Since John Clay seems to pay attention to arguments that involve actual physics, maybe I can explain something to him that he already knows, if he only thought about it.
John, have you ever seen a steel bar? Very strong, yes? Try to bend it, and it’s very hard to bend. Now, if someone wants to bend that bar, what do they do? They heat it up. The bar doesn’t have to melt, it doesn’t get cut in two. The blacksmith just puts the bar of steel in the furnace for a while, it gets hot (but well below the melting point of steel!), then the takes it out and bends the metal easily.
Now imagine you have a steel building. The steel supports are very strong. Then you start a fire in the steel building. The steel doesn’t melt, that would take temperatures of 1500 C, which requires specialized blast furnaces. But just heating the steel makes it easier to bend. And so if you’ve got a building with steel beams that can hold ten tons, but the building is designed so that each beam only supports 5 tons, the building is very strong. But now you’ve flown a plane into the building and cut a lot of steel beams. No problem, the rest of the beams are strong enough to hold it up. But there’s a fire. The fire weakens the strength of the steel beams, just like a horseshoe in a blacksmith’s furnace is weakened by fire.
Each beam gets weaker and weaker, until some of them start to fail. And when one fails, the beam next to it needs to be strong enough to support twice as much weight. Which means that it is very likely to fail, and when it fails the next beam needs to support more weight. But that beam has been weakened by fire also, and so it fails.
And so you have the entire top of the building falling onto the lower part of the building, which causes the entire building to collapse.
What causes the collapse isn’t the fire, or the cut beams. It’s gravity. And what direction does gravity pull? Straight down. This is why when buildings collapse they collapse down, because gravity pulls them down and they aren’t strong enough to hold together like a tree that’s been cut through. And maybe you’ve noticed that when dead trees get cut it’s extremely dangerous because the tree might break apart and fall on you, because the dead tree might be weak enough to break apart and if it does gravity pulls the pieces straight down.
This is simple physics that you can demonstrate to yourself. Heat weakens steel. Gravity pulls straight down. A given steel bar can support a given weight at 50 C, but is weaker at 200 C and weaker still at 500 C.