Discovery channel program on reasons the towers fell as quickly as they did yesterday was outstanding.
They provided detailed analysis from structural engineers who worked on the building and even displayed the original architecture documents. The WTC towers weren't constructed using standard building practices at the time.
They did a lot of unorthodox things to make the project work.
What many believe now is that the trus system that they used to construct the floors was easily compromised by the impact of the jet and fire. An MIT professor did a demonstration using a model. Showed how if any area of the trus system didn't have fire protection as the fire burned, it would weaken greatly. They used this trus system because a lot of the prefab could be done on the ground and the sections of flooring put into place by cranes. They also used this system because due to the elevator system implemented, the trus system allowed much more open floor space on each floor.
They also showed how the stairways were built too close to each other and provided no redunancy and thats why all the people above where the planes impacted couldn't get out.
This design is considered a major no-no by todays standards.
Until I watched those shows on National Geographic, I didn't know that much information was available about the murderers who skyjacked the planes on 9-11 and murdered those people. I thought I already knew all that there was to know about 9-11. Those shows on NGC took me to school.