The WTC was actually designed to survive the energy involved in a 707 sized aircraft strike. An airliner had previously hit a skyscraper in crappy weather so while a terrorist attack was not envisioned, an aircraft crashing into the WTC was an anticipated event.Notso_Ken:Personally, I was amazed it took as long as it did for a 767 to bring down a building. The kinetic energy in a 300,000 pound airplane traveling at 300-400 mph is quite high. When I saw it on TV, I was amazed the buildings had not collapsed right away.
The engineers had the impact forces figured properly, but were they got it wrong was that they did not take into account the jet fuel on board the aircraft and the effects of the heat of the resulting fire on the structure. This oversight was due in part to failing to consider the possibility of the fire mains being severed in the impact allowing the fire to burn more or less unabated. Grouping all the elevators, ducts and pipes in the center was space efficient but it put all the eggs in one basket so to speak.