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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.
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| 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.
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| 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.
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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