What Happened to the World Trade Centere Towers?

Jet fuel burns at around 21 hundred degrees fahrenheit, while steel doesn't melt unti 28 hundred. Steel begins to lose its strength at around 750 degrees. By the time it reaches 11 hundred degrees, it's lost about 50 % of its strength. And if it reaches 18 hundred degrees, which is cooler than what jet fuel burns at,. It retains only ten% of its strength after two hours. The argument here is that as the buildings collapsed, the lower floors should have put up resistance, slowing the fall of the buildings.