FACT: Jet Fuel Burns at 1400F WHEN ITS BEING BURNED IN A JET ENGINE!! otherwise in free air it burns at around 500 You can get every scientist on earth to agree on that one, it cannot be disputed. The reason it burns hotter in the jet engine is because of this little thing called Stoichiometric efficiency....
http://en.wikipedia.org/wiki/Air-fuel_ratio and the fact that air is being compressed at about 4.5 to 1 before having fuel applied. So at 500 degrees, the steel is experiencing 0 loss of strength. Lets also not forget that the beams are all completely covered with one of the beast heat insulators known to man...Asbestos. There are posts here of thermal images taken while the buildings burned. None of those fires got hot enough, there were no 1800 degree pockets.
This is certainly true for REINFORCED CONCRETE, unfortunately for you they did not use any of that for structural use in these buildings, They were made of steel walls, a MASSIVE steel core and steel floors, in essence a GIANT heat sink, ready to dissipate every floor a firey inferno's heat. Just like all the other steel framed buildings do when fire completely and totally engulfing every single floor to the point where it is not even distiguishable as a building, none of them fell, not even the ones that burned for 10 times longer than the towers did.