It has been said that the location of the explosion, on the fifth floor from the top of the building, was critical to the collapse. If it had been much higher, the lesser momentum of the debris falling onto the floor of the explosion flat would not have caused its collapse, even though it might have sustained some damage, and the progressive collapse of the floors beneath would therefore not have occurred. If it had been much lower, the friction of the joints, under the great weight of the building above, would have prevented the wall panels from blowing out, and there would have been no structural damage at all. The reason that an explosion lower down the block would not have affected the load walls is that the combined mass of all the load above it creates force downwards due to gravity that would have required far greater lateral forces to create catastrophic structural failure and there are specific equilibrium equations used by civil engineers to calculate such forces. The principal layman explanation is this: the lateral force required to blow out the upper floor walls is in linear proportion to the vertical load above multiplied by the lateral frictional resistance, and Mohr's circle calculations bear this out.