Every column was weakened by the impact and the high temperature of the fire. Was it hot enough to melt it? No, but it didn't need to be. At those temperatures steel loses half of its strength. Here's a guy with a forge to explain it, visually.
"Was it hot enough to melt it? No." Molten steel can be seen flowing from the building before it fell, and molten steel was found in pockets of the wreckage. So yes, something needed to be hot enough to melt steel.
http://ine.uaf.edu/wtc7
So every column decided to fail at the same time?
Every column was weakened by the impact and the high temperature of the fire. Was it hot enough to melt it? No, but it didn't need to be. At those temperatures steel loses half of its strength. Here's a guy with a forge to explain it, visually.
https://www.youtube.com/watch?v=FzF1KySHmUA
"Was it hot enough to melt it? No." Molten steel can be seen flowing from the building before it fell, and molten steel was found in pockets of the wreckage. So yes, something needed to be hot enough to melt steel.