NEW GUIDELINES to prevent progressive collapse in US buildings outline an 'objective performance measure' to help designers assess the risk of collapse.
Five years after the progressive collapse of the World Trade Center (WTC) towers in New York following the terrorist attacks on 11 September, the US National Institute of Standards & Technology (NIST) has proposed a calculation that it hopes will better identify the risk of catastrophic collapse in tall buildings.
Dr Shyam Sunder, deputy director of building and firre research at NlST - who oversaw the official report into the WTC collapse - told NCE that the new guidance should help ll current gaps in design knowledge.
'We don't have an objective measure of how buildings perform on progressive collapse at the present time, ' he said.
'There is a very big need for this, particularly in very tall buildings where there is less and less structural redundancy.' The draft guideline 'Best practices for reducing the potential for progressive collapse in buildings' was published by NIST in August and calls for comments by the end of September.
It highlights the use of an objective performance measure to enable the development of a 'performance-related' code that would allow for different solutions to prevent progressive collapse depending on the building type.
This differs from existing 'prescriptive' codes - as in the UK - which require the same solutions for all tall buildings.
Don Dusenberry, senior principal of US engineering consultant Simpson Gumtertz Heger, who chairs an ASCE committee on structural integrity, also backed the new performance measure.
'Most private clients are looking for guidance from engineers but there is little information on technical performance of buildings for progressive collapse, ' he said.
Sunder added that a data bank of information would begin to take shape in forthcoming reports into progressive collapse from US bodies the National Council of Structural Engineers Associations; the Council for Tall Buildings & Urban Habitats; and the American Society of Civil Engineers.
'My general sense is that new guidelines will focus more on the development of a performance-related code than a prescriptive code, ' added Sunder.
The guidelines also call for a 'common definition' of progressive collapse. A suggested definition in the document is: 'The spread of local damage, from an initiating event, from element to element, resulting eventually in the collapse of an entire structure or a disproportionately large part of it, also known as disproportionate collapse.
To view the draft guidelines, visit www. nceplus. co. uk.