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."
I don't think there should be any liability extended to the architects and engineers responsible for the construction of this building. Current jets are much larger than those in existance when the WTC was designed, so both the crash load and fuel are much greater hazards.
That being said, is there any way a hypothetical WTC replacement could be protected from such dangers?
D
Believed to Be Safe, the Towers Proved Vulnerable to the Intense Heat of a Jet Fuel Fire
September 11, 2001
Televised images of the attacks on the World Trade Center suggest that explosives devices caused the collapse of both towers, a New Mexico Tech explosion expert said Tuesday.
The collapse of the buildings appears "too methodical" to be a chance result of airplanes colliding with the structures, said Van Romero, vice president for research at New Mexico Institute of Mining and Technology.
"My opinion is, based on the videotapes, that after the airplanes hit the World Trade Center there were some explosive devices inside the buildings that caused the towers to collapse," Romero said.
Romero is a former director of the Energetic Materials Research and Testing Center at Tech, which studies explosive materials and the effects of explosions on buildings, aircraft and other structures.
Romero said he based his opinion on video aired on national television broadcasts. Romero said the collapse of the structures resembled those of controlled implosions used to demolish old structures.
"It would be difficult for something from the plane to trigger an event like that," Romero said in a phone interview from Washington, D.C.
Romero said he and another Tech administrator were on a Washington-area subway when an airplane struck the Pentagon.
He said he and Denny Peterson, vice president for administration and finance, were en route to an office building near the Pentagon to discuss defense-funded research programs at Tech.
If explosions did cause the towers to collapse, the detonations could have been caused by a small amount of explosive, he said.
"It could have been a relatively small amount of explosives placed in strategic points," Romero said. The explosives likely would have been put in more than two points in each of the towers, he said.
The detonation of bombs within the towers is consistent with a common terrorist strategy, Romero said.
"One of the things terrorist events are noted for is a diversionary attack and secondary device," Romero said.
Attackers detonate an initial, diversionary explosion that attracts emergency personnel to the scene, then detonate a second explosion, he said.
Romero said that if his scenario is correct, the diversionary attack would have been the collision of the planes into the towers.
Tech President Dan Lopez said Tuesday that Tech had not been asked to take part in the investigation into the attacks. Tech often assists in forensic investigations into terrorist attacks, often by setting off similar explosions and studying the effects.
I really think it is likely that structural engineers contributed to the plan.
Perhaps the destructive power of a fueled jumbo jet is so much that my thoughts on a secondary explosion in the building to bring down the plane crash damaged structure is unnecessary.
I agree with your conclusion. As a former firefighter and fire investigator, I have seen what a "normal fire" will do to the structural steel in a building. Having the intense heat of hundreds of gallons of jet fuel burning in an enclosed space will generate a tremendous amount of heat in a short period of time. The structural members on the interior of the building were exposed to the highest temperatures. Once enough of the interior supporting members failed, the sudden weight of the floors above collapsing would pancake the building. No additional explosives would have been needed.
Here we go again. For pete's sake, use your brains Romero. If they had bombs in the building, they wouldn't have to hit it with a plane! They would use the plane to hit another target!You can see on the video exactly what happened. There were no secondary explosions. This is poppycock.
This makes as much sense as the thread that said the pilots were in with the terrorists. Duh... If the pilot wanted to crash the plane, why would they need terrorists in the back to make him crash the plane?
I remember the old joke that Civil Engrs aren't allowed to take Dynamics classes because if their work is in motion, something went wrong.
The static load of a building is nothing compared to the kinetic energy of half of the building under freefall accelerated by gravity onto the rest of the building below.
Try this experiment!
1. Balance a brick on your head.
2. Note that it doesn't feel all that heavy.
3. Now gently tip that brick off the top of your head and let it fall on your bare foot.
Note any difference???
I own an Apple Titanium G4 computer; it's reassuring to note that the titanium in it would survive this fire even if the plastics that make up the bulk of the machine wouldn't :-(.
For what it's worth, I was amazed at how similar the collapse was to photographs of controlled demolition; that was exactly the thought that crossed my mind when I saw the pictures. However, I don't think there was any reason for the terrorists to do that; the building would clearly not be safe to occupy after the fires, and the image of two 110 story dead hulks would, if anything, create a more dramatic impact on the city.
It is my opinion that the controlled demolition effect may have even been unintentional; they knew they would mortally wound the structures for the purpose of human habitation simply through the fire, and there would really be no point in going any further.
D
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