New Approved Documents A and C come into force; structural Eurocodes arrive

               
New Approved Documents to Building Regulations Part A (structure) and Part C (site preparation and resistance to contaminants and moisture) came into use in England on 1 October 2013.
While the Building Regulations themselves remain unchanged, the revised Approved Document for Part A signals the formal adoption of Eurocodes in place of British Standards for Structural Design, with the entire list of structural design standards now updated accordingly.

               
The changes in the two Approved Documents, follows a DCLG Circular (30 July) that acknowledges that structural designs are expected to be completed using the withdrawn standards for some years, and instructs Building Control bodies to accept that as an appropriate approach. At the same time, there is a warning that safety could be compromised if designs combine Eurocodes and the withdrawn British Standards in an inappropriate manner.
The most significant changes to the Approved Document for Part C reflect updated guidance on radon protection and revised radon maps that have increased the areas at risk. References to the Contaminated Land (England) Regulations and CDM Regulations are also updated.

Below is a detailed summary of the changes:

Approved Document A

Eurocodes
The most significant change to Approved Document (AD) A 2013 is the adoption of the Eurocodes in place of the withdrawn British Standards for structural design. The change affects every part of AD A, but is most obvious in section 1, where the entire list of structural design standards has been revised.
A DCLG circular letter (30 July 2013) acknowledges that structural designs will continue to be completed using the withdrawn standards for some years and instructs Building Control Bodies to accept that as an appropriate approach. However, it warns of a safety risk if designs inappropriately mix new and withdrawn standards. The letter includes a table of correspondences between the old and new standards.
Maximum building heightThe method for calculating the maximum height of a building set out in subsection 2C16 of AD A now follows BS EN 1991-1-4:2005. The basic steps are similar but there are some changes:

1. The wind speed map at Diagram 6, Figure 1 has been revised. Wind speeds to the East of England have generally come down (e.g. Norwich is now 22.5 m/s, from 24.5 m/s) while those to the West have increased slightly (e.g. Plymouth is now over 23.5 m/s, previously under 23 m/s)
2. The topographic zone has been replaced by an orographic zone. The definitions of the zones are largely unchanged, although on cliffs and escarpments zone 3 now extends for 1.5 times the slope length, instead of 1.2 times
3. Table a (Diagram 7) now gives Factor O. The three categories of slope have been expanded to four, by the addition of a category for shallow terrain (<1 3="" 6="" alternative="" an="" determining="" diagram="" factor="" figure="" for="" li="" method="" o="" offers="" slope="">
4. Table b – Factor A – has been extended to a maximum altitude of 500 m
5. In Table c – Maximum allowable building height – distances to the coast have been revised, with break points at 2 and 20 km, rather than 10 and 50 km. The maximum heights have been revised slightly upward.

Overall, it seems likely the revisions to the map will make conditions more onerous in the western parts of England and slightly less onerous to the East.
WallsWall ties for cavity walls must now conform to BS EN 845-1 (section 2C19 and Table 5) and ties must be type 1, 2, 3 or 4 to PD 6697:2010. The tie length requirements are unchanged. The recommended spacing of ties remains 900 mm horizontally and 450 mm vertically: alternative arrangements are permitted, providing there are at least 2.5 ties/m².
The minimum depth to the underside of strip foundations when building on shrinkable clays with plasticity index equal to or greater than 10% was 0.75 m in AD A 2004. The recommended minimum depth has been revised:

• Low shrinkage clay soils – 0.75 m
• Medium shrinkage clay soils – 0.9 m
• High shrinkage clay soils – 1.0 m.

Where wall cladding acts as pedestrian guarding, additional imposed loading must be taken into account at vertical drops of more than 600 mm in dwellings or at more than the height of two risers, or 380 mm, in other buildings.
Roof coveringsMaterials used to cover roofs must be capable of withstanding the concentrated imposed loads specified in BS EN 1991-1-1:2002. There is a new exemption for transparent or translucent covering materials which need not meet the loading requirement, provided they are not accessible for normal maintenance and repair, and are non-fragile or suitably protected against collapse.
The boundary for a significant change in loading on re-covering a roof is still 15%, but it is important to consider whether the roof covering being replaced is the original as-built covering.
Disproportionate collapseThe requirements for the prevention of disproportionate collapse of buildings (section 4) have been modified at several points:

• The building classes defined in table 11 are now referred to as 'consequence classes': the overall classification is unchanged
• It is now clear that buildings in consequence classes 2a and 2b require horizontal and vertical ties in addition to the measures required for buildings in consequence class 1
• The maximum area of floor at risk of collapse as a result of the notional removal of a structural element is now the smallest of 15% of floor area or 100 m² (up from 75 m²)
• When testing key elements, the accidental design loading of 34 kN/m² is now applied simultaneously with all other design loadings (wind and imposed loadings) rather than one third of such loadings.

Seismic design
A new section 5, seismic design, recommends that for buildings in consequence class 3 the risk assessment should consider whether seismic design should be undertaken.

Approved Document C

Radon protection
The guidance on radon protection has been updated to refer to the 2007 edition of BR 211 Radon: Guidance on protective measures for new buildings and the revised radon maps produced by the Health Protection Agency and the British Geological Survey. The maps show far more areas with a risk from radon.
Basic protection measures are required where 3-10% of homes are predicted to have radon above the Radon Action Level of 200 Bq/m³ and full protective measures where more than 10% of homes are predicted to be above the action level.
Other changesA number of minor changes have been made throughout AD C:

• Annex A (Guidance on the assessment of land affected by contaminants) has been withdrawn
• References to the Contaminated Land (England) Regulations and CDM Regulations have been updated to the latest versions, dated 2006 (amended 2102) and 2007 respectively
• References to standards and other documents have been revised
• References to Planning Policy Guidance Notes have been deleted and replaced by references to the National Planning Policy Framework.