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| (Image
courtesy of the Pentagon Building Performance Report) |
A long-awaited report
on the performance of the Pentagon in the aftermath of the
Sept. 11, 2001, terrorist attack contains few surprises and
confirms observations made following the event. The Pentagon
Building Performance Report concludes that original design
features–aided by recent upgrades–of the 6.5-million-sq-ft
home of the Dept. of Defense were key to limiting collapse
after a hijacked airplane slammed into the building.
"The Pentagon's structural performance
during and immediately following the Sept. 11 crash has validated
measures to reduce collapse from severely abnormal loads,"
states the report released Jan. 23 by the American Society
of Civil Engineers, Reston, Va.
The six-person study team of experts
in structural, fire and forensic engineering asserts that
if a building is required to resist progressive collapse,
it should be designed to include structural continuity and
redundancy. And the building's frame should also have energy-absorbing
capacity and reserve strength.
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| (Image
courtesy of the Pentagon Building Performance Report) |
In the Pentagon, continuity refers
to the use of the extension of bottom beam reinforcement through
girders and bottom girder reinforcement through columns. Redundancy
refers to the structure's two-way beam and girder system.
Spirally reinforced columns provided energy-absorbing capacity.
Reserve strength was provided by the original design for live
load in excess of service.
The first major renovation of the
five-story facility, completed in 1943, was under way at the
time of the attack. The first phase of the work in Wedge One–the
point of impact–included blast-resistant windows and
was near completion on Sept. 11. "The structural upgrades
of the exterior wall performed reasonably well, considering
they were not specifically designed for aircraft impact,"
the report states. "It really is remarkable that it wasn't
worse," says team leader Paul F. Mlakar, a technical director
with the Army Corps of Engineers.
The study recommends additional
research into progressive collapse mitigation and deformation
capacity of spirally reinforced columns subjected to lateral
loads over their height. The study's authors say they are
in no way implying that buildings should be expected to survive
such catastrophic events.
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