Any building that is taller than a 10 stories has to have a differnt design. Building under 10 stories have a static load that is supported by strong
columns. Tall building have a dynamic load system, this allows them to transfer the load to the ground. This is explained in the "How Stuff Works:
Skyscraper" link that I posted.
You have to keep in mind all of the evnets, and remember that each played a part on the destruction of the system.
First, you have two 400,000 pound aircraft that act as giant hammers, hitting the large structures, and that forces was sent into the entire
structural system, causing damage to most of the support structure.
The fire alone might not have been enough to weaken the strucutre, but if it was shattered, cracked or damaged due to a high impact from a 400,000
aircraft, then the there might have been heat expanstion in cracks that normally that do not exist. Heat & Fire resistance coating that was more than
30 years old may have been knocked free off and away from the surface it was to protect.
Then when the initial failure of the first floor occured, due to the 400,000 aircraft knocking out the support columns, and the buring jet fuel, each
colapse of every floor was another shock to the weak system. Each drop of every floor another few 100 tons slamming and shocking the structure.
These shock wave tranfer throughtout the entire frame, and into the foundation that is connected to the ground. Just as sound travel better through a
solid than air, these constant shockwaves were disperced to the surrounding buildings. Right after 9/11 there were several building that were kept
evacuated until there were inspected. This is due to the transference of engery from two aircraft slamming inot two buildings, and several hundred
floors falling ot the ground.
Ever play with those ball bearings on a string? You move one and watch the last one fly up as the otehr two remain still? That is not magic, it is
the transfer of energy throught those ball bearings. All of the arguments for the explosives do not take into consideration the system of forces upon
the building.
Everyone talks about the fire and just the fire, or the law of free falling objects. Yes, the fall time should be slower, if the system was not
already weak from a 400,000 aircraft shocking the system. Yes, otehr bulding with fire do not fall down. Yes buiding 7 was not hit by a plane but
that does not mean it was not effected by the shockwave.
The same goes for the images of the hole that do not fit a plane. You have to remeber that the plane is not a solid metal object, it is a series of
thin metal frames with a thin metal frame. It is designed similat to a skyscraper. This is due to the forces of wind on anr aircraft as it is in the
air. It has to bend and move to distribute the load. Since an aircraft is a high drag tube, with high lift wings, which provide all of the lift,
often the first thing to snap off is the tail or wings due to high stress.
Here is a NOVA interview With Dr. S. Sunder of NIST
NOVA Interview
NOVA Website