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Wednesday, September 21, 2011

An inexcusable omission...

Dear Editors of Scientific American,

I read with great interest (and disappointment) Mark Lamster’s article “Castles in the Air” in your September 2011 issue. In presenting the changes that have occurred in the design and construction of skyscrapers in the decade since September 11th 2001, Mr Lamster (apparently along with many of the top names in the architectural and structural engineering communities) has completely missed the most important issue.

While highlighting the structural threats to skyscrapers, Mr Lamster notes three issues: aircraft impact, earthquakes, and wind. The claim is made that structural engineers are now able to design against these threats with a very high level of confidence and safety. As a structural engineer I agree completely.

Unfortunately, the Twin Towers did not collapse due to aircraft impact, earthquake, or wind; they collapsed due to fire. Nowhere in the article is the structural threat posed by fire explicitly mentioned. I agree completely with Mr Robertson's statement that the responsibility is “to keep airplanes away from the buildings and not to design the buildings for that circumstance,” but again this misses the point entirely.

On 9/11 we clearly saw, for the first time, that fires can cause entire modern highrise buildings to collapse. Indeed, in addition to the Twin Towers a steel-framed highrise (Building 7) on the WTC site also collapsed on 9/11. Building 7 was not struck by an aircraft; it collapsed due to the structural impacts of an uncontrolled office contents fire, ignited by debris from the Twin Towers.

The most important engineering lesson of the 9/11 collapses is that in order to ensure the safety of occupants (and property) in ever taller and more optimized and sustainable tall buildings, the potential impacts of uncontrolled fires need to be explicitly considered during the structural design process. These impacts need to be considered with the same care as earthquakes and wind. The engineering community has the technical knowledge to begin to do this, yet it is still not required by building codes and happens only very rarely in practice. While changes in escape stair width, firefighter communication systems, and the addition of skybridges (all noted by Mr Lamster) can only improve life safety in tall buildings, they do nothing to prevent structural collapse due to fire.

It is both sad and worrying that the structural engineering community has, with a few notable exceptions, failed to understand the full significance of the events of 9/11 for the design of tall buildings. We have vividly seen what uncontrolled fires can do to tall buildings yet we continue to design them using decades old, outdated knowledge. In addition to the measures noted in Mr Lamster's article, preventing another 9/11 requires that the structural engineering and architecture communities own up to this reality and take the necessary actions.

Perhaps Scientific American should consider publishing an article highlighting the significant and considerable technical progress in structural fire engineering being made by engineers around the world.

Kind regards,

Dr Luke Bisby
Senior Research Fellow in Structures in Fire
The University of Edinburgh

NOTE: Mark Lamster's original Scientific American article is available here


Hotstuff said...

To suggest that the aircraft impact did not influence the ensuing collapse is naive. Even the most imaginative structural engineer considering the disproportionate collapse of the building could not have foreseen the level of damage that resulted.

Obviously the buildings were sufficiently designed for impact, otherwise collapse would have occurred near instantaneously.

The impact did however result in an increase in utilisation for the columns that remained. This coupled with the fire and associated 2nd order effects is what I believe to be the root cause of the collapse.

To hypothesize that the buildings would have collapsed if the same fire had occurred, without the associated impact, is conjecture at best. Historical evidence suggests that WTC7 is a bit of an anomaly.

Luke Bisby said...

With respect to WTC1 and WTC2 I agree with these comments. However, "Hotstuff" has also completely missed the point. The original post never suggested that the aircraft impact did not INFLUENCE the collapse; it said it did not CAUSE the collapse. There is a subtle but important difference here and I stand by my comments on the SciAm article. Many structural fire engineers would agree that the WTC towers would have collapsed due to an uncontrolled multiple floor fire even without the aircraft impact. This is the important point that the community seems to be missing.

With respect to WTC7, invoking the "argument from ignorance" supported by historical "evidence" of other fires in steel buildings, which may have had very different structural layouts etc, totally glosses over the issue of how real multi-floor fires might impact real, modern buildings. Now that is naive.

Guillermo Rein said...

Conjecture at best?? The resulst supporting that WTC1 and 2 would have collapsed if the same fire had occurred without the associated impact was published in the Fire Safety Journal in 2003. See open access copy here:

Full reference:
A.S., Usmani, Y. C. Chung, J. L. Torero (2003), How did the WTC towers collapse: a new theory, Fire Safety Journal, Volume 38, Issue 6.

And I reproduce here the abstract:
"This paper uses a finite element model to investigate the stability of the Twin-Towers of the World Trade Center, New York for a number of different fire scenarios. This investigation does not take into account the structural damage caused by the terrorist attack. However the fire scenarios included are based upon the likely fires that could have occurred as a result of the attack. A number of different explanations of how and why the Towers collapsed have appeared since the event. None of these however have adequately focused on the most important issue, namely ‘what structural mechanisms led to the state which triggered the collapse’. Also, quite predictably, there are significant and fundamental differences in the explanations of the WTC collapses on offer so far. A complete consensus on any detailed explanation of the definitive causes and mechanisms of the collapse of these structures is well nigh impossible given the enormous uncertainties in key data (nature of the fires, damage to fire protection, heat transfer to structural members and nature and extent of structural damage for instance). There is however a consensus of sorts that the fires that burned in the structures after the attack had a big part to play in this collapse. The question is how big? Taking this to the extreme, this paper poses the hypothetical question, “had there been no structural damage would the structure have survived fires of a similar magnitude”? A robust but simple computational and theoretical analysis has been carried out to answer this question. Robust because no gross assumptions have been made and varying important parameters over a wide range shows consistent behaviour supporting the overall conclusions. Simple because all results presented can be checked by any structural engineer either theoretically or using widely available structural analysis software tools. The results are illuminating and show that the structural system adopted for the Twin-Towers may have been unusually vulnerable to a major fire. The analysis results show a simple but unmistakable collapse mechanism that owes as much (or more) to the geometric thermal expansion effects as it does to the material effects of loss of strength and stiffness. The collapse mechanism discovered is a simple stability failure directly related to the effect of heating (fire). Additionally, the mechanism is not dependent upon failure of structural connections".

Guillermo Rein said...

The letter of Luke has been published in the December 2011 issue of Scientific American: