Well with a building that is already damage it would not take much.
Such as a couple beams cut and small kicker charges placed.
But that is just 1 of several ways to do it.
Behold, the full retreat of the 9/11 truth movement.
Instead of a building made of kryptonite that could withstand everything, and needed to have a full blown controlled demo done with all the CD bells and whistles, we now have a theory that the building was almost completely gone and only a small push was required to send it over the edge.
Ultimas theory predicts a collapse that would be
completely indistinguishable from an otherwise "natural" fire and impact induced collapse.