Part A – falling behind?

TRADA Technology Principal Engineers Dr Hugh Mansfield-Williams and Dr Keerthi Ranasinghe discuss the impact of the Eurocodes on Part A.

Part A of Schedule 1 to the Building Regulations and the corresponding Approved Document A (ADA) relate to structural requirements. Although ADA states "Approved Documents are intended to provide guidance" and "there may well be other alternative ways of achieving compliance" most practitioners act as if the Approved Documents are the legal requirement, particularly for "small buildings of traditional construction". This is no accident, and benefits all concerned, since there is little argument as to how the requirements should be fulfilled. Designers, specifiers and regulators are entitled to presume that compliance with the Approved Documents is deemed to satisfy the requirements of the Building Regulations themselves. This useful presumption rests on the advice and guidance provided to the Department of Communities and Local Government (CLG) by the Building Regulations Advisory Committee (BRAC) and other experts when CLG revises the documents, and it has stood us in good stead.

In 2010 Part A faced the greatest challenge yet to the presumption that it could be deemed to satisfy, with the withdrawal of the existing system of familiar BS structural design codes and its replacement with the Eurocodes. This withdrawal was undertaken by BSi in accordance with a long standing agreement between European standardisation bodies, so the Eurocodes became the new British Standards on 1 April 2010. However, Part A was not updated. Under the CLG's periodic review programme, a revised version will be published in 2013. Until then, the construction industry must cope with an Approved Document A which refers to structural design codes that BSi has withdrawn and where the only mention of Eurocodes is a short section in the foreword, unchanged since 2004.

This situation is unsatisfactory. Engineering companies that have spent time and money in training staff and developing software and processes in anticipation of Eurocodes have found that their old systems must be maintained for longer, while those companies that made few plans for the transition may feel they can continue to delay. Developers are unsure as to which set of codes they should use in preparing tenders, and are concerned that others may gain an unfair advantage by using different codes. Regulators must be prepared to assess submissions under either set of codes and may have to decide if a design is safe when different parts of it are designed to different codes.

Both CLG and BSi warn against inappropriate "mixing and matching" between superseded British Standards and Eurocodes. There is no real need to do so, as Eurocodes and their National Annexes are now fully published, but the concern persists that some designers may seek to design different structural members to conflicting codes within a structure. There are boundaries within a structure where large scale systems or components can meet although they are designed to different standards, provided the correct assumptions are made about load transfer across the boundaries. An example of this is where trussed rafter roofs designed to the timber code are fixed to masonry or concrete structures. However, mixing and matching of codes is potentially dangerous where the basis of design and the relevant safety factors are not fully considered.

Designing to EC5 – Sheffield Winter Gardens

The CLG understood the potential confusion and issued a Circular Letter on 29 January 2010. This listed in detail the Eurocodes that were to be introduced, a total of 10 codes with 58 parts, and the superseded standards that were to be withdrawn. CLG advised Building Control Bodies (BCBs) that the Eurocodes could be used immediately, while pointing out that the then current British Standards were already no longer maintained, so "may not necessarily be suitable ... in the medium to long term". CLG acknowledges that Eurocodes will replace the now withdrawn British Standards, but seem to accept that the transition will take longer than originally planned.

Irrespective of the exact timescale and the revision status of Approved Document A, the construction industry must accept that Eurocodes will fully replace the withdrawn BSs in the near future. A lack of relevant expertise in the UK will be no great barrier, since such expertise will be available throughout Europe and beyond, wherever the Eurocodes are implemented. BCBs will have to be satisfied that the requirements of the Building Regulations are met, regardless of the design codes employed. This will be a change, since the withdrawn British Standards often adopted a prescriptive approach that was widely considered to be relatively easy to refer to, while the Eurocodes, with their strict numerical approach, are somewhat harder to understand and apply.

For designers, Eurocodes provide greater flexibility, which should encourage innovation. They are less prescriptive and ask designers to use their own professional judgement in making key decisions. If properly applied, this flexibility should ensure economic designs with no compromise on safety. One recurrent criticism of the Eurocodes, however, is that the calculation processes are too complex, particularly within Eurocode 0: Basis of structural design. In time, this complexity will be overcome, through training, familiarity and the development of design tools. In the meantime, the expected economies may not be realised, particularly within the current economic climate. Designers and BCBs should be wary of relying on "Eurocodes Compliant" software if they do not understand the basis of calculation, since it is still the duty of the designer to ensure that the outputs from such software are correct if they are to be relied upon in structural design.

What else should be changed in ADA in 2013, apart from the introduction of the Eurocodes as the basis of design? Well it is reasonable to expect that the ADA should not conflict with the Eurocodes, so subjects such as wind loads – wind actions in Eurocode terminology – and disproportionate collapse should be checked. We can also hope for clarification on some sections: for instance Class 3 buildings in Section 5 on disproportionate collapse might be expected to comply at least with all the requirements for Class 2B buildings, but this is not stated. Other technical refinements will probably be addressed too.

However, CLG noted in its recent report Future changes to the Building Regulations – next steps that the increasing complexity and scope of the Approved Documents (particularly Part L) was a concern, and are unlikely to introduce new technical provisions without a clear benefit. So, for instance, although CLG acknowledges that some would like to see the scope of the ADA extended to include flood resilience, they are not convinced that national regulation is the most effective route.

Perhaps the most interesting potential development in Part A raised by the report that CLG will examine is the potential for a voluntary self-certification scheme for structural designers, allowing them to declare that their design complied with the Building Regulations. Such a scheme would be of interest to qualified practising engineers and could have the real benefit of strengthening the role of the competent person within the design process. Such self-certification would not extend beyond the Part A requirements.

Related NBS information:

Articles:

• NBS and the structural Eurocodes
• Compliance with Eurocodes
• Calibration report for Eurocode 1 on wind actions
• Video: Eurocode 7: Geotechnical Design

Selected links:

• Civil and structural engineering, general

April 2011

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