9/11 fire-fighters: explosives in building, page-20

"Fire at the 60th floor does not buckle the supporting beams for every floor in a building at the same time"

North and south tower collapsed upon itself from the top down, caused by the weight of the buildings above where the planes hit overcoming the ability of the heat damaged steel to support what was above. The reason why the south tower, hit well after the north tower, collapsed first was that the plane hit the south tower lower down, so there was more weight above the damaged area, resulting in the structure failing sooner.

Explosives cutting structural supports near ground level would have caused the building to collapse from the bottom up, which didn't happen. Furthermore, unless expertly placed, cutting the structural supports near ground level would have (more than likely) caused the building to topple, not collapse in upon itself.