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Fire Safety Journal 38 (2003) 589–591
Letter to the editor
A suggested cause of the fire-induced collapse ofthe World Trade Towers
I am writing in response to the short communication by Quintiere, Marzo andBecker, published in your journal No. 37 2002, titled ‘‘A suggested cause of the fire-induced collapse of the World Trade Towers’’. Subsequent interest in the work bythe US and UK press, has led me to comment on this short communication,particularly in light of the extensive technical work carried out in the field ofstructural fire engineering in the last 15 or more years.
The short communication does not address the now well-understood phenomenonof full structural response to real fire conditions. It instead relies on the traditionalsingle-element approach to structural design for fire. In other words, the shortcommunication relies on the assumption that single-element failure times or failuretemperatures are indicative of the global failure time/temperature of a structure.Additionally, the work is based on the assumption that fire resisting insulationremains in place after impact.
Extensive research in the last 15 years (see Refs. [1–5]) has clearly identified thatsingle-element analysis does not capture the real structural responses in fireconditions. Full-scale testing and complex analysis such as was part of theCardington Large Building Test Frame program in UK, effectively dismissed thesetheories and design codes are now moving away from these assumptions. Therefore,the time or temperature at which failure is calculated of a single truss elementdoes not necessarily imply the actual failure time or failure temperature of anentire floor system and certainly not the entire structural system. Therefore, theauthor’s conclusion that the failure times derived in this short communicationshould be comparable with the failure times on the day is not technically possible tojustify. It is suggested here instead, that one means of analysing real structuralresponse to fire is a geometrically non-linear analysis including thermal expansioneffects.
Although the total effect of impact on the presence of structural fire protectionmay not be known, even if the fire protection was no longer in place in the fire regionas the authors state, the calculated failure time of 10–15min for a single, bare steelchord bears no relation to the overall response of the full structural system. It ispossible that a mechanism developed that did not rely on local buckling of acompression element in the trusses and this is why the actual failure time exceeds theauthors calculated 10–15min. It is unreasonable to conclude in this way that fire
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0379-7112/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0379-7112(03)00058-4
protection was therefore in fact in place because the 10–15min calculated time wasexceeded.
The authors have not attempted to address the resulting global structural state inthe towers on failure of some or all of the trusses. Until this state is understood, it isnot technically justifiable to deduce that a failure temperature of a steel truss impliesa generalised failure temperature for the entire tower or is in fact the initiatingcollapse temperature of the tower. The importance of the truss response to fire mustfirst be established—i.e. were the Tower structures entirely reliant on these trussesfor stability? In addition, any resulting load-carrying mechanism on loss of some orall of the trusses must be deduced. Similarly, the importance of the columns andcores in terms of restraining any possible tensile or catenary action must then bedetermined. If indeed these mechanisms even occurred. It is possible some newmechanism in fact took place. A sequence of possible failure mechanisms as a resultof the combined fire and impact damage must be established.
The short communication is not representative of the state-of-the-art under-standing. Discussions that relate the total failure time of a building to specificthickness of protection raise false expectations/concerns for the public. Morespecifically, the conclusion from the single-element calculations as presented that theassumed protection thickness gives ratings of 67–79min by temperature inASTME119, but that this is less than the authors calculated fire duration of 80–100min, implies some correlation is possible. In fact, it is well known thatASTME119 heating (or any standard fire resistance test such as BS 476, ISO 834)does not represent the range of possible real fire conditions. The total fire durationand the rate of heating to a maximum (or several peak points) in any fire can result inentirely different structural responses (Ref. [5]). It is, therefore, not possible to solelycompare ASTME119 durations with real fire durations and then derive somesignificance in the context of applied fire protection thickness. A robust design of astructure for fire is combination of passive fire protection and specific detailing of thestructure to withstand fire-induced responses.
For such an important topic, we should as a profession be demonstrating to thosenot expert in the field of structural fire engineering, the state-of-the-art under-standing and skills now available. By doing this, we can encourage a comprehensiveand meaningful program of work to understand the response of the World TradeCenter towers to the extreme events of 9–11, and from this work develop guidancefor robust designs for all buildings in the future.
References
[1] Bailey CG, Moore DB. The behaviour of full-scale steel framed buildings subject to compartment fires.
Struct Eng 1999;77(8):15–21.
[2] Huang Z, Burgess IW, Plank RJ. Non-linear modelling of three full-scale structural fire tests. In: First
International Conference, Structures in Fire, Copenhagen, June 2000.
[3] The University of Edinburgh. Final report of the DETR-PIT project: behaviour of steel framed
structures under fire conditions. Technical Report, The University of Edinbutgh, 2000. www.civ.ed.
ac.uk/research/fire/project/main.html
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[4] Usmani AS, Rotter JM, Lamont S, Sanad AM, Gillie M. Fundamental principles of structural
behaviour under thermal effects. Fire Safety J 2001;36(8):721–44.
[5] Lamont S, Lane B. Robust design of tall buildings in fire—ANALYTICAL APPROACH.
Proceedings of the CIB-CTBUH International Conference on Tall Buildings, Malaysia, 8–10 May
2003. http://www.cibklutm.com/abstracts received.htm. Or [email protected]
Dr. Barbara LaneArupFire 13, Fitzroy Street London W1T 4BQ, UK
E-mail address: [email protected]
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