

How?

What made New York's twin towers collapse, dooming thousands? Skokie's W. Gene Corley, who heads the official U. S. inquiry, lays out the theories





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Examining the disaster



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Collapse scenarios (Chicago Tribune)



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W. Gene Corley (Tribune photo by Chris Walker )





One of the photos studied by Corley



Tribune architecture critic





Many of his fellow structural engineers were stunned when the twin towers of the World Trade Center crashed to the ground on Sept. 11. But not W. Gene Corley, the suburban Chicago structural engineer who is heading the federal investigation into the collapse.



Corley, senior vice president at Construction Technology Laboratories in north suburban Skokie, is an old hand when it comes to figuring out why buildings fall down. His roster of past investigations includes the fatal 1993 fire that occurred during the FBI's raid of the Branch Davidian compound in Waco, Texas; the 1995 bombing and collapse of the Alfred P. Murrah Federal Building in Oklahoma City; and the 1999 collapse of a construction crane at Milwaukee's Miller Park. You might even call him a master of disaster, though he's far too reserved to use such a jaunty, self-promoting term.















When the hijacked Boeing 767 jets hit the Trade Center that morning, Corley was in his Skokie office, part of a Bauhaus-like compound of low-slung modern buildings at 5420 Old Orchard Rd., just west of the Edens Expressway. Alerted to the disaster in the making, he went to The Washington Post Web site and found still pictures of black smoke and red flames shooting out of the lower Manhattan landmark.



"I said: 'At least it hasn't fallen yet,' " Corley told the Tribune in his first extensive interview on the investigation. "'But if they don't get the fire out, it will.' "



For years, the 65-year-old Downstate native has quietly gone about the business of structural sleuthing, amassing bits and pieces of evidence to figure out why structures that are supposed to stand up don't. Now he's carrying out that task on a grand stage, heading the high-profile inquiry that is trying to pinpoint why the 110-story towers fell.



In a sense, everybody already knows what happened on Sept. 11: When the hijacked planes, gorged with fuel for cross-country flights, struck the towers, the rupture of the fuel tanks started an extremely hot fire that weakened the buildings' structural steel and caused the Trade Center's deadly crumpling. No building could have survived such a blow, expert after expert has said.



But things are hardly that simple. No one has authoritatively explained why One World Trade Center remained standing for 1 hour and 40 minutes while Two World Trade Center crashed to the ground just 56 minutes after impact. Another mystery is why smaller, nearby structures remained standing even though fire and steel from the twin towers rained down on them and even ripped huge chunks out of their facades.



Could the Trade Center's original architects and engineers have done anything differently that would have prevented such a large loss of life -- more than 3,300 victims, according to current estimates? Or is there reason to be satisfied that the buildings stood up as long as they did, allowing roughly 25,000 people to escape?



Grappling with the issues



These are some of the issues that Corley and his team of 23 specialists, who include experts in such esoteric fields as blast effects, will be grappling with until their investigation wraps up in March or April. The inquiry is co-sponsored by the American Society of Civil Engineers, an industry group based in Reston, Va., and the U.S. Federal Emergency Management Agency, the government entity that tries to prevent, as well as deals with, disasters.



Much more than solving a structural mystery is at stake. With America fighting a war on terrorism, highly symbolic structures both tall and short may require new armor to protect them against fanatics like the ones who destroyed the twin towers and damaged the Pentagon on Sept. 11.



"For those buildings that can be identified as being particular targets of terrorists," Corley says, "we expect to have recommendations that will allow the designers to do things differently that will enhance the safety of the people in the building."



The investigators are looking at several spots where the collapse might have begun -- exterior columns, interior columns, or the lightweight steel trusses that supported the building's floors. They also are challenging conventional wisdom by entertaining this hypothesis: The planes so damaged the twin towers that a regular fire of 1,100 to 1,800 degrees Fahrenheit, not a superhot fire stoked by jet fuel and reaching at least 2,000 degrees Fahrenheit, was sufficient to weaken the skyscrapers' structural steel to the point of collapse.



Whatever its outcome, the investigation is proceeding very differently than the one Corley led in Oklahoma City. There, the nine-story Alfred P. Murrah Building fell almost instantly, denying investigators a real-time record of the steps leading to its collapse. The much longer interval between blast and collapse at the World Trade Center resulted in the accumulation of a wealth of images (more than 150 hours of videos, as well as myriad photographs) that are crucial pieces of evidence.



In trying to determine what piece of the skyscrapers gave way first, for example, the investigators are searching for an aerial-perspective video that would show the fall of One World Trade Center. If the communications antenna atop the building started dropping before the perimeter of the building, that would offer a clue that the interior steel columns in the building's core (which supported the antenna) began collapsing before the perimeter columns.



In contrast, Corley says, "the only thing we had in Oklahoma City was an audio tape. There was a hearing on water rights going on right across the street. They had turned on the recorder at 9 a.m. At 9:01 a.m., the bomb went off. We were able to analyze the tape and determine from the sound that it took roughly 3 seconds for the building to collapse."



Specialist in concrete



After Corley was named to head the Trade Center investigation, there was some grumbling among steel industry leaders because the Skokie engineer is known as a specialist in concrete. The steel people fretted that Corley would blame the collapse on the steel frame of the Trade Center towers and say that structures that relied more on concrete, such as the Petronas Towers in Malaysia, currently the world's tallest buildings, would have fared better.



The criticism is nonsense, according to Jim Rossberg, a director at the American Society of Civil Engineers, who is working with the investigative team. Because Corley led a similar team in Oklahoma City, "it was very natural that Gene's name came to the forefront," Rossberg says. "Gene has tremendous experience in looking at and organizing studies and getting the right people involved in the effort. This issue is not a concrete issue or a steel issue. It's a building issue."



Building is in Corley's blood. His father, a contractor in Shelbyville, Ill., a small town about 60 miles southeast of Springfield, built single-family homes, shopping malls and grocery stores. "I was on building sites even before I was big enough to crawl," he says. Only one of his father's projects collapsed -- a farm equipment warehouse that couldn't bear the weight of heavy snow on its roof. But that was the designer's error, not his father's, Corley hastens to add.



He enrolled in the architecture school at the University of Illinois at Urbana-Champaign in the 1950s, but opted for engineering instead. "I cannot draw a picture of a person that looks like a person," he says. "I figured I would flunk my art courses."



He got his bachelor's, master's and PhD in engineering at Illinois (his graduate adviser, Chester Siess, also tutored the late Fazlur Khan, the noted structural engineer who worked on both Sears Tower and the John Hancock Center). Then Corley joined the U.S. Army Corps of Engineers, for whom he developed a collapsible aluminum and steel bridge that would pop out of a tank and provide a river crossing for tanks and troops.



NASA vs. concrete



During the mid-1960s, the National Aeronautics and Space Administration asked him to work on its lunar rover program, but he took a less glamorous route, joining the Portland Cement Association, a concrete industry group that in 1997 spun off Construction Technology Laboratories to become a for-profit research and engineering company. At both Portland Cement and Construction Technology Laboratories, Corley explored the effects of fire on various materials, a body of knowledge that is proving essential in the Trade Center investigation.



"The aircraft hits wounded the buildings, but they could have recovered if it weren't for the fire," says Corley, who has been to Ground Zero four times since Sept. 11.



The inquiry centers on three areas:



Fireproofing: Investigators are trying to figure out whether the impact of the planes dislodged the fireproofing that was sprayed onto the structural steel of the twin towers -- an outcome that would have made the steel far more vulnerable to fire.



To determine what happened, the team will work backward, estimating the temperature of the fire just before the collapses occurred by taking samples of the salvaged steel that now sits in junk yards in New Jersey and a New York City dump. (Surviving chalk marks on that steel indicate where each piece went in the building.) Investigators then will use computer modeling to ascertain how long the buildings should have stood up, both with fireproofing intact and with it knocked off.



"That should give us a pretty good clue" about whether fireproofing stayed on or not, Corley says.



Escape routes: Based on media accounts showing that the vast majority of the dead were trapped on or above the floors where the planes hit, it appears that the stairwells in the Trade Center were wide enough to allow people on lower floors to get out and probably would have been wide enough to accommodate those on the upper floors as well. Elevators apparently operated right up to the time of collapse, though only on the floors below where the jets struck. (Not all elevators served the upper floors, and those that did were likely inoperable.)



As a result, Corley says, investigators are focusing on what happened on the floors where the planes hit and above. Did the jets smash through the stairwells, blocking access to escape routes on these floors? Or did the stairwells remain intact and fire prevented people from reaching them?



Structure: Corley's team is seeking to determine which structural elements of the twin towers gave way first and whether the skyscrapers' unique structural system played any role in the collapse.



Designed by the late architect Minoru Yamasaki and structural engineers Skilling, Helle, Christiansen, Robertson, the twin towers were each largely supported by a "tube" of closely spaced perimeter steel columns that formed a rigid box. That contrasts with a more typical building, in which an internal cage of columns and beams support a non-load-bearing exterior "curtain wall."



In another innovation, lightweight steel trusses (rather than heavier steel beams) supported the buildings' floors and connected the perimeter columns to the steel columns of each building's core. Finally, a pair of bolts (instead of multiple bolts) connected each truss unit to each perimeter column and core column.



Unusual structural system



The towers' structural system, which was economical because it used less steel and took less time to erect, was "unusual and not really repeated" in other buildings, says one member of the team, who asked not to be identified. There was "a very dense frame on the outside -- a good thing. But on the other hand, the [trusses] -- that's what you see in Kmart roofs -- they're not common."



Corley is less critical of the support network. "The structural system is a big part of the reason the towers held up after being hit by an airplane. [The investigators] don't think this was a highly vulnerable system. We think it did quite well."



The big question is whether this system was more prone to collapse than a conventional structural network after the planes hit and the fires started raging. The investigators don't know yet, but they are looking at two ways that the collapse might have occurred.



After impact, the weight of the columns would have been transferred to adjacent surviving columns on the perimeter of the building, with the surviving columns temporarily forming a kind of arch over the hole made by the plane. Because the remaining columns would have been supporting more weight than usual, Corley says, "they don't have to be heated as much before you have a collapse."



A variation on this theme is that the core columns collapsed first. Then, they would have dragged down the floor trusses and the exterior columns with them. This would have resembled a classic demolition scheme, Corley says, in which explosives "take out the interior part first."



Focusing on the trusses



The collapse also could have occurred another way: The floor trusses would have sagged because of the heat of the fire, and the trusses might have broken free because of the relatively weak connections between them and the columns. The weight of the trusses would have dropped onto the floors below, dangerously adding to their load. At the same time, the perimeter columns would have had no lateral support, causing them to buckle and fly outward.



In each case, one floor would have "pancaked" on top of another, with the weight from above bringing the entire building down. The only difference is which element gave way first -- the floors or the columns.



Some engineers have speculated that One World Trade Center stood longer than its counterpart because it was struck at a higher point -- around the 90th floor rather than 70th floor, as was the case at Two World Trade Center. Therefore, the columns beneath the hole had less weight to support. Such an explanation is plausible, Corley says, but it might founder because of the fact that the lower columns in the Trade Center towers were beefier than the higher columns, meaning that they might be able to handle the added stress on them.



"There is the possibility," he says. "that the [evidence] won't be conclusive and that we'll never know."



However the investigation turns out, Corley is always likely to remember how it got started -- with an incredible coincidence that brought one of member of the team dangerously close to one of the Sept. 11 terrorist attacks.



Calm conference call



About 8 a.m. that morning, Corley calmly began a conference call with Rossberg and a New York blast effects expert, Robert Smilowitz, about design standards they had been working on to make skyscrapers and other non-government buildings better able to resist the impact of terrorist bombs. Rossberg, who lives in Virginia and was driving to downtown Washington, was speaking on his cell phone.



One World Trade Center had been hit about 15 minutes earlier, but though the engineers were aware of this, they continued with their call, thinking that a small private aircraft had accidentally hit the massive high-rise. They even kept going when Smilowitz, who was in his SoHo office, reported that another plane had struck Two World Trade Center.



By a twist of fate, meanwhile, Rossberg had run into traffic jams and pulled into the parking lot of the Pentagon for respite. Around 8:40 a.m., he heard a tremendous explosion. The third hijacked plane had just burrowed itself into the side of the America's military hub, perhaps 1,500 feet from Rossberg's car.



"At that point, we ended our phone call," Corley says.



Copyright © 2002, Chicago Tribune

