the planes hit it right on the spot!

the heat of burning jet fuel had heated the steel to collapsing point. the steel insulations had helped in prolonging the time for the steel to resist heat but the jet fuel heat intensity is way too high compared to normal building fires.

the weakening due to fire induced chemical alteration of the steel combined with the buckling stress induced by the upper floors' weight had caused the columns in direct contact to flame to collapse.

and the rest is history now..... :(