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Common sense suggests that steel beams should not yield under burning jet fuel without presence of other substances that produce very high temperatures when burning, such as thermite. So can jet fuel melt steel beams?

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Given adequate oxygen, certainly. From here, for instance, you can get an approximate maximum flame temperature for kerosene burning in air, and a higher concentration of oxygen will increase the temperature. At 3800 F, this is about 1000 F above the melting point of steel, so melting steel with jet fuel (kerosene) is entirely possible.

Of course, "Common sense suggests that steel beams should not yield" suggests that you've been visiting 9-11 conspiracy sites, and it's important that you realize that melting beams is not remotely required to bring down a building. All you need to do is a) destroy some of the existing beams in order to increase the load on the survivors, and b) heat some of the survivors enough to reduce their strength to the point that they can no longer support their load. When some start to deform, this will throw extra load on the others, and a chain reaction of failure is certainly possible. And steel will certainly lose strength at high temperatures - typically about 50% at 1000 F, and 75% at 1100 F.

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    $\begingroup$ Thanks a lot, that's very interesting, I will think about it. $\endgroup$ – Max Ruuli Sep 11 '15 at 14:27
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    $\begingroup$ Check out warehouse fires. Steel turns to taffy in even a modest fire. 911 conspiracy nutjobs are forever telling lies about stuff like this. $\endgroup$ – John Duffield Sep 13 '15 at 15:17
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In addition to WhatRoughBeast's excellent answer let me also debunk the 'Thermite myth' that's so pervasive in 9/11 conspiratorial thinking.

This misconception that burning Thermite could cause steel beams to melt is based on a poor comprehension of Heat Transfer. Adherents of the Thermite thesis start from the correct knowledge that burning Thermite mixtures can reach temperatures in excess of 2,500 centigrade, well above the melting point of steel.

Unfortunately for their claims, that is only half of the story.

Thermite mixtures typically release an amount of heat energy (Enthalpy) that if expressed in $kJ/kg$ (kilojoule per kilogram of Thermite mixture) is always the same. In order for the burning Thermite to heat up the steel beam to above its melting point heat now must flow from the burning Thermite to the colder steel beam.

Much of the heat generated by the burning Thermite however is lost to radiative losses (light and infra-red radiation), convective losses and other losses. Only a small part of the heat energy released per kilogram of the Thermite mixture actually causes the beam to heat up.

The consequence is that impractically large amounts of Thermite mixture would be needed to reach the melting point of the beam. This is what both theory and empirical tests show very clearly.

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