Engineering Report on Pentagon Disaster
When the Pentagon
in Washington D.C. was hit by a hijacked plane on September 11, 2001,
the damage and the loss of life were appalling. But the destruction was
less severe than might have been expected from such an impact. About
20,000 people were at work in the U.S. Department of Defense
headquarters, the largest office building in the world. Yet according
to casualty reports, only 125 Pentagon employees were killed along with
the 64 from the ill-fated airliner.
As often happens after natural disasters
such as earthquakes or hurricanes, an engineering investigation was
immediately launched to learn exactly how structures succomb to
extraordinary forces. Such investigations may lead to further
refinements in U.S. building codes.
"The Pentagon Building Performance Report" has now been released by the American Society of Civil Engineers
(ASCE). After seven months of study, a team of six prominent
structural, fire protection, and forensic engineers have drawn
conclusions about the building's structural resistance to progressive
collapse during and immediately following the airliner crash. The
report may have far-reaching implications for engineering design
Constructed during World War II as office
space that would store unusually large quantities of files and
documents, the Pentagon was designed to support heavier loads than an
ordinary office building. The investigative team observed that the
direct impact of the crash destroyed approximately 50 columns on the
ground floor and six columns along the exterior walls of the floor
above. The structural system immediately redistributed the weight of
the building and its contents onto the columns left standing, thereby
limiting the collapse of floors above the point of impact.
The subsequent fire ignited by aircraft
fuel, and fed by the aircraft components and building contents, caused
moderate damage to the reinforced concrete frame in relatively small
areas on those two floors, which later collapsed.
The engineers found that the original
structural design helped arrest the progression of collapse and
resultant loss of life. These structural design elements include short
spans between structural supports, continuity of floor reinforcement,
and redundant load paths, allowing one support system to carry the load
if another fails.
Many columns withstood extreme lateral
loads from the impact of the aircraft, and the floor system was capable
of significant load redistribution without collapse when several
adjacent supporting columns were removed. Moreover, the blast-resistant
windows installed as part of the Pentagon Renovation Program performed well in resisting the aircraft impact and pressure from the massive ignition of fuel.
The team recommends several design
measures that could reduce the chance of structural collapse in other
buildings. These measures include continuity, such as the extension of
floor reinforcements through structural supports; redundancy in design
and construction, such as two-directional framing of the floors;
spirally steel-reinforced concrete columns that absorb energy generated
from lateral loads; and design for higher-than-normal loads.
The team urges further research and
development in the prevention of progressive collapse and structural
response to improbable events. They specifically recommend research
into the load-carrying capacity of columns and other structural
elements after severe deformation, the vertical load on floor systems
resulting from horizontally deformed columns, and the energy-absorbing
capacity of concrete elements when subjected to impact loads that
result in large deformations.
Founded in 1852, the ASCE represents
130,000 civil engineers worldwide and is the oldest national
engineering society in the United States.
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