look @ the rubble, u cant c floors stacked on eachother..hell u cant even find a computer in it..not just that if a building started 2 collapse you need momentum 2 keep it coming down(hence an explosion)..so even if a few floors did collapse(still not possible cuz out of 47 inner core columns only 10 were gone and the building can stand alone on just half)the collapse will cause the building to lose energy going down in which it will stop falling & only bombs can cause the momentum 2 increase.