Zorba
Lifer
- Oct 22, 1999
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Originally posted by: straightalker
fell into their own footprints
How else would the buildings collapse? Why would it fall sidewise, or any other way? Gravity acts directly downward so what force would pull the building any where except in their own foot print?
at freefall speed within what?
Very simple impact dynamics. If you drop an object from an infinitesimal height the force of its impact will be twice its weight. Therefore, dropping an object from any higher than a very small height will create a load of more than twice the weight of the object. A typical factor of safety on a building is less than 1.5. Therefore as soon as a large part of the building falls on another part the structure is going to fail instantaneously upon impact.
When in the past 100 years not one skyscraper has ever collapsed like that due to fire. Let's do the math. 100 years none. 1 day 3.
How many of the other skyscraper fires where combined with MASSIVE structural damage? None! When you knock out structural support the load isn't distributed evenly over the remaining supports. You end up with beams that are carrying far more load than they were ever designed to carry, then you anneal and soften them with fire making the situation worse.
You also have the effect of thermal expansion. If a beam is heated, it expands. If there is something keeping it from expanding it adds massive stresses to the beam. For example take the floor trusses into account. In the WTC they linked the outer structure to the core. These trusses heat up, expand, and press outwards on the outside columns, adding a massive bending moment to the columns. The trusses also add buckling nodes to the columns, you remove/weaken those floor trusses and the outer columns are much more likely to buckle.
Next, let?s consider how professional demolishing actually works. They knock out much of the structural support on a handful of floors, causing the building to fall on top of itself, within its own foot print at free fall speed.
At the WTC a lot of the structure was knocked out by the aircraft and the remaining support couldn't handle the load while being weakened by fire. Causing a couple floors to fail, which resulted in a chain reaction on every floor impacted by the falling mass.
