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Forest of columns kept Pentagon standingReport: Structural system saved lives September 11, 2001
WASHINGTON (CNN) -- A forest of steel-reinforced concrete pillars in the Pentagon saved many lives and prevented more serious damage to the building during the attack September 11, 2001, structural engineers said in a report released Friday. When the hijacked jumbo jet plowed into the Pentagon near ground level, it penetrated 310 feet through the building's outer three rings in less than a second, according to the report by the American Society of Civil Engineers. That sent debris through the west side of the building like an avalanche, according to the engineers. The attack killed 189 people -- 64 aboard the aircraft and 125 inside the Pentagon. "While this loss of life exceeded that of the Oklahoma City bombing, it is very remarkable that it was not worse," said Paul Mlakar, a team leader for the study. The bombing of the federal building in Oklahoma City, Oklahoma, killed 168 people April 19, 1995. The study gives most of the credit to the engineers who went beyond required standards 60 years ago, when they designed and built the structure as a document warehouse. The Pentagon has served as the nation's military headquarters since the end of World War II. The Pentagon was built of cast-in-place reinforced concrete, and the floors consisted of a slab, beam and girder system supported on spiral-steel-reinforced columns. Clues on limiting 'progressive collapse'That structural system redistributed the weight of the 29-acre Pentagon among the columns left standing after the plane hit, which limited the "progressive collapse" of the floors above, according to the study. A progressive collapse is the failure of a structure in which relatively localized damage to an element leads to a larger breakdown. "An understanding of its specific features will be valuable to engineers who need to design structures to resist progressive collapse," Mlakar said. Analysis also revealed that the plane's wings severed exterior columns but were not strong enough to cut through the second-floor slab. The impact turned the Boeing 757 inside out, igniting the jet's fuel and setting fire to exposed areas of the Pentagon. As the fire weakened the structural supports, a small part of the building collapsed after 20 minutes. Sprinklers in a recently renovated section helped prevent the fire from spreading. Recent safety modifications under the Pentagon Renovation Program also included blast-resistant windows, which helped limit casualties and saved lives, according to the report. Mike Sullivan, acting director of the Pentagon Renovation Program, said the construction and safety measures used at the Pentagon are significant to building professionals and the public. The engineers' report "explains what structural features and mechanisms contributed to the Pentagon's resiliency and how these features and mechanisms performed during the impact and ensuing fire," he said. "The report further underscores the usefulness and applicability of these systems in future building designs and other structures in which resistance to progressive collapse is critical."
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