Buildings made of steel beams and girders don't fall straight down.
Not unless there is a controlled explosion causing the load bearing parts of the structure to be removed. The only way for a building to fall straight down is via explosives placed in key structural points in a building. It requires months of planning for a demolition team to take a building down like that.
Especially buildings designed to carry incredible loads. The World Trade Centre buildings in NYC were designed to carry weights many times their own, and the steel beams supporting the structure had a melting point far above the burning temperature of jet fuel.
Plus the steel beams were covered in a flame retardant, asbestos - which the WTC needed to remove sometime because asbestos causes a variety of health problems including cancer.
In other words there was no way that the WTC buildings could collapse straight down, without controlled explosions / implosions at multiple key points in the buildings' structures.
Anyone who is a structural engineer will be able to tell you, there is no way those buildings collapsed like that due to "jet fuel" melting the steel girders. Jet fuel simply is not hot enough. Not even close. Not even half. (Think of it this way, if jet fuel burned hot enough to melt or weaken steel, then airplanes would be crashing all the time as the engines would weaken, melt and fall apart.)
Thus the jet fuel certainly cannot cause steel girders further below to collapse due to "added weight" when the structure of the building was designed to carry many times the extra weight.
In other words "terrorists" could have crashed 20 planes into the WTC buildings and they still would not have collapsed. The only way to bring a building like that straight down is a demolition team with months of planning and tonnes of explosives.
It is a bit like crashing a plane into the Hoover Dam and expecting the dam to break. It won't happen. The Hoover Dam is just too big. It would just be a scratch on the surface.