Posted on 09/11/2001 8:06:15 PM PDT by summer
September 12, 2001
THE BUILDINGS
Believed to Be Safe, the Towers Proved Vulnerable to the Intense Heat of a Jet Fuel Fire
By JAMES GLANZ
The cause of the twin collapse yesterday of the World Trade Center towers in Downtown Manhattan was most likely the intense fire fed by thousands of gallons of jet fuel aboard the two jetliners that crashed into the buildings, experts on skyscraper design said.
The high temperatures, of perhaps 1,000 to 2,000 degrees, probably weakened the steel supports, the experts said, causing the external walls to buckle and allowing the floors above to fall almost straight down. That led to catastrophic failures of the rest of the buildings.
The towers were built to withstand the stresses of hurricane-force winds and to survive the heat of ordinary fires. After the 1993 trade center bombing, one of the engineers who worked on the towers' structural design in the 1960's even claimed that each one had been built to withstand the impact of a fully loaded, fully fueled Boeing 707, then the heaviest aircraft flying.
No engineer could have prepared for what happened yesterday, the experts said. "No structure could have sustained this kind of assault," said Richard M. Kielar, a spokesman for Tishman Realty and Construction Company, the construction manager for the original project.
The enormous heat from the jet fuel fire probably caused the steel trusses holding up concrete-slab floors and vertical steel columns to bend like soft plastic, said Jon Magnusson, chairman and chief executive of Skilling Ward Magnusson Barkshire in Seattle, a structural engineering firm that worked out the original design.
The skyscrapers had two means of defense against normal fire damage, Mr. Magnusson said. One, thick layers of insulation sprayed onto the steel beams, could have been breached by the initial crash, he said. Another, the building's sprinkler system, may have been disabled as well, or it may simply have been useless in the heat of the jet fuel fire.
Although they resisted collapse immediately after the planes' first impact, the hundreds of steel columns spaced around the outer facing of each tower eventually failed.
"They buckled outward and then the floors came down," said Mr. Magnusson, who warned that no conclusions could be reached yesterday since the information available was so sketchy.
Other experts agreed that the extreme conditions caused by the fire, and not unusual vulnerabilities of the buildings, were the likely causes of the collapse.
"There isn't anything particularly vulnerable about it," said Aine Brazil of Thornton-Tomasetti Engineers in New York, a structural engineering firm that worked on the Petronas Towers, the world's largest buildings, in Malaysia.
Buildings are simply not designed to withstand "the extreme levels of heat that would be found in the situation with the amount of jet fuel and the explosion that occurred," Ms. Brazil said.
Mr. Kielar, the Tishman spokesman, said it was too early to piece together a precise train of events, but he agreed that weakening by fire, followed by catastrophic collapse of the floors, was the most likely possibility. "As the structure warped and weakened at the top of each tower, it — along with concrete slabs, furniture, file cabinets and other materials — became an enormous consolidated weight that eventually, progressively crushed each tower below," he said in a statement.
The later collapse of the smaller 7 World Trade Center could have been caused by a combination of falling debris and a less intense fire — one not accelerated by jet fuel — lasting several hours, said Brian McIntyre, chief operating officer of Skilling Ward. Such a building is "basically designed to resist heat buildup for three hours," he said.
The structural design of the two towers, fairly common now, was considered innovative in its day. Instead of the heavy internal bracing and heavy exterior masonry of, for example, the Empire State Building, the designers of the trade center towers chose a light, glass-and-steel facing threaded by steel columns. Those columns, 61 on each side, gave the towers most of their stiffness and largely held them up, said Dr. John Schuring, a professor and chairman of civil engineering at the New Jersey Institute of Technology.
"The major strength of the building is in its skin," Dr. Schuring said.
There was also a cluster of columns in the center, supporting structures like the stairs and elevators, he said. A network of steel trusses ran between the two sets of columns, holding up each concrete floor and providing further strength to the buildings.
A special set of plates on each floor ran among the trusses, serving to dampen stresses on the buildings caused by winds of up to 200 miles per hour, said Dr. Jack Cermak, president of Cermak Peterka Peterson in Fort Collins, Colo., the firm that did the wind-tunnel testing for the design of the towers.
Dr. Cermak agreed that the impact of the crash itself probably could not have collapsed the massively reinforced building on its own.
"I presume, without knowing the details, that that collapse was caused by weakening of the structure due to the heat," Dr. Cermak said.
Matthys Levy, an architect at Weidlinger Associates and the author of "Why Buildings Fall Down" (Norton, 1992), watched the first tower collapse while standing at Seventh Avenue and Houston Streets, some 20 blocks away.
"I saw the beginning of the top moving down, and the whole thing collapsed in a cloud of smoke," Mr. Levy said. "From what I saw, it seemed to come straight down."
Mr. Levy said the situation was much different from the one that occurred in 1945 when a much smaller plane slammed into the Empire State Building.
That plane, a bomber with a smaller impact and less fuel, ripped a 20-foot hole in the structure, but the building remained standing.
There was some disagreement yesterday about whether, decades later, the trade center towers had been designed to withstand an impact from an airliner filled with fuel.
The engineer who said after the 1993 bombing that the towers could withstand a Boeing 707, Leslie Robertson, was not available for comment yesterday, a partner at his Manhattan firm said.
"We're going to hold off on speaking to the media," said the partner, Rick Zottola, at Leslie E. Robertson Associates. "We'd like to reserve our first comments to our national security systems, F.B.I. and so on."
But Anthony G. Cracchiolo, director of priority capital programs for the Port Authority of New York and New Jersey, which owned the buildings, said little thought had been given to the possibility of a plane crash into the towers.
"We never were asked to consider trying to protect the building from such a threat," said Mr. Cracchiolo, who was among those who coordinated the reconstruction after the 1993 bombing. "As structural engineers, there is nothing we could have done to protect the building from a direct impact from a plane as large as these."
Melvin Schweitzer, a member of the Port Authority board of commissioners from 1993 to 1999, said, however, that the board repeatedly inquired about that possibility. "We were just told that architects had explained that the building was designed to withstand a jet," Mr. Schweitzer said. "Frankly, when we raised that question, most of us were thinking of a small plane."
The architectural firm for the trade center, Minoru Yamasaki Associates of Rochester Hills, Mich., declined to answer specific questions about the collapse, and issued only a brief statement.
"The company has been in contact with law enforcement authorities, and we will provide any assistance we can to aid the rescue efforts," the statement said. "In this time of national emergency, we believe that any speculation regarding the specifics of these tragic events would be irresponsible."
With all due respect, YOU flagged him to this thread. I can see it with my own eyes.
I cannot believe you had him banned...........how childish of you. Do what the rest of us do when somebody is bugging us, BE A BIG GIRL AND IGNORE THEM.
Now, you may proceed to have ME banned if you'd like.
The fire sprinkler system on those floors ceased to exist after the initial explosions.
Gravity tanks on the roof and fire pumps sending water to no longer existing sprinkler systems contributed to the weight on the beams, whose 2-3 hour rated fireproofing was obviously not up to the initial 2000 degree heat.
I imagine the fire burned less hot after the first 15 minutes though.
It is horrible and unfortunate that nobody had the foresight to tell rescue personnel about the possibility of a collapse within 90 minutes of impact.
We need to invite big ern. He's on her banishment list too.
I don't know what strings you pulled to get HalfIrish banned but this makes me sick.
During the explosion and fire and the prolonged evacuation, six people died and 1042 people were injured, though there was a real potential for many more deaths and severe injuries. Many of the injured occupants suffered from smoke inhalation but, remarkably, none died from the exposure. This occurred primarily because there was a limited amount of combustibles that were initially ignited, and because of the basement floor collapse, there was limited fire spread to adjacent materials. Because of the limited burning there was a significant dilution of the products of combustion as they moved through this massive building complex. Had there been a more continuous burning or less dilution of the smoke, it is likely that the loss of life in this tragedy would have been far greater than the six casualties who were directly exposed to the force of the explosion.
The facts of the World Trade Center incident and the lessons learned should serve as an impetus for the fire safety community to re-examine the current design philosophies and future directions for high-rise fire technology. Prior to the incident there was no attempt to treat "mega-high-rises" such as the World Trade Center differently from other high-rise buildings with regard to reliability of fire protection system design. However, since such mega-high-rises can simultaneously expose tens of thousands of people to life-threatening conditions from a "single event," more reliable performance of fire protection components may be appropriate. Further warranting consideration is where the structures contain critical, private enterprises or government agencies, as was the case in the World Trade Center. Although fire protection designers normally would include a "single event" scenario in their performance criteria of fire protection systems, this single event at the World Trade Center complex has initiated an active discussion as to what a "single event" scenario should include.
The NFPA believes fire protection designers and code officials should perhaps broaden their responsibilities to include security issues or other such subtle changes in our society in achieving dependable and reliable fire protection system performance for such critical occupancies.
The complexity of this incident also demonstrates the importance of compliance with fundamental fire protection requirements for high-rise buildings. The growing inventory of high-rise buildings in this country should be reviewed to ensure that minimum levels of fire protection are present. Further, recent fatal high-rise building fires have demonstrated the importance of inspection and maintenance of these systems. One of the more recent high-rise fires resulted in three fire fighters losing their lives, further underscoring the importance of fire safety requirements for these buildings. Finally, owners, insurers, security, fire protection professionals, and code developers may want to re-examine fire protection designs in high-rise structures, especially where those structures accommodate large numbers of people and the economic impact is great, to ensure that the protection systems achieve a high degree of reliability during a "single event" occurrence. The NFPA's Life Safety Code® addresses occupant safety in both new and existing buildings; an important consideration in fundamental fire safety principles in the built environment.
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