For Gods sake, the fire did not have to melt the steel, heat exposure over a period of time weakens steel but that wasn't the only factor that lead into failure.
Heat also causes expansion of steel, the steel beams expanded inducing thermal stresses that added to the stress placed on the steel from the weight of the building. The stresses were increased due to a significant number of the supports being taken out by an aircraft travelling 600 mph so there were less beams supporting the load.
Here is a chart showing the strength of steel as it gets hotter, note this chart is only for 30 minutes exposure to the heat, it gets weaker the longer it is exposed to the heat. It is from MIL-HBK-5 which was the standard for aerospace metal properties, it has now been commercialized and called MMPDS, but you can find another source for this info if you like such as Marks Standard Handbook for Mechanical Engineering or probably some civil engineering reference (sorry not a civil engineer).
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