https://www.scientificamerican.com/article/fahrenheit-2777/

In an article in the Journal of the Minerals, Metals, and Materials Society and in subsequent interviews, Thomas Eagar, an engineering professor at the Massachusetts Institute of Technology, explains why: steel loses 50 percent of its strength at 1,200 degrees F; 90,000 liters of jet fuel ignited other combustible materials such as rugs, curtains, furniture and paper, which continued burning after the jet fuel was exhausted, raising temperatures above 1,400 degrees F and spreading the inferno throughout each building. Temperature differentials of hundreds of degrees across single steel horizontal trusses caused them to sag--straining and then breaking the angle clips that held the beams to the vertical columns. Once one truss failed, others followed. When one floor collapsed onto the next floor below, that floor subsequently gave way, creating a pancaking effect that triggered each 500,000-ton structure to crumble. Conspiricists argue that the buildings should have fallen over on their sides, but with 95 percent of each building consisting of air, they could only have collapsed straight down.
All the 9/11 conspiracy claims are this easily refuted. On the Pentagon "missile strike," for example, I queried the would-be filmmaker about what happened to Flight 77, which disappeared at the same time. "The plane was destroyed, and the passengers were murdered by Bush operatives," he solemnly revealed. "Do you mean to tell me that not one of the thousands of conspirators needed to pull all this off," I retorted, "is a whistle-blower who would go on TV or write a tell-all book?" My rejoinder was met with the same grim response I get from UFOlogists when I ask them for concrete evidence: Men in Black silence witnesses, and dead men tell no tales.