World Trade Center (WTC) collapse investigators have this week called for fire to be considered as a load case in the design of tall buildings worldwide.
The recommendation follows National Institute of Standards and Technology (NIST) findings that the thermal expansion of a composite flooring system exposed to fire was to blame for the collapse of World Trade Center 7 (WTC7).
The 47-storey WTC7 collapsed just after 5pm on 11 September 2001. It was the third building in the WTC complex to collapse that day, but it was the only one that had not been hit by a terrorist-hijacked passenger jet.
NIST claims the destruction of the building is the first known instance of fire causing the total collapse of a tall building.
“Structural engineers design for gravity, wind and earthquake load cases but not fire,” said NIST WTC lead investigator Shyam Sunder.
“Architects specify fire proofing from a catalogue, based on building code requirements. What is missing is a connection between the disciplines. No one evaluates the structural response to fire.”
Halcrow fire safety engineering director Fathi Tarada said adding another element to the structural design process would significantly raise costs and only a change in design codes would make engineers switch from the current method of specifying the fire resistance of individual components of the structure.
“Finite element analysis of a whole building is expensive and it’s quite rare due to resource reasons,” said Tarada.
“Computers are now much quicker but it needs motivation to get done. If codes are changed to say that if you have long spans you need to consider thermal expansion, you will have more people reaching for their computers and doing more calculations.”
Seattle Department of Planning and Development principal engineer John Siu meanwhile queried the necessity of changing design codes for what could be seen as a one-off event.
“[NIST] recognises that adding collapse prevention in an infrequent event versus looking at life safety in a ‘normal’ event is a big change in philosophy, and I don’t think everyone in the code world has bought into the NIST premise,” said Siu.
Even if codes were changed to include a more analytical approach to fire, added Siu, there would still remain the question of what type of fire should be designed for.
“I think engineers should be aware of the issues, such as long beam spans and ‘unbalanced’ loading on girders due to expansion of the beams under fire conditions.
“However, I do not think there is enough of a defined design case for structural engineers to use to evaluate their designs – do you assume open office plan, or enclosed offices, do you assume normal combustibles or more paper than ‘normal’ offices, etcetera. Or do you assume ‘worst case’ for everything? What is ‘worst case’? What length of time should be used for high temperature exposure? There are lots of other questions besides these that
would need to be answered before this could actually be implemented.”
The WTC7 report has also raised the issue of over reliance on sprinkler systems. British Standards currently allow the design fire load to be reduced when sprinklers are installed in the building. However, sprinklers have been known to fail and, in this instance, this was a factor leading to the collapse of the building.
“There are a number of buildings in the UK whose structure has been fire engineered based on the reduction of the fire load due to sprinklers,” said Tarada.
“For example, Abbey Mill House in Reading is a 16-storey tower whose structural fire rating has been reduced from 120 minutes to only 60 minutes. This reduction was partly justified on the basis of sprinklers reducing the fire load by 61%. Based on the WTC7 collapse, we have to question the assumption that sprinklers will always be available to protect the structure.”