All materials weaken with increasing temperature and steel is no exception. Strength loss for steel is generally accepted to begin at about 300ºC and increases rapidly after 400ºC, by 550ºC steel retains about 60% of its room temperature yield strength. This is usually considered to be the failure temperature for structural steel.
Now think of the damage done by a plane moving at 500 MPH+ into a building with no internal structural pillars except for the inner core (for elevators, stairs, etc. The structure was carried on the outer skin of the buildings. Add to that a fire fueled by a plane full of fuel and the contents of the building. Now add the immense weight on top of this damaged area.
I was just at Dkos and even tho I see a big ad promoting the Climate Rally, I do not see much else on Dkos concerning this event.
Also.... explain this to me, why did we have wall to wall coverage of the tea baggers in DC and even heavy promotion of it by faux news.... but I have heard NOTHING on the news about the Climate Rally that is happening NOW in DC.