9-11: Why the Buildings Fell Down


In the months immediately following the 9-11 attacks, the going theory for why both towers collapsed was that a massive fire, fed by jet fuel, warped the lightly constructed and inadequately fireproofed floors of each tower, causing a cascading collapse.

Not so, says a $16 million federal study that made its technical findings public Tuesday.

The National Institute of Standards and Technology found that, in fact, the twin towers’ design performed fairly well, all things considered. When the planes blew out some support columns, the weight transferred to other floors. The floors, held in place by a “truss” system that some suspected might have failed in the collapse, largely held. The fireproofing, where it remained in place after the shock of the planes’ impact, worked.

And the fire that brought the towers down wasn’t caused by jet fuel — that all burned up in the initial crash — but by all the flammable stuff inside the buildings, fed by air rushing in through the holes made when the planes hit the building. The initial impact also shook loose fireproofing on several floors, severed the sprinkler system, and damaged ceilings that might otherwise have prevented heat from traveling up the building’s structure.

So what did bring the towers’ down? The NIST, using computer models and mathematical analysis based on video and photographic evidence, says the fires were so hot they caused the outer walls on the impact side of each of the buildings to bow inward. Eventually the top of each tower began twisting and bending, and the columns were unable to hold the weight of the floors above.

“The buildings would likely not have collapsed under the combined effects of aircraft impact and the subsequent jet-fuel ignited multi-floor fires, if the fireproofing had not been dislodged or had been only minimally dislodged by aircraft impact,” the report—which specifies that it cannot be used in any lawsuit—finds. “The existing condition of the fireproofing prior to aircraft impact and the fireproofing thickness on the WTC floor system did not play a significant role in initiating collapse on September 11, 2001.”

As for the emergency response to the attacks, the NIST amplifies some of the points made by the 9-11 commission. For one thing, “A preponderance of evidence indicates that lack of timely information sharing and inadequate communication capabilities likely contributed to the loss of emergency responder lives.” The NIST also concludes that the loss of the Office of Emergency Management headquarters, located at 7 World Trade Center, “detrimentally affected” rescue operations, but competition between cops and firefighters had only a “minimal effect.”

But the report leaves open the big question hanging over all previous reviews of what happened on 9-11, which is what happened when you are trapped above a high-rise fire—which is where about 1,970 of the civilians who died on 9-11 were located.

A roof rescue was not an option: The FDNY only considers roof rescues a “last resort” for “medical emergencies,” the report says. And the NIST says that, “even if it had been possible for a helicopter to gain access to the roof, possibly only a very small fraction of the large number of people trapped above the impact zone could have been rescued.” So going up was not an option. And the way down was blocked by heat, smoke and flame.

Even for people below the impact zones, going down was no piece of cake, with problems like “evacuation initiation delay, evacuation interruption, and encountering obstacles in the evacuation path (environmental cues) such as smoke, water, or debris.” Hence the chilling statement that if the buildings had been full, “it is possible that as many as 14,000 people may have lost their lives based on rough estimates using existing models.”