re: Explain what caused the towers to collspse.

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to evenly weaken the whole top, to make it drop straight down,
What fails first? Columns or beams?

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Here’s how the columns work. Each column spans two floors, as that’s the limit to trucking. Each column is attached at three points- top (where two columns meet at top floor), mid column, and bottom (where two columns meet bottom floor). The joint in the middle failed due to heat stress after the intumescent foam was blown away by impact, and the column failed there. The column , at that time, was reduced to 25% of its original load bearing capacity.

Take a four foot metal ruler, and grab it tightly top and bottom. Have a second person grab it at midpoint, and hold on tightly. Now have a third person try to wiggle the column, and note it’s flexure.

Now repeat the process without the person holding the column midpoint, and note it’s flexure. That’s what happened to WTC - the columns failed. The beams were just dead load at that point, no more of a contributor than the concrete deck.