It would not take a lot of explosives to breakaway an entire floor constructed with steel joists like WTC; it is often done in planned demolitions with a small charge placed at each joist support -- customarily a shelf angle either continuous or segmented. What would do it though without explosives is the twisting torque and vertical flexing in the tower tubes caused by an aircraft hitting off center of the tower's axis at 300 mph. Survivors tell of the tower swaying several feet, but they are not likely to have perceived the torque as such. The hit on the South Tower (the second hit) was more off center than that on the North Tower. Tall buildings are designed with some torque resistance against turbulent weather and earthquake but probably not for torque coupled with high impact. The WTC towers were made of two tubes, the exterior wall system (the more flexible) and the interior core (the more rigid), the two tubes connected by light-weight floor structures and at about 1/3 intervals, heavy-weight transfer and mechanical equipment floors.

From examining the wreckage the steel floor joists rested on shelf angles welded to vertical supports. These angles are designed to support vertical loads not much twisting.

The joists could have lost their support by tower twisting of no more than a few inches, the shelf angle welding beads popping, or the joists slipping off the angles that did not break loose (as photos show many did not). If the floor slab concrete was not reinforced it would have crumbled easily under torque. Again, most building structure is designed to resist vertical loading not substantial twisting. (Structural connections are designed to resist relatively minor twisting caused by static and dynamic loading.) There is a valid question of why the buildings did not collapse immediately if floor structures were damaged and serial pancaking set in motion. The raging fires surely contributed to further weakening of the building structure, but probably due to adverse effect on floor structures and in particular on the light-weight shelf-angle welded connections between the floor system and the heavy-weight vertical supports. It is at these connections that demolition designers place light-weight charges, as well as heavy-weight cutting charges on vertical supports. For comparison, a WTC shelf angle may be 1/2 to 3/4 inches thick welded with a bead about the same while a vertical columnon WTC could average 2-3 inches thick, increasing in thickness from top to bottom. The WTC designers claim the buildings were capable of withstanding a 727 hit, asserted at original building and after the 1993 bombing. But the design criteria for that protection have not been published so it cannot be determined what crash scenario was used for design.