Last week the National Institute of Science and Technology (NIST) issued its draft report on the fire and collapse of World Trade Center 7. This document is very interesting reading for all tall building designers and it can be found at www.nist.gov
Tower 7 collapsed as a result of the fire that was ignited during the 9/11 terrorist attack. The report concludes that the collapse was solely a result of the fires that started on ten levels following the initial attack. The failure occurred approximately eight and half hours after the first attack. The collapse of the WTC1 and 2 severed the water mains and the firefighting effort was abandoned after the collapses of the twin towers.
The NIST report recommends that in future buildings should be designed so that they do not collapse, even in an extreme fire, even if the sprinkler system fails or is overwhelmed by the fire. The approach recommended by NIST is essentially a performance based approach which explicitly checks the performance of structure in fire. Although this approach has already been used on many tall buildings it is much less common on normal towers, and it marks a substantial change of design philosophy, from the prescriptive code method of applying a thickness of fire protection to all elements.
The NIST report, in draft format, has been issued for public comment and I hope you will read it and comment. I for one am not satisfied with the report. It does not deal with the failure in enough detail and what is really missing is an explanation of what types of details and configurations create poor performance, and why.
The fire induced failure of WTC 5/6 shows that short slotted holes in primary beams do not work. What did the failure of WTC 7 show and what can be done to make a similar building perform better? The NIST report says that the failure was due to a primary beam, but does not explain:
- If the primary beam had shear studs would it have failed?
- If the girders had fin plates or end plates would the building have survived?
- Did the floors fail on the heating or cooling cycle… and theoretically what was worse?
- How effective was the slab to tie the floors over the column… what were the catenary forces and how effective was the reinforcement?
The Council will be holding a technical discussion forum on this issue on our website and I would encourage you to participate. This discussion forum should be up and running in the next few days and our plan is to solicit views and opinions from our members in order to provide a formal CTBUH opinion to NIST.
To get involved in the CTBUH discussion on our forum, click here.
NIST notes that the failure was caused by the effects of thermal expansion. While fire engineers are well aware that the effects of thermal expansion and thermal contraction (during the cooling phase) are often substantially more significant than the effects of heat reducing the strength of materials. This understanding needs to extend to architects and engineers that are involved in the high rise industry and an introduction to the subject is described in the paper that I presented at the NIST Chicago workshop. The paper “Fire Induced Progressive Collapse “ by Scott, Lane and Gibbons can be found online here.
Readers outside of the US will be surprised to see that the NIST report spends quite a bit of effort disproving the Conspiracy Theory, that the towers were a result of a planned demolition. While more than 99.9% of the US tall building community believes that the Conspiracy Theory is complete garbage, NIST has been compelled to spend considerable effort disproving the theory; partly because the engineering community has not spoken out against the theory as strongly as we could. . From my perspective there is not one aspect of the failure of WTC1, 2, 5, 6 or 7 that cannot be explained by engineering phenomenon.
Finally, I am pleased to see that the CTBUH Seismic Design Guide has now been published and is in the post to all Organizational Members. An electronic version can be downloaded from the Council’s website, here.