That is what I said and agree with the experts, but fuel will not cause steel to melt in a normal fire because it doesn't get hot enough.
Steel doesn't have to melt to loose so much strength as to be not able to support a weight. Blacksmiths have known this for centuries. After all, it's part of how they work.
The bottom floors way below the fire collapsed, the elevator shaft columns were holding up the entire building, but collapsed and melted from the walls falling down? At no time in history has a metal building collapsed from a fire.
-1997, three 4-story buildings at the Kader Toy Factory in Singapore caught fire. All three collapsed from fire alone under two hours.
-2000, the Dogwood Elementary School in Virginia caught fire, resulting in the fire-affected sections collapsing in under half an hour.
-2005, fires broke out on the steel-framed Mumbai High North Platform, causing it to completely collapsed in two hours.
-2005, the top 11 floors of the Windsor Building in Madrid, the only part of the building that was steel-framed, was the only part to collapse and from fire alone (except the concrete inner core).
-April 2007, sections of Interstate 580 collapsed from fire alone.
The fact that fire can, indeed, collapse steel-framed structures (as demonstrated most dramatically in the WTC) is the reason why the World Financial Center, the new
WTC7, and the Freedom Tower are being designed with a concrete core and concrete-engased outer columns. All of these structural engineers are putting their money where their mouth is.
I know and explainned how bar joists can weaken and slip off their mountings like the experts say, but there is no way steel columns can collapse that hold up an entire building that were not even in a fire.
You do know that fire spreads, right? Especially when there's no sprinkler system to douse them (because of cut water mains).
Also, when the fire-heated parts collapse, the load above goes from dead to live load, which is significantly harder to deal with than a static load that the columns were actually designed to support. The potential energy of the towers were enormous, and as towers get higher, the relative structural strength of the materials shrink.