When I was doing research for our terrorism response plan, one of the things I found out was that the bomb in the last World Trade Center bombing was actually strong enough to bring the building down, and the planners had it figured out fairly well. The problem was that the people who carried out the bombing put the vehicle in the wrong place.
The original plan was to take out the base of one corner of the "cube" that provided the infrastructure support. If the bomb had been parked in the right place, it would have collapsed one tower into the other.
I suspect that the reason the WTC has been targeted multiple times is because the terrorists believed the nature of the structure would allow a complete collapse under the right circumstances.
I did not realize this, and appreciate the insight.