Now you are suggesting that normal office fires, that move organically along every 20 minutes or so as the fire load is consumed, can cause heavy structural steel to reach a high enough temperature to cause it to fail catastrophically. You have to know thats wrong. Your previous input here tells me that you know thats wrong. It has never happened before or since - yet did that three times in one day. And don't bring up the 'plane damage' and 'plane fuel' points because you would then be arguing against NIST who discounted those as being the cause of collapse of the three buildings. They said fire and gravity did it. However, if the steel was heated to a high enough temperature, as could be caused by a thermitic attack, and then if the attacked area was persuaded by a small concussive charge to move aside then what we saw would match the event. As you say, steel fails under heat, but not from an office desk and a hat-stand burning.