Reply #52: Damage assessment from the Silverstein report

[Bolo Boffin]

The team determined that the initial hits destroyed 33 of 59 perimeter columns in the north face of One WTC and 29 of 59 perimeter columns in the south face of Two WTC. Computer analysis showed that the impact of the planes also destroyed or disabled some 20 of 47 columns in the center of the core of One WTC and some five of 47 columns in the southeast corner of the core of Two WTC. The crashes stripped fireproofing from columns in the path of debris created by the planes penetrating the buildings, it continues.

As the fireproof-compromised columns buckled, they compromised the columns they were connected to - the same columns that were suddenly supporting more weight because of the buckling columns around them.

And here's a simply stated explanation at HowStuffWorks.com:

http://people.howstuffworks.com/wtc7.htm

The heat expanded, twisted and buckled the steel support structure, gradually reducing the building's stability. Any number of things could have happened during this period. For example, connections between vertical columns and floor trusses probably broke, dropping sections of floor on lower levels and breaking connections between the core and the perimeter wall, possibly causing columns along the perimeter to buckle outward. Every broken connection or buckled length of steel added to the force acting on connected steel segments, until the entire structure was weakened to the point that it couldn't hold the upper section of the building.

When this happened, the top part of each building collapsed onto the lower part of the building. Essentially, this was like dropping a 20-story building on top of another building. Before the crash, this upper structure exerted a constant downward force -- its weight -- on the superstructure below. Obviously, the lower superstructure was strong enough to support this weight. But when the columns collapsed, the upper part of the building started moving -- the downward force of gravity accelerated it. The momentum of an object -- the quantity of its motion -- is equal to its mass multiplied by its velocity. So when you increase the velocity of an object with a set mass, you increase its momentum. This increases the total force that the object can exert on another object.

When the upper structure of each tower fell down, its velocity -- and therefore its momentum -- increased sharply. This greater momentum resulted in an impact force that exceeded the structural integrity of the columns immediately underneath the destroyed area. Those support columns gave way, and the whole mass fell on the floors even farther down. In this way, the force of the falling building structure broke apart the superstructure underneath, crushing the building from the top, one floor at a time.

As the Silverstein report shows, the floor trusses aren't the factor in collapse initiation that the FEMA report posited. Core column failure was enough to explain collapse initiation.

The page lists two factors for the South Tower falling first. One, the impact was lower in the South Tower, adding that much more weight to the South Tower's upper section. Two, since Flight 175 was travelling faster than Flight 11 on impact, there was greater initial damage to the South Tower, especially to the external tube structure.

No need for explosives - the airplane impacts and resulting fires explain the whole thing.