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That is, in fact, a testimonial to how well designed the towers actually were. No steel frame building is capable of standing indefinitely in the face of severe fire, and the original designers of the towers had a goal of avoiding catastrophic collapse for at least 60 minutes after an overwhelming fire started -- and they achieved that. One tower stood 62 minutes, the other 103. And because of that, most of the people were able to evacuate, and 24,000 people were saved. Engineers always study their failures in detail, to learn as much as they possibly can from them. Everyone who took high school physics has seen the film of the Tacoma Narrows Bridge when high winds destroyed it; that failure was extensively studied and all bridges built since then are resistant to those kinds of failures, and there has been no comparable bridge collapse anywhere in the world since then. Generally speaking, civil engineering is extremely good. We take it for granted, but shouldn't. Consider how little actual damage was done in San Francisco by its major earthquake a few years ago: a few buildings on landfill in NE SF collapsed when the ground they were built on liquified; one segment of one deck of the Oakland Bay Bridge went down, and a stretch of elevated freeway in East Bay collapsed. That was pretty much the extent of it. It could have been far, far worse; none of the large buildings there were even slightly damaged, for instance, and the Golden Gate Bridge came through with nary a scratch. And even the damage to the Bay Bridge was slight considering how large it is. They were able to repair it and put it back into service. That overall reliability of a huge number of structures in the Bay area (well over 99% survival rate) was due to analysis of prior failures; the buildings in SF survived because of studies done of all the buildings which had collapsed in the big one in 1905. The WTC towers used a relatively innovative design where the outer wall of the building was load bearing. As a result it didn't have internal columns and this gave it more useful room inside as well as large open spaces on each floor. It also had a different effect: when the collapse finally came, it went straight down. Given the amount of damage and the ferocity of the fire, a collapse was unavoidable. But if the frame had been designed differently, the building might have leaned to the side and set off a far greater catastrophe: a domino-like sequence of buildings falling into each other and going over. Had that happened, the death toll might have reached a hundred thousand. In retrospect, these buildings did nearly everything you could have hoped that they'd do in the face of intolerable damage, up to dying in a way that didn't destroy their neighbors. It isn't possible to build a large building that cannot be destroyed. But these buildings actually survived the initial insult extremely well, especially considering how much of the external load-bearing structure was destroyed in the initial impact. The remainder took up the load; had the insult been confined to the impact, the buildings would not have come down. It was the fire which did them in; the remainder of the load-bearing steel weakened in the heat, and the impulse from the top part of the structures falling then accordioned the rest of the structure. What lessons will engineers learn from this and what will they do in future? It is, of course, too early to tell. It will take years for the analysis to be completed. Some things stand out already: For buildings the size of these, a 60-minute survival rate in major fire for the steel is not adequate. It needs to be twice or three times that. It probably isn't practical to extend it much further, though. And there probably should be more and better emergency staircases. The ones in the Petronas towers are particularly well built and are maintained at an atmospheric overpressure to keep smoke out, and if it is not already, then that will become standard design practice. The steel structure will probably be beefed up and a bit more redundancy added, so that it can sustain more damage without immediate catastrophic failure. I think that, on analysis, this will not repudiate the basic tube structure used; indeed, the fact that the buildings collapsed straight down instead of falling laterally into neighboring buildings will come to be seen as a plus. (It's been commented on that they came down nearly as cleanly as if they had been deliberately imploded.) On a different note, though, the "Mine is longer than yours" race will probably come to an end soon. Even if someone is willing to build another huge tower, there will be more reluctance by people to rent space in it, and no single com 
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