SCI PUBLICATION P304

Guide to the Major Amendments

in BS 5950-1:2000

M Heywood MEng PhD CEng MICE

Published by: The Steel Construction Institute Silwood Park Ascot Berkshire SL5 7QN Tel: 01344 623345 Fax: 01344 622944

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2001 The Steel Construction Institute

Apart from any fair dealing for the purposes of research or private study or criticism or review, as permitted under the Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the UK Copyright Licensing Agency, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organisation outside the UK.

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Publications supplied to the Members of the Institute at a discount are not for resale by them.

Publication Number: SCI P304

ISBN 1 85942 131 8

British Library Cataloguing-in-Publication Data. A catalogue record for this book is available from the British Library.

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FOREWORD

The design Standard for structural steelwork, BS 5950-1, is used in the design calculations for the majority of constructional steelwork in the United Kingdom. Structural engineers are very familiar with the 1990 issue of this Code and have used it routinely to design safe and efficient structures in a fast and cost-effective manner. BS 5950-1 has been amended recently and there has been genuine concern within the industry that unfamiliarity with the revised code could result in poor productivity, increased costs and design errors, seriously damaging the competitiveness of the construction industry. BS 5950-1:2000, as the revised code is known, came into effect on 15 August 2001.

The purpose of this publication is to ease the transition from BS 5950-1:1990 to BS 5950-1:2000, by highlighting Clauses that have undergone significant technical changes and explaining how these changes will affect the design of steel building structures. This should enable designers to adopt the new Code quickly and with the minimum of disruption, thereby minimising the potential cost to the industry resulting from reduced productivity. This guide is not intended as an in-depth commentary to BS 5950-1, as it deals only with the major changes, however it does cover all the changes with important safety implications, allowing structural engineers to continue to use BS 5950-1 with confidence. This publication was written by Dr Martin Heywood of The Steel Construction Institute, with contributions to the worked examples from the late Mr Paul Salter, Mr Abdul Malik, Mr David Brown and Mr Charles King. Funding for the preparation of this guide was gratefully received from the Department of the Environment, Transport and the Regions (DETR) and Corus.

The Steel Construction Institute has produced a comprehensive guide to the amendments in BS 5950-1:2000. Available on CD, the guide contains a Clause-by-Clause comparison of the 1990 and 2000 editions, a description of all changes, interactive design paths, worked examples, a keyword search facility, and the facility to print a paper copy of the Standard.

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Contents Page No.

FOREWORD iii

SUMMARY vii

1 INTRODUCTION 1 1.1 Background 1 1.2 Scope of this publication 1 1.3 Summary of the changes 1

2 LIMIT STATES DESIGN 4 2.1 Load factors 4 2.2 Stability 4 2.3 Brittle fracture 7 2.4 Structural integrity 8

3 PROPERTIES OF MATERIALS AND SECTION PROPERTIES 10 3.1 Grades of steel 10 3.2 Section classification 10 3.3 Effective plastic modulus 11 3.4 Slender cross sections 11

4 DESIGN OF STRUCTURAL MEMBERS 13 4.1 Members subject to bending 13 4.2 Lateral-torsional buckling 15 4.3 Plate girder webs 20 4.4 Design of stiffeners 24 4.5 Tension members 27 4.6 Compression members 28 4.7 Combined moment and axial force 30 4.8 Column bases 32

5 CONTINUOUS STRUCTURES 35 5.1 Column bases 35 5.2 Frame stability 35 5.3 Portal frames 37 5.4 Multi-storey frames 39

6 CONNECTIONS 42 6.1 Bolted connections 42 6.2 Pin connections 47 6.3 Welded connections 48

7 REFERENCES 51

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WORKED EXAMPLES 53 Sway stability 55 Choosing a steel sub-grade 57 Restrained beam 59 Unrestrained beam 63 Plate grider 67 Web bearing and buckling 71 Compression member 75 Axial load and bending 77 Baseplate 83

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SUMMARY

BS 5950 Structural use of steelwork in building, Part 1: Code of practice for design - Rolled and welded sections has undergone major amendment. Almost every Clause of this widely used Standard has changed in some way; some of the changes are technical in nature, others are editorial and do not alter the recommendations for building design. The revised Standard, referred to as BS 5950-1:2000, became effective on 15 August 2001.

The aim of this publication is to ease the transition to BS 5950-1:2000 by guiding designers through the major technical amendments to the Standard. A short description of each important change is provided, and simple worked examples illustrate the revised design procedures.

The major amendments to BS 5950-1 include revised rules for checking the stability of frames, changes to the method for selecting an appropriate steel sub-grade and the introduction of the effective-area method for class 4 slender sections. On the subject of member design, the greatest change relates to lateral-torsional buckling, where the n-factor method has been removed. Changes have also been made to the Clauses on shear buckling, stiffener design, tension members, compression members, combined axial load and bending, and the design of column bases. Elsewhere, important changes have been made to the rules relating to the in-plane stability of portal frames, prying forces and the transverse strength of fillet welds.

Recueil des principales modifications la norme BS 5950-1

Rsum

La norme BS 5950 Usage structural des constructions en acier dans les btiments, Partie 1: Code de pratique pour le dimensionnement - Sections lamines et soudes, a subi dimportantes modifications. Pratiquement tous les articles de cette norme, fort utilise en pratique, ont subi des modifications; certaines ont un simple caractre ditorial mais dautres, par contre, apportent des modifications techniques importantes. La norme rvise, rfrence BS 5950-1:2000, est devenue dapplication le 15 aot 2001.

Le but de cette publication est de faciliter la transition vers la nouvelle norme en guidant les utilisateurs travers les modifications majeures. Une courte description de chaque changement important est donne; des exemples simples illustrent les procdures de dimensionnement rvises.

Les modifications majeures incluent la vrification de la stabilit des portiques, la mthode de slection des nuances dacier appropries et lintroduction dune mthode daire effective pour les sections de classe 4. Concernant le dimensionnement des lments, la modification la plus importante a trait au dversement, o la mthode du coefficient n a t supprime. Des modifications sont aussi apportes aux articles traitant du voilement par cisaillement, du dimensionnement des raidisseurs, des lments en traction, en compression et en combinaison charge axiale-flexion ainsi que du dimensionnement des pieds de poteaux.

Dautre part, des modifications ont aussi t apportes aux rgles relatives la stabilit dans leur plan des portiques, aux forces de levier et la rsistance transversale des soudures dangles.

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Leitfaden fr die wichtigsten nderungen in BS 5950-1

Zusammenfassung

BS 5950 Stahlbauten, Teil 1: Vorschrift zur Berechnung - Gewalzte und geschweite Querschnitte hat sich wichtigen nderungen unterzogen. Fast jeder Satz dieser allgemein verwendeten Vorschrift hat sich in gewisser Weise gendert; manche nderungen sind technischer Art, andere sind redaktioneller Art und ndern die Empfehlungen fr die Berechnung nicht. Die berarbeitete Norm, jetzt mit BS 5950-1:2000 bezeichnet, ist seit 15 August 2001 gltig.

Das Ziel dieser Publikation ist es, den bergang zu BS 5950-1:2000 zu erleichtern, indem der Ingenieur durch die wichtigsten nderungen gefhrt wird. Eine kurze Beschreibung jeder wichtigen nderung ist enthalten, und einfache Berechnungsbeispiele illustrieren die berarbeiteten Berechnungsverfahren.

Die wichtigen nderungen in BS 5950-1 beinhalten berarbeitete Regeln zur Prfung der Stabilitt von Tragwerken, nderungen zur Auswahl einer geeigneten Stahlgte und die Einfhrung der Methode der wirksamen Flche bei schlanken Querschnitten der Klasse 4. Bezglich der Bauteilberechnung ergibt sich die grte nderung beim Biegedrillknicken, hier wurde die n-Faktor Methode gestrichen. nderungen gibt es auch bei folgenden Themen: Schubbeulen, Berechnung von Steifen, Zug- und Druckglieder, Normalkraft und Biegung, Sttzenfe. An anderer Stelle wurden wichtige nderungen vorgenommen beim Stabilittsverhalten in Tragwerksebene von Portalrahmen, bei Sttzkrften und bei der Festigkeit von Kehlnhten in Querrichtung.

Guida alle principali modifiche della BS 5950-1

Sommario

La norma BS 5950 Carpenteria strutturale negli edifici, Parte 1: regole progettuali - Sezione laminate e saldate ha subito un importante aggiornamento. Quasi ogni punto di questa norma, ampiamente diffusa ed utilizzata, stato in qualche modo variato. Alcune di queste modifiche sono di natura tecnica, altre editoriale e non alterano le raccomandazioni relative al progetto degli edifici. La norma revisionata, denominata BS 5950-1:2000, entrata in vigore il 15 Agosto 2001. Scopo di questa pubblicazione facilitare il passaggio alla nuova BS 5950-1:2000, guidando i progettisti attraverso le principali modifiche tecniche che sono state effettuate. Viene fornita una breve descrizione di ogni variazione rilevante, e semplici esempi applicativi illustrano le procedure di progettazione aggiornate.

I principali emendamenti alla BS 5950-1 includono una revisione delle regole di verifica della stabilit dei telai, del metodo di selezione dellidonea classe di acciaio e lintroduzione del metodo dellarea efficace per le sezioni snelle della classe 4. Per quanto concerne la progettazione degli elementi, le maggiori variazioni si riferiscono instabilit flesso-torsionale, ove stato eliminato il metodo del fattore-n. Sono state apportate modifiche anche ai punti relativi allimbozzamento da taglio, alla progettazione degli irrigidimenti, agli elementi tesi ed a quelli compressi, alla combinazione di azione assiale e flettente e al progetto delle basi delle colonne. In altre parti, sostanziali modifiche sono state effettuate alle regole riguardanti la stabilit nel piano dei portali, le forze di contatto e la resistenza trasversale delle saldature a cordone dangolo.

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Guia de correcciones mas importantes a la BS 5950-1

Resumen

La norma BS5950 Uso de acero estructural en edificios, Parte 1: Reglas de buena prctica para el proyecto Perfiles Laminares y soldados ha sufrido importantes correcciones. Casi cada Clusula de esta popular Norma ha cambiado de algn modo en el aspecto tcnico aunque tambin hay cambios en el aspecto editorial que no afecta las recomendaciones de proyecto. La Norma Revisada, titulada BS 5950-1:2000, se hizo efectiva el 15 de agosto de 2001. El propsito de esta publicacin es facilitar la transicin guiando a los proyectistas a travs de las correcciones ms importantes de la Norma. Se da una breve descripcin de cada cambio importante y los procedimientos revisados de proyecto se ilustran desarrollando ejemplos sencillos. Las correcciones a la BS 5950-1 incluyen reglas de comprobacin de la estabilidad de prticos, cambios al mtodo de seleccin de una subclase de acero adecuada al problema y la introduccin del mtodo del rea efectiva para las secciones esbeltas de clase 4. Sobre el tema de proyecto de piezas el cambio mayor se refiere al pandeo lateral y de torsin donde se ha eliminado el mtodo del factor n. Tambin se han hecho cambios en las clusulas sobre la abolladura por cortante, proyecto de rigidizadores, piezas a traccin y a compresin, flexin combinada con axil y proyecto de basas de columnas. Adems se han llevado a cabo importantes cambios en las reglas relativas al pandeo en su plano de prticos fuerzas de retraccin y resistencia transversal de cordones de soldadura.

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1 INTRODUCTION

1.1 Background Since its introduction in 1985, BS 5950 Structural use of steelwork in building, has gradually (although not completely) replaced BS 449 in the design office and is now the established Standard for the design of steel-framed buildings in the UK and several other countries. Part 1 of BS 5950 (referred to here as BS 5950-1) is the Code of practice for design using rolled and welded sections. It gives recommendations for the safe design of general building structures, including the specification of the appropriate steel sub-grade, the classification of sections, design for stability, the design of members subject to bending, tension and compression, stiffener design and the design of column bases and connections.

BS 5950-1 has undergone a major amendment, affecting the majority of the Clauses to some extent. The revised Standard, BS 5950-1:2000[1], became effective on 15 August 2001. Because this Standard is so widely used for the design of structural steelwork, it is hardly surprising that news of this amendment was greeted with some trepidation among designers, many of whom are very familiar with the recommendations of its predecessor BS 5950-1: 1990[2]. The Steel Construction Institute recognised that some guidance was required during the period of transition, as designers familiarise themselves with the content and layout of the amended Standard. This publication provides a concise guide to the changes, together with advice on the implementation of the revised Clauses.

1.2 Scope of this publication The purpose of this publication is to guide designers through the major technical amendments to BS 5950-1, by means of a short description of each important change and simple worked examples. It is not a commentary to BS 5950-1: 2000 and does not, therefore, attempt to give the theoretical background to the Clauses or any justification for the amendments. This publication is limited to those Clauses from Sections 2 to 6 of BS 5950-1:2000 that have undergone a major or significant technical amendment. Section 7 (which deals with testing) and the Annexes are beyond the scope of this publication, as are the numerous minor changes. For convenience, guidance on the modified Clauses has been grouped together into five sections, with numbered headings matching those used in BS 5950-1:2000. The numbered sub-sections do not correspond to the Sub-sections in BS 5950-1:2000, but Clause numbers are stated in all cases.

1.3 Summary of the changes The 2000 amendment to BS 5950-1 has affected almost every Clause in the Standard to some extent, even though many of the changes are only editorial in nature (i.e. the technical recommendations are unchanged). Users of BS 5950-1:2000 will notice immediately that the familiar two-column format has been replaced with full-width pages, giving the impression that this is a completely new document. This impression is reinforced by the renumbering of many of the Clauses and the extensive re-drafting of much of the text.

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Fortunately, this impression is deceptive and many of the technical requirements are completely unchanged. Even where changes have been made to the values and equations in the Standard, the majority of design procedures are the same as in BS 5950-1:1990. However, there have also been a number of significant technical changes and designers will need to familiarise themselves with several new methods of design.

One of the most important changes is the extension of the scope of BS 5950-1 to include cold formed structural hollow sections. The Steel Construction Institute first recommended that BS 5950-1 could be used for design using cold formed structural hollow sections in Advisory Desk article AD185[3] and offered advice on how such sections could be designed using a Standard written principally for hot rolled steel. These recommendations have now been incorporated into BS 5950-1. However, designers must note that the inclusion of cold formed structural hollow sections in BS 5950-1 does not mean that they can be used in direct substitution for a similar-sized hot finished member, because there are important differences between the two types of section in terms of section properties and residual stresses. Designers wishing to substitute cold formed for hot finished structural hollow sections must redesign the members using the appropriate strut curves, d/t limits (for section classification) and section properties. Other types of cold formed section should still be designed according to BS 5950-5[4].

Within Section 2, important changes have been made to the rules for checking the stability of all types of framed structure, including braced frames. All of the stability rules, apart from those for portal frames, can now be found in Clause 2.4.2, reducing the risk of the common misconception that only continuous frames need be checked. In fact, most of the changes to this Clause have been made to clarify the intent of the Standard and the technical recommendations are largely unchanged. The rules for brittle fracture have also been amended, resulting in a revised method for calculating the maximum allowable thickness of steel. Compared with BS 59501:1990, the new Standard includes a greater variety of details and temperatures (down to 45C). Other changes to Section 2 include a few new load factors and changes to the rules for structural integrity and disproportionate collapse.

In Section 3, numerous minor changes have been made to the limiting width-to-thickness ratios used in the classification of cross sections, although the general principle remains unchanged. By far the greatest change to Section 3 of BS 5950-1 relates to the treatment of class 4 slender sections. BS 5950-1:2000 recommends the use of the effective-area method, in which the reduction in capacity due to local buckling is allowed for by the use of effective section properties, as an alternative to the conservative approach of reducing the assumed design strength. Section 3 of BS 5950-1:2000 also introduces the effective plastic modulus, Seff, which may be used instead of Z for class 3 semi-compact sections (the use of Z is over-conservative in many cases).

Section 4, which deals with the design of structural members, has undergone many significant changes, particularly in relation to members subject to bending. The rules governing the design of both restrained and unrestrained beams have been modified, although most of the changes will only affect the design of class 3 semi-compact and class 4 slender beams. The only significant change relevant to the design of Universal Beams under pure bending (which are usually class 1 plastic or class 2 compact) is the removal of the n-factor

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1 INTRODUCTION

1.1 Background Since its introduction in 1985, BS 5950 Structural use of steelwork in building, has gradually (although not completely) replaced BS 449 in the design office and is now the established Standard for the design of steel-framed buildings in the UK and several other countries. Part 1 of BS 5950 (referred to here as BS 5950-1) is the Code of practice for design using rolled and welded sections. It gives recommendations for the safe design of general building structures, including the specification of the appropriate steel sub-grade, the classification of sections, design for stability, the design of members subject to bending, tension and compression, stiffener design and the design of column bases and connections.

BS 5950-1 has undergone a major amendment, affecting the majority of the Clauses to some extent. The revised Standard, BS 5950-1:2000[1], became effective on 15 August 2001. Because this Standard is so widely used for the design of structural steelwork, it is hardly surprising that news of this amendment was greeted with some trepidation among designers, many of whom are very familiar with the recommendations of its predecessor BS 5950-1: 1990[2]. The Steel Construction Institute recognised that some guidance was required during the period of transition, as designers familiarise themselves with the content and layout of the amended Standard. This publication provides a concise guide to the changes, together with advice on the implementation of the revised Clauses.

1.2 Scope of this publication The purpose of this publication is to guide designers through the major technical amendments to BS 5950-1, by means of a short description of each important change and simple worked examples. It is not a commentary to BS 5950-1: 2000 and does not, therefore, attempt to give the theoretical background to the Clauses or any justification for the amendments. This publication is limited to those Clauses from Sections 2 to 6 of BS 5950-1:2000 that have undergone a major or significant technical amendment. Section 7 (which deals with testing) and the Annexes are beyond the scope of this publication, as are the numerous minor changes. For convenience, guidance on the modified Clauses has been grouped together into five sections, with numbered headings matching those used in BS 5950-1:2000. The numbered sub-sections do not correspond to the Sub-sections in BS 5950-1:2000, but Clause numbers are stated in all cases.

1.3 Summary of the changes The 2000 amendment to BS 5950-1 has affected almost every Clause in the Standard to some extent, even though many of the changes are only editorial in nature (i.e. the technical recommendations are unchanged). Users of BS 5950-1:2000 will notice immediately that the familiar two-column format has been replaced with full-width pages, giving the impression that this is a completely new document. This impression is reinforced by the renumbering of many of the Clauses and the extensive re-drafting of much of the text.

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2 LIMIT STATES DESIGN

2.1 Load factors Clause 2.4.1 BS 5950-1:2000 contains load factors for several new load combinations. Values of f are now given for storage tanks (full and empty), earth and groundwater loads, exceptional snow loads and various combinations of dead, imposed, wind and crane loads. A new case of dead load whenever it counteracts the effects of other loads has also been added, with gf = 1.0, to take account of the fact that in some cases the dead loads are actually beneficial (similar to, but more general than, dead load when restraining sliding, overturning or uplift in BS 5950-1:1990).

In addition, the load combinations that the designer must consider are now given explicitly in the Code. Previously, these combinations were listed in Table 2 along with their f values, but there was no compulsion to consider any or all of them (although it was assumed that competent designers would know that they had to consider all load combinations to determine the worst case for their buildings).

According to BS 5950-1:2000, the following principal load combinations need to be considered:

For buildings without cranes:

Load combination 1 dead load and imposed load (gravity loads)

Load combination 2 dead load and wind load

Load combination 3 dead load, imposed load and wind load.

For buildings with overhead travelling cranes:

Crane combination 1 dead load, imposed load and vertical crane loads

Crane combination 2 dead load, imposed load and horizontal crane loads

Crane combination 3 dead load, imposed load, vertical crane loads and horizontal crane loads.

In BS 5950-1:2000, the wind loading on outdoor overhead travelling cranes that are not in operation is now obtained from BS 6399-2 instead of CP3. For all cranes under working conditions, reference should be made to BS 2573-1.

2.2 Stability Clause 2.4.2

The requirements for stability, which were previously contained in several Clauses in different Sections of the Code, have been brought together into Clause 2.4.2 in BS 5950-1:2000. The basic requirements have not changed, but the entire Clause has been rewritten to clarify which checks are required and to distinguish between the various modes of failure that are covered by the term

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stability limit state. There have also been several technical changes to the methods of analysis.

BS 5950-1:2000 recommends that structures be checked for the following:

Static equilibrium

Resistance to horizontal forces

Sway stiffness.

The requirement for static equilibrium, as set out in Sub-clause 2.4.2.2, is simply that the most unfavourable realistic combination of the factored loads should not cause the structure, or any part of it, to slide, overturn or lift off its seating. This is similar to the recommendation in Sub-clause 2.4.2.2 in BS 5950-1:1990, except that the emphasis used to be on overturning, with no mention of sliding.

Sub-clause 2.4.2.3 outlines the requirements for providing resistance to horizontal forces. The purpose of this Sub-clause is to ensure that designers consider the possibility of incidental horizontal loads acting on the structure and provide a practical level of robustness against their effects. This is particularly important in cases where the structural actions are dominated by gravity loads and there is a risk that the need to resist horizontal loading will be overlooked, leaving the structure vulnerable to horizontal impact or other accidental loading.

In load combination 1, the gravity loads should be accompanied by the notional horizontal forces (see Sub-clause 2.4.2.4) to allow for imperfections in the structure. In load combinations 2 and 3, the structure should be designed to withstand the horizontal wind loading, as in BS 5950-1:1990. However, in BS 5950-1:2000, there is now a minimum wind load of 1% of the factored dead load, to ensure that a minimum horizontal resistance is provided, even in cases where the wind load is very small or non-existent. The important difference between this minimum wind load and the notional horizontal force of 1% of the factored dead load in BS 5950-1:1990 is that the notional horizontal forces are not taken to contribute to the net reactions at the foundations, whereas the wind loads are. In BS 5950-1:1990, it was possible to design a structure with no allowance for horizontal foundation loads. In BS 5950-1:2000, this is no longer possible, because the 1% of the dead load considered in load combinations 2 and 3 is carried through to the foundations.

The notional horizontal forces are not externally applied loads in the way that the dead, imposed and wind loads are, but are a convenient means of taking into account the effects of imperfections, such as columns being out of plumb, on the performance of a structure. In reality, such imperfections exist in all structures, causing lateral forces to be induced in the structure under the action of gravity loads. For this reason, the notional horizontal forces must always be applied simultaneously with the gravity loads. In BS 5950-1:1990, the notional horizontal forces are taken as the greater of either 1% of the factored dead load or 0.5% of the dead plus imposed vertical loads. In BS 5950-1:2000, the 1% of factored dead load has been removed (except as a minimum wind load, see above) and the notional horizontal force is always 0.5% of the dead plus imposed vertical loads.

The final check in this Clause relates to the sway stiffness of the structure and in particular whether it is safe for the second-order (P-delta) effects to be

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neglected. One significant change to this subject is that all of the recommendations have now been placed together in this one Clause (in Sub-clauses 2.4.2.5 to 2.4.2.8). In BS 5950-1:1990, sway stiffness was only referred to very briefly in Section 2, while the majority of the recommendations was in Section 5, along with the rules for continuous construction. This gave the impression that sway stiffness was only important in continuous structures while, in reality, it is something for which all structures should be checked.

The degree of sway stiffness is obtained by applying the notional horizontal forces at the floor or roof level under consideration and calculating the deflection at this level relative to the storey below by elastic analysis. An approximation to the sway mode elastic critical load factor of the frame cr is then determined from

cr = d200

h

where h is the storey height.

For clad structures in which the stiffening effect of the cladding has been neglected in the analysis, if cr 10, it is safe to assume that the second-order effects are small enough to be ignored and the frame may be classed as non-sway.

If cr

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2.3 Brittle fracture Clause 2.4.4

The method used to select the sub-grade of steel in order to avoid brittle fracture has changed. This is a major technical change.

To prevent the sudden catastrophic collapse of a building without warning, it is necessary to ensure that structural steelwork is resistant to brittle fracture. Resistance to brittle fracture depends not only on the toughness of the steel (expressed as a Charpy impact value at a specific test temperature), but also on the actual temperature of the steel, the level of stress in it, the type of detail, the rate of loading and the thickness of the element. BS 5950-1 expresses the requirement for resistance to brittle fracture by giving a limiting thickness (maximum) dependent on the material toughness and the service conditions. This approach is essentially unchanged in BS 5950-1:2000, but the method by which the maximum thickness is calculated has been modified. In BS 5950-1:1990, for steel subjected to the normal UK minimum temperatures of 5C and 15C for internal and external steelwork respectively, the maximum thickness could be obtained directly from Table 4, for all of the commonly used grades of structural steel. The effects of tensile stress and the type of detail were taken into account by the use of the factor K, obtained from Table 3. Alternatively, designers could use the empirical equation in Sub-clause 2.4.4.3 to calculate the required Charpy impact value at the minimum service temperature for a particular thickness, yield strength and value of K.

Although the use of Table 4 in BS 5950-1:1990 had the advantage of being very simple, it was limited to a minimum temperature of 15C and to the two values of K given in Table 3. The method in Sub-clause 2.4.4.3 could be used for any temperature (because it simply involved specifying a Charpy value at the required temperature), but was still limited to the values of K in Table 3.

In BS 5950-1:2000, Table 4 has been extended to include lower temperatures of 25C, 35C and 45C, in addition to the usual internal and external conditions. The steel grades have also been amended to bring the table into line with the current product standards (e.g. S275JR and S275J0 to BS EN 10025). Note that Table 4 only applies to plates, flats and rolled sections. For structural hollow sections, reference should be made to Table 5.

Table 3 has also been expanded and now accommodates seven types of detail and three levels of tensile stress. The result is a wide variety of K values, compared with the two values in BS 5950-1:1990.

Such an expansion in the range of temperatures and K values would have resulted in a very large, complicated Table 4, if the method had remained unchanged. Consequently, in BS 5950-1:2000, Table 4 only gives maximum thicknesses corresponding to K = 1 (denoted t1) and the maximum thickness for any other value of K is given by

t = Kt1

Alternatively, t1 may be obtained from one of two empirical equations given in this Clause. The temperature T27J referred to in these equations is the test temperature for which a minimum Charpy impact value of 27 Joules is specified in the appropriate product standard, or the equivalent value given in Table 7.

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An additional limitation in BS 5950-1:2000 is that the maximum thickness of the component should not exceed t2, as given in Table 6. This is the maximum thickness at which the full Charpy impact value given in the product standard applies.

The procedure for choosing a suitable steel sub-grade is illustrated by Worked Example 2 of this publication.

2.4 Structural integrity Clause 2.4.5

Clause 2.4.5, which deals with structural integrity and the avoidance of disproportionate collapse, has been revised in line with the current Building Regulations Approved Document A[5]. The principal changes are described below.

In BS 5950-1:1990, it was stated that ties should be capable of carrying a factored tensile load of not less than 75 kN at floor levels and 40 kN at roof level. In BS 5950-1:2000, this requirement has been generally amended to 75 kN at all levels. However, there is no need to provide horizontal ties at roof level if the steelwork carries only imposed roof loads, wind loads and cladding that weighs not more than 0.7 kN/m2.

With regard to the avoidance of disproportionate collapse, the requirements of the Building Regulations may be assumed to be satisfied if the five conditions given in Sub-clause 2.4.5.3 are met. There have been two significant changes to these conditions.

Firstly, there has been a relaxation in the tying force to be resisted when the tie is a primary beam. The two equations for the tying force presented in BS 5950-1:1990 (for internal and edge ties) are unchanged in BS 5950-1:2000, but there is now an additional sentence which states that, in the absence of other loading, the General tying condition may be assumed to be satisfied if the member and its end connections are capable of resisting a tensile force equal to its end reaction under factored loads (but not less than 75 kN). In the case of a primary beam supporting secondary beams, the end reactions of the primary beam under factored loads could be as little as half the tying force given by the equation for internal ties.

The second change relates to column splices. According to BS 5950-1:1990, column splices should be capable of resisting a tensile force of not less than two-thirds of the factored vertical load applied to the column from the floor level immediately below the splice. In BS 5950-1:2000, splices should be designed for a tensile force equal to the largest factored vertical dead and imposed load reaction, applied to the column at a single floor level, located between the column splice under consideration and the next column splice down.

In BS 5950-1:1990, where any of the five conditions was not met, the building had to be checked at each storey to see whether any individual column, or beam carrying a column, could be removed without causing collapse of more than a limited proportion of the building. If it was found that the removal of a

9

member would result in disproportionate collapse, that member had to be designed as a key element.

In BS 5950-1:2000, the test of removing one column at a time has been restricted to cases where one or more of the first three conditions is not met (those relating to tying of columns and continuity of columns). If condition d, resistance to horizontal forces, is not met, disproportionate collapse should be checked by removing each element of the bracing system in turn. In both cases, disproportionate collapse is defined as the collapse of a portion of the building exceeding 15% of the floor or roof area or 70 m2, whichever is less, at the relevant level and at one level immediately above or below. As in BS 5950-1: 1990, if the removal of any member results in disproportionate collapse, that member should be designed as a key element. In BS 5950-1:2000, all key elements should be designed to withstand the accidental loading specified in BS 6399-1.

10

3 PROPERTIES OF MATERIALS AND SECTION PROPERTIES

3.1 Grades of steel Clause 3.1.1

BS 5950-1 has been amended to take account of the introduction of the new European product standards. Under these standards, all grades of structural steel referred to in BS 5950-1:2000 conform to a common system of designation as illustrated by the example:

BS EN 10025 S275

In this example, the first term is the product standard, the S stands for structural and 275 means a minimum yield strength of 275 N/mm2 (for thickness not exceeding 16 mm).

As a result of this change, reference is made to steel grades S275, S355 and S460 throughout BS 5950-1:2000, in place of the old BS 4360 grades 43, 50 and 55.

As before, the design strength py depends not only on the grade of steel but also on the thickness. Values of py are given in Table 9 (formerly Table 6) for three common grades of structural steel and a range of thicknesses. This Table has been expanded and now includes design strengths for 150 mm thick S275 and S355 steel and two new thicknesses of S460 steel.

3.2 Section classification Clause 3.5.2

The rules for section classification have undergone several technical changes, although the general approach of using limiting width-to-thickness ratios is unchanged.

In BS 5950-1:2000, the limiting width-to-thickness ratios used for section classification are given in Table 11 for sections other than circular hollow sections and rectangular hollow sections and Table 12 for circular hollow sections and rectangular hollow sections. Although these tables are similar to Table 7 in BS 5950-1:1990, several important changes have been made and designers need to familiarise themselves with the revised layout of the tables. Many of the limits have changed and several new categories of section type have been introduced.

In BS 5950-1:2000, the classification of the web of an I, H or box section in Table 11 or rectangular hollow sections in Table 12 depends on the level of axial load in the member. This is achieved by the use of the stress ratios r1 and r2 in determining the limiting d/t value. Formulae for r1 and r2 are given in Clause 3.5.5 for three types of section.

11

It is important to recognise that this dependence on the level of axial load might result in a section changing its classification as the axial load changes. For example, consider the web of an I section with d/t = 79e. When there is no axial compression, the web is class 1 plastic (d/t < 80e), but if there is a compressive force equal to 20% of the squash load of the web (i.e. r1 = 0.2), the section becomes class 3 semi-compact. For this reason, it is essential to reclassify the web whenever there is a change to the axial load.

As a conservative alternative to the use of these ratios, the limit of 40e, for I sections, H sections, hot rolled rectangular hollow sections and box sections, or 35e, for cold formed rectangular hollow sections, may be used.

3.3 Effective plastic modulus Clause 3.5.6

Clause 3.5.6 presents equations for calculating the effective plastic modulus, Seff, which may be used as an alternative to the elastic section modulus Z for class 3 semi-compact sections.

Unlike plastic or compact sections, semi-compact sections are not able to develop their full plastic capacity because of local buckling. In BS 5950-1: 1990, this is allowed for by limiting the moment capacity to pyZ. However, in many cases, this approach is conservative, because the moment capacity of a semi-compact section can be anywhere between pyZ and pyS (i.e. above the moment at first yield but below the fully plastic moment). In BS 5950-1:2000, the moment capacity may be taken either conservatively as pyZ or more accurately as pySeff. This new approach gives a less conservative result by utilising the additional capacity beyond first yield, but it does involve significantly more computational effort.

This Clause contains equations for Seff for I or H sections with equal flanges, rectangular hollow sections and circular hollow sections. Two values of Seff are given for each case. The first applies when the web is the critical element (i.e. more slender) and the second applies when the flange is critical. Designers wishing to use the new approach for I or H sections with unequal flanges, subject to bending in the plane of the web, should refer to Annex H.3.

3.4 Slender cross sections Clauses 3.6.1 to 3.6.6

The rules for slender cross sections have been changed to allow the use of effective areas, as an alternative to the old approach of reducing the assumed design strength. This is a major technical change with considerable implications for designers.

A slender section is one in which the stress at the extreme compression fibre cannot reach the design strength due to local buckling. Consequently, whenever such sections are subjected to axial compression, bending or a combination of the two, the effect of local buckling on the capacity of the section needs to be taken into account.

12

In BS 5950-1:1990, the local buckling of slender sections was allowed for by limiting the yield stress assumed in the design to such a level that the elements of the cross section would not buckle. This approach is over-conservative, especially for the case of Universal Beams used as columns, as the reduced strength is applied to the whole cross section, even though it is often only the web that is slender.

In BS 5950-1:2000, a new method is presented in which the reduction in capacity due to local buckling is allowed for by using an effective area equal to the semi-compact limit. This approach is valid for I sections, rectangular hollow sections, angles, channels etc. but account must be taken of the shift in the centroid where appropriate. Designers using this new approach will find that, in many cases, the calculated section resistance is noticeably different from that determined using the old design rules, often leading to greater economy. However, designers are under no obligation to use this new method and, if they prefer, may still use the reduced design strength, as described in Clause 3.6.5.

The analysis of doubly symmetric cross sections with class 4 slender elements is considered in Clause 3.6.2. The effective area, Aeff, of such sections should be determined from the effective cross sections shown in Figure 8a. The effective section modulus, Zeff, should be obtained from Figure 8b, for sections whose webs are not slender under pure bending (i.e. only the flanges are slender), and from Figure 9, if the web is slender under pure bending.

The effective widths obtained from Clause 3.6.2 may also be used for class 4 slender singly symmetric and asymmetric cross sections, provided that account is taken of the additional moments induced in the member due to the shift in the centroid of the effective cross section compared with that of the gross cross section. A method for calculating these moments is described in Clause 3.6.3.

Hot rolled equal-leg angles may be treated as asymmetric sections and analysed using the method presented in Clause 3.6.3 or, alternatively, their effective section properties Aeff and Zeff may be obtained from the simple but conservative formulae given in Clause 3.6.4.

Formulae for the effective section properties of circular hollow sections are given in Clause 3.6.6.

13

4 DESIGN OF STRUCTURAL MEMBERS

4.1 Members subject to bending Clause 4.2.2

BS 5950-1:1990 stated that there is no need to consider the lateral-torsional buckling of a member when full lateral restraint is provided. In BS 5950-1: 2000, resistance to lateral-torsional buckling can only be deemed adequate if, in addition to full lateral restraint, there is also nominal torsional restraint at the supports. Such restraint may be provided by web cleats, partial depth end plates, fin plates or, in continuous beams, by the continuity with the next span. Because all of these methods of restraint are common details in typical building structures, designers should have no difficulty in complying with the amended Clause. Nevertheless, this is a significant amendment.

Clause 4.2.5

Clauses 4.2.5 and 4.2.6 of BS 5950-1:1990, which deal with moment capacity for the low shear and high shear cases respectively, have been merged so that all of the expressions for moment capacity are now contained in a single Clause. Several significant changes have been made and two new Sub-clauses have been added covering notched ends and bolt holes.

In BS 5950-1:1990, the moment capacity for class 1 plastic or class 2 compact sections with low shear (i.e. Fv 0.6Pv) was given by

Mc = pyS but 1.2pyZ

The limit of 1.2 pyZ at ultimate limit state corresponds to approximately 80% of the elastic capacity of the section (i.e. 0.8 pyZ) at serviceability state and ensures that the section remains elastic at the working loads, allowing for residual stresses. If S $ 1.2Z, the 1.2 constant could be replaced by the ratio of the factored load to the unfactored load.

In BS 5950-1:2000, the moment capacity of a class 1 or class 2 section with low shear is still given by

Mc = pyS

but Mc is now limited to 1.5pyZ generally and to 1.2pyZ for simply supported beams and cantilevers.

In BS 5950-1:1990, the moment capacity of class 3 semi-compact sections with low shear was given by

Mc = pyZ

This equation is conservative, as it prevents any additional moment from being carried above the point of first yield, despite the fact that some class 3 sections have a significantly higher moment capacity. To allow this additional capacity to be utilised, BS 5950-1:2000 includes the following alternative equation:

Mc = pySeff

in which Seff is the effective plastic modulus, as defined in Clause 3.5.6.

14

This alternative approach requires greater computational effort compared with the original method, because of the need to calculate Seff, but the reward for this additional effort is a more efficient design, which utilises the sections elastic-plastic capacity beyond the point of first yield, i.e. up to the limit dictated by local buckling. Of course, designers may still wish to use the conservative capacity based on the elastic modulus Z.

The moment capacity of class 4 slender sections with low shear was given in BS 5950-1:1990 by

Mc = pyZ

where py is the reduced design strength obtained using the appropriate reduction factor from Table 8.

In BS 5950-1:2000, class 4 slender sections are designed using effective section properties instead of reduced design strengths (see Sub-section 3.6) and, consequently, the moment capacity of a class 4 section is now given by

Mc = pyZeff

where Zeff is the effective section modulus as defined in Clause 3.6.2 and py is the design strength. This change should result in an increase in the moment capacity of slender sections, because the previous method of reducing the design strength was over-conservative.

The high shear case (i.e. Fv > 0.6Pv) has also been revised. In BS 5950-1: 1990, the moment capacity was given as follows:

For plastic and compact sections:

Mc = py(S Svr1) but 1.2pyZ

where r1 = 5.15.2

v

v -P

F

For sections with equal flanges, Sv is the plastic modulus of the shear area Av. For sections with unequal flanges, Sv is the plastic modulus of the gross section less the plastic modulus of that part of the section remaining after the deduction of the shear area.

For semi-compact and slender cross sections, the moment capacity was the same as in the low shear case, i.e.

Mc = pyZ

using a reduced py for slender sections. This was clearly wrong, as it neglected the effects of high shear completely for class 3 and class 4 sections.

In BS 5950-1:2000, this error has been corrected and the new equations are as follows:

For class 1 plastic or class 2 compact sections:

Mc = py(S rSv)

For class 3 semi-compact sections:

Mc = py(Z rSv / 1.5) or Mc = py(Seff rSv)

15

For class 4 slender sections:

Mc = py(Zeff rSv / 1.5)

In all three equations:

r = [2 (Fv / Pv) 1]2

Note: Although the moment capacity equation for class 1 plastic and class 2 compact sections is unchanged, the expression for r is different in BS 5950-1: 2000.

The design of restrained beams is illustrated by Worked Example 3 of this publication.

4.2 Lateral-torsional buckling Clause 4.3.5

Clauses 4.3.5 and 4.3.6 of BS 5950-1:1990 have been merged to form Clause 4.3.5 of BS 5950-1:2000. This new Clause covers the effective lengths of both simple beams and cantilevers for lateral-torsional buckling. A number of important changes have been made and a new Sub-clause on double curvature bending has been added.

Clause 4.3.5 of BS 5950-1:1990 dealt specifically with the effective lengths of simple beams. Most of these requirements are unchanged, although the text has been completely rewritten. However, there have been amendments to the effective length values given in Table 13 (formerly Table 9).

Firstly, a new symbol LLT, standing for segment length, has been introduced to distinguish this dimension from the member length L. The segment length is the length between restraints, whether these are intermediate restraints or supports.

Secondly, several new restraint conditions have been added to the Table, allowing designers to model their structures more accurately. These are compression flange fully restrained against rotation on plan and compression flange partially restrained against rotation on plan. Note that the effective lengths of a beam with lateral and torsional restraint at one end and both flanges partially restrained against rotation on plan at the other end have been reduced to 0.8LLT and 0.95LLT for the normal and destabilising loading conditions respectively.

Clause 4.3.6 of BS 5950-1:1990 considers the effective lengths of cantilevers. The content of this Clause has undergone several significant changes, with important implications for the design of these members.

The first change relates to cantilevers with intermediate lateral restraints. In BS 5950-1:1990, the lengths between restraints were treated as beams and their effective lengths obtained accordingly using the provisions of Clause 4.3.5. In BS 5950-1:2000, provided that the cantilever is restrained laterally and torsionally at both ends (i.e. cases c4 and d4 in Table 14), an effective length of 1.0L is given for the normal loading condition, where L is the length of the relevant segment between adjacent lateral restraints. However, for the

16

destabilising loading condition, the effective length should be obtained from Table 14. In this case, L is taken as the length of the cantilever, unless the there are intermediate lateral restraints to the top flange.

Secondly, the rules for determining the effective length of cantilevers without intermediate restraints are unchanged (i.e. LE obtained from Table 14), apart from the case where the cantilever has a moment applied to its tip. BS 5950-1: 1990 deals with such moments by treating the cantilever as a beam, as for cantilevers with intermediate restraints. In BS 5950-1:2000, the effective length is obtained from Table 14 then increased by either 30% or 0.3L, whichever is greater. The effective length values in Table 14 of BS 5950-1:2000 are unchanged from those given in Table 10 of BS 5950-1:1990, except that a new category of restraint has been added.

Finally, the new Clause 4.3.5 contains a new Sub-clause on the subject of beams with double curvature bending. This Sub-clause has been added to emphasise that special consideration needs to be given to beams that have both hogging and sagging regions. Design rules are given for beams with intermediate lateral restraints to each flange, beams with intermediate lateral restraints to the compression flange in the sagging region only and beams directly supporting a concrete or composite floor or roof slab.

Clause 4.3.6

Clause 4.3.6 contains the design rules, tables and equations needed to calculate the buckling resistance moment Mb of unrestrained beams susceptible to lateral-torsional buckling. Although the general method is unchanged from that in BS 5950-1:1990, there have been a number of significant changes to the individual steps in the procedure. Overall, this Clause has undergone a major technical amendment.

The Clause begins by listing the situations in which there is no need to check for lateral-torsional buckling. These are:

bending about the minor axis

circular hollow sections, square rectangular hollow sections or circular or square solid bars

rectangular hollow sections when LE/ry does not exceed the limiting value from Table 15

I, H, channel or box sections when lLT lL0.

The first of these cases was not listed explicitly in BS 5950-1:1990, but this is not a technical change, because lateral-torsional buckling occurs about the minor axis as a result of major axis bending. The second and third cases were noted in BS 5950-1:1990, but the table of limiting slenderness values for rectangular hollow sections has been extended to include 12 values of D/B, compared with the four values given in Table 38 in Appendix B.2.6 of BS 5950-1:1990. In the final case, L0 is the limiting slenderness obtained from the last row of Table 16 or Table 17. The inclusion of the L0 values in these tables is new to BS 5950-1:2000, although a formula for L0 was included in Appendix B.2.5 of BS 5950-1:1990.

17

Every segment length of an unrestrained beam, subject to a major axis moment Mx, should satisfy the following conditions:

Mx Mb / mLT

and

Mx Mcx

where:

Mcx is the moment capacity of the section from Clause 4.2.5

Mb is the buckling resistance moment

mLT is the equivalent uniform moment factor for lateral-torsional buckling, which takes account of the fact that the theory from which Mb is obtained assumes a uniform moment throughout the segment.

These requirements are unchanged from BS 5950-1:1990, although they have been rewritten and there is no longer any reference to the equivalent uniform moment .M

Except for hot rolled angles, which are considered in Clause 4.3.8, the buckling resistance moment Mb may be obtained either using the conservative method described in Clause 4.3.7 or from the expressions below.

For class 1 plastic or class 2 compact cross sections:

Mb = pb Sx

For class 3 semi-compact cross sections:

Mb = pb Zx or Mb = pb Sx,eff

For class 4 slender cross sections:

Mb = pb Zx,eff

In BS 5950-1:1990, Mb was taken as pbSx irrespective of the classification of the cross section. This was based on the false assumption that lateral-torsional buckling and local buckling do not interact.

The term pb in these equations is the bending strength and is obtained from Table 16 for rolled sections and Table 17 for welded sections, for given values of design strength py and equivalent slenderness LT. These tables are equivalent to Table 11 and Table 12 in BS 5950-1:1990 and have been amended to include new columns corresponding to different values of py and a new row containing values of the limiting slenderness lL0. In all other respects, these tables are unchanged.

Designers familiar with BS 5950-1:1990 will recall that there used to be two approaches to analysing lateral-torsional buckling, one using the slenderness correction factor n (for loading between lateral restraints), the other using the equivalent uniform moment factor m (for cases with end moments only). In BS 5950-1:2000, there is a major change in that the n factor method has been removed from this Clause, leaving the latter method to be used for all cases (n

18

is still used for tapered or haunched members in Annex B.2.5). Consequently, the equation for LT has changed.

In BS 59501:1990, LT was given by

lLT = nuvl

where:

l is the minor axis slenderness LE / ry

u is a buckling parameter

v is a slenderness factor.

In BS 5950-1:2000, lLT is now given by

lLT = Wbluv

where bW is the ratio of section moduli, as described below.

For class 1 plastic or class 2 compact sections:

bw = 1.0

For class 3 semi-compact cross sections:

bw = Zx / Sx or bw = Sx,eff / Sx

For class 4 slender sections:

bw = Zx,eff / Sx

The slenderness ratio, v, for sections with two plain flanges, may be obtained from Table 19 for various values of /x and where x is the torsional index of the section and is given by

=

ytyc

yc

II

I

+

where Iyc and Iyt are the minor axis second moments of area of the compression flange and tension flange respectively.

The only difference between Table 19 in BS 5950-1:2000 and its equivalent in BS 5950-1:1990, is that the two columns at either end of the range, i.e. those corresponding to = 1.0 and = 0.0 (T sections), have been deleted. In BS 5950-1:2000, designers wishing to use T sections must instead refer to Annex B.2.8.

As an alternative to Table 19, the slenderness ratio v may be obtained from the equations in Sub-clause 4.3.6.7. In BS 5950-1:1990, similar expressions could be found in Appendix B.2.5.

The buckling parameter u and torsional index x may be obtained from the formulae in Annex B.2.3 (B.2.5 in BS 5950-1:1990) or from published tables of section properties. Alternatively, for rolled I and H sections with equal flanges, the following conservative approximations may be used:

u = 0.9 and x = D/T

19

Sub-clause 4.3.6.8 has not been affected by the recent amendments, other than the change in Sub-clause number.

As noted above, the two approaches to analysing lateral-torsional buckling in BS 5950-1:1990 have been replaced by a single method using the equivalent uniform moment factor mLT. Previously, for the normal loading condition, if a member was loaded between adjacent lateral restraints, m was taken as 1.0 and the appropriate value of n was obtained from Table 15 or Table 16. If the member was not loaded between its restraints, n was taken as 1.0 and m was obtained from Table 18. Thus, the allowance for the shape of the bending moment was made using either the n factor or the m factor, depending on the location of the loading, but never both. The change in BS 5950-1:2000 to the use of mLT for all cases has necessitated a substantial extension to Table 18.

In BS 5950-1:1990, m values were provided for segments with end moments only, i.e. only linear variations in bending moment were considered. In BS 5950-1:2000, values are also given for four specific cases with transverse loads applied between restraints and a general formula is provided from which mLT may be obtained for more complex bending moment diagrams.

Note:

1. Slight changes have been made to the values of mLT for segments with end moments only, compared with those in BS 5950-1:1990.

2. BS 5950-1:2000 distinguishes between mLT, the equivalent uniform moment factor for lateral-torsional buckling, and mx, my, myx, the equivalent uniform moment factors for flexural buckling. Values of mLT are obtained from Table 18, while values of mx, my, myx are given in Table 26.

The design of unrestrained beams is illustrated by Worked Example 4 in this publication.

Clause 4.3.7

Clause 4.3.7 gives a simple but conservative alternative approach to the method presented in Clause 4.3.6 for determining the buckling resistance moment of a plain rolled I, H or channel section with equal flanges. There have been two changes to this method compared with BS 5950-1:1990.

Firstly, in BS 5950-1:1990, Mb was taken as pbSx irrespective of the classification of the cross section. This was based on the false assumption that lateral-torsional buckling and local buckling do not interact. This has been corrected in BS 5950-1:2000, in which Mb is only taken as pbSx if the section is class 1 plastic or class 2 compact and is taken as pbZx for class 3 semi-compact sections.

The second change relates to the table of pb values (Table 20 in BS 5950-1: 2000 and Table 19 in BS 5950-1:1990). In BS 5950-1:1990, pb was given as a function of x (= D/T) and the slenderness l (= LE/ry), whereas in BS 5950-1: 2000, pb is given in terms of D/T and yEw / rLb . However, the numbers in

Table 20 are unchanged, so for all class 1 and class 2 sections, for which bw = 1, there will be no change at all to pb, or indeed Mb.

20

Clause 4.3.8

Clause 4.3.8, which deals with the buckling resistance moment for single angles, has been revised resulting in a new basic method and a significant change to the existing simplified method.

In BS 5950-1:1990, the calculation of the buckling resistance moment for single angles was very straightforward, as it simply involved one of the following equations:

Mb = 0.8pyZ for L / rvv 100

Mb = 0.7pyZ for L / rvv 180

Mb = 0.6pyZ for L / rvv 300

where:

rvv is the radius of gyration about the v-v axis

L is the unrestrained length.

This simple approach has been shown to be non-conservative for some cases (for low L / rvv) and has been replaced in BS 5950-1:2000 by two alternative methods.

The first method, known as the basic method in BS 5950-1:2000, is applicable to equal and unequal angles and involves resolving the applied moments into their components about the principal axes u-u and v-v. The buckling resistance moment should then be obtained using an equivalent slenderness lLT obtained from Annex B.2.9.

The second method, referred to as the simplified method, provides a simple alternative to the basic method, but it is only applicable to equal angles with b / t 15e. In this case, the buckling resistance moment is given by

Mb = 0.8pyZx

when the heel of the angle is in compression or

Mb = pyZx

e

-e1625

1350 vE rL but 0.8pyZx

when the heel of the angle is in tension.

4.3 Plate girder webs Clause 4.4.4

If the web of a plate girder is susceptible to shear buckling (i.e. d / t > 62e), the moment capacity of the cross section should be obtained using one of the methods given in Sub-clause 4.4.4.2.

BS 5950-1:1990 presented three alternative methods of analysis. The first option was to assume that the moment and axial load are resisted by the flanges alone, leaving the web to resist only the shear force. In the second method, the moment and axial load are assumed to be resisted by the whole section and the web is designed for shear and longitudinal stresses, using the method in H.3. The third method was a combination of the first two.

21

The methods presented in BS 5950-1:2000 are similar, except that there is now an additional option for low shear. Where the applied shear is no greater than 60% of the simple shear buckling resistance obtained from Sub-clause 4.4.5.2, it is acceptable to obtain the moment capacity using the rules given in Clause 4.2.5. Where the applied shear exceeds 60% of the simple shear buckling resistance, the moment capacity should be obtained either by assuming that all of the moment is resisted by the flanges or by using the rules in H.3 to design the web for the applied shear plus any moment beyond the flanges only moment capacity. These two high shear methods are essentially the same as methods a and c in BS 5950-1:1990.

BS 5950-1:2000 also contains a new Sub-clause relating to axial loads. This states that where a member is subject to an axial load combined with a moment, reference should be made to the design rules in Clauses 4.8.1 to 4.8.3. In this case, when using the flanges only method, it should be assumed that the moment and the axial force are both resisted by the flanges alone, with each flange subject to a uniform stress not exceeding pyf.

Clause 4.4.5

Clause 4.4.5, which considers the shear buckling of plate girder webs, has been rewritten for clarity and replaces the over-conservative simple method with the one used in BS 449. This is a major technical change.

Clause 4.2.3 of BS 5950-1:1990 states that if the d / t ratio of a web exceeds 63, the shear buckling resistance of the web should be checked in accordance with Clause 4.4.5. In BS 5950-1:2000, this limit has been replaced by two new limits: 62e for a welded section and 70e for a rolled section.

BS 5950-1:1990 and BS 5950-1:2000 both present two alternative methods for calculating the shear buckling resistance of a web. The first method, known in BS 5950-1:2000 as the simplified method, does not allow for the beneficial effects of tension field action and may be used for webs with or without intermediate stiffeners. The second method, referred to as the more exact method in BS 5950-1:2000, does make use of tension field action and its use is restricted to webs with intermediate transverse stiffeners. For webs with longitudinal stiffeners, or as an alternative for webs with intermediate transverse stiffeners, reference should be made to BS 5400-3[6].

The simplified method in BS 5950-1:2000 is very similar to the Design without using tension field action in BS 5950-1:1990, although the two methods give slightly different results.

In BS 5950-1:1990, the shear buckling resistance, Vcr, of a stiffened or unstiffened panel is given by:

Vcr = qcr dt

where:

d is the depth of the web

t is the web thickness

qcr is the critical shear strength from Table 21.

22

In BS 5950-1:2000, the shear buckling resistance Vb of a web is taken as the simple shear buckling resistance Vw given by:

Vw = dtqw

where:

d is the depth of the web

t is the web thickness

qw is the shear buckling strength of the web from Table 21.

While these two methods may appear to be identical, apart from the choice of symbols, they give different results because the values of qw in Table 21 of BS 5950-1:2000 are not the same as the values of qcr in Table 21 of BS 5950-1: 1990.

In the more exact method, the difference between the two versions of BS 5950-1 is more apparent.

In BS 5950-1:2000, if the flanges are fully stressed, the shear buckling resistance, Vb, equals the simple shear buckling resistance, Vw, given by:

Vw = dtqw

If the flanges are not fully stressed, Vb is taken as the sum of the simple shear buckling resistance, Vw, and the flange-dependent shear buckling resistance, Vf, i.e.

Vb = Vw + Vf

where Vf is given by

Vf = ( ) ( )

)/(15.01

/1/

pfpw

2yffv

MM

pfadP

+

-

where:

ff is the mean longitudinal stress in the smaller flange due to moment and/or axial force

Mpf is the plastic moment capacity of the smaller flange about its own equal area axis perpendicular to the plane of the web

Mpw is the plastic moment capacity of the web about its own equal area axis perpendicular to the plane of the web

Pv is the shear capacity from Clause 4.2.3

pyf is the design strength of the flange

pyw is the design strength of the web.

By comparison, in BS 5950-1:1990, the shear buckling resistance of a stiffened panel is given by:

Generally:

Vb = qb dt

23

If the flanges are not fully stressed:

Vb = ( )dtKqq ffb + but 0.6pydt where:

qb is the basic shear strength obtained from Table 22

qf is the flange-dependent shear strength factor from Table 23

Kf is a factor given by

Kf =

-

yfpw

pf1

4 pf

M

M

(The f term here has the same meaning as ff defined above.)

Note that the basic shear strength qb used in the design of stiffened panels, using tension field action, is higher than the critical shear strength qcr used when designing panels without tension field action, giving a higher value of Vb, even when the flanges are fully stressed. By contrast, in BS 5950-1:2000, both methods use qw and therefore yield exactly the same result in the case of fully stressed flanges.

The tension field action induced in the plate girder web produces a horizontal anchor force, Hq, at the end of the girder, as shown in Figure 4.1.

It is generally necessary to provide an end anchorage to resist this horizontal force, however BS 5950-1:2000 gives the following two cases when an end anchorage is not necessary:

1. If the shear capacity, rather than the shear buckling resistance, is the governing design criterion, i.e. Vw = Pv.

2. If sufficient shear buckling resistance is available without forming a tension field. The existence of this condition is indicated by:

Fv Vcr

where Vcr is the critical shear buckling resistance and Fv is the maximum shear force. Vcr may be obtained from Annex H.2 or using the formulae in this Clause.

These exemptions are new to BS 5950-1:2000.

Hq

Figure 4.1 Tension field action in a plate girder

24

Note that the provisions dealing with the design of the end anchorage, which used to be in Clause 4.4.5, have been moved to Annex H.4 in BS 5950-1:2000.

The design of plate girders is illustrated by Worked Example 5 in this publication.

4.4 Design of stiffeners Clause 4.5.1

Clause 4.5.1 considers the design of web bearing and buckling stiffeners. Sub-clause 4.5.1.3, which deals with the stiff bearing length, has been amended and Sub-clause 4.5.1.5, on the subject of hollow sections, has moved to this Clause from Clause 4.5.12. In other respects, the technical content of this Clause is unchanged.

In BS 5950-1:1990, the stiff bearing length b1 (i.e. the length that cannot deform appreciably in bending) was calculated by taking the angle of load dispersion through the steel to be 45 (see Figure 8 of BS 5950-1:1990). BS 5950-1:2000 also assumes a load dispersion of 45, but there is no longer a need to analyse the geometry of the section, because b1 may be obtained directly from the formulae given in Figure 13.

Clause 4.5.12 of BS 5950-1:1990 is now Sub-clause 4.5.1.5 in BS 5950-1: 2000. There is an additional sentence referring designers to a design procedure given in Steelwork Design Guide to BS 5950-1:2000, Volume 1: Section Properties and Member Capacities[7] but the general requirements for the design of hollow sections subject to concentrated loads are unchanged.

Clause 4.5.2

The provisions in BS 5950-1 relating to the bearing capacity of an unstiffened web have been revised.

In BS 5950-1:1990, the bearing capacity of an unstiffened web was given by

(b1 + n2)tpyw

where:

b1 is the stiff bearing length (see Clause 4.5.1)

n2 is the length obtained by dispersion through the flange to the flange/ web connection at a slope of 1 in 2.5

t is the web thickness

pyw is the design strength of the web.

In BS 5950-1:2000, the bearing capacity of an unstiffened web is given by

Pbw = (b1 + nk)tpyw

where n is taken as follows:

except at the end of the member:

n = 5

at the end of the member:

n = 2 + 0.6 be / k, but n 5

25

where be is the distance to the end of the member from the end of the stiff bearing.

k is given by:

for a rolled I or H section:

k = T + r

for a welded I or H section:

k = T.

Therefore, in BS 5950-1:2000, the bearing capacity of an unstiffened web depends on the proximity of the end of the stiff bearing to the end of the member, whereas in BS 5950-1:1990, the bearing capacity was always determined assuming a dispersion of 1:2.5 across the flange thickness.

Where the applied load or reaction exceeds the bearing capacity of the unstiffened web, bearing stiffeners should be provided. These should be designed to carry the applied force minus the bearing capacity of the unstiffened web. This requirement is unchanged from BS 5950-1:1990.

The bearing capacity of a web is considered in Worked Example 6 of this publication.

Clause 4.5.3

The provisions in BS 5950-1 relating to the buckling resistance of an unstiffened web and the design of load-carrying stiffeners have been revised.

In BS 5950-1:1990, the buckling resistance of an unstiffened web was given by

Pw = (b1 + n1)tpc

where:

b1 is the stiff bearing length

n1 is the length obtained by 45 dispersion through half the depth of the section

t is the web thickness

pc is the compressive strength obtained from Table 27c.

In BS 5950-1:2000, the buckling resistance of the web, Px, is obtained directly from the bearing capacity, Pbw, and the geometry of the section. There is no longer a need to refer to the strut curve (i.e. Table 24c) in BS 5950-1:2000. Three equations are presented for Px, depending on the restraint of the flange and the location of the applied load relative to the end of the member.

If the loaded flange is effectively restrained against rotation relative to the web and against lateral movement relative to the other flange, Px, is given by:

When ae 0.7d:

Px = ( ) bw1 25

Pdnkb

t

+

e

26

When ae < 0.7d:

Px = ( ) bw1e

254.1

7.0P

dnkb

td

da

+

e+

where:

ae is the distance from the applied load or reaction to the end of the member

d is the depth of the web

Pbw is the bearing capacity of the unstiffened web obtained from Clause 4.5.2

n is a dispersion factor obtained from Clause 4.5.2

k is taken as follows:

for a rolled I or H section:

k = T + r

for a welded I or H section:

k = T

If one or both of the restraint conditions given above is not met, a reduced buckling resistance must be used given by

Pxy = x7.0

PL

d

E

In BS 5950-1:1990 and BS 5950-1:2000, the buckling resistance of a load-carrying stiffener is given by

Px = As pc

where:

As is the effective area of a cruciform section, consisting of the stiffeners and an effective width of web on each side of the centreline of the stiffeners

pc is the compressive strength from Table 27c in BS 5950-1:1990 or Table 24c in BS 5950-1:2000.

The important change here is that in BS 5950-1:1990 the effective width on each side of the stiffener was taken as 20t, whereas in BS 5950-1:2000 it is limited to 15t. In other respects, the buckling check for a load-carrying stiffener is unchanged.

Perhaps more importantly, BS 5950-1:1990 also included a bearing check for load-carrying stiffeners (Sub-clause 4.5.4.2). This stated that load-carrying stiffeners should be designed to resist 80% of the total applied force, irrespective of the capacity of the unstiffened web, i.e.

A > ys

x8.0p

F

27

where:

A is the area of the stiffener in contact with the flange

Fx is the applied load

pys is the design strength of the stiffener.

As a result of the reduction in the effective width of the web from 20t to 15t, this bearing check has been removed from BS 5950-1:2000. This is a key change because, in most practical cases, the size of the stiffeners was governed by this rule. There is still a requirement to check the bearing capacity, as BS 5950-1:2000 states that load-carrying stiffeners should also be checked as bearing stiffeners. However, this requirement is not as onerous as the previous 80% rule, because bearing stiffeners are only designed to carry the external load minus the bearing capacity of the unstiffened web, not the full external load.

The buckling resistance of a web is considered in Worked Example 6 in this publication.

Clause 4.5.4

Clause 4.5.4 (formerly Clause 4.5.7) has been expanded to provide greater detail on the design of tension stiffeners. Two cases are presented for which tension stiffeners are provided and a separate method of design is given for each case.

If the applied load or reaction exceeds the tension capacity of the unstiffened web at its connection to the flange, the tension stiffener should be designed to carry that portion of the load which exceeds the tension capacity of the unstiffened web. If, on the other hand, tension stiffeners are needed because the applied load or reaction exceeds the tension capacity of the unstiffened flange, the proportion of the load assumed to be carried by the stiffener should be consistent with the design of the flange. This latter case was not considered in BS 5950-1:1990.

4.5 Tension members Clause 4.6.3

New formulae have been introduced for calculating the tension capacity of a simple tie consisting of an angle connected through one leg only, a channel connected only through the web, or a T section connected only through the flange.

In BS 5950-1:1990, the tension capacity of a single angle, a single channel or a T section was calculated using an affective area equal to the net area of the connected leg plus the area of the outstanding leg multiplied by

21

1

3

3

aa

a

+

where a1 is the net cross-sectional area of the connected leg and a2 is the cross-sectional area of the unconnected leg.

In BS 5950-1:2000, the tension capacity, Pt, for single angles, channels or T sections with bolted connections is given by

28

Pt = py(Ae 0.5a2)

and for welded connections by

Pt = py(Ag 0.3a2)

in which Ae is the sum of the effective net areas ae (see Clause 3.4.3), Ag is the gross area of the cross section and

a2 = Ag a1

where a1 is the gross area of the connected element.

The reason for this change is that the formulation in BS 5950-1:1990 is non-conservative for grade S460 steel. The 3a1 / (3a1 + a2) factor was derived from tests on mild steel and allows for the effects of eccentricity and strain hardening, assuming Us / Ys to be the same for all grades of steel. In BS 5950-1: 2000, eccentricity and strain hardening are considered separately, by first calculating the effective area in Clause 3.4.3 (multiplying the net area by Ke) then using the formulae in this Clause to allow for eccentricity. Consequently, compared with BS 5950-1:1990, there has been an increase in capacity for S275, a small decrease for S355 and a significant decrease for S460. For welded angles, there is a very small decrease in capacity for all grades of steel.

Similarly, there are two new formulae for double angle, channel and T section members, where the components are connected to both sides of a gusset plate and are interconnected by bolts or welds. In this case, the tension capacity for bolted connections is given by

Pt = py(Ae 0.25a2)

and for welded connections by

Pt = py(Ag 0.15a2)

Note: If the components of the tie are both connected to the same side of the gusset plate or are not interconnected as described above, the member should be treated as if it were a single angle, channel or T section.

4.6 Compression members Clause 4.7.1

An additional paragraph has been added to Clause 4.7.1 giving the design loading for bracing systems. As with the intermediate restraints for lateral-torsional buckling (see Clause 4.3.2), bracing systems that supply positional restraint to more than one member must be designed to resist the sum of all of the individual restraint forces from each member reduced by the factor kr, given by

kr = (0.2 + 1 / Nr)0.5

where Nr is the number of parallel members restrained.

Clause 4.7.2

Sub-clause 4.7.3.2 of BS 5950-1:1990 gave maximum slenderness values for different types of member. These have now been removed as it was felt that the limits presented in the Code were not universally applicable and in many

29

cases were inappropriate, leading to the rejection of perfectly acceptable members. However, it is important for designers to recognise that there may still be practical limits on the slenderness of a member, for example to limit sag due to self-weight, and these must be considered carefully.

One additional requirement is that it is now necessary to increase by 20% the slenderness of single-angle struts that have lateral restraints to their two legs alternately.

Clause 4.7.4

The means by which the compression resistance of members with slender cross sections is calculated have changed, resulting in the removal of the anomalies leading to the over-conservative design of slender cross sections, such as Universal Beams used as columns.

In BS 5950-1:1990, the compression resistance Pc of a member with a slender cross section was given by

Pc = Ag pcs

where:

Ag is the gross cross-sectional area of the section

pcs is the reduced compressive strength based on the slenderness and the reduced design strength pyr.

In BS 5950-1:2000, the recommended method for slender cross sections uses effective areas equal to the semi-compact limits, instead of a reduced design strength (see Sub-section 3.6 for details). Consequently, the compression resistance Pc of a member with a slender cross section is now given by

Pc = Aeff pcs

where Aeff is the effective area obtained from Sub-section 3.6 and pcs is the compressive strength based on the design strength py and a reduced slenderness of

l 5.0

g

eff

A

A

The design of compression members is illustrated by Worked Example 7 in this publication.

Clause 4.7.5

Changes have been made to Table 23 (formerly Table 25) regarding the choice of strut curve for different types of section and to Table 24 (formerly Table 27) from which the compressive strength pc is obtained.

The most significant change to this Clause is the inclusion of cold formed structural hollow sections in BS 5950-1:2000, for which Table 24c should be used for buckling about both axes. In addition, changes have been made to the choice of strut curve for rolled I sections. In BS 5950-1:1990, the compressive strength pc of all rolled I sections about the major and minor buckling axes was obtained from Table 24a and Table 24b respectively. In BS 5950-1:2000, the

30

use of Table 24a and Table 24b is limited to sections with a maximum thickness not exceeding 40 mm. Table 24b and Table 24c should be used for rolled I sections with a maximum thickness greater than 40 mm. This change also applies to rolled I sections with welded flange cover plates within the range 0.25 < U / B < 0.8 (see Figure 14 of BS 5950-1:2000).

The strut curves themselves are unchanged, although the range of design strengths in Table 24 has been amended. Compressive strengths pc for py values of 225 N/mm2, 305 N/mm2, 320 N/mm2, 340 N/mm2, 395 N/mm2, 415 N/mm2 and 450 N/mm2 have been deleted and replaced by values corresponding to design strengths of 235 N/mm2, 315 N/mm2, 345 N/mm2, 400 N/mm2, 440 N/mm2 and 460 N/mm2. In addition, a few of the other pc values have been revised slightly, although the majority is unchanged.

No technical changes have been made to the text of this Clause, but much of it has been rewritten, with Figure 14 in BS 5950-1:2000 replacing Table 26 in BS 5950-1:1990.

4.7 Combined moment and axial force

Clause 4.8.3

Clause 4.8.3 deals with members that are subject to combined compression and bending. It has undergone a major technical change.

The capacity of a member subject to combined compression and bending is dependent on the local cross-section capacity and the overall buckling resistance of the member. This Clause contains checks for both of these modes of failure. Although both checks have been amended, only the overall buckling check has undergone a major change.

BS 5950-1:1990 presented two alternative methods for checking buckling resistance. In the simplified method, the following relationship had to be satisfied:

yy

y

b

x

cg Zp

mM

MmM

pAF

++ 1

where:

F is the applied axial force

Ag is the gross cross-sectional area

pc is the compressive strength

m is the equivalent uniform moment factor

Mb is the buckling resistance moment

Mx is the applied moment about the major axis

My is the applied moment about the minor axis

py is the design strength

Zy is the elastic section modulus about the minor axis.

31

Alternatively, designers could opt for the more exact method by satisfying the following relationship:

ay

y

ax

x

M

mM

MmM

+ 1

in which Max and May are the maximum buckling moments about the major and minor axes respectively in the presence of axial load.

BS 5950-1:2000 also presents designers with the option of using either a simplified or a more exact approach. In the case of the simplified approach, the single equation in BS 5950-1:1990 has been replaced by the following two expressions:

yy

yy

xy

xx

c

c

Zp

Mm

ZpMm

P

F++ 1

yy

yy

b

LTLT

cy

c

Zp

Mm

MMm

P

F++ 1

In applying these equations, the following points need to be noted.

1. In the first term of the first equation, Pc is the smaller of the compression resistance for buckling about the major axis Pcx and the compression resistance for buckling about the minor axis Pcy. However, in the second equation, Pcy is always used, whether it is smaller than Pcx or not.

2. In the second term of the second equation, MLT is used in place of Mx. MLT is the maximum major axis moment in the segment length between restraints against lateral-torsional buckling.

3. The equivalent uniform moment factor m used in BS 5950-1:1990 has been replaced by mx, my and mLT. mLT is the equivalent uniform moment factor for lateral-torsional buckling for the pattern of major axis moments over the segment length LLT. It is obtained from Table 18. mx and my are the equivalent uniform moment factors for flexural buckling about the major and minor axis respectively. Both are new and are obtained from Table 26 using the appropriate moment pattern between the relevant flexural buckling restraints.

4. For cantilever columns and members in sway sensitive frames, BS 5