AUSTROADS BRIDGE DESIGN CODE - PROGRESS REPORT

AUSTROADS BRIDGE DESIGN CODE - PROGRESS REPORT

R. WEDGWOOD, BE, MEngSc, MIEAust Chief Bridge Engineer Roads and Traffic Authority, NSW

SUMMARY

The AUSTROADS Limit States Bridge Design Code will provide the benefit of significant economies and rationalities both for new bridges and for the assessment of existing bridges. The strength of bridges will be determined on a consistent, rational basis for the applied loadings and material properties. The serviceability requirements will minimise maintenance resource requirements by ensuring the performance of new bridges throughout their life and by identifying potential inadequacies in existing bridges at an early stage, allowing more effective remedial treatment. The Code has a background in overseas bridge code developments and also through links with complementary Australian Standards. Significant variations in the Code provisions from the existing NAASRA 1976 Bridge Design Specifications are described. The new Code will be a valuable resource to assist bridge designers to provide even better solutions to the bridge engineering challenges of the future.

KEYWORDS

Bridge Design Code, Limit States Design, Design loads, Materials properties, Structural analysis, Member strengths

ACKNOWLEDGEMENT

The author acknowledges the permission of the Chief Executive of the Roads and Traffic Authority of NSW to publish this paper. The author also wishes to acknowledge the contributions of the many dedicated contributors to the AUSTROADS Bridge Design Code, from both within the State Road Authorities and outside, in particular the Convenors of the Working Groups for each section. Many of these contributors have devoted a considerable amount of their own time to this work. Special acknowledgement is also due to Mr Gil Marsh, former Director Design MRD, Western Australia, the original Project Manager for the Code Working Group and currently Technical Editor, and Mr Albert Contessa, former Deputy Chief Engineer (Structures), MRD, Queensland, Project Manager from 1985 to 1988.

R.1 Heywood (Editor) Bridges - Part of the Transport System Pages 77-88

Ray Wedgwood graduated from Sydney University in 1963. His twenty nine years experience in bridge engineering with the Roads and Traffic Authority of NSW has included design, construction and maintenance activities on many of the State's bridges. He has been involved in the preparation of design standards and construction specifications for bridges and is currently the AUSTROADS Project Leader for the Bridge Design Project Group. He has been the Chief Bridge Engineer of the Roads and Traffic Authority, NSW since 1987.

78 AUSTROADS Conference Brisbane 1991

INTRODUCTION

The preparation of the Limit States AUSTROADS Bridge Design Code has been a major undertaking for the AUSTROADS Bridge Engineers. The origins of the Code are in the development world wide of the use of limits states design concepts in an attempt to achieve more rational and consistent strength characteristics, as related to the accuracy with which load effects (viz self weight, dead loads, live loads) and material properties can be predicted, together with a recognition of the need to satisfy seiviceability criteria, such as deflections, cracking, fatigue and vibration, which could limit the performance of a structure over its design life.

Applied to the design of new bridges, limits states design offers potential economies in:

consistency in use of materials; ii rationalisation of load effects, related to the confidence of prediction of

the load; iii) the control of maintenance costs by ensuring serviceability and durability.

Limit states design concepts also offer the opportunity to assess existing bridges more rationally to identify or confirm serviceability inadequacies and allow early remedial action, and also to identify possible strength reserves, which is important in providing for the continual demand for increases in vehicle load limits.

In this regard, it is important to recognise that the valuable reserves of strength available in the majority of our older bridges, which have allowed our road system to keep functioning under loadings well in excess of the original design loads, will not be available in new bridges designed using limit states concepts. The realistic prediction of maximum live loads into the future is therefore a very important consideration in the implementation of this new design code.

BACKGROUND TO CODE

For prior editions of the NAASRA/AUSTROADS Bridge Design Code, most of the information has been based on specifications issued by the American Association of State Highway and Transport Officials (AASHTO). For the 1976 and the new edition of the AUSTROADS Code, an effort has been made to relate the Code provisions to Australian conditions. However, significant use has still been made of AASHTO publications and research.

Much of the pioneering work in the development of a bridge design code in limit states format has been carried out by the Ontario Ministry of Transport and Communications and the influence of the Ontario Highway Bridge Design Code is acknowledged.

Also, acknowledgement is made to the Standard Association of Australia, with which AUSTROADS is developing a memorandum of understanding regarding the development of standards. AUSTROADS representation on SAA design standard committees has been aimed at developing consistency between the codes, and the contributions of the SAA design standards to this Code have been most important.

FORMAT OF CODE AND PUBLICATION PROGRAM

The code is to be published as a series of separate but complementary Sections, the provisions of which have to be applied consistently between Sections to determine appropriate responses to the Design Loads. For each Section, a separate Commentary is to be published.

RJ Heywood (Editor) Bridges - Part of the Transport System 79

The Sections making up the Code are:-

Section 1: General Section 2: Design Loads Section 3: Foundations Section 4: Bearings and Deck Joints Section 5: Concrete Section 6: Steel Section 7: Temporary Works

In order that the valuable effort put into the development of these sections can be utilised as early as practicable, it is intended to publish Sections 1 to 5, together with their commentaries, as soon as they are ready. This will be in early 1992. Sections 1 to 5 have sufficient coverage to provide for the major part of bridge design requirements in Australia. The publication of Sections 6 and 7 will follow as soon as resources allow their completion. In the interim, design of steel bridges can be to the existing rules or be based on reference to the British Bridge Code BS 5400, at the discretion of the State Road Authority, and the design of temporary works can be to relevant Australian Standards.

CODE SECTIONS

Some comments on the sections of the code follow.

GENERAL

Section 1, General, outlines general principles for the use of the Code in Article 1.1. This article is reproduced here, together with the Commentary for that article, in Appendix A, as it provides good information for understanding the basis of the limit states design process. The information provided also gives some comfort in that the determination of the serviceability limit states will generally follow existing design practices.

The Article on Waterways and Flood Design Principles introduces the requirement for the need to ensure strength and stability under the design ultimate flood effects, including debris. This article also introduces the concept of the need to ensure that the total flooded cross section (i.e. the bridge and approach embankment system) does not act as a dam for the design ultimate limit state flood (2000 year return period).

The article on traffic barriers defines four levels of service, with the Level 1 Barrier being required where absolute containment of vehicles is required for the protection of occupants and also for the protection of other persons or property. Level 2 barriers are for an intermediate level of service, corresponding to the design requirements for traffic barriers in the 1976 Bridge Design Specification. Level 3 barriers correspond to flexible W-beam guard-rails and Level 4 is no barrier at all.

DESIGN LOADS

Section 2, Design Loads, sets out the design loads, forces and effects. Provision is made for the design engineer to vary loads on the basis of engineering measurements and calculations, provided the general principles of design in Section 1 are complied with.

This Section nominates the load factors to be used for design actions for both serviceability and ultimate limit states.

AUSTROADS Conference Brisbane 1991

In comparison with the 1976 Bridge Design Specification, the T44 Truck Loading remains unchanged and the L44 lane loading is similar except that the associated concentrated load has been set at 150 kN for both moment and shear effects. The A14 axle has been deleted and replaced by a W7 wheel load for localised effects.

The abnormal vehicle load has been replaced by a heavy load platform (HLP) which is significantly heavier. A HLP gross weight of 320 tonne is applied as the general case for principle routes. The 400 tonne HLP is applicable to special designated 'routes, as determined by the Road Authority. The load factor applicable to the HLP loadings depends on the degree of control over the actual mass of permit vehicles and their passage exercised by the State Road Authority. The State Road Authority may vary the load factor depending on the degree of control exercised.

In comparative designs carried out with the above design loads, in some structures the HLP loading has been the controlling load.

The impact factor has been replaced by the dynamic load allowance, adapted from the Ontario Code. The dynamic load allowance has a maximum value of 0.4, depending on the natural frequency of the superstructure.

Braking forces are significantly increased on the 1976 values and are a function of that length of the structure resisting the force. In addition, a minimum lateral restraint requirement has been introduced to resist accidental lateral forces not catered for in the design.

Loadings are introduced to account for collisions on bridge supports from road traffic, rail traffic and shipping.

Earthquake loadings are at this stage unchanged from 1976 but these will be reconsidered when the SAA revision of the Earthquake Code is complete. Earthquake effects can be increased by upgrading the zone classification.

FOUNDATIONS

The major change in this Section is the development of a philosophy of emphasising the nexus between the extent of foundation investigation and the confidence in the assessed foundation capacity. By this means, the designer has the flexibility to balance the cost of investigation work with the cost of the foundation, with an appropriate confidence in the performance of the foundation. Such an approach takes advantage of improved technology both in foundation investigation and also in the assessment of driven pile capacities.

This approach will result in more responsibility on the designer in assessing the nature of the site and the consequences of variations, rather than relying on the lowest common denominator approach previously applied to standard piles. I believe a consequence of the philosophy will be an increased use of 'test piles' to assess capacity at the pre-tender stage.

BEARINGS AND DECK JOINTS

As well as a conversion to limit state format, Section 4 represents a major updating and revision of the rules for the design of bridge bearings and deck joints.

The rules for elastomeric bearings have been completely revised. The shear deflection capacity has been significantly increased. The vertical load capacity (rated load) for 'thin' bearings has been significantly increased. However for 'tall' bearings the rated load has remained unchanged.

RI Heywood (Editor) Bridges - Part of the Transport System 81

The rotation capacity of elastomeric bearings has been increased. The NAASRA 1976 rules forced designers to select tall bearings to satisfy rotation limits, but this sometimes caused other problems due to their low shear stiffness and relative instability.

The new rules should result in the selection of smaller, more economical bearings. The design specifications for pot bearings and contact sliding surfaces have been updated to reflect current practice and allow increased load capacities.

The loading and design requirements for deck joints have been clarified, particularly for finger plate joints and other fabricated joints, where the distribution of load to the anchorages has been more fully considered.

'The allowable open gap width for deck joints has been increased to conform to international standards. This will permit, where large movement capability is required, the selection of modular (multi-element) deck joints with a smaller number of elements.

CONCRETE

Section 5 of the new Code unifies design rules for reinforced concrete, 'partially prestressed' concrete and 'fully prestressed' concrete. This Section is also compatible and to a high degree uniform with the AS 3600 (1988), Concrete Structures

The emphasis on certain articles has been changed. Rules that were presented in a fragmented way or where insufficient guidance was provided by the NAASRA 1976 Code, such as durability, materials properties and analysis of beams, slabs and columns, are treated prominently in the new Section 5.

Durability

Rules for design for durability of concrete in the new Code will probably result in more dramatic changes of design practice than any other provisions included in this document. Since the publication of the 1976 NAASRA Bridge Design Specification some 15 years ago, durability has become a major issue in the industry because of problems associated with corrosion of reinforcement and tendons and degradation of concrete surfaces, particularly in areas of high exposure, e.g. structures exposed to salt water or salt spray.

All the durability provisions, previously scattered throughout various Articles of the Code, are concentrated and placed at the beginning of the concrete design section to signify the effect the new rules are expected to have on design. The durability requirements can result in concrete strengths which in many cases will be higher than would otherwise be required for consideration of strength alone. The strength required for durability is therefore determined initially. It may then be appropriate to enter the strength design calculations with the minimum acceptable concrete strength as determined for durability.

Unlike the previous Code, the new provisions for durability are related to a wide range of defined exposure classifications. In general the rules governing durability are tighter than comparable rules in the previous Code; for higher exposure classifications the new requirements are much more stringent. The new rules respond to the development of the last 10-15 years related to the gradual changes in the composition of concrete mixes which now, as a result of the much more finely ground cementitious materials used, contain less cementitious materials by weight for concrete of a given strength, and therefore do not provide the same degree of protection to reinforcement. For higher exposure classifications it is now mandatory to specify minimum cement content and maximum water/cement ratio. Although the latter is difficult to measure in the field, it


is still recognised as a reliable measure of the ability of concrete to protect reinforcement.

Materials Properties

The new Code attempts to avoid the tendency of the previous Specification to give precise values of material properties. This has been done to avoid misinterpretation by less experienced users as to the accuracy of these values. The Code introduces a concept of guaranteed minimum capacity to resist loads by linking the concept of Strength Reduction Factors and Ultimate State Load Factors. For Serviceability Limit States the emphasis is on materials test values or current local knowledge; the curves or formulae are given with the emphasis of bands of uncertainty for all material properties. The Code and the accompanying Commentary encourages the designer to examine the sensitivity of possible variations in material properties. The information on creep and shrinkage of concrete emphasizes the dependence, among other influences, on the type of aggregate used.

Methods of Structural Analysis

The new Code requires that analysis must be carried out for all relevant limit states; the difference is particularly obvious for reinforced concrete which, to satisfy the requirement of NAASRA 1976, was analysed for working stresses only.

For reinforced concrete the new Code allows redistribution of moments at the Strength Ultimate Limit State of up to 30% of design moment. The removal of the stress limitation at serviceability limit states is expected to result in significant savings of reinforcing steel.

Other provisions of the new Code include:

An increase of the limit of the stress increment in prestressing steel to 200 MPa (The 1981 Addendum to NAASRA 19/b limited this increase to 120 WO

Design of reinforced concrete is completely different from NAASRA 1976 in that is designed for the ultimate strength limit state and there are no particular checks for stresses at the serviceability limit state. There are checks, however, for other serviceability limit states, viz. cracking and deflection.

Design for shear, torsion and suspension reinforcement are very similar for both prestressed and reinforced concrete. Truss analogy methgd is used and for shear and torsion, a flatter truss angle is allowed than the 45 implied in NAASRA 1976. This should result in more economical design as a proportion of stirrups may be replaced by additional longitudinal reinforcement which is more economical to supply and fix.

Design of Columns

The working stress method of design in NAASRA 1976 had been abandoned by many designers long before the drafting of the relevant parts of the new Code. The method required by the new Code is similar but somewhat different to the one published in AS 3600. The majority of framed structures will be analysed by first-order methods of linear analysis, e.g. it will be assumed that changes of geometry under loading will have negligible effect on bending moments, axial forces and shear forces. Second-order method of analysis, of iterative nature, will be used for slender columns where changes of geometry cannot be safely ignored.

RJ Heywood (Editor) Bridges - Part of the Transport Sysiem 83

Summary

Section 5 provides a basis for rational and economical design of concrete bridges. The integration of design rules for reinforced, partially and fully prestressed concrete will remove the remaining artificial barriers between 'partially and 'fully prestressed' concrete.

STEEL

Section 6, Steel, is closely based on AS 4100, Steel Structures, although the tiered approach of the Australian Standard has only been incorporated for bridge specific design situations. The rules for composite design, which are relevant and necessary for most modern steel bridges, have been taken from a number of sources, including Eurocode 4 (Section 7), the British Bridge Code BS 5400, Part 5, and the Recommendations of the European Convention on Constructional Steelwork (ECCS) on Composite Structures. At this stage, there is no Australian Standard for limit states design of composite structures.

General Design Requirements

The design conditions to be considered by the designer are specified, viz. stability, strength, serviceability, brittle factor and fatigue, together with requirements for corrosion resistance/protection and camber.

Materials capacity reduction factors are listed in separate tables for both the strength and serviceability states.

The additional requirements for corrosion resistance and protection are based on BS 5400.

Methods of Structural Analysis

Three methods of structural analysis are allowed: elastic analysis will be most commonly used, with rigorous structural analysis and structural testing being adopted for special situations.

Plastic analysis has been excluded because, in AS 4100, its application is limited to compact doubly symmetric I-sections which are not subject to impact or fatigue loads.

Composite Beams

The calculation of the section moment capacity is different for positive and negative moment regions. For positive moment regions a plastic section analysis is required, whereas for negative moment regions, the section capacity is governed by the compactness of the steel section.

CONCLUSION

The AUSTROADS Bridge Design Code has had a long gestation as a result of a number of factors, including the need for harmonisation with SAA Codes, and the fluctuations in resources available from member authorities to contribute to the Code.

It will he pleasing to see the results of the labours of many dedicated contributors come to fruition in the Bridge Design Code which, used sensibly and with professional judgement, will allow bridge designers to develop even better solutions to the bridge engineering challenges of the future.


1.1

APPENDIX A

ARTICLE 1.1 OF AUSTROADS BRIDGE DESIGN CODE

CODE REQUIREMENTS AND ASSOCIATED COMMENTARY

General

1.1

1.1.1

General Principles

Aim of Design

The aim of design shall be the achievement of acceptable probabilities that the structure being designed will not become unfit for use during its design life, having regard to economic, physical, aesthetic and other relevant con-straints.

Limit States Design is the procedure that has evolved to enable the "aim of design" to be accomplished in a logical manner, and has been adopted in this Code.

1.1.2 Bases of Design

The design shall be based on scientific theories, experi-mental data and past experience, interpreted statistically as far as possible. The safety and service performance of a

structure depends also on the quality control exercised in fabrication, supervision on site, the control of unavoidable imperfections and the qualifications, experience and skill of all personnel involved. Adequate attention shall there-fore be given to these factors. In addition adequate management control and supervision by experienced engi-neers shall be required at all stages of design and construc-tion to prevent the occurrence of gross errors. Control over the conditions of use of the structure during its life was assumed in drafting this Code.

This Code shall be applied to the design of road traffic bridges in Australia. For long spans (> 100 m) and unusual structures, special conditions may apply to loading or strength requirements. In general, restrictions and quali-fications are noted in the relevant Sections of this Code.

1.1.3 Design Life

The basic design life of bridge structures shall be assumed to be 100 years. The design life of elements such as bearings and expansion joints shall be the same as the main structure, but where experience has shown a shorter life may occur in practice, provision shall be made for easy replacement and any fixings shall be detailed to be reus-able.

For the design of ancillary elements such as light poles and sign structures a shorter design life may be assumed by the Road Authority (refer to Section 2).

1.1.4 Limit State

A structure, or structural element, is considered unfit for use when it reaches any one of a number of states at which it no longer satisfies the criteria governing its performance or use. All appropriate limit states shall be considered in design. Limit states are classified as either

(a) ultimate limit states, or

(b) serviceability limit states.

1.1.5 Other Considerations

In general, bridge structures are not designed for all possible loads and conditions, such as those arising in war. However, accidental effects that can reasonably be fore-seen shall be considered in the design.

1.1.6 Ultimate Limit States

The ultimate (safety-related) limit states can be both strength and stability limit states, and include:

(a) loss of static equilibrium by sliding, overturning or uplift of a part, or the whole of the structure;


1.2 AUSTROAOS 13ndpe Design Code

(b) a post-elastic or post-buckling state in which the collapse condition is reached at one or more sections

of the structure. Plastic or post-buckling redistribu-

tion of actions and resistance shall only be considered when the design engineer has adequate knowledge of

the associated deformation characteristics of the

structure from theory and tests;

(c) failure of any foundation material causing excessive movement in the structure, or failure of significant

pans of the structure;

(d) sufficient deterioration of strength occurring due to

fatigue and/or corrosion such that the collapse strength of the damaged section is reached. The

design engineer shall carefully consider the implica-

tions of fatigue damage or any other local failure when die number of elements carrying the loading is small, and failure of one element significantly in-

creases the loading of the remainder.

1.1.7 Serviceability Limit States

The serviceability limit states to be considered include:

(a) permanent deformation of foundation material or a

major load carrying element, of sufficient magnitude

that the strucnire is unfit for use, or such that the public would become concerned as to the safety of the structure,

(b) permanent damage due to corrosion, cracking or fatigue which significantly reduces structural strength or useful service life;

(c) vibration leading to structural damage or public concern;

(d) flooding of the road network and surrounding land and scour damage to the channel bed, banks and road embankments.

1.1.8 Analysis Methods

Analyiis for all limit states shall be based on linear elastic

assumptions except where non-linear methods are specifi-

cally implied elsewhere in this Code or approved by the Road Authority.

1.1.9 Design Actions or Loads

An action is a system of:

(a) concentrated or distributed forces (direct actions), or

(b) imposed or constrained deformations (indirect ac-tions)

applied to a structure due to a single cause.

The design actions S• arc spuified in Section 2 for serv-iceability and ultimate limit stales.

A serviceability action is defined as one having a 5%

chance of exceedance per year.

An ultimate action isde fined as one which hasa5%chance

of exceedence during the design life.

1.1.10 Design Resistance or Strength

The design resistance or strength (or capacity) R• is de-

fined in the various material sections of the Code and is

derived from the nominal strength of the element and a

material (or strength reduction or capacity) factor, #.

Where the resistance is derived from dead loads of pan or

all of a structure, the resisting dead load shall be reduced

by the appropriate load farim given in Section 2.

1.1.11 Verification of Limit States

For ultimate limit states, the following relationship must

be satisfied -R• 2 S•

For serviceability limit states, the stress, deflection,

cracking or vibration level must satisfy the limits set out in

the appropriate Sections of this Code.

A USTROA DS Conference Brisbane 1991

C1.1

C1 General Commentary

C1.1 General Principles

C1.1.1 Aim of Design

Thc structure and all its components must be designed to

resist all loads, deformations and environmental condi-

tions likely to occur during construction and normal use,

and to have adequate durability. Design loads, actions and

strengths arc intended to product a very low probability of

failure during the design life.

C1.1.2 Bases of Design

This is a simple statement of current good practice in

design. Statistical analysis of design information is essen-

tial if consistent levels of safety and performance are to be achieved. Much of the current design practice is explicitly

based on statistical analyses, eg concrete strengths, wind

loading, etc. This Code is intended to encourage the use of

statistical methods where practical and where data is available.

Foundation investigations rarely yield sufficient data to

allow a full statistical treatment, and characteristic values

for design must be based on test results and the experience

and judgement of the geotechnical engineer.

In general, it is uneconomical to attempt to build structures

with no imperfections, and construction supervision is

aimed at controlling imperfections, so that they lie within

acceptable tolerances.

Experienced qualified persNns arc needed at all levels to ensure the work conforms to current practice and to detect

gross errors, as the "safety factors" incorporated in this Code arc not intended io protect against such mistakes. In

particular, the design check should be carried out by

personnel not involved in the original design, to avoid

repetition of incorrect assumptions, methods and data. Construction must be supervised by experienced engi-

neers who can recognise major mistakes and have them

remedied.

It follows also that the conditions of use of bridges must be controlled, particularly for traffic loads, in a manner

consistent with the design assumptions.

Limit States Design is not a new or radically different

design process. It is merely a logical statement of current good practice, namely, identifying all the constraints of

loading, environment and service performance, and de-

signing logically to satisfy all those constraints.

This Code should be adequate for the design of spans up to 100 m. For unusual structural forms and spans longer than 100 m, the design engineer should also refer to overseas

codes and technical literature, and should carryout testing

where information is doubtful.

C1.1.3 Design Lite

The selection of 100 years is somewhat arbitrary. BS 5400

selects 120 years, while buildings are often assumed to

have a 50 year design life. Historically, bridges have been

one of the more permanent types of structures and a useful

life far in excess of 100 years can be envisaged for most bridges unless they are replaced for other reasons, such as

road realignment, width limitations or they are made of

less durable material such as timber.

This assumption of a "fixed" design life does not mean that the bridge will no longer be fit for service when it reaches

that age, or that it will reach that age without adequate and

regular inspection and maintenance. Steel bridges require

regular maintenance of their corrosion protection and

concrete bridges often require repair of construction de-

fects which cause minor local deterioration or of defects

arising from carbonation or chloride ingress.

Common sense dictates that bridge elements which arc

subject to movement, impact and wear, such as bearings, guardrails and expansion joints, should be easy to replace.

Bolted attachment is preferable to permanent fixing where

possible. Sockets or bolts cast into concrete should be highly resistant to corrosion to ensure reuse.

Light poles and minor roadside sign structures arc seen as

less durable elements which arc more economically manufactured with a shorter design life. Ease of replace-

ment is a major factor in determining a suitable design life.

Major sign gantries erected over roads should be designed

fora 100 year life.

C1.1.4 Limit States

The limit states specified in this Code arc analogous to the

working stress and ultimate strength and stability checks used previously. In theory, a change in state occurs in the

structure when the "limit" is reached. Usually, the design

calculations arc based on the most "critical" limit state

(determined by experience) and other limit states are checked. Simple "deemed to satisfy" requirements suffice

for some limit states.

C1.1.5 Other Considerations

It is uneconomical to design for every eventuality, such as the use of high explosives in war. Accidents can be

foreseen to a certain extent, and provision is made for various accidental collision forces. The design engineer

should also foresee other events such as gas or water

leakage in a closed cell, and either relocate, ventilate or

drain as necessary. The design engineer must ensure that minor damage to a structural component or services car-

ried on the structure, does not lead to major damage out of

all proportion to the original cause.


Cl.?

C1.1.6 Ultimate Limit States

Reaching an ultimate limit state can lead to catastrophic failure which could endanger the lives of workmen and the public. Hcncc it is sometime referred 10 as a "safety" limit

sutc. The provisions of this Code arc similar to the

previous (1976) edition. Regular inspection and mainte-nance should prevent deterioration due to corrosion or

fatigue tr,coming a cause of collapse. The design engineer

should be careful to avoid situations where only a small

number of elements carry the main load in a non-redundant (single load path) structure and where fatigue fracture can

cause a rapid and significant increase in the loads carried

by other elements, leading to a progressive collapse.

Plastic redistribution, based on the formation of a collapse

mechanism, is not allowed in calculating design resistance unless adequate test information is available to ensure a "plastic plateau" is achieved rather than a "brittle" failure.

It is essential that the structural Clement as a whole (eg beam. column or box girder) acts "plastically', not just its material components (eg steel, concrete). Note that mo-

ment redistribution implies a redistribution of shear forces as well, and this must be checked.

C1.1.7 Serviceability Limit States

Loss of serviceability due to local yielding and deflection

of structural elements has been controlled by limiting the

"working** stresses, as in previous editions of this Code. These limits arc intended to have the same effect on the

design as a specified higher load being equated to achiev-

ing "yield" stress in the member. Simple ratios of loads and stresses arc valid as the structure is linear elastic under such loads.

Vibration is mainly of concern -when pedestrians use the structure.

C1.1.8 Analysis Methods

This requirement is based on the fact that simple analytic tools arc adequate and conservative in most cases. More

complex techniques using non-linear analysis are allowed

when sufficient experimental proof of the method is avail-able and the Road Authority approves.

"Working stress" design methods are considered appro-priate for limit state design as they describe the behaviour

of the structure under service loads, and provide a margin

against non-linear behaviour and permanent deforma-

tions. Consistency of design methods in the transition between code editions is also desirable.

Where flexural action controls, the ultimate limit state is

defined as the formation of the first moment hinge. Where the behaviour of a section is known from both theory and

tests, an analysis including both moment and shear re-

distribution maybe used in design.

In general, a particular analytical method to derive a

preliminary design concept which satisfies one set of limit

states will be chosen. This design will then be checked

against the other relevant limit states and modified as

necessary to reach a final design satisfying all require-

ments. Within the limits set out in this Code. the choice of preliminary design method lies with the design engineer.

C1.1.9 Design Actions or Loads

Since strains caused by creep, shrinkage etc. are not

actually "loads" but rather alter the distribution of stresses

in a structure, the generic term "action" is used to include

both loads and imposed deformations.

In some cases a "characteristic" value of the action or load

is specified, and then "load factors", to convert the charac-

teristic value to the "design" serviceability or ultimate

effect or load, arc given. However, some load factors can

have regional variations if a constant "safety factor" is to be maintained. In such cases the Code specifics the

"design" actions or loads directly.

Based on the assumed 100 year design life, the definitions

of the limit states imply the conditions set out in Table

C1.1.9 for transient loads.

Table C1.1.9 Definition of Limit States

Limit State

Return

Interval of load

Probability of exceertance in

any 1 year

Probability of

exceedance in

100 years

Servicea- bility

20 years 0.05 0.99

Ultimate 2000 years 0.0005 0.05

Such definitions are common in relation to natural phe-

nomenon such as floods, wind, earthquake, etc. They are

not commonly used for traffic loads. However, traffic loads arc defined so as to achieve the same probability of

exceedance as for natural load effects.

C1.1.10 Design Resistance or Strength

The provisions of this Code are similar to ultimate strength

requirements in previous editions where the design resis-

tance or strength is assessed after considering:

(a) the variability of material properties,

(b) the reliability of strength calculation methods,

(c) a possible reduction in strength due to fabrication and

construction tolerances,

(d) the type of failure being considered, eg "ductile" or

"brittle".

C1.1.11 Verification of Limit States

This equation is a simple statement that the design resis-tance or strength should equal or exceed the design actions

or loads, and that the probability of this not being achieved

is very small. Note that when the design ultimate load does

occur, there is still only a small probability that the design

strength will be exceeded.

88 A UST ROA DS Conference Brisbane 1991

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AUSTROADS BRIDGE DESIGN CODE - PROGRESS REPORT