re: Explain what caused the towers to collspse.

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At that point, you might ask “ why didn’t the building collapse down to the point where this condition (joint failure at center column point) then stop?” And the answer to that is simple - the columns were designed to carry static loads, not dynamic. If you stood on my shoulders, my knees are designed to support your weight, if you jump off the roof onto my shoulders, my knees fail. Hope this helps.

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This

The simplest way I could explain it..place 10lb weight on a coke can..it will support it. Now take the weight and hold it 1ft above the can, then drop it. The Can will get crushed flat.

When the beams gave, the 20+ floors above came straight down and flattened the floors below the impact zone.