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Research May Never Pinpoint Sequence of Events on 9/11

Officials leading the federal investigation into the destruction of the World Trade Center say the $16-million study may never determine the exact sequence of events that led to the collapses triggered by terrorist plane attacks on Sept. 11, 2001. But the National Institute of Standards and Technology says it is most likely that columns of the twin 110-story towers bulged out and yielded first, as a result of floor trusses heating up and expanding in length.

VALIDATION Truss and column assemblies were burned to assess accuracy of agency’s fire dynamics simulator.

The result of "very focused calculations is that columns will yield first," says S. Shyam Sunder, Gaithersburg, Md.-based NIST’s lead World Trade Center investigator.

The scenario trumps the hypothesis that room-temperature floor trusses, adjacent to the fire floors, pulled out from their columns because of tension. The least likely scenario, say researchers, is that compressive forces in the heated trusses caused them to buckle.

The two-year investigation’s "final draft" report is not due until fall but NIST released some preliminary findings last month (ENR 12/15/03 p. 10).


One under further study concerns wind loads. Wind tunnel tests performed in 2002 seem to show higher wind loads than were apparently used in the WTC design, done in the 1960s. One test indicated wind loads varied between 15 and 30% higher; the other was 66% higher. NIST is still analyzing 1960s wind-tunnel test results to estimate wind loads considered in the design.

Sunder says recent wind tunnel tests in general indicate higher loads. "It’s not clear if increases are due to better scientific understanding of the wind effects or an increase in the conservatism" of persons conducting tests, he says. The inconsistent test results might suggest a need for better wind tunnel test procedures, says Sunder.

Much attention is on the effect of fire on structure. Sunder says NIST needs to study how much of the buildings’ performances were due to "unique circumstances," and how much were due to the "intrinsic fire protection." NIST also wants to determine factors that would have delayed or prevented the "fire-induced collapses," he says.

RANKINGS Floor buckling, under fire loads, is rated least likely scenario. fires burned.

NIST is estimating how many people in the towers were still alive at the time of the collapses. Some of the 2,752 people killed died from plane impact, smoke inhalation, fire and jumping.

NIST has created controversy by using the term "composite bar-joist floor system" in its December report. The term bar joist was avoided in the WTC Building Performance Study by the American Society of Civil Engineers-Federal Emergency Management Agency, the basis for NIST’s research.

"I hadn’t heard of trusses being called joists until after 9/11, when some structural engineers wanted to ‘degrade’ the structural systems in the WTC," says Saw-Teen See, managing partner of Leslie E. Robertson Associates. The Manhattan firm’s founding partner, Leslie E. Robertson, was the WTC’s project manager.

The discussion is considered important because bar joists are often disparaged and because of New York City’s intention to ban bar joists in buildings over 75 ft tall, pending further research. The action is a result of 9/11.

Lawrence G. Griffis, president of the structures division in the Austin office of Walter P. Moore Associates Inc., and chair of the ASCE task committee on wind loads, is offended by the looming ban, calling bar joists a "high-quality, premanufactured product" with an "astonishingly low" failure rate. But he "absolutely" agrees the element used in the WTC was not a bar joist. "It was custom-designed, not picked from a book," Griffis says, adding that he objects to any criticism of the WTC design or con- struction, not just to joist bashing. "Those buildings were ahead of their time by a long shot," he says.

Robert Hackworth, managing director of the Steel Joist Institute, Myrtle Beach, S.C., calls the WTC system a "bar truss." It was not an approved SJI design, though it was supplied by an SJI member, he says. Because it was not SJI-approved, James M. Fisher, vice president of Computerized Structural Design, Milwaukee, and technical consultant to SJI, says they are not bar joists but trusses.

FLAME PROGRESS Computer model simulates fire spread on the 97th floor of One WTC.

Engineers describe a bar-joist system as a one-way system of closely spaced trusses connected only with bridging. When there is a concrete slab, it is not composite with the member. The floor or roof deck spans perpendicular to the joists and uniformly loads the top chord.

The WTC floor system was a two-way truss system. In addition, the concrete slab was cast composite with the trusses through a steel knuckle. And the metal floor decking was parallel to the primary trusses, not perpendicular, thereby loading the truss in point loads.

The federal Occupational Safety and Health Administration defines a steel joist as an open-web, secondary load-carrying member of maximum 144 ft length. It is designed by the manufacturer, not the engineer of record. A joist girder is defined as an open-web, primary load-carrying member, also designed by the manufacturer. A steel truss is defined as an open-web member designed of structural steel components by the project structural engineer of record.

The WTC’s structural engineer, Skilling Helle Christiansen Robertson, called the member a floor truss on the drawings. "All sizes of all members of all trusses were provided in the drawings," says Robertson, currently LERA’s director of design.

Sunder concedes there is ambiguity and suggests calling the system a two-way, composite floor-truss system that utilizes components typical of open-web steel joist assemblies.

Griffis and some other practitioners think the $16 million for NIST’s WTC investigation could be better spent elsewhere. There is no need to consider code changes as a result of 9/11 except for exiting and "particularly vulnerable [structural] members," Griffis says.

(Images courtesy of NIST)

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