OK so with jet fuel by itself is not hot enough. What about other combustibles in the building. The materials/stuff on each floor can burn. Now I don’t know if the buildings had any natural gas lines threw them but lets say they did. Natural gas burns around 3000F +/-100 degrees F or 1648.8888888888c. Lets say it burns at 3100F that’s comes out to 1704.444444C witch surpasses your 1688 C. Witch mean melted titanium. This is of course of the buildings had gas lines running threw them.
The point im trying to make is there wer other materials burning in the building besides jet fuel that could have gotten hot enough to make the buidling fall.
http://www.newton.dep.anl.gov/newton/askasci/1995/eng/ENG49.HTM
http://www.wbuf.noaa.gov/tempfc.htm