CITY—The Commerce Department’s National Institute
of Standards and Technology (NIST) today presented its analysis
of how the World Trade Center (WTC) towers collapsed after
two aircraft were flown into the buildings by terrorists on
Sept. 11, 2001. This is the most detailed examination of a
building failure ever conducted.
most building collapses, these events were the result of a
combination of factors,” said Shyam Sunder, lead investigator
for the agency’s building and fire safety investigation
into the WTC disaster. “While the buildings were able
to withstand the initial impact of the aircraft, the resulting
fires that spread through the towers weakened support columns
and floors that had fireproofing dislodged by the impacts.
This eventually led to collapse as the perimeter columns were
pulled inward by the sagging floors and buckled.”
collapse sequences, which update and finalize hypotheses released
by NIST last October, were presented by Sunder at a press
briefing in New York City.
factors in the collapse sequences relevant to both towers
(the sequences vary in detail for WTC 1 and WTC 2) are:
aircraft severed perimeter columns, damaged interior core
columns and knocked off fireproofing from steel as the planes
penetrated the buildings. The weight carried by the severed
columns was distributed to other columns.
fires began that were initiated by the aircraft’s
jet fuel but were fed for the most part by the building
contents and the air supply resulting from breached walls
and fire-induced window breakage.
fires, in combination with the dislodged fireproofing, were
responsible for a chain of events in which the building
core weakened and began losing its ability to carry loads.
floors weakened and sagged from the fires, pulling inward
on the perimeter columns.
sagging and exposure to high temperatures caused the perimeter
columns to bow inward and buckle—a process that spread
across the faces of the buildings.
are supported by extensive computer modeling and the evidence
held by NIST, including photographs and videos, recovered
steel, eyewitness accounts and emergency communication records.
Additionally, this information was used to document a variety
of factors affecting the performance of the buildings, the
efforts of emergency responders and the ability of occupants
to escape prior to the collapses. In turn, NIST has identified
a number of future practices and technologies that potentially
could have enhanced building performance and life safety capabilities
on 9-11 had they been available for implementation. All are
being considered for NIST’s upcoming recommendations.
also released drafts of 15 reports from three projects of
the investigation: analysis
of building and fire codes and practices; occupant
behavior, egress and emergency communications; and fire
service technologies and guidelines.
for improvements to building and fire codes, standards and
practices derived from these and the other five projects in
the investigation will be released for public comment in June,
along with the draft of the final investigation report and
drafts of 27 reports from the remaining five projects.
WTC investigation’s goal is to recommend improvements
in the way people design, construct, maintain and use buildings,
this release are selected portions of Sunder’s
presentation at today’s press briefing that detail
the findings on structural and life safety factors, and the
future technologies/practices by which these factors might
have been enhanced on 9-11 had they been available. Sunder’s
full presentation (including the complete probable collapse
sequences for both WTC towers), the text of the 15 reports
issued, and all previous WTC investigation findings are available
non-regulatory agency of the U.S. Department of Commerce’s
Technology Administration, NIST develops and promotes measurement,
standards and technology to enhance productivity, facilitate
trade and improve the quality of life.
SHEET—Findings on Structural and Life Safety Factors
Factors that Enhanced Building Structural Performance
on Sept. 11, 2001
unusually dense spacing of perimeter columns, coupled with
deep spandrels, that was an inherent part of both the architectural
and structural design of the exterior walls, resulted in
a robust building that was able to redistribute loads from
severed perimeter columns to adjacent intact columns.
wind loads used for the World Trade Center (WTC) towers,
which governed the design of the perimeter frame-tube system,
significantly exceeded the prescriptive requirements of
the New York City building code and selected other building
codes of the era (Chicago, New York State), including the
relevant national model building code (BOCA).
robustness of the perimeter frame-tube system and the large
dimensional size of the WTC towers helped the buildings
withstand the aircraft impact.
composite floor system with open-web bar joist elements,
framed to provide two-way flat plate action, enabled the
floors to redistribute loads without collapse from places
of aircraft impact damage to other locations, avoiding larger
scale collapse upon impact.
hat truss resisted the significant weakening of the core,
due to aircraft impact damage and subsequent thermal effects,
by redistributing loads from the damaged core columns to
adjacent intact columns and, ultimately, by redistributing
loads to the perimeter walls from the thermally weakened
core columns that lost their ability to support the buildings’
a result of the above factors, the buildings would likely
not have collapsed under the combined effects of aircraft
impact and the subsequent jet-fuel ignited multi-floor fires,
if the fireproofing had not been dislodged or had been only
minimally dislodged by aircraft impact. The existing condition
of the fireproofing prior to aircraft impact and the fireproofing
thickness on the WTC floor system did not play a significant
role in initiating collapse on Sept. 11, 2001.
Practices and Technologies that Potentially Could Have Improved
Building Structural Performance on Sept. 11, 2001 (Requires
not dislodged or only minimally dislodged by aircraft impact.
columns and floor framing with greater mass to enhance thermal
and buckling performance.
passive and active fire protection features (e.g., compartmentation
to retard spread of building fires; thermally resistant
window assemblies to limit air supply and retard the spread
of fires; fire-protected and structurally hardened elevators
for firefighter access with continuous, redundant water
supply for standpipes).
with improved high-temperature properties (e.g., yield strength
and stiffness) and creep behavior.
There is far greater knowledge of how fires influence structures
in 2005 than was the case in the 1960s. The analysis tools
available to calculate the response of structures to fires
also are far better now than they were when the WTC towers
that Enhanced Life Safety on Sept. 11, 2001
the buildings were occupied by only about one-third of the
building’s full capacity of 25,000 occupants, the
egress capacity (number and width of exits and stairways)
was adequate for those survivors seeking and able to reach
and use undamaged exits and stairways.
elevators in WTC 2 enabled nearly 3,000 occupants to self-evacuate
prior to aircraft impact.
greater remoteness of stairwells in the impact areas of
WTC 2 enabled one of the stairwells to remain marginally
passable after aircraft impact.
large number (two-thirds) of surviving occupants participated
in a fire drill in the prior 12 months, with almost all
of those (93 percent) instructed about the location of the
were made to the life safety system components after the
responders provided evacuation assistance to building occupants.
a result of the above factors, approximately 87 percent
of the WTC tower occupants, including more than 99 percent
below the floors of impact, were able to evacuate successfully.
Practices and Technologies that Potentially Could Have Improved
Life Safety on Sept. 11, 2001 (Requires Analysis)
performance to delay or prevent building collapse.
stairwell integrity via increased remoteness of stairwells
and/or enhanced structural integrity of stairwell enclosures.
communications to occupants and among first responders via
improved systems and timely information sharing.
command and control for large-scale incident management
(e.g., location of command posts and physical assets; interagency
evacuation training (e.g., practice stairwell evacuation,
roof rescue not presently feasible as a standard option,
existence of transfer hallways).
life safety features (e.g., fire protected and structurally
hardened elevators available for occupant use during emergencies;
vibration-protected elevators such as those used in seismic
regions; self-evacuation capability for mobility impaired
occupants; operational smoke and fire control systems).
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