Tuesday, August 26, 2008


As a structural engineer, I've received a lot of questions from friends and relatives about the collapse of the World Trade Center towers back on 9/11. I never was convinced by any of the arguments put forth by conspiracy theorists concerning the collapse of the buildings. The report released by the NIST provided a perfectly legitimate explanation for the collapse of the towers.

The attack occured while I was still in engineering school, so I could hardly have called myself an expert at the time. However, disregarding any of the expertise I have gained in the years that followed as a practicing engineer, I have yet to read from any legitimate expert on structural engineering, construction, demolition, or any other related field who agreed with any of the claims of the "truthers." That, in itself, made me automatically suspicious of any claims for conspiracy. Oh sure, there have been plenty of folks with Ph.D.'s behind their names claiming to have "proven" that the collapse of the towers had to have been caused by controlled demolition explosives, but dig a little deeper and you'll find that these "experts" are college professors with knowledge about many valuable things...cold fusion, economics...but certainly not structural engineering or construction.

We now have an official report from NIST on the collapse of WTC 7, the smaller building adjacent to the two towers. This building sustained damage from the collapse of the twin towers, and eventually experienced a catastrophic failure itself. Many of those behind the "President Bush Authorized and Planned 9/11" movement pointed to the collapse of this building, which was not directly damaged by either plane, as an indication that foul play was involved. After reading through a synopsis of the report and looking at the engineers' methods of analysis and creation of the original structure's model, I see nothing to cause suspicion.

In a nutshell, falling debris from the collapse of the adjacent towers ignited fires on at least 10 floors within the WTC 7 structure. Because of the fact that the original collapse disabled the water main, the sprinkler systems in WTC 7 did not function, causing some of these fires to burn uncontrolled for up to 7 hours. Despite what many of our esteemed "experts" would have you think, uncontrolled office fires can produce enough heat to cause significant thermal expansion within long-span floor framing systems. (These fires burned between 570 and 1100 F, temperatures which would have certainly caused significant thermal expansion, and even a reduction in strength and stiffness at the upper range). Most beam-to-column connections are not designed to resist the lateral loads induced by thermal expansion, so a failure occured at one of the critical columns, causing a progressive collapse. This link answers many of the questions raised by some of our previously mentioned conspiracy theorists.

That's not to say that there aren't some issues I'd like to investigate further. As mentioned in the report, this is the first known case of fire causing the total collapse of a tall building. That's a pretty big development for structural engineers, as we're generally more concerned with wind, earthquakes, and typical service loads. It'll be interesting to see what changes are made in the building codes over the next few years to address this issue.

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