Approved Document A and the Eurocodes

Structural engineers have had nearly two years to come to grips with the withdrawal of all parts of the structural design standard BS 5268, referenced in Approved Document A, but it has not been plain sailing, as Dr Hugh Mansfield-Williams, TRADA Technology's Principal Consultant, explains.

The structural Eurocodes are a set of harmonised European design standards which provide a common approach to the structural design of buildings and civil engineering works across the European Union. They are intended to remove potential barriers to trade that exist when countries have different design rules.

For timber and wood-based products, BS 5268-2 is superseded by Eurocode 5 (EC5) or to be more precise BS EN 1995 - Design of Timber Structures. It applies to the design of buildings and civil engineering works in timber. It complies with the principles and requirements for the safety and serviceability of structures and the basis of design and verification given in BS EN 1990:2002, the key code for all the harmonised Structural Eurocodes. Through its various parts, it covers the requirements for mechanical resistance, serviceability, durability and fire resistance of timber structures.

The Department of Communities and Local Government (DCLG) contacted local authorities, Building Control, approved inspectors and other interested bodies on 29 January 2010* to inform them about the introduction of the Eurocodes, and the associated withdrawal by British Standards Institution (BSI) in March that year due to conflicting BS design standards. Some of these design standards, including BS 5268, are referenced in the Building Regulations Approved Documents, particularly Approved Document A (Structure).

As DCLG explained, there are 10 Eurocodes made up of 58 Parts that were adopted in all EU Member States in 2010. Each Part is implemented nationally with a National Annex. These Annexes contain information on Nationally Determined Parameters to be used for the design of building and civil engineering works to be constructed in the country concerned, addressing, for example, particular national safety parameters, geographical and climatic conditions, and procedures.

When assessing compliance with the Building Regulations, Building Control Bodies (BCBs) were advised in the DCLG communication of 29 January 2010, to ‘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, which formally become the new national standards in April 2010 reflecting the changes made by the standards organisations.

The withdrawn British Standards remain available from BSI, but BSI committees had already stopped updating those British Standards well before they were withdrawn, and so, said DCLG, ‘they may not necessarily be suitable for aspects of structural design in the medium and long term’.

It is this part of the communication from DCLG which appears to have caused some confusion. During 2011, TRADA Technology was contacted by several builders who had been advised by Building Control that they might need to rebuild roofs and other structures if built using the BS 5268 version of its span tables for solid timber members in dwellings, instead of the EC5 version.

Until 2004, these span tables were included in Approved Document A (ADA). Then TRADA Technology was asked to take over their publication and ADA 2004 references the TRADA Technology document, now in its second edition. Five years later, to prepare for the advent of EC5, TRADA recalculated the span tables to comply with Eurocodes and published a 3rd edition, in anticipation of a further updated version of ADA. Because BS 5268 is still referenced in Approved Document A, however, the 2nd and 3rd editions of the span tables have been sold concurrently, for calculations to BS 5268 and Eurocode 5 respectively. This was done to ease the transition from design standard to another but in the knowledge that designers would be reluctant to change overnight.

Although TRADA naturally wants to encourage a complete move to EC5 technically – we strongly discourage designers from ‘mixing and matching’ the old and the new standards - it would be incorrect to say that builders cannot build to the withdrawn standards because the revised version of ADA will not be in place until 2013.

We believe that this is the correct interpretation of the DCLG guidance, which stated, ‘When an Approved Document makes reference to a named standard, the relevant version of the standard is the one listed at the end of the publication. However, if this version has been revised or updated by the issuing standards body, the new version may be used as a source of guidance provided it continues to address the relevant requirements of the Regulations.’

When the communication was issued, DCLG was evaluating Parts A (Structure) and C (Site preparation and resistance to contaminants and moisture) and their Approved Documents. It said, ‘We are looking at these Parts together as they target similar building elements. The evaluation has identified a number of issues that Parts A and C will need to reflect, but the full implications of these issues will not become clear until later this year at the earliest, ie too late to inform changes in 2010. We have concluded, therefore, that we should continue our work reviewing Parts A and C, with a view to proposing appropriate changes for the next periodic amendment in 2013. We expect to be updating at that time the references in the Approved Documents that relate to structural design standards.’

Importantly, the communication affirmed: ‘However, we would not expect this rescheduling to affect or deter the take up of the new national standards (BS ENs), based on Eurocodes. We continue to work with industry including professional institutions and representative bodies, to assist as they prepare for this change to the British Standards.’

BSI has published the Eurocode (EN) standards as British Standards (BS ENs). BSI has also published the National Annexes. The 10, with the number of Parts in each, are:

• BS EN 1990 Basis of Structural Design 1 Part
• BS EN 1991 Actions on Structures 10 Parts
• BS EN 1992 Design of Concrete Structures 4 Parts
• BS EN 1993 Design of Steel Structures 20 Parts
• BS EN 1994 Design of Composite Structures 3 Parts
• BS EN 1995 Design of Timber Structures 3 Parts
• BS EN 1996 Design of Masonry Structures 4 Parts
• BS EN 1997 Geotechnical Design 2 Parts
• BS EN 1998 Design of Structures for Earthquake Resistance 6 Parts
• BS EN 1999 Design of Aluminium Structures 5 Parts

Find out more

TRADA Technology provides a wide range of commercial and training services to TRADA members and to the timber and construction industries.

Related information:

• Compliance with Eurocodes
• NBS and the structural Eurocodes
• Part A – falling behind?

Written January 2012

 

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