I doubt that the fire would be hot enough to melt steel, but even at 1300F the strength of steel is seriously degraded. These structural steels used in civil engineering are not intended for high temperature. The fire weakened the steel, and the upper floors crashed into the lower floors and the structure pancaked. I agree with your conclusion. As a former firefighter and fire investigator, I have seen what a "normal fire" will do to the structural steel in a building. Having the intense heat of hundreds of gallons of jet fuel burning in an enclosed space will generate a tremendous amount of heat in a short period of time. The structural members on the interior of the building were exposed to the highest temperatures. Once enough of the interior supporting members failed, the sudden weight of the floors above collapsing would pancake the building. No additional explosives would have been needed.
Can a stronger steel be used, or one more fire resistant? How about some combination of titanium/stronger steel for some of the beam and regular steel for others? I know its not possible to plan for everything, nor is it cost efficient. It seems as though these days it may be prudent to spend a little more if you want to build a landmark building.
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