Why the Twin Towers fell

NIST’s account has four steps. The planes severed and damaged columns, knocked fireproofing off the steel and spread jet fuel over several floors. The fires that followed, fed mostly by office contents and reaching about 1,000°C, heated the unprotected steel. The floors sagged and pulled inward on the perimeter columns, which bowed in. The south face of the North Tower and the east face of the South Tower failed first. NIST says photographs, video and reports from the New York police aviation unit in the half hour before each collapse support that sequence. [1][2]

When the bowed columns buckled, the whole section of the building above the impact zone began to move down. NIST says that once that happened, the structure below could not stop it. The collapse began at the impact and fire floors and nowhere else, and the time it took to start, 56 and 102 minutes, depended on how much damage each plane did and how long the fires took to reach the critical areas. [2]

About 200 experts worked on the investigation. They reviewed tens of thousands of documents, interviewed more than 1,000 people, studied 7,000 segments of video and 7,000 photographs, and analyzed 236 pieces of recovered steel. [2]

NIST’s simulations stop at the moment the collapse begins. It says the energy released by the falling top section was so great the rest followed. Critics, led by Architects & Engineers for 9/11 Truth, say this leaves the speed and near-symmetry of the collapse unexplained by any published model. [1]

NIST’s answer is that the first exterior panels hit the ground about 11 seconds (North) and 9 seconds (South) after the collapses began, and that neither the seismic records nor the video, obscured by dust clouds, show how long each full collapse took. [1] The engineer Zdeněk Bažant and colleagues published a peer-reviewed analysis in 2007 arguing that a collapse of that kind, once started, could not be stopped by the floors below. [2]

FEMA’s earlier, smaller study, published in May 2002, called for further investigation, which NIST then carried out. [3] The “pancake” explanation many people heard in 2001 is not NIST’s: NIST found the sagging floors pulled the columns in before the collapse began.