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Yes they were. They were built to withstand raging carbon fires.
They were built to withstand getting hit by the biggest planes known to be in service at the time the buildings were designed. Since then, the planes have got bigger, their fuel capacity has got bigger, and the fuel itself has become more powerful and more explosive.
This is a perfect example of the conspiracy-buffs' transparently selective and inadequate grasp of reality. To say nothing of common sense.