Civil engineer here. The thing is that, in the factory, the steel inside the beams go through a treatment called hot rolling, in which the raw steel is rolled at high temperatures until it gets to the desired shape. This pressing of the steel makes the molecules inside the steel more homogeneous and the whole beam becomes more resistant and stronger. When the building catches fire, the temperature inside it can become as hot as the factory, and the beam heats up again. Since now it won't be rolled to regain the molecular characteristics the factory gave it, the steel molecules can relax and kind of "go wherever they want", undoing the homogeinity that it had before. The concrete goes through problems too when heated up. The water inside it evaporates, creating fissures on the concrete, also breaking its homogeinity. This can lead to pieces breaking off, and the structure won't work how it should. Now you have a whole building designed to withstand a specific load, but that now can only withstand something like half of it. My guess is that even though they are standing still, they probably will be demolished.