Structural steel design: BS449 v. BS5950
Over many years we have periodically been contacted when an over-zealous building control officer has told a SuperBeam user that calculations to BS449 are not acceptable and BS5950 should be used instead, especially following the introduction of the revised Part A Approved Document in December 2004.
Whilst having a long overdue purge of old magazines and papers I came across this announcement on p.28 of BSI News, August 1992:
"Following the widespread concern over the proposal to declare British Standard 449: Part 2: 1969 'obsolescent', Sub-committee B/525/31 (formerly CSB/27) has decided not to proceed further with the implementation of this action at this time and the standard maintains its present status.In coming to their decision, the members of Sub-committee B/525/31 were mindful of the fact that BS449: Part 2: 1969 continues to be used by engineers and is a standard referred to in Approved Document A to Building Regulations. The Sub-committee conforms to the policy view of BSI that its future work lies with the development of Eurocodes and supporting standards."
- 15+ years on, the definitive Eurocodes and supporting National Annexes have finally been published, BS449 Part 2 being finally withdrawn on 30 March 2010 BSI catalogue
In the 2004 Part A Approved Document (available as a free download from the Planning Portal web site, BS449 was removed from the list of approved codes. Replacing BS449 with BS5950 within SuperBeam would mean that in many cases (e.g. a loft conversion ridge beam bearing on a timber post) limit-state and permissible stress calculations would be intermixed in the same set of calculations - something less than desirable. When we pointed this to ODPM (as was), their response was to confirm that the fact that a code does not appear in the Part A Approved Document does not mean that it cannot be used, and that the continued use of BS449 in domestic scale structural calculations would not necessarily be impermissible. To quote the new Part A AD "There is no obligation to adopt any particular solution contained in an Approved Document if you prefer to meet the relevant requirements in some other way".
'Structural Engineer' 15 March 2005, p.38 includes a letter from Geoff Harding
of ODPM: "... Regarding the use of withdrawn Codes, building control
authorities will need to be satisfied that such documents continue to provide
satisfactory guidance for any structures under consideration. In this context reference
should be made to the safety aspects given in paragraph 0.2 on page 6 of the AD.
In the case of BS449 its recommendations were considerably enhanced by amendment No. 8 prior to the document being declared obsolete several years ago. The Code will therefore no doubt continue to provide satisfactory guidance at least for the design of simple beams and other minor works.
We now produce, EuroBeam™, a Eurocode-based program which we expect will ultimately replace SuperBeam and ProSteel - see our EuroBeam site for more information.
On 29 January 2010 DCLG issued a letter stating that the Part A Approved Document A was unlikely to be amended until 2013 (see below), thus the present AD listing BS5950 (and other codes) as acceptable. Quote:
WHAT THIS MEANS FOR BUILDING CONTROL BODIES (BCBs)
When assessing compliance with the Building Regulations, BCBs should continue to consider the appropriate use of relevant standards on a case by case basis. This may include the use of the new BS ENs [Eurocodes], which formally become the new national standards in April 2010 reflecting the changes made by the standards organisations. There is no need to wait until April 2010.
The British Standards to be withdrawn on 31 March are and will remain available from BSI. But BSI committees have already stopped updating those British Standards, and so they may not necessarily be suitable for aspects of structural design in the medium and long term.
BCBs will need to be aware of the risk of designs inappropriately mixing new design standards based on the BS ENs and withdrawn BS design standards.
Emphasis is ours. Full letter here [PDF]
For now we hope that Building Control checking engineers will continue to accept the use of BS449 for structures such as loft conversions that involve an intimate mix of steel and timber structural elements, and not unreasonable to use this code for simple steelwork calculations in domestic scale structures. For other work we would agree that BS5950, on which our ProSteel program is based, is the better code to use. Registered users of SuperBeam can now buy ProSteel for just £100+VAT.
As familiarity with Eurocodes builds it will become increasingly possible to use them with confidence. Note in particular that for unrestrained steel beams EC3 often shows a much higher resistance to lateral torsional buckling than the earlier codes; since the steel has got no stronger, the Eurocode approach is delivering a lower factor of safety (but hopefully still adequate) than the older codes, so using them is not risking building safety, rather the opposite.
In January 2012 DCLG has published several consultation documents on possible changes to the Building Regulations and Approved Documents. One covers Parts A, B, C, K, M and N [free download here] and says
Implementation of Eurocodes changes
46. The proposed changes to Approved Document A are intended to come into force in 2013. As the Impact Assessment accompanying this consultation document recognises, there are one-off, transitional costs to business associated with a move to a structural design approach based on the Eurocodes. A proportion of industry has already incurred this cost wholly or to some extent. However, a significant proportion of industry has not and Government accepts that many of the firms that might potentially be impacted upon are smaller businesses.
47. It is important to remember, however, that the proposed Eurocodes-related changes affect only the guidance contained in the Approved Document and not the functional requirements contained in the Building Regulations themselves. Further, the functional nature of the Building Regulations means that this guidance does not necessarily need to be followed, that is, alternative approaches may well be acceptable, subject to them being shown to satisfy the functional requirements of the Regulations.
48. In order to ensure this flexibility of approach is properly understood by business and building control bodies, we therefore propose to supplement guidance in the Approved Document with additional advice, perhaps through an accompanying Circular with the final changes, that clarifies:
- that until withdrawn British Standards become significantly outdated, probably some time after 2015, use of currently-referenced structural design standards could still be used to demonstrate compliance with the Building Regulations
- that beyond 2015 use of these withdrawn standards may in some circumstances still be acceptable and that building control bodies should accept an approach where a designer is able to demonstrate it is appropriate for the particular building structure proposed. This is likely to be particularly appropriate for smaller-scale building development.
49. This will assist business, and small firms in particular, in two ways. Firstly, it will provide at least an additional two years for firms to prepare for, and spread the costs over, the switch to a regime based on Eurocodes-based structural design. Secondly, it will also allow certain types of firm, principally those very small firms whose work is made up of smaller-scale buildings in the domestic sector, to continue to use the currently referenced withdrawn British Standards, thereby avoiding the additional costs associated with a switch to Eurocodes-based British Standards.
The new Part A Approved Document referred above took effect on 1 October 2013. As expected, Eurocodes are now the codes listed for structural design. To quote the DCLG's circular letter of 30 July 2013:
The main change to Approved Document A (Structure) has been updating of the references to structural design standards, so these refer to the current British Standards for structural design, replacing references to standards withdrawn by the British Standards Institution (BSi) on 31 March 2010. The current British Standards are based on a suite of common European design standards, the Eurocodes, which provide a unified approach to structural design across the European Union.
It is likely that structural designs will continue to be completed using the withdrawn British Standards for some years. This approach is considered appropriate given the functional nature of Building Regulations. Building Control Bodies assessing compliance with the Building Regulations will need to consider the use of standards on a case by case basis. Where the use of the withdrawn British Standards is proposed, it should be remembered that whilst BSI might not be maintaining withdrawn British Standards, typically a British Standard is only subject to review and revision every five years. The use of withdrawn standards may therefore continue to remain acceptable for the foreseeable future, particularly for smaller, less complex designs. It would be appropriate to refer to the relevant guidance in the 2004 edition of Approved Document A where the use of recently withdrawn standards is proposed.
Building Control Bodies will need to be aware of the safety risk of designs inappropriately mixing structural designs based on both the new and the withdrawn British Standards.
Our inference is that whilst official policy must be in favour of the adoption of Eurocodes, for small scale works the use of the older codes should be accepted for a good while yet.