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["Carter, Charlie" <carter(--nospam--at)aisc.org>]

> Was there a recommendation for protecting the
> buildings' sprinkler systems? If these remained in
> service collapse could have been avoided. 

The FEMA report is soon to be released and is quite extensive, covering much more than just the towers. There is also a similar report that will be released on the Pentagon. It is notable how similar the conclusions are given the differing structural systems and types. As one example: fire protective material was blown off in the WTC and the concrete cover (i.e., the fire protective material) was blown off in the Pentagon. In both cases, the steel was then exposed to fire with none of the protection we conventionally assume.

It is clear (to me at least) that systems such as these (fire protection, sprinklers, egress, etc.) were destroyed. That is different in my mind than failure. Nadine Post expressed this very well in the editorial she wrote in the April 8, 2002 issue of ENR.

    http://www.enr.com/new/editorials040802.asp

Given that the fires were essentially instantaneously ignited 5-acres of fire in each tower, I personally believe it is doubtful that any of the fire protection, suppression, egress and similar systems we have ever put in any building would have made any difference. I also personally find it to be amazing in retrospect that the structures themselves initially survived the attacks. One of the biggest difficulties of the 9/11 WTC experience is adjusting your mindset to the enormity of every aspect of this situation. It is an experience unlike any other and our conventional concepts of what should happen often do not apply. I believe this is true of both the extent of the structural damage and the fires.

I was both impressed and disappointed by the NOVA program. On the one hand, they gave the best synopsis to date of the damage that was likely and the eventual collapse modes for the two towers. Also, they did not pander to fire department officials who seem to be very willing to offer opinions and conclusions on structural design and performance without a shred of understanding or knowledge of structural engineering.

On the other hand, the NOVA program perpetuated a few significantly erroneous bits of information. The most glaring example is the misconception that the floor system connection into the perimeter and core framing was only a two-bolt connection. In fact, those were erection bolts and the actual structural connection was a large welded gusset that would have been capable of developing significant catenary action, particularly given that the truss floor framing was a two-way truss system. The design configured by Skilling, Robertson and others is being unjustly criticized, in my opinion.

Most of all, I was disturbed by the direct and indirect indications of one notable engineer, based largely on the two-bolt misconception, that the designers should have somehow known or done more and that the building performance was a failure, not a destruction. I shudder to think what the average structure would experience being hit by one of those planes. There may be no other structure that could have initially withstood it, much less survived it.

Charlie