they were built to be the strongest buildings. thats why they have a steel exo-skeletal design. have a quick look at this site quite interesting http://www.liveleak.com/view?i=069_1180900563
But not strong enough to take the velocity of an acre of steel and concrete crashing down onto one floor after another, which as I stated got to 120mph. Even the strongest steel will eventually buckle and give way under heat and pressure, but add velocity as well and nothing will stop the collaspe.
The term "the straw that broke the camel's back" comes to mind!

Velocity indeed is a very powerful force, and on steam railway engines, to add another completely unusual angle to the argument, the clack valve that allows water, at atmospheric pressure, from the tender into the boiler, which can be at 250lb's psi, is done by mixing low steam pressure with the water while it passes through the cones of an injector.
This adds velocity to the water, that is not under anything like 250lb's psi, to do the seemingly impossible in science and lift the clack valve to allow the water in against the massive boiler pressure.

In the case of the twin towers, architects have proved that this type of velocity, let alone the assistance of the weight of the floors would have taken out the whole building at the speed it was.
