http://www.clemusart.com/exhibcef/mu...ng,%201998.jpg
From this picture, let's say that the botom 4 floors were destroyed by the bomb. The fifth floor would have no support under it, so it would colapse. The sixth floor is connected it that one, so it would follow. Then when the fifth floor hits the ground with the weight of all of the floors above it, would there be a pause, resistance, or slowing down. Or would it be like dropping a bowling ball onto a pop can? Wouldn't the top most floors have some large pieces of them left?
What caused the Oklahoma Fed building to stay up? Is it because the fire wasn't there? Here is the video of the implosion. You can hear the explosives, but I bet there are quiet explosives that just get very hot...
http://www.cnn.com/US/OKC/reporters/.../implosion.mov
There isn't much resistance in the parts that had explosive charges, and the places that didn't get blasted, were slower to topple over.
It looks like it took twice as long for the OK Fed building to come down than from the animated gif movie above. I'm not sure if that is because the buildings were constructed differently or what.