Eli5 why if the towers collapsed because the support structures were weakened by fire then why weren’t the main verticle columns still standing?
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Because everything started collapsing down on top of them. Modern engineering is an optimisation game of what’s the least material required to achieve the structural requirements (including an acceptable margin for error), meaning it’s not intended to survive everything above it coming down with added force from gravity.
Because it fell from the top down. The added force of everything falling and piling more to fall further was enough to tear it all down.
The columns only have the strength to hold all the floors above then because every floor braced the columns against buckling.
You can experiment with this effect if you have a ruler/flat stick – if you try to compress it it’ll buckle out sideways, if you cut it in half and then try to compress it again it’ll take much much more force to get it to buckle. Same principle applies to columns.
So once those floors aren’t holding the column against buckling the column no longer has the strength to hold the floors up so will come right down with them.
There’s a few other effects also:
1. The impact force from a few dozen floors falling and landing on the undamaged column below is enormous, much more than the load the column was designed for, so would immediately overwhelm the column and buckle it.
2. In the event it didn’t what’s happening is the floors are landing on the floors (instead of the column). The beams are connected to the columns for a force based on the load of one floor. A few dozen floors landing on the floor below is much more force than this so will immediately fail that connection, ripping the beam from the column, and then proceed to the next floor down where the same thing happens again. In this scenario you might have a longish stretch of column temporarily pushing up through the falling floors, but once it gets long enough to buckle the falling floors will just pull/push it down as they land on it.
Demo charges? There was an old video of youtube that showed some flash and expansion (dust) of the charges.
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