>>2988169 (lb)

The buildings construction allowed for that type of collapse. The support structure was inside the buildings at the elevator shafts and stairwells.

After one floor collapses they

floor below can not support the added weight.

>>2988208

>>2988209

See the picture on the right, it shows why it fell straight down. The support is in the middle on the buildings not on the outside.

Once a single floor above drops the impact on the floor below shears the beams underneath.

The result is seen in the videos.