Excerpts from a discussion on the architectures involved FWIW, kendall:

Very tall buildings are hard to build, not only for the obvious strength and safety reasons, but also because needed services (i.e. elevators, plumbing, wiring, stairs, air-conditioning ducts) require increasing amounts of overhead as the building gets taller. In addition to bigger beams and columns, tall buildings need more elevators, bigger pipes, etc. than short campus-style buildings. As building height increases, these overhead items grow faster more quickly than the usable floor space, making very tall buildings hard to justify economically. Making very tall buildings make business sense requires a significant architectural and engineering effort.

The World Trade Centers existed because of their architecture and the supporting engineering. Unlike other skyscrapers, the exterior walls of the World Trade Centers were load bearing. [See for a brief discussion]. The whole building was a vertical truss, and the interior was column free. Without this design, it is unlikely that the WTC could have been built on that site. The architecture enabled the existence of the building.

As we saw in the painful to watch news footage of September 11, 2001, when the load bearing walls were damaged, the whole structure became unstable and collapsed.

Simply telling the engineers to make the building stronger is not a viable answer. Of course they could make it stronger, add redundancy, or both. But at the cost not only of the material and labor to add the strength, but at the cost of substantially increasing the overhead of the building itself. If the building's internal overhead becomes larger, the economics of the project quickly disappear. There are hard numbers of dollars behind the decisions not to build buildings bigger than the Empire State Building up until the WTC towers, and if the WTC architecture is not feasible or acceptable, then the density it enables will not be possible.

The implications for the city are huge. Successful public transportation requires that large numbers of commuters go to the same place at about the same time. Similarly, the density of people is what enables the wonderful shops, markets, theaters and clubs of New York. The lack of density is a direct cause of the decline in quality of life.

The attacks on the World Trade Centers caused a horrible loss of individual human lives today, and there is a significant chance it will cause a significant loss of life of the city in the future.

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Date: Tue, 11 Sep 2001 13:25:30 -0700 From: John Young

The '45 Empire State Building crash is oft studied in architectural and structural engineering to learn why the building withstood the hit. The plane was a B-24, I believe, but in any case a much smaller craft than the ones which hit the WTC and the Pentagon. The '45 plane's engines did penetrate the building, shooting out the far side and falling to the ground and killing passersby, but most of the plane remained inside the structure for it was made of far more fragile materials than a building. A relative small amount of damage was done to the structure of the building though fire was devastating, especially from flaming gasoline cascading inside.

The fireball that shot from the second WTC tower hit, opposite where the jetliner penetrated, blew out windows and perhaps part of the latticework exterior structure. Flaming fuel probably cascaded down the shafts of elevators and ductwork and stairwells whose fire-protection enclosures would have been destroyed by the explosive crash and ballistic heavy plane parts. These fuel flames, and fires started from them, would have weakened interior structural support beyond protection provided by code-required fireproofing. Once the interior structural supports were weakened, and the exterior lattice lost its integrity collapse was inevitable.

I modify my first evaluation to speculate that the interior supports appear to have given way before the exterior lattice (whose girdle of closely-space columns and thin vertical windows between gave the buildings a unique look compared to use of large panes of glass elswhere) The lattice amazingly contained the interior collapse and the whole mess dropped vertically, almost, as newscasters report, as if executed by a demo expert.

I did not expect the Twin Towers to collapse. To suffer terrible fires and localized interior damage but not total collapse. The first was unbelievable, and as I said, I thought only the portion above the crash fell. Then the smoke cleared momentarily to show the totality. Then the second tower, collapsing in a near-perfect copy of the first. The sudden dropping of the floors above the crash, that impacting load overpowering the remaining system, and the straight drop collapse, neither tower falling much to the side, indicated what had happened.

Close-ups of the exterior show the latticework bridging the crash penetrations, reminding of sales pitches from the 19th Century when cast-iron manufacturers promoted their architecture with structural compoments missing with no apparent destabilization -- the load automatically shifting to remaining components. Their prognostications failed at the first intense fire which overheated and cracked the cast iron, sometimes collapsing more quickly than predecessor masonry bearing wall and wood floor system composites.