The steel didn't melt. It was weakened enough to the point that the steel was softened up.. not melted. Around 1000'F it would have lost ruffly 50% strength. Steel is very conductive to electricity and heat. So an area around the support beams on fire with jet fuel would be absorbed by the beams. No 2 towers are built like they were with an internal frame support. Weight from above would have been enough to stress the weakened steel, then kinetic energy would do the rest. And do we really know if every beam was UL tested? Contractors do like to cut corners if it saves them more money. Look at Bostons "Big dig" and the incident that happened in the tunnel this last summer. Paneling fell off from a tunnel and crushed a car. People were cheap and didn't assemble it correctly.