Comment: The flame was confined, and

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In reply to comment: With massive bellows (see in situ)

The flame was confined, and

The flame was confined, and the windows at that height were the perfect bellows.... I'd GUESS.

The steel didn't need to melt, just weaken.

I don't know how hot jet fuel burns in open air but I'd guess hotter than coal, which can melt steel... I think around 1500 F.

Thermite is aluminum and rust. It works by acting as an accelerant. It has an extremely hot flash point.