Question

At what temperature do steel beams melt, And what temperature is jet fuel when exposed to fire. The jet fuel temperature is not enough to melt steel beams, So how did the twin towers fall.

Answer 1

Steel beams have different grades but in general they melt around 2500F. Jet fuel burns at 800° to 1500°F, not hot enough to melt steel. But the prolong exposure of steel beams to the high temperature fire from jet fuel weakened them. With the beams weakened, the buildings collapsed eventually.

Answer 2

When burning, Yes.

A normal fire in a steel-frame building can soften the structure to the point of collapse.


Building laws / regulations require that all the structural steelwork is either covered in a protective coating (such as intumescent paint) or boxed in with fire-resistant plaster, so the fire can be extinguished before the building is weakened - or at least give people time to get out.

If that coating or plaster is damaged by impact or an explosion, the steel is exposed and the building can collapse relatively quickly.

(The common intumescent coatings just look like paint until exposed to fire, so the steelwork may appear to have no particular protection - but it always does).

Information:
http://www.steelconstruction.info/Fire_p...

See the images below - small buildings with steel-frame roofs after fires; you can see the amount of "sagging" and distortion on structures that have no particularly high loads.
These have burned long enough to destroy any protection, or they did not have any as the structure does not support occupied space.

http://www.champnews.com/Picture_Library...
http://thelincolnite.co.uk/wp-content/up...