28 October 2008

Wind-load annex gives green light to Eurocode designs

BSI British Standards has published the much-awaited national annex for part 1-4 of Eurocode 1 covering wind actions, such that Eurocode building designs can now be accepted by UK local authorities as complying with the Building Regulations.

NA to BS EN 1991-1-4: 2005 is a significant national annex as it modifies the Eurocode considerably – added to which modern, lightweight building designs are increasingly sensitive to wind loads. As such the annex has had a difficult birth, taking over two years from being issued as a draft for public comment to finally seeing the light of day on 30 September 2008.

The main problem has been calibrating the annex against existing wind code BS 6399-2, which will be replaced by 2010. A study commissioned by the Department for Communities and Local Government (DCLG) concluded in December 2007, ‘The calibration has confirmed that BS EN 1991-1-4 in conjunction with the draft national annex will generally lead to acceptable results comparable to current practice.’

However, the study recommends BS 6399-2 values for external pressure coefficients be used for cladding loads, ‘until better justification for the EN coefficients are available’. Also, for flat roofs with parapets or curved eaves, the BS scaling parameter for defining different zones should be used instead of the EN parameter. A copy of the report can be downloaded from the BSI Eurocodes website (link below).

It is understood DCLG will shortly clarify the status of Eurocodes with regard to demonstrating compliance with the Building Regulations in a letter to all building-control authorities. The Institution of Civil Engineers and UK Wind Engineering Society are also planning a series of lectures for design engineers on the background to the new wind code.

BSI British Standards has so far published 31 of Britain’s likely total of 55 national annexes – including all those for Eurocode 0 (basis), Eurocode 2 (concrete), Eurocode 4 (composite), Eurocode 5 (timber) and Eurocode 6 (masonry).

The remaining national annexes will be published between now and spring 2009. Annexes currently out for comment include that for BS EN 1990 Annex 2 (basis for bridge design) and for BS EN 1993-5 (steel piles).

All BSI Eurocodes and national annexes can be bought via the BSI Eurocodes website (link below).

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