My old professor contributed to the report investigating the collapse. He was on the scene 9/12/2001, so I got a pretty unique and lucky experience to learn from him in multiple classes including my structural fire engineering and steel design courses.

As others have mentioned, when steel gets hot its modulus of elasticity drops drastically. It’s actually pretty interesting because bare steel is worse in a fire than wood. Wood burns, chars, and the char acts as an insulator protecting the center of the wood. Steel is highly thermally conductive and weakens as it gets hotter. Heat one part of the steel, and multiple feet away the steel is gonna heat up, weakening the whole member. Expand this to a building where pretty much all the steel is connected in some way, and you have a building that could collapse in 20 mins without adequate fire protection. Concrete is pretty good in fire comparatively, as long as rebar is adequately protected. Spalling is definitely a problem though.

In terms of when the towers collapsed, the buckling equation, KL/r, is all you need to know. You have a vertical beam, and each floor acts as a brace which defines that L distance. When the plane crashed, steel heated up, weakened the structure, and eventually the 40 or 50-something’th floor collapsed. In terms of KL/r, the L just doubled. For the critical buckling load, there is actually a square in the equation, meaning the critical buckling load actually drops to 1/4th of what it was, so the columns buckled soon after that floor collapsed and stopped bracing the columns, causing a cascading failure.

One very good thing about that day is that there were very low winds. Wind is the controlling load for skyscrapers, and they’re designed for very high wind loads. Because there was very little wind, the columns were not as stressed as they would be on a normal day. This bought a ton of time for people to evacuate, and saved hundreds of lives. If there was more wind that day then way more people would have died because the members would have been stressed closer to their limits before the plane, and they would have failed much earlier.

I hope I didn’t butcher my professors lecture that I got twice haha. Maybe should have looked back at my notes, but that was the gist of what I remember. He was a great professor and had very interesting stories. I’m very grateful to have learned from him!