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Why
The Towers Collapsed
The World Trade
Center Building Performance Study
Following the September 11, 2001, attacks on New York City's
World Trade Center (WTC), the Federal Emergency Management Agency (FEMA) and the
Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE),
in association with New York City and several other Federal agencies and
professional organizations, deployed a team of civil, structural, and fire
protection engineers to study the performance of buildings at the WTC site.
The
events following the attacks in New York City were among the worst building
disasters in history and resulted in the largest loss of life from any single
building collapse in the United States. Of the 58,000 people estimated to be at
the WTC Complex, over 3,000 lost their lives that day, including 343 emergency
responders. Two commercial airliners were hijacked, and each was flown into one
of the two 110-story towers. The structural damage sustained by each tower from
the impact, combined with the ensuing fires, resulted in the total collapse of
each building. As the towers collapsed, massive debris clouds consisting of
crushed and broken building components fell onto and blew into surrounding
structures, causing extensive collateral damage and, in some cases, igniting
fires and causing additional collapses.
In total, 10 major buildings experienced
partial or total collapse and approximately 30 million square feet of commercial
office space was removed from service, of which 12 million belonged to the WTC
Complex. As each tower was struck, extensive structural damage, including
localized collapse, occurred at the several floor levels directly impacted by
the aircraft. Despite this massive localized damage, each structure remained
standing. However, as each aircraft impacted a building, jet fuel on board
ignited. Part of this fuel immediately burned off in the large fireballs that
erupted at the impact floors. Remaining fuel flowed across the floors and down
elevator and utility shafts, igniting intense fires throughout upper portions of
the buildings. As these fires spread, they further weakened the steel-framed
structures, eventually leading to total collapse. The collapse of the twin
towers astonished most observers, including knowledgeable structural engineers,
and, in the immediate aftermath, a wide range of explanations were offered in an
attempt to help the public understand these tragic events. However, the collapse
of these symbolic buildings entailed a complex series of events that were not
identical for each tower. The structural damage sustained by each of the two
buildings as a result of the terrorist attacks was massive. The fact that the
structures were able to sustain this level of damage and remain standing for an
extended period of time is remarkable and is the reason that most building
occupants were able to evacuate safely. Events of this type, resulting in such
substantial damage, are generally not considered in building design, and the
ability of these structures to successfully withstand such damage is noteworthy.
Preliminary analyses of the damaged structures, together with the fact the
structures remained standing for an extended period of time, suggest that,
absent other severe loading events such as a windstorm or earthquake, the
buildings could have remained standing in their damaged states until subjected
to some significant additional load. However, the structures were subjected to a
second, simultaneous severe loading event in the form of the fires caused by the
aircraft impacts. The large quantity of jet fuel carried by each aircraft
ignited upon impact into each building. A significant portion of this fuel was
consumed immediately in the ensuing fireballs. The remaining fuel is believed
either to have flowed down through the buildings or to have burned off within a
few minutes of the aircraft impact. The heat produced by this burning jet fuel
does not by itself appear to have been sufficient to initiate the structural
collapses. However, as the burning jet fuel spread across several floors of the
buildings, it ignited much of the buildings' contents, causing simultaneous
fires across several floors of both buildings. The heat output from these fires
is estimated to have been comparable to the power produced by a large commercial
power generating station. Over a period of many minutes, this heat induced
additional stresses into the damaged structural frames while simultaneously
softening and weakening these frames. This additional loading and the resulting
damage were sufficient to induce the collapse of both structures.
Credit: Steve Greenberg, Ventura County Star, Calif.; posted
with artist's permission
Greenberg
Art
The ability of
the two towers to withstand aircraft impacts without immediate collapse was a
direct function of their design and construction characteristics, as was the
vulnerability of the two towers to collapse a result of the combined effects of
the impacts and ensuing fires. Several building design features have been
identified as key to the buildings' ability to remain standing as long as they
did and to allow the evacuation of most building occupants.
These included the
following:
- robustness and redundancy of the steel framing system
- adequate
egress stairways that were well marked and lighted
- conscientious
implementation of emergency exiting training programs for building tenants
Similarly, several design features have been identified that may have played a
role in allowing the buildings to collapse in the manner that they did and in
the inability of victims at and above the impact floors to safely exit. These
features should not be regarded either as design deficiencies or as features
that should be prohibited in future building codes. Rather, these are features
that should be subjected to more detailed evaluation, in order to understand
their contribution to the performance of these buildings and how they may
perform in other buildings.
These include the following:
- the type of steel
floor truss system present in these buildings and their structural robustness
and redundancy when compared to other structural systems
- use of
impact-resistant enclosures around egress paths
- resistance of passive fire
protection to blasts and impacts in buildings designed to provide resistance to
such hazards
- grouping emergency egress stairways in the central building core,
as opposed to dispersing them throughout the structure all.
Several other buildings, including the Marriott Hotel
(WTC 3),
the South Plaza building (WTC 4), the U.S. Customs building (WTC 6), and the
Winter Garden, experienced severe damage as a result of the massive quantities
of debris that fell on them when the two towers collapsed. The St. Nicholas
Greek Orthodox Church just south of WTC 2 was completely destroyed by the debris
that fell on it. WTC 5, WTC 7, 90 West Street, the Bankers Trust building, the
Verizon building, and World Financial Center 3 were impacted by large debris
from the collapsing twin towers and suffered structural damage, but arrested
collapse to localized areas.
The debris from the collapses of the WTC towers also initiated
fires in surrounding buildings, including WTC 4, 5, 6, and 7; 90 West Street;
and 130 Cedar Street. Many of the buildings suffered severe fire damage but
remained standing. However, two steel-framed structures experienced fire-induced
collapse.
WTC 7 collapsed completely after burning unchecked for approximately 7
hours, and a partial collapse occurred in an interior section of WTC 5. Studies
of WTC 7 indicate that the collapse began in the lower stories, either through
failure of major load transfer members located above an electrical substation
structure or in columns in the stories above the transfer structure. The
collapse of WTC 7 caused damage to the Verizon building and 30 West Broadway.
The partial collapse of WTC 5 was not initiated by debris and is possibly a
result of fire-induced connection failures. The collapse of these structures is
particularly significant in that, prior to these events, no protected
steel-frame structure, the most common form of large commercial construction in
the United States, had ever experienced a fire-induced collapse.
http://www.house.gov/science/hot/wtc/wtcreport.htm
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