.

EC4-1-1 does not give separate values of effective slab width for slabs spanningperpendicular to and parallel to the supporting beam. A subtle distinction between the twocases is given in BS 5950: Part 3, where the effective width of a slab spanning parallel to thebeam is limited to 0.8 times the beam spacing.

In contrast to BS 5950: Part 3, EC4-1-1 makes allowance for the shrinkage of concrete, in theserviceability limit state, as well as cracking of concrete, creep, the sequence of constructionand any pre-stressing.

The effects of creep are dealt with using the modular ratio for short-term loading modified by acreep coefficient depending upon the age of the concrete at the moment considered, t , andthe age at loading, t 0 , and a creep multiplier which can be used to account for the effects of concrete shrinkage. In practice, the effects of curvature due to shrinkage of normal weightconcrete may often be ignored (see clause 7.3.1(8), EC4-1-1 for details). This is a littledifferent to the approach used in BS 5950: Part 3, where the modular ratio is determinedconsidering the proportion of long-term to short-term loading.

In common with BS 5950: Part 3, EC4-1-1 considers the effects of cracking on the flexuralstiffness of composite beams in two ways.

Involved Method An initial un-cracked analysis is carried out assuming the un-crackedstiffness, E aI 1 , throughout. In areas where the extreme fibre tensile stressin the concrete is twice the concrete strength, the stiffness of the section isreduced to the cracked flexural stiffness, E a I 2. An updated distribution of internal forces is then determined by re-analysis, termed the crackedanalysis .

Simple Method The effect of cracking can be modelled by taking a reduced flexuralstiffness over 15% of the span on each side of each internal support, withthe un-cracked flexural stiffness taken elsewhere. This method may beused for continuous beams where the ratio of the adjacent spans(shorter/longer) is greater than or equal to 0.6.

The more complicated method given in BS 5950: Part 3, is basically the same as the simplemethod given in EC4-1-1, where a cracked section is assumed over 15% of the span on eachside of each internal support, with the un-cracked section assumed elsewhere.

The simplified method given in the BS 5950: Part 3 involves carrying out an elastic analysis,assuming all members are un-cracked. The resulting negative moments over the supportsand at mid-span can then be re-distributed in accordance with guidance given in Table 4 of BS 5950: Part 3, which effectively models the reduced stiffness of the member over thesupports. EC4-1-1 also allows some limited redistribution, in accordance with Table 5.1, withboth cracked and uncracked analysis for buildings, for the verification of all limit states other than fatigue.

5.1.5 Beams Ultimate Limit State

In EC4-1-1 the resistance moment of a composite cross-section with full interaction betweenthe structural steel, reinforcement and concrete is given by plastic theory. It is assumed that


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the effective area of the structural steel is stressed to its design yield stress, f yd , in either tension or compression and the effective area of concrete in compression resists a stress of 0.85f cd (which is constant over the whole depth between the plastic neutral axis and the mostcompressed fibre of concrete). The value f cd is the design cylinder compressive stress whichis determined according to the following expression (given in EC2-1-1):

f cd = cc f ck / C

Where: cc is a coefficient that takes account of long term effects on compressive stress andunfavourable effects due to the way the load is appliedf ck is the cylinder compressive stress C is the concrete partial factor

EC2-1-1 allows the relevant National Annex to specify a value for cc , however, the guidancegiven in EC4-1-1 has been developed using cc equal to one. Therefore where f cd is given inEC4-1-1 it represents f ck / C. See reference [23] for further discussion on this topic. Inprinciple, this is exactly the same approach as that taken in BS 5950: Part 3. However, theconcrete cube compressive stress is used with the material safety factor included in BS 5950:Part 3. Therefore in BS 5950: Part 3 the concrete is assumed to resist a stress of 0.45f cu over the whole depth between the plastic neutral axis and the most compressed fibre of concrete.

In keeping with the other Eurocodes, EC4-1-1 does not give any guidance for thedetermination of the effective or equivalent span, LE .

EC4-1-1 outlines limits on the degree of shear interaction required, including the requirementthat full shear interaction is attained when the effective span is greater then 25m. Theminimum degree of shear interaction for spans less than 25m is determined based upon theyield stress of the steel section and effective span and should always be greater than 0.4.

BS 5950: Part 3 gives similar guidance, but stipulates that full shear interaction is required

when the span is greater than 16m, and shear interaction must be greater than 0.4 for spansup to 10m. For intermediate spans the minimum degree of shear interaction is given by thesimple equation (L-6)/10 0.4 .

The vertical shear strength is based on that of the bare steel section in exactly the same wayas BS 5950: Part 3.

5.1.6 Beam serviceability limit state

Serviceability requirements are specified in relation to limiting deflections and concretecracking. Elastic analysis must be used for the serviceability limit state and the effective widthof the concrete flange, considered in beam design, is as defined for the ultimate limit state.

EC4-1-1 refers the user to EN1990 A1.4.4 for criteria reflecting to the dynamic properties of floor beams. Unlike BS5950, stress limits under construction loading are not given (theseneed only be checked if fatigue is a consideration).

EC4-1-1 states that the effect of cracking of concrete in regions subject to hogging momentsshould be taken into account by adopting appropriate global analysis methods. This is incontrast to BS 5950: Part 3, where the gross uncracked section is used when calculatingdeflections.

Although no specific procedures are stated in EC4-1-1, the effects of creep must be includedwhen calculating deflections. It is therefore necessary to consider relevant values of the

modular ratio when calculating the equivalent second moment of area of the gross sectionand distinguishing between shorter term and long term loading. This effect is covered in BS


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5950: Part 3, by the use of an effective modular ratio, for the proportion of total loading that islong term.

EC4-1-1 does not make allowance for increased deflections in beams with partial shear interaction (provided the degree of shear interaction is above 0.5). This is in contrast to BS5950: Part 3, where the deflection of a beam with partial shear interaction is increased fromthat with full interaction, based upon the degree of shear interaction provided. EC4-1-1 doesnot provide any guidance on the procedure to be used if the shear interaction is between 0.4and 0.5.

5.1.7 Lateral torsional buckling

The possibility of the onset of lateral torsional buckling is covered in more depth in EC4-1-1than in BS 5950: Part 3.

In general, the methods outlined in EC3-1-1 (discussed in section 4.1.8) can be adoptedwhen checking the lateral torsional buckling resistance of the steel section duringconstruction, and EC4-1-1 outlines a method applicable to composite beams with uniformcross-sections classified as Class 1, 2 or 3. This method basically decreases the compositemoment resistance of the section using a reduction factor based on the relative slendernessof the section, LT .

EC4-1-1 also outlines some detailing rules which can be used to prevent lateral torsionalbuckling. These include ensuring that adjacent spans do not differ in length by more than20%, the top flange of the steel section is connected to a reinforced concrete or compositeslab which is in turn connected to another member approximately parallel to form an invertedU frame, and by laterally restraining the bottom flange of each member and stiffening the webat each support. No such advice is presented in BS 5950: Part 3.

5.1.8 Members in compression

The first point to note is that at present there is no specific British Standard that covers thedesign of composite columns in building structures. Whilst BS 5950: Part 3 Section 3.1 refersto BS 5950 Part 3 Section 3.2, Code of Practice for the Design of Composite Columns andFrames, it has never been published. BS 5400: Part 5 Code of practice for the design of composite bridges [24], does, however, cover the design of concrete encased sections andconcrete filled circular and square hollow sections, although clearly this is for use with bridgestructures.

EC4-1-1 covers the design of composite columns and composite compression members withconcrete encased sections, partially encased sections and concrete filled rectangular and

circular tubes. It should be noted that EC4-1-1 only covers isolated non-sway columns inframes where all other structural members within the frame are also composite or steel. TheEurocode considers elements constructed with grade S235 to S460 steel and with normalweight concrete with grades between C20/25 and C50/60. It should be noted that the upper concrete strength limit is less than that for other design guidance contained within EC4-1-1.

EC4-1-1 provides two methods for the calculation of the resistance of composite columns; theGeneral Method and the Simplified Method.

General Method This takes explicit account of both second-order effects andimperfections. The method is relatively complex and requires the use of numerical computational tools. Whilst EC4-1-1 includes a description of

the processes to be considered it does not include detailed rules for thegeneral method. It is not covered at all in BS 5400.

LT .


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design in the UK is generally based on manufacturers load capacity tables/software whichare based on experimental testing (the semi-empirical m-k method). Informative Annex B of EC4-1-1 outlines testing procedures on composite slabs, which are not the same astraditionally used in the UK. The national annex may comment on this difference. Pleaserefer to Johnson and Anderson [19] for more details. In addition, EC4-1-1 allows the design

longitudinal shear resistance to be determined using both the m-k method and the partialinteraction method. This is in contrast to BS 5950: Part 3.

5.2 Part 1-2: Structural fire design

EN1994-1-2 [25] (hereafter referred to as EC4-1-2) covers the fire design of composite steeland concrete structures. In essence, it identifies the differences between the fire design andthe ambient design methods and provides supplementary information for the design for fire.This is similar to the scope of BS 5950 Part 8, except that the current British Standard coversboth bare steel and composite construction in fire.

EC4-1-2 is not applicable to uncommon material grades, such as concrete grades lower thanC20/25 and higher than C60/75 and LC60/75. EC4-1-2 provides design guidance for sometypes of element that are not common in the UK, such as partially encased concrete beamsand composite columns.

5.2.1 Fire exposure

The nominal fire exposure given in EC4-1-2 is similar to BS 5950: Part 8, where the exposureto standard and hydro carbon fire curves is adopted. However, EC4-1-2 also allows theconsideration of parametric fire exposure, which considers real behaviour (as discussed insection 4.2).

5.2.2 Material Partial FactorsThe recommended material partial factors given in EC4-1-2 are the same as those given inBS 5950: Part 8 except for concrete. EC4-1-2 recommends a value of 1.0 for concretecompared with 1.1 given in BS 5950 Part 8. However, it should be noted that the Eurocodesconsider cylindrical compressive stress compared with cube compressive stress used in theBritish Standards. The resulting difference between the concrete design values is small, withthe Eurocode value being approximately 2% greater than the BS5950 Part 8 value.

5.2.3 Structural analysis

While the assessment methods given in both EC4-1-2 and BS 5950: Part 8 assume that thestructural members are individual elements, EC4-1-2 also provides brief guidance on globalstructural analysis and the use of an analytical model which takes into account secondaryeffects and whole building behaviour. Recommendations on the validation of these advancedcalculation models are also given. Such guidance promotes the use of performance designfor structural fire design.

5.2.4 Design procedures

EC4-1-2 provides three design methods:

Tabulated data Simple calculation models Advanced calculation models


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The tabulated design data is provided for some structural types which are not easilyaddressed by simplified calculation methods. They are not common types of construction inthe UK. The following elements are included:

Simply supported beams Composite beams comprising a steel section and concrete upper flange. Composite beams comprising a steel beam with partial concrete encasement. Encased steel beams, for which the concrete has only an insulating function.

Columns Composite columns comprising totally encased steel sections Composite columns comprising partially encased steel sections Composite columns comprising concrete filled hollow sections.Note: The tables only cover the case where columns at the level under consideration are fullycontinuous with the columns above and below, and the fire is limited to only a single storey.

The simple calculation models provided in EC4-1-2 are more akin to the concept adopted for BS 5950 Part 8.