Fireproofing, sprinkler systems and the water supply for hoses all largely failed in the World Trade Center towers on Sept. 11 in the face of a blaze so intense that it drove temperatures as high as 2,000 degrees and generated heat greater than the energy output of a nuclear power plant, a federal report on how the towers fell has concluded.
The fire, combined with those failures, brought down the towers even after they had shown surprising and lifesaving resiliency to massive structural damage caused by the impact of two hijacked airliners, the report says.
The report’s findings detail for the first time the horrific events that led to the collapse of two of the world’s tallest buildings. They are contained in a draft of a report commissioned by the Federal Emergency Management Agency and the American Society of Civil Engineers.
The draft describes a structure that showed both remarkable strength and critical weaknesses. The towers sustained the initial impact of the planes and were able to redistribute loads away from damaged columns so well that they probably could have remained standing indefinitely if not for the fires, a major earthquake or an overwhelming windstorm, the report said.
Team members still are debating the delicate question of whether the tremendous fires could have brought the towers down on their own.
“The ability of the two towers to withstand aircraft impact without immediate collapse was a direct function of their design and construction characteristics, as was the vulnerability of the two towers to collapse as a result of the combined effects of the impacts and ensuing fires,” the report concludes.
The report, a copy of which was obtained by The New York Times, is not due to be officially released until late April or early May.
What is clear is that the jet plane fuel played a somewhat different though still critical role than some experts had speculated. For example, after the planes slammed into the towers, the fireballs that burst over lower Manhattan consumed perhaps a third of the 10,000 gallons of fuel on board each plane, but did little structural damage themselves, the report says.
Like a giant well of lighter fluid, though, the remaining fuel burned within minutes, setting ablaze furniture, computers, paper files and the planes’ cargo over multiple floors and igniting the catastrophic inferno that brought the towers down.
Other buildings susceptible?
Under normal circumstances, fire-suppression systems are designed to allow a high-rise blaze to burn itself out before the building collapses. But the report concludes that there were across-the-board failures in the fire-suppression systems inside the towers, raising disturbing questions about the safety and integrity of other tall buildings in out-of-control fires.
The ultimate significance of those failures, however, is extremely difficult to gauge, the report says, because of the extraordinary circumstances of attacks.
In fact, besides setting the fires, the impact of the jets may have jarred loose the light, fluffy fireproofing that had been sprayed on steel columns, and flying debris almost certainly sliced through the vertical pipes that supplied water for the hoses and sprinklers.
Because of those uncertainties, the report says, the possibility of changes to building codes and engineering practices should receive extensive further study, a step the federal government already is planning with a $16 million, two-year inquiry by the National Institute of Standards and Technology now getting under way. The report’s final version may recommend specific changes in building codes and standards.
The report also is significant for what it does not include. Except for a few torn, contorted steel beams from 5 World Trade Center, a nine-story office building on the site, not a single piece of evidence collected from the piles of debris contributed in a meaningful way to the report’s conclusions.
About 60 pieces of trade center steel are being sent to the National Institute of Standards and Technology for the continuing investigation, so it is possible that future analysis of the steel remnants could provide additional answers.
The draft report also does not contain any discussion of what could become an explosive new issue in attempts to explain why the south trade tower, though struck after the north tower, fell first. That issue involves a program, started after the 1993 bombing of the towers, to increase the thickness of the fireproofing on the lightweight steel joists that held up the floors in the trade center.
Whatever its thickness, much of the fireproofing probably was dislodged by the impact of the planes, the investigators concluded. One official knowledgeable about the fireproofing said that even with the wipe of a finger, the woolly, mineral-based material could be brushed away.
Steel soft as licorice
Stripped of its fireproofing, a steel column heats up much more quickly in a fire. The hotter the steel, the less it is able to support loads, as it eventually becomes soft as licorice. Investigators believe the structural steel also was greatly imperiled because the sprinklers and standpipes supplying water for firefighting were almost certainly disabled, their supply pipes cut by flying debris in the initial crash.
“Damage caused by the aircraft impacts is believed to have disrupted the sprinkler and fire standpipe systems,” the report says.
The report cites the tightly clustered exit stairways, three per tower, as a factor that may have made it easier to cause damage to all of them with one blow. The exit stairwells also had relatively lightweight gypsum-board sheathing, providing little armor. Partly for those reasons, thousands of people above the floors of impact were trapped and killed.
The report–assembled with data collected at ground zero, in scrap yards, in laboratories, by analyzing more than 100 hours of videotape and by talking to witnesses–turned up the greatest amount of detail on the south tower attack.
United Airlines Flight 175, its wings slightly canted, angled into the south facade of the south tower, slicing through about 30 of the 59 exterior columns on that face. The immediate damage, probably including unseen devastation to the steel core, stretched from the 78th to the 84th floor.
The impact of the plane, which had been traveling as fast as 586 m.p.h., was so great that it gathered office material like a snowplow and apparently forced it toward the northeast corner of the building. Parts of the plane came to rest there and others punctured the far wall, soaring as far as six blocks to the north before hitting the ground.
A fuel-fed fireball emerged from three sides of the tower and consumed roughly one-third of the estimated 10,000-gallon supply on the plane. Some of the rest flowed down the face of the building and into elevator shafts and stairwells. What remained burned ferociously.
The incredible energy generated by the blaze was estimated at its peak to be 3 to 5 gigawatts. A typical nuclear power plant generates about 1 gigawatt. All of that energy was converted to deadly heat that began weakening the steel.
Fifty-six minutes and 10 seconds after it was hit, the top of the south building tilted horribly and initiated the collapse of the entire tower.
Unfolding at a slower pace, the disaster at the north tower will require study before it can be explained in such detail.
