BS5950-1 -2000

David Nethercot

BS5950 is the principal source of guidance for the design of structural steelwork in the UK. It is also widely used overseas e.g. , Malaysia, Singapore, etc., as well as for work designed in the UK but intended for export. Under the current BSI rules it is subject to care and maintenance so that its provisions remain both safe and appropriate. It must therefore recognise the need for change and improvement

through a controlled process of updating. Having carefully considered the position of the version of the Part 1 issued in 1990, the BSI Sub-Committee responsible

(B/525/31) took the decision in 1996 to prepare and issue a substantial amendment. The result, in the form of a reprinted version of the Code was finally published in May 2001 incorporating Corrigendum No.1. An earlier issue had been published in December 2000 but was withdrawn due to typesetting problems.

Once the Sub-Committee decision had been taken, actual preparation of the document entailed the ten separate steps listed below. The most important of these was the issue of the Draft for Comment and the subsequent revision of the document in the light of the comments received. Not only were comments gathered through the usual postal process, the Sub-Committee organised a one-day seminar at the Institution of

Structural Engineers in July 1998 as a way of both highlighting the background to the major changes and encouraging comments. Of course, not every change was

welcomed – some were criticised quite forcefully. The event was, however, regarded by the Sub-Committee as being particularly helpful in providing a users view; all comments were recorded, some were augmented by the contributors in writing and the views stated on that day were carefully considered during the re-drafting process.

Key stages in preparing the Amendment

1. BSI agreement

2. Sub-committee establishes Steering Panel

3. Steering Panel oversees preparation of draft AMD 4. Draft AMD approved by sub-committee 5. Published as a Draft for Comment

6. Comments received and responses prepared. 7. Sub-committee approved responses 8. Redrafting overseen by Steering Panel

9. Revised document approved by Sub-committee 10. Publication by BSI.

Whilst keen students of the new document will, no doubt, be able to spot numerous minor changes, it is considered that the most important modifications relate to the 39 topics listed below. Not all have been subject to the same degree of change;

something like a dozen represent significant technical improvement over the treatment in the 1990 version.

Topics amended in BS5950-1 : 2000

Notional forces, non-sway frames, minimum horizontal forces Brittle fracture

Disproportionate collapse Steel grades Bolt grades

Electrode classes

Use of notes leading to a Bibliography, e.g. wind moments and semi-continuous braced frames

8. Classification of webs

9. Cold formed rectangular hollow sections 10. Slender cross-sections 3.6

11. Moment capacity – notches etc.

12. Effective lengths for lateral torsional buckling (extra cases) 13. Lateral torsional buckling – general use of m 14. Angles in bending 15. Shear buckling

16. Load bearing stiffeners 17. Curtailment of stiffeners

18. Use of Ae effective area for angles etc. 19. Compression members – slender sections 20. Combined axial load and moment 21. Wind loads on purlins and rails 22. Base plates

23. Missing rules from BS449 (4.16 and 4.17) 24. Plastic design – member buckling out of plane 25. Portal frames – in-plane stability

26. Multi-storey frames braced compared to non-sway 27. Multi-storey frames – sway moments 28. Eaves corner of a portal 29. Block shear

30. Bolts through packing 31. Bolts in tension

32. Welded connections to unstiffened flanges 33. Deep penetration welds

34. Fillet welds – directional method

35. Lateral torsional buckling of T-sections and angles (Annex B) 36. Internal moments at splices (B.3, C.3 and I.5) 37. Effective lengths for mezzanine floors

38. Appendix G – three flange haunches and correction of example 39. Appendix H – edge loading.

The need for the Amendment was to a very great extent externally driven, with the list of issues and problems with the existing version reported by the Advisory Services of the BCSA, Corus and the SCI; widespread revision of the supporting Product Standards for steel, bolts and welds; matters raised but not included in the 1990 Amendment; users queries to BSI; and points to emerge as a result of the EC3

calibration all forming major inputs to the process. The overriding consideration was improved safety – taken to include lack of clarity as well as situations in which rules known to be over-conservative tended to be ignored. Thus many of the changes are 1. 2. 3. 4. 5. 6. 7.

intended to reduce misunderstanding of rules and time-wasting disagreements where different parties presume different meanings. Much of the time spent in drafting the Amendment has been devoted to ensuring that the printed words convey the intended meaning.

The treatment of brittle fracture and selection of steel grades has been brought into line with the revised approach of the steel bridge code. Classification of cross-sections has been simplified, with the over conservative method of Table 8 that

penalised many UBs when used as columns due to their slender webs being replaced by a more rational effective area approach.

The scope of the m-factor approach for dealing with laterally unrestrained beams has been extended to cover all cases, thereby eliminating the need for the alternative n-factor method. Treatment of the shear buckling of plate girder webs has been simplified through adoption of an approach similar to that used in EC3. More space has been devoted to the treatment of members under combined bending and

compression – although the fundamental treatment remains essentially the same – as a way of making the application of the rules to specific cases more straightforward. Drawing on work of the BCSA/SCI Connections Group, several changes have been made to Section 6. Arguably the most significant is the inclusion of an explicit method of allowing for prying action, in which higher bolt resistances may be used, providing the enhanced tensile forces due to prying are actually calculated. The simpler alternative of ignoring prying and using reduced bolt resistances in tension remains as an alternative. Block shear and the reduced strength of bolts passing through packing are both covered. The dependence of the strength of fillet welds on the direction of loading may now be recognised. For joints between structural hollow sections the previous assumption that intersection of centrelines will be the common case has been replaced by recognition that gaps or overlaps are more likely.

Major changes to ensure the production of safe structures have been made to the

material dealing with portal frame construction. The Sub-Committee was well aware of both the importance of this market of the UK steel construction industry and the fact that BS5950 represented the first attempt to codify rules for portal frames. The previously rather fragmented and often difficult to follow treatment of out of plane buckling and associated provision of lateral restraint to portal frame members has been recast into a more logical and progressive treatment. Guidance is provided on the need (or, more frequently, the absence of the need) to reduce the moment capacity of a column in the presence of high local shear stress in the column web within the panel zone at the eaves joint. An important change concerns the in-plane stability of single storey pitched roof portal frames, for which recent studies by Professor J.M. Davies have clearly demonstrated how modern arrangements are more sensitive to reductions in strength due to second order effects than the 1990 rules would suggest. Part of this is due to insufficient clarity in the present wording, and part to an

assumption that simplifications that were derived for arrangements in which rafter axial forces were comparatively low could be extrapolated to the current, shallower pitch frames. Considerable attention has been given to this topic and the rules of the Code will be augmented by a forthcoming SCI publication dealing exclusively with portal frames.

Preparation of the Amendment has not been an easy task. The Steering Panel and the Sub-Committee have sought carefully to balance the need to ensure that the Code remains safe and relevant to modern forms of steel construction and that, as far as possible, it neither appears radically different to the user nor is its actual use in practice changed unduly. By far the most important consideration has been greater clarity of intent – a feature exposed all too frequently through enquiries, misunderstandings and disagreement.

Now that the Code has been published, it will shortly be followed by a Guide to the Amendment, revised versions of the accompanying Blue and Red handbooks and the Green Connections Guide dealing with simple connections. Companies responsible for steelwork design software are also known to be preparing up-dated programs for release in 2001.

As designers become familiar with this somewhat changed environment, it is worth reflecting on the following quotation.

“The onset of new or revised regulations invariably heralds a trying period for the unfortunate people who have to work to such regulations. This applies both to those who have to comply with, and those have to administer, such regulations”.

It was not made at the IStructE Seminar, nor does it come from any of today’s engineers. Rather, it refers to the introduction of the 1959 version of BS449 – a document that, when BS5950 was first introduced, was lamented by many.

Professor David A. Nethercot is Chairman of BSI Sub-committee B/525/31, and Head of Department of Civil and Environmental Engineering at Imperial College, London.

This article is an updated version of an article published in New Steel Construction, March/April 2001.