Actually, I believe that what happened is called a "Cascade Failure." It's similar to that concrete walkway collapse that occurred in Kansas City about 10 years ago. Once the top floor collapsed onto the one below it, the weight just kept on gaining as it hit each subsequent floor, like dominoes falling over. The effect was the same you see when demolition contractors implode a building. They study the statics of the building to see which members are key and need to be taken out. I'm amazed that the building imploded, as opposed to falling over, which would have resulted in many more casualties.
What many people also don't understand is that steel structural members heat up much like your stove burners and the heat transfer which occurs is devastating. As each member heats up, it ignites flammables such as wood which are in proximity to it....
Now, does anyone know all the businesses that were housed in these buildings?
Now, does anyone know all the businesses that were housed in these buildings?
The only cascading failure in Kansas City was horizontal, as the supporting-rod/box-beam joints failed and caused the load they'd supported to be transfered to other joints on the same level which failed in turn.
Unlike a "pancake" collapse where the lower level supports the upper level, and the collapse of the other level hits the lower level with an unsurvivable dynamic load, the balconies in Kansas City were all supported from above, with the upper balcony directly supporting the lower balcony [note: this represented a change from the original plan]. The joints connecting the upper balcony to the rods from above could not withstand the combined wait of both balconies so the upper balcony fell. The lower balcony fell not because the upper balcony hit it, but because there was no longer anything supporting it.