The Twin Towers were skscrapers, a very tall building, which have to withstand heavy winds and crazy temperatures. This means lots of thought definetly went into the structural design of the World Trade Center.
Engineers worked for years working towards the structural integrity of the Twin Towers in order to make sure specifically that in case of emergency, it could even withstand a plane crashing into it. The interior of the original buildings had a core filled with thick steel pipes to ensure that it wouldn't topple over by air currents or harsh weather. The exteriors were built with thousands of intertwining pieces of metal to create an insanely strong structure. Weak architectural structures for a governmental building is highly unlikely because the plans would have been made by a few hundred professional engineers and looked over by several others and they would not have made the same mistake twice.
There have also been video recordings from news stations and channels of small implosions coming out the windows of each floor in one of the Twin Towers a few seconds before it would continue with the pancake collapse. The other Twin Tower collapsed inwards from the center which perfectly resembles the demolition of a building. This means that the collapse was planned.
In addition to this, the workers who survived all claimed that in the last few weeks before the attacks, the drug dogs were all removed from the premises, and there suspicious people in hazmat suits "fixing the floors and ceilings, etc". Several people believe that they might have been putting in explosives which would ultimatly aid in the collapse of the Twin Towers.
Finally, in the shrapnel and remains of the Towers, there were parts of the large metal pieces, mading up the pipes and exterior walls , that were completely bent. Metal pieces of that size and thickness require insane heat, at least 1000 degrees Farenheit, to become pliable. However when the planes crashed into the buildings, they were smoking. Smoke only arises if there was a fire put out, not while a fire is still burning. Smoke by itself cannot reach 1000 degrees. In addition to that, most survivors claimed that the building was not actually that hot and that there was no major fire, thus debunking the common theory that the Twin Towers collapsed due to heat and fire.
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