steel is a crystalline matrix so when you spread the matrix out with heat it gets weak fast.
ASCE: Books
anyone ever seen a steel cable snap? i have, it's quick - one second the truck is coming out of the mud and the next moment the cable is just gone. good thing you weren't standing there! does anyone really expect huge towers to gracefully sag their way to the ground like they're taking a nap???
due to impact we have fewer core supports. due to fire we are constantly weakening those remaining members that were never intended to have this increased load.
all it takes is one or two of the secondary supports to give and that's going to create enough momentum to let the whole level drop down AND pull the ones above it. once that level hits the next you've doubled your total "falling" mass. remember E=mc^2?? by doubling the mass you have more than squared the energy, not just doubled it!! there is no building in the world that could put the brakes on 15 or 20% of its own mass coming down on it. for each level that initially slipped 12 feet you're giving the levels below many many many times more energy/weight than it was made to take. keep in mind that this happened in the blink of an eye. one or two supports gave instantly, the mass was so large and there were so many secondary supports that it took a little while for the rest of the building to catch up but once the velocity was introduced to core supports it was all over.
also, those buildings were cheaply made. there could have been inferior steel involved, they might have been testing some kind of high strength concrete (known to be weaker than normal concrete when heated) during construction, who knows/cares???
the real problem is this: OUR GOVERNMENT HAD INFO THAT THIS WAS GOING TO HAPPEN!
it happened and the government response was to increase bureaucracy, increase the layers or agencies, increase the spending!
we would demand a refund if we bought this shit-pile government at a store!
they don't get it!
if there was any steel that looked like it had been cut someone would have grabbed a piece or snapped a pic and it would be on full public display. think of all the people sifting through that pile, all the engineers looking at the rubble. there was no blast, there was no demolition. seriously, just get mad at the obvious and real tragedy about our government's failure.. that should be enough to get you through to a very old age.
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