Structural damage from the falling debris of the towers.
This was really insufficient to bring it down afaik
edit: AI says
The National Institute of Standards and Technology (NIST) concluded that the collapse was caused by uncontrolled fires fueled by office furnishings, which led to the failure of a critical interior column and initiated a progressive collapse. This was the first known instance of a tall building collapsing primarily due to fire.
"First known instance"... aka did not happen because they were built to withstand fires
This was really insufficient to bring it down afaik
edit: AI says
"First known instance"... aka did not happen because they were built to withstand fires
It was sufficient to damage the spider legs that supported the rest of the building and when they failed the building came down.
The spider legs were out to the edge of the building unlike normal building which have internal core supports that go into the ground.
This was distributed weight over another building... Completely unorthodox just to save money and keep the other building below.