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After reaching the yield strength, structural steel components continue to have significant reserve capacity, thus allowing for load redistribution to other components that are still in the elastic range.
On September 11, the towers were subjected to in-service live loads, which are considered to be approximately 25 percent of the design live loads.
On September 11, the wind loads were minimal, thus allowing significantly more reserve capacity for the exterior walls (demand on exterior columns was about 1/5 their capacity).
Floor 97 contained about 33,603 cubic meters of air at the moment of collapse. That is a factor of almost 2.5 times the normal volume of air inside floor 97. This finding corroborates the work of another researcher who measured the entire cloud volume created by the WTC 1 collapse, not just of a single floor as I am doing. In his research he estimated the building cloud volume expanded over 3.4 times. Considering analysis of the entire building introduces more error than analyzing the first floor of collapse, his estimates are credible. It begs the question: Where is all this air volume coming from?
Let us pause for a moment. This is simply a phenomenal amount of air volume. To visualize, a floor of the WTC is about 1/3 of a city block. As one researcher put it, you don't need a wall of degrees to be an expert in common sense. Could a gravity collapse somehow explain this? Science says no way. There is no phenomenon in a natural gravity collapse that can account for this large volume of air. It is artificially produced. What phenomenon can produce this amount of air?
It may be argued that air pressure inside the building was higher because of fire. The WTC 1 generally had an open-floor plan. There was a large crash hole. Many windows were broken. This is like a big bag of air with holes in it. If pressure built up in the building where is it going to go? It will follow the path of least resistance and go out the holes. Substantial pressure would not be able to build up.
It may be argued that the additional air volume was created by the core collapsing first. Take two paper plates and push them together. There is only so much air between those plates, it doesn't mater if you push the plates in the middle. There has to be another force, such as intense heat to expand the air to the volume documented.
On September 11, the towers were subjected to in-service live loads, which are considered to be approximately 25 percent of the design live loads.
On September 11, the wind loads were minimal, thus allowing significantly more reserve capacity for the exterior walls (demand on exterior columns was about 1/5 their capacity).
That means the yield strength was 5 times greater than the actual applied load on these columns on 9/11, not considering load redistributions after impact. On a typical day, or on 9/11 specifically, the perimeter columns had a generalized reserve strength ratio of 5.
I'm assuming they're talking gravity loads. Apparently the lateral loads (or lack thereof) affect the capacity for gravity loads? Griff, if you're reading this, does that make sense? I guess it works the same way as any other deflection or buckling causing losses in vertical load-bearing capabilities but I could be wrong.