a reply to:
Bordon81
Plenty of real conspiratorial stuff went on around/during 9/11. The buildings were not brought down with explosives. A basic grasp of physics, and
general understanding of the towers designs are key to understanding their collapse. If you don’t have those two things, you shouldn’t enter into
discussions on the subject.
It is known that structural steel begins to soften around 425°C and loses about half of its strength at 650°C. This is why steel is stress relieved
in this temperature range. However, a 50% loss of strength is still insufficient, by itself, to explain the WTC collapse. The WTC, on this low-wind
day, was likely not stressed more than a third of the design allowable, which is roughly one-fifth of the yield strength of the steel. Even with its
strength halved, the steel could still support two to three times the stresses imposed by a 650°C fire.
The additional problem was distortion of the steel in the fire. The temperature of the fire was not uniform everywhere, and the temperature on the
outside of the box columns was clearly lower than on the side facing the fire. The temperature along the 18 m long joists was certainly not uniform.
Given the thermal expansion of steel, a 150°C temperature difference from one location to another will produce yield-level residual stresses. This
produced distortions in the slender structural steel, which resulted in buckling failures.
Thus, the failure of the steel was due to two factors:
loss of strength due to the temperature of the fire, and loss of structural integrity due to distortion of the steel from the non-uniform temperatures
in the fire.
As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also
fell. The floor below (with its 1,300 ton design capacity) could not support the roughly 45,000 tons of ten floors (or more) above crashing down on
these angle clips. This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of
200 km per hour. If it had been free fall, with no restraint, the collapse would have only taken eight seconds and would have impacted at 300 km/h.
It has been suggested that it was fortunate that the WTC did not tip over onto other buildings surrounding the area. There are several points that
should be made.
First, the building is not solid; it is 95 percent air and, hence, can implode onto itself.
Second, there is no lateral load, even the impact of a speeding aircraft, which is sufficient to move the center of gravity one hundred feet to the
side such that it is not within the base footprint of the structure.
Third, given the near free-fall collapse, there was insufficient time for portions to attain significant lateral velocity.
To summarize all of
these points, a 500,000 ton structure has too much inertia to fall in any direction other than nearly straight down.
It’s simple math. Any “architects and engineers” that throw their hats into the rings of the controlled demolition theory should probably have
their credentials checked by someone of authority before they build anything.