I know that the structure was 30 plus year old steel.
Some of the steel was rusting.
What you don’t understand is the affect of catylysts.
Molten aluminum, plus concrete, plus water, plus thousands of gallons of jet fuel would cause explosions and get hot enough to melt steel.
The towers werea tually built to withstand a plane crash in 1970, but no one figured the size of the jumbo jet at that time.