"Prolonged heating can get steel to its melting point.
But what's important is the mode of failure of a particular structure.
The towers in question had an inner concrete core which carried the bulk of the vertical loads, and an outer steel frame which are good for resisting lateral loads - like wind or earthquake.
When the plane struck the building, one side of the outer steel frame of 3 or 4 stories was knocked out.
No problem, the building 'stood', as expected.
It is possible for the concrete core to lose strength with prolonged heat at the affected levels.
But it is highly unlikely for the whole core to crumble vertically straight downwards, as had happened over there.
The upper part of the structure should have 'tumbled over' laterally, with the lower part remaining relatively unscathed."
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