The collapse of WTC7 makes more sense when you consider the unusual structural design of the building. WTC7 was built over a ConEd substation. When the substation was built it was anticipated that a skyscraper would be built over it, so sizeable foundations were laid around it. In WTC7 support columns from these foundation areas supported a complex system of transfer girders and structural floors in what became floors 5-7 of the final building. The design that was picked for WTC7 was larger than that originally planned when the substation was built, and the columns of the building above the 7th floor didn’t line up with those below the 5th, so the floor 5-7 structure essentially had to support the weight of the entire building above it.
The floor 5-7 region was cramped, with heavy beams and structural floors and walls. What space was in there was used for mechanical equipment - generators, diesel tanks for those generators, electrical rooms, ventilation and air conditioning. Additional diesel fuel tanks in the building’s basement fed generators in this area through pressurized feed lines, fed by pumps which were set to automatically start in the case of a power outage and would keep running until the tanks ran dry. The total amount of diesel fuel available was comparable to the fuel load in the planes which hit the towers. There were also vents on the sides of the building in this region to let in air for the diesel generators.
When the North tower collapsed, large amounts of debris struck WTC7. It’s difficult to determine exactly how bad the damage is, mostly due to the thick smoke which was billowing out of the building for most of the afternoon. From photos, video, and eyewitness account it appears that damage extended from the roof to ground floor, including a gouge running from the roof at least halfway down the southern face of the building. Photos, videos, and eyewitness accounts also indicate that multiple major fires were started in the building and burned until it collapsed.
It’s difficult to know exactly how the fires progressed or where the internal damage was. The firefighters determined fairly early on that under the circumstances the fires could not be fought and the building was in danger of collapse, and cleared everyone out of the area. What video and photo evidence we have is taken from a distance and obscured by dust and smoke. We do know that when the rubble was finally cleared the basement diesel tanks were empty, with no evidence of the fuel having leaked into the ground below. This suggests that the pumps kicked in and ran at full capacity, probably due to a rupture in the fuel line, and that this fuel was all pumped elsewhere in the building. A fuel line rupture and fire in the floor 5-7 region would be consistent with the available evidence, although it’s impossible to do more than speculate about that right now.
Shortly before the final collapse, the eastern mechanical penthouse on the roof of the building sank into the building. This mechanical penthouse was a sizeable structure in its own right, directly supported by columns running down to the floor 5-7 transfer structure. Its collapse was probably due to the failure of a single one of those columns, and once it started falling it wouldn’t stop until it hit the transfer structure. The impact of that structure falling 40 floors, hitting a girder structure already weakened by fire and debris impact, probably caused the entire transfer structure to fold inwards, pulling the support out from under all the remaining structural columns holding the building up.
