>>1176114802 main things happen when steel is heated.
It becomes more ductile and it expands.
The beams pushed outward against supporting concrete columns and began to sag.
Concrete is good against compressive forces but less so against shear forces and has terrible tensile strength.
That's why its reinforced by steel. But severe impacts can cause the concrete to fracture leaving gaps of exposed metal.
Which expands and starts to buckle, further cracking the concrete.
People forget that it took an hour for one tower and over an hour and a half for the other tower to collapse.
During all that time, it's not just fuel that's burning, its the contents of multiple floors all ablaze.
Since heat rises, the holes from the airplanes acted as the oxygen supply and the floors above accumulated heat with limited "exhaust" ventilation to release the hot gasses.
Flashovers can occur at high temperatures when the byproducts of combustion become ignited themselves, causing multiple localized structural failures.
When the collapse started, people say it pancaked, but it's more like a vertical domino effect.
Gravity assisted impacts.
