Engineers Explain WTC Collapse
by B.J. Novitski
After seven months of wide speculation about the causes of New York's World Trade Center collapses on September 11, 2001, a report has been issued based on physical evidence and a thorough engineering analysis.
The report, World Trade Center Building Performance Study: Data Collection, Preliminary Observations and Recommendations, was produced by the Federal Emergency Management Agency (FEMA), the American Society of Civil Engineers (ASCE), and other organizations. The report explains why the buildings failed — but didn't fail right away, enabling 99 percent of people below the floors of impact to escape.
The authors attribute each tower's collapse to three separate but related "loading events." The first event was a Boeing aircraft hitting the building, cutting through the exterior structure and creating a fireball that immediately consumed some of the estimated 10,000 gallons (38 kiloliters) of jet fuel. The highrises' structural systems were sufficiently redundant, however, that this major damage by itself did not cause the collapse. According to the report, "most of the load supported by the failed columns is believed to have transferred to adjacent perimeter columns through Vierendeel behavior of the exterior wall frame."
The second event was the continuing fire, fed both by the remaining jet fuel and the office contents of furniture and paper. This fire heated and weakened the structural systems, adding stress to the damaged structure. Meanwhile, the sprinklers were not operating as designed. "Even if these systems had not been compromised by the impacts," says the report, "they would likely have been ineffective... the initial flash fires of jet fuel would have opened so many sprinkler heads that the systems would have quickly depressurized and been unable to effectively deliver water to the large area of fire involvement."
The third event was a progressive collapse: "As the large mass of the collapsing floors above accelerated and impacted on the floors below, it caused an immediate progressive series of floor failures, punching each in turn onto the floor below, accelerating as the sequence progressed. Freestanding exterior walls... buckled at the bolted column splice connections and also collapsed."
The Public Broadcasting Service's Nova series from WGBH Boston presented a popular account based on the engineering report. The companion Web site offers additional links to more information about the towers.
The complete story may never be known. Indeed a theme throughout the report is a call for additional research. However, the authors caution, it will be prohibitively expensive, if not impossible, to protect all structures from all hazards simply by strengthening the building codes.
B.J. Novitski is managing editor of ArchitectureWeek.
After the initial impacts, the most heavily loaded columns were probably near, but not over, their ultimate capacities. The structure successfully redistributed the building weight to the remaining elements and maintained stability long enough for a life-saving evacuation.
Heat caused steel in the floor trusses to expand, promoting buckling in columns, at the same time that the heat softened the steel and the aircraft debris contributed to gravity loads, leading to progressive collapse.
Engineers searched through piles of steel for pieces of the World Trade Center. In particular, they looked for columns exposed to fire or aircraft impact, connections, bolts, and floor trusses.
Photo: Structural Engineers of New York (SEAoNY)
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