This weeks issue of Professional Engineering contains a very interesting analysis, based mainly upon work by a Prof. Ed Galea from the University of Greenwich. There are some interesting points in it, since I'd get in trouble if I posted it, here are the bare bones of it: -
- After the 1993 bomb, 52% of people took over 1 hour to exit the building, some took up to 3 hours.
- Emergency services would have been aware of the risk of collapse, but had no reason to expect it to happen so soon and without warning.
- The building structure consists of steel cores with concrete cladding. The cladding would have been destroyed by the impact, and the fire would have roughly halved the strength of the steel.
- The building would probably have withstood the impact alone, it's the fire that almost certainly caused the collapse. The article quotes 24,000 gallons of fuel.
Speaking as an Engineer, I'm unsurprised that the failure mode was rapid once it started, the structure was based upon a stiff outer shell, not a central core. This (like carefully standing on a coke can then tapping it) will fail rapidly and in a fairly symmetric manner. Equally, the heat must have very rapidly reduced the strength of the steel, and more to the point it's stiffness - the Euler buckling load would have come right down very fast.
However, you can hardly blame anybody for not sitting there and working all this out at the time.
G