Originally posted by GenRadek
Did you ever once stop and think what was meant when it was said of the WTC that they were tube-in-tube designs? That means it was built without a conventional box-frame skeleton as you would find in say, the Empire State Building, or other conventionally buildings and highrises. Those have skeleton frames of steel I-beams connected in box shapes giving rigid vertical and horizontal stability. Floors are laid on the I-beams and the whole things stands up pretty solidly.

The WTCs were tube-in-tube. Rather than a whole skeleton of I-beams, they used box-columns for an interior core of vertical columns arranged in a box shape. This would give the core for the stairs and elevator shafts and have the floor trusses something to attach to. The exterior involved another set of columns, however these were arranged in a puzzle like set, with column trees interconnected. These formed an exterior that is more flexible. Each column tree had three small columns connected together by three larger flat plates. Then each tree was interconnected in a staggered pattern, and then bolted together at each end. The floors consisted of light steel trusses, similar to those used in warehouse roofs. These were connected at each end to the exterior and interior columns with 1" and 5/8" bolts as well as dampeners on the exterior columns, to allow for flexibility when the wind blows. The floors had a pan of light concrete poured out and these were placed on the trusses.

When the tower began to collapse, the top 30+ floors tilted as one unit, while the floors where the failure began failed and sagged down. One side stayed intact while the other sagged down and began the tilt and collapse. Pretty soon the floors below began to collapse downward, and the tilting section began to plummet down telescoping into the rest of itself. The floors just sheared away as the connections severed from the incredible amount of force above.

Well look at that. I honestly did not even see this post. I thought I was just brushed off completely.

Ok, I have read this entire answer now and it only leaves me with one question.

Would you mind explaining what aspect of the building's construction would cause that tilt to correct and the vector to turn straight down?

Originally posted by K J Gunderson

Well look at that. I honestly did not even see this post. I thought I was just brushed off completely.

Ok, I have read this entire answer now and it only leaves me with one question.

Would you mind explaining what aspect of the building's construction would cause that tilt to correct and the vector to turn straight down?

No sir I try to be prompt and answer the questions as best to my knowledge. No harm no foul. Its easy to overlook things.


To answer your question, that would be the tube design itself. When one end failed and buckled in on itself, the other end was still connected. Thats why it tilted. What caused it to stop tilting and start falling? When the hinge broke. Once the hinge failed, gravity took over and down it went. It would not have continued to tilt over as it was going down.

You must remember, there was no external force pushing agianst the side of the building when it tilted over. It tilted into itself from gravity alone. Once the hinge broke, it just went down for the ride. The floors below have already started to collapse and "pancake" down. (note: the floors did pancake, but they were not the initiators of the collapse.) The core columns would have snapped apart at the connections and down it went.