DEP Invoice or Financial Record, September 30, 2001

NYC 9/11 Public Portal Document

unms were made of steel, and the

Defending intense heat would have caused the girders to expand, distorting their sha^pe and s^ipang their strength, leading to the collapse.

Skyscrapers .-“It’s better to build in reinforced ^concrete,” said Dr. Mir M, Ali, a professor of architecture at the Uni­ f Against Terror versity of Illinois. “If there is an impact, crash or explosion, it can absorb the energy better. That makes the building less vulnerable.” But reinforced concrete — con­ By KENNETH CHANG crete with steel bars inside — is heavier. When construction began on No one designs a skyscraper to withstand the direct hit the World Trade Center in the late of a fully fueled 767, and construction engineers agree that 196O’s, concrete was not a viable such an attack would have doomed almost any high-rise. option because it would have re­ Each World Trade Center tower absorbed the impact of quired huge, unwieldy pillars to sup­ a jet with a shudder, as each was designed to do, and stood. port the towers’ weight But high- Inside, though, 2,000-degree infernos started burning, fed strength concrete developed in re­ by thousands of gallons of jet fuel cent years has made it more practi­ It then became a question of time. Would the fuel bum up cal. first or would the steel columns weaken and buckle under the “The trend is toward more con­ heat? crete,” Dr. Mir said. “The technol­ For the people on floors above the crash site, there was ogy has substantially Improved. An another critical factor: an ordinary fire would take two or all-concrete structure would have three hours to bum through the gypsum wallboard around lasted longer.” the stairwells — but projectiles of plane wreckage almost Each of the two Petronas Towers certainly pierced through, letting in the fire and smoke. That has an outer ring of 16 7-foot-wlde trapped people on the upper floors. columns made of concrete and at the The south tower collapsed 56 minutes after impact The center of each tower is a 75-foot by north tower lasted an hour and 40 minutes. 75-foot concrete core — almost a Someone probably could build a fortress skyscraper. building within a building — that “Given enough money, we can design anything,” said Dr. houses the stairwells and elevator Charles H. Thornton, chairman of the Thomton-Tomasetti shafts. Group Inc. of New York City, the structural engineering firm Concrete — a mix of cement, sand that worked on the 1,483-foot-taII Petronas Towers in Kuala and gravel — is not impervious to Lumpur, Malaysia. heat, rae cement expands at a differ­ But no one would pay to build one, and no one would want ent rate than the sand and gravel, to work there. Such a building would probably have the causing cracks. Under intense heat, aesthetic appeal of a containment vessel of a nuclear power some types of concrete can flake plant, which is designed to survive the crash of a falling 747. apart at about three-quarters of an In the decades since the World Trade Center was built, inch an hour, eventually exposing the however, new materials and building techniques — some steel inside. used on the more recent super skyscrapers like the.Petronas “It ultimately would lose its Towers — may have given people more time to escape. strength,” Dr. Thornton said. The key would have been slowing the fir'es. The sprinkler The concrete core of the Petronas systems offered little help. Towers may have remained intact Even if the pipes survived the impact, the sprinklers of a under a similar crash and provided a typical skyscraper put out a few hundred gallons of water a better escape route than the gypsum­ minute for half an hour. Dr. Thornton said, and water would walled stairwells of the World Trade have been useless against a fuel fire in any case. (Water and Center. oil don’t mix; droplets of water sink “In our buildings, most of the into the fuel, turn into hot steam and stairways are in the core, which is a explode, and the fuel continues burn­ very safe haven,” Dr. Thornton said. ing.) The cores of the Petronas Towers By contrast, an anti-fire system at are also pressurized to keep smoke an aircraft hangar can unleash a and fire out of the stairwells. deluge of 120,000 gallons a minute of In addition to building more fire- water and foam — which sticks to resistant structures, another protec­ burning fuel for two hours tion against crashing airplanes straight, Dr. Thornton said. would be to keep the jet fuel from Since extinguishing the fire is im­ entering the interior, of the building. possible, “You have to build a more At the University of California at rugged building,” said Dr. R. Brady Berkeley, Dr. Abolhassan Astaneh- Williamson, an emeritus professor of Asl, a professor of structural engi­ civil engineering at the University of neering, has been developing a new California at Berkeley. construction technique — bolting The main ingredients of any sky­ half-inch steel plates to six-inch con­ scraper are steel and concrete. Both crete wadis — to create buildings that are strong, but in different ways. Concrete bears more weight; steel can bend without breaking. The World Trade Center’s supporting col-

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