The impact of two passenger jets and the raging fires they ignited were not enough to bring down the World Trade Center towers, according to a comprehensive study of the towers’ collapse released Tuesday.
But when the planes hit the twin towers on Sept. 11, 2001, they dislodged the fireproofing material protecting the steel columns and trusses that held the towers together. That proved to be the key factor, according to the National Institute of Standards and Technology.
Stripped of protection, the steel sagged following prolonged exposure to temperatures of at least 1,000 degrees Fahrenheit, triggering the towers’ pancaking collapse. The attacks killed 2,749 people in the towers and on the two planes.
But lead investigator Shyam Sunder said the failure of the fireproofing material was not a result of improper installation when the towers were constructed more than 30 years ago or lax maintenance since then.
“You would not expect fireproofing to be installed to withstand the impact of an airplane,” Sunder said, speaking at a media briefing at a Times Square hotel. “We are not suggesting here that people should design buildings to withstand airplanes.”
Based on mathematical modeling that pushed the boundaries of computer science, the federal agency’s conclusion is the most authoritative statement likely to emerge about the sequence of events on Sept. 11.
The institute’s investigation is examining not only the engineering issues behind the collapse but also the evacuation of the towers, the response of police, firefighters and other emergency personnel, and technical issues. The study is intended to find out if current building codes, construction practices and emergency procedures must be revised to take account of the threat of terrorism.
3 reports released
At Tuesday’s briefing, the agency released three reports on which it has completed work, including a study of how occupants evacuated the building and how communications hampered firefighters.
To re-create the impact of the two fuel-laden Boeing 767 passenger jets, investigators created extremely detailed mathematical models of the planes and the towers.
The planes broke apart into thousands of fragments within one second of striking the towers, Sunder said. The impact was so powerful that the south tower swayed for four minutes.
The jet fuel burned up in a few minutes, but it ignited the contents of the floors, such as papers and furniture. Even so, the towers likely would have remained standing if the impact had not dislodged the sprayed-on fireproofing material that covered the steel columns and floor trusses, the study found.
Since the attacks, several theories have been put forward about the sequence of events that caused the towers to collapse. Questions arose about the reliability of the towers’ design, an innovative arrangement of exterior columns linked by floor trusses to an inner core of columns. Some architects and engineers speculated that the trusses separated from the outer columns in the fire and led to the collapse.
Sunder said the investigation concluded that, apart from the impact areas, the trusses mainly stayed in place. But subject to high heat and stripped of fireproofing material, they sagged and pulled the fire-weakened outer columns inward.
Among other findings, the agency determined that more lives might have been saved if stairwells in the north tower had been more widely separated. All three emergency staircases were severed by the hijacked jet, preventing anyone above the impact zone from escaping. In the south tower, where staircases were farther apart, one remained intact.
Communication problems
The institute also found that documented problems with radio communication and information-sharing among firefighters, police and others on the scene probably led to avoidable deaths among the rescuers.
With at least 700 emergency personnel on the scene, radio frequencies were overloaded with people trying to talk, and inside the stairwells the radios often did not work. Many firefighters in the north tower who survived said they did not hear the order to evacuate that tower after the south tower collapsed.
The institute also determined that current calculations of how quickly people walk down stairwells in an emergency are based on a “phased” evacuation, not the mass evacuation on Sept. 11.
But the towers were only about one-third occupied when the planes struck, with the result that 87 percent of the estimated 17,400 people at the trade center survived. If the buildings had been full, with 50,000 people, the evacuation would have taken four hours. As the towers collapsed less than two hours after they were struck, the agency said that 14,000 people might have been killed.
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Study details collapse sequence
The National Institute of Standards and Technology on Tuesday released findings from the most detailed study yet on the collapse of the twin towers after the Sept. 11 terrorist attacks.
Two World Trade Center
One World Trade Center
SEQUENCE OF EVENTS
1 When the airplanes struck the towers, they severed perimeter columns and damaged interior columns, shifting weight elsewhere. The impact shook fireproofing from the buildings’ steel structures.
2 Intense heat from the fires made the now unprotected steel floor trusses weaken and sag, causing the exterior columns to bend inward and buckle across the faces of the buildings.
3 The buildings collapsed.
EXTERIOR COLUMNS
Each building face contained 59 steel columns, which were preassembled into panels that were 36 feet high and 10 feet wide.
INTERIOR COLUMNS
There were 47 columns in each building’s core, which housed the elevators and stairwells.
STEEL TRUSSES
Fireproofing sprayed onto steel beams and trusses
Source: National Institute of Standards and Technology
Chicago Tribune
– See microfilm for complete graphic.
