The
Commerce Department’s National
Institute of Standards and Technology (NIST) today released
the latest findings from its building and fire safety investigation
into the World Trade Center (WTC) disaster of Sept. 11,
2001. These include the leading collapse sequence for each
of the two WTC towers; details from the analysis of first-person
interviews of nearly 1,200 WTC occupants, first responders
and families of victims; and new information gleaned from
the analysis of emergency response and evacuation procedures/actions.
WTC lead
investigator Shyam Sunder presented the findings today
at a public meeting of the National
Construction Safety Team (NCST) Advisory Committee, the
panel of building and fire experts established to advise NIST
in its conducting of technical building failure investigations
as authorized under the NCST Act. Previous WTC investigation
findings, including those documented in two interim reports,
are available at http://wtc.nist.gov.
Sunder’s presentation today, along with the presentations
of WTC investigation project leaders, may be found at the
same Web address.
The NIST
WTC investigation’s goal is to recommend improvements
in the way people design, construct, maintain and use buildings,
especially high-rises.
The findings
released today still may be revised and additional findings
still may be included in the team’s final report, scheduled
for release as a draft document for public comment in December
2004 or January 2005. NIST is not making any recommendations
at this time. All recommendations will be made in the final
report.
Key findings
not previously reported by the WTC investigation team include
the following:
- Leading
hypotheses for the collapse of WTC 1 and WTC 2 (the towers)
developed. These hypotheses identify the chronological
sequence of major collapse events for each tower and identify
specific load redistribution paths and damage scenarios.
Previously, a single working hypothesis was defined for
both towers without identification of the load redistribution
paths and damage scenarios resulting from aircraft impact
and the subsequent fires. The two hypotheses are detailed
in the attached sheet.
- Time
delay between the collapses of the WTC towers explained.
Although the WTC towers were attacked by virtually identical
aircraft, WTC 1 stood for 103 minutes before it collapsed—nearly
twice as long as WTC 2, which survived for 56 minutes. The
buildings themselves, although not identical, had many similarities.
The time delay between the collapses was due primarily to:
(1) the asymmetrical structural damage of the aircraft impact
to WTC 2 compared to the aircraft damage to WTC 1; (2) the
time it took for heat to soften, buckle and shorten core
columns that had fireproofing dislodged by debris impact;
(3) the structure’s ability to redistribute loads
as the core columns shortened; (4) the time it took for
fires to traverse from their initial location to the face
of the towers where perimeter columns were bowing inward
(as seen only minutes before the collapse of each tower);
and (5) the time it took for heat to soften and buckle those
columns.
- Post-impact
capabilities of the WTC towers assessed. Demand
to capacity ratios—the calculations indicating whether
or not structures can support the loads put on them—showed
that for the floors affected by the aircraft impacts, the
majority of the core and perimeter columns in both towers
continued to carry their loads after the impact. The loads
from damaged or severed columns were carried by nearby undamaged
columns. Although the additional loads strained the load-bearing
capabilities of the affected columns, the results show that
the columns could have carried them. This shows that the
towers withstood the initial aircraft impacts and that they
would have remained standing indefinitely if not for another
significant event such as the subsequent fires. NIST previously
reported that the towers had significant reserve capacity
after aircraft impact based on analysis of post-impact vibration
data obtained from video evidence on WTC 2, the more severely
damaged tower.
- Fire-induced
core column shortening detected. Due to heating
from fires following the aircraft impacts and subsequent
buckling, there was a shortening of core columns seen in
both towers on floors at or near the fire-affected impact
sites. Shortening of the core columns caused the floor system
to pull the perimeter columns inward—the observed
inward bowing that was seen minutes prior to the collapse
of each tower. Significant thermal sagging of the floor
system exacerbated the inward pull on the perimeter columns
in WTC 2. Vertical loads carried by shortened columns were
redistributed to perimeter columns, putting additional strain
on their load-bearing capabilities.
- Role
of fireproofing determined. The structural components
that became weakened due to the fires and eventually caused
the towers to collapse had their fireproofing dislodged
by debris from the aircraft impact. The region of dislodged
fireproofing was determined from the predicted path of the
debris. Had the fireproofing not been dislodged, the temperature
rise of the structural components would likely have been
insufficient to cause the global collapse of the towers.
Fireproofing dislodged by debris left the components more
sensitive to heat than any areas where there was missing
or thin fireproofing before the aircraft impacts.
- Majority
of steel found stronger than minimum requirements.
Approximately 87 percent of the recovered WTC steel specimens
tested exceeded the required minimum yield strengths specified
in the building design criteria; some 13 percent did not.
However, the safety of the towers was most likely not affected
by the small percentage of steel below the minimum. Building
designs routinely allow structures to withstand greater
loads than are expected by including significant factors
of safety. Moreover, the structural loads on Sept. 11, 2001,
were well below this design level.
- Full-building
evacuation presented challenges for occupants.
Based on first-person interview data, an assessment of WTC
1 and 2 occupant preparedness concluded that in both towers:
-
Occupants often were unprepared for the physical challenge
of full building evacuation;
-
Occupants often were unprepared to encounter transfer
hallways during the stairwell descent; and
-
Mobility challenged occupants were not universally identified
or prepared for full building evacuation.
- Movement
in WTC 1 stairwells perceived as a problem. Although
a number of persons who evacuated WTC 1 reported that they
perceived a problem with counterflow (the movement of firefighters
in the opposite direction) on the stairwells, it was determined
not to be a significant factor in the total evacuation time
of WTC 1 occupants when compared to other factors including
delays in evacuation initiation, evacuation interruption
and encountering obstacles in the evacuation path (such
as smoke, water and debris).
- Based
on first-person interviews, NIST estimates the average surviving
occupant spent 48 seconds per floor descending the stairwell,
which is about half as fast as previously reported for non-emergency
evacuations. NIST also estimates that each stairwell door
exited about 37 people per minute, which is comparable to
the slowest rate previously reported for non-emergency evacuations.
In other words, the average surviving occupants moved slower
down stairs and through stairwell exits than previously
reported for non-emergency evacuations.
Firefighters
and other first responders reported difficulty in climbing
the stairs due to crowding by evacuating occupants. Based
on first-responder interviews, NIST estimates that they
took an average of 1.4 to 2 minutes per floor to climb
up to their maximum height (mostly to floors in the 20s
and 30s). Therefore, it would have taken more than two
hours for a first responder wearing personal protection
equipment and carrying gear to reach the 60th floor using
the stairwell, while it would have taken about 1-1/2 hours
to do so without equipment and gear.
- Evacuees
did not receive coordinated or informative communications.
During interviews, survivors said that they felt emergency
communications could have been more helpful during the evacuation
of the towers. Specific knowledge about the location of
fires and aircraft impact damage was only occasionally communicated
to occupants who requested the information. Those communications
were apparently uncoordinated.
Additionally,
some contradictory announcements—first to return
to offices and then to start an orderly evacuation—were
heard by occupants in WTC 2 immediately prior to the aircraft
impact on that tower.
- Mobility
impaired occupants faced special evacuation challenges.
About 6 percent of the surviving occupants reported a pre-existing
limitation to their mobility. Examples of these limitations
include obesity, heart conditions, pregnancy, advanced age
and recent surgery.
Firefighters
and police officers found 40 to 60 mobility impaired occupants
on the 12th floor of WTC 1 as they attempted to clear
each floor on their way out. The impaired individuals
had been placed on this floor to await rescue in an attempt
to clear the stairway. Emergency responders were assisting
approximately 20 of these persons down the staircase just
prior to the tower’s collapse.
- First
responder command and control was hampered. While
a significant amount of evidence showed that the different
first responder agencies were, for the most part, working
together, they were hampered by inadequate information,
dispatch and unit assignment records. This included:
- First
responders, including key incident commanders, who did
not have adequate information (voice, video and data)
on conditions in the WTC towers or an overall perspective
of what was happening elsewhere at the WTC site.
- Large
numbers of firefighters who were dispatched to the WTC
site before adequate command posts and staff could be
assembled to manage them.
-
Self-dispatch by first responders and ambulances that
further complicated command and control at the site.
-
Self-dispatch by EMS and private/volunteer ambulance
units that contributed to clogging of the streets, making
it difficult for assigned responders to get through.
- Overwhelming
of the system for maintaining records of unit assignments
at each fire command posts because of the large numbers
of units and personnel.
In his
presentation to the NCST Advisory Committee today, Sunder
outlined the approach NIST is taking to formulate its recommendations
for improvements based on the lessons learned from the WTC
investigation. NIST, he said, is considering the following:
- Findings
related to building performance, evacuation and emergency
response, and procedures and practices;
- Whether
findings relate to the unique circumstances surrounding
the terrorist attacks of Sept. 11, 2001, or to normal building
and fire safety considerations, including evacuation and
emergency response;
- What
technical solutions are needed, if any, to address potential
risks to buildings, occupants and first responders, considering
both identifiable hazards and the consequences of those
hazards; and
- Whether
the risk is in all buildings or limited to certain building
types (e.g., a distinct height and area, or type of structural
system), buildings that contain specific design features,
iconic/signature buildings, or buildings that house critical
functions.
Sunder
also stated that, based on the investigation findings, NIST
has identified issues related to practices, standards and
codes that are the foundation for the WTC team’s final
recommendations (see Sunder’s presentation at http://wtc.nist.gov/media/NCSTACWTCStatusFINAL101904WEB2.pdf
for a detailed list of these issues).
The issues
being addressed have been grouped as follows:
- Increased
structural integrity;
- Enhanced
fire protection, including passive and active systems;
- Improved
building evacuation, including egress system design, emergency
communication to occupants, occupant preparedness and egress
technology; and
- Improved
emergency response, including access and firefighting, emergency
communications, and command and control.
Additionally,
the issues in each group have been more specifically defined
by categorizing under the following three levels:
Level
1
- Practices
- Standards,
codes and regulations
- Adoption
and enforcement
- Research
and development/further study
- Education
and training
Level
2
- All
tall buildings (buildings over 10 stories in height)
- Selected
tall buildings (buildings over 10 stories in height that
are at risk due to design, location, use, symbolism, contents,
etc.)
- Selected
other buildings (other buildings that are at risk due to
their use, content, historic status, symbolism, location,
etc.)
Level
3
- Related
to 9/11 outcome
- Unrelated
to 9/11 outcome
As a
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.
FACT
SHEET
Leading
Hypotheses for the Collapses of the World Trade Center Towers
The National
Institute of Standards and Technology (NIST) World Trade Center
(WTC) investigation team has formulated the chronological
sequence of major events leading to the eventual collapses
of the WTC 1 and WTC 2 towers as a result of the terrorist
attacks of Sept. 11, 2001. The leading collapse hypothesis
for each tower is based on evaluations of the building’s
innovative structural system; the effects of the aircraft
impact and subsequent fire; the post-impact condition of the
fireproofing; the quality and properties of the structural
steel used in construction; and the relative roles in the
collapse scenario played by the perimeter and core columns,
and the composite floor system (including connections).
The two
collapse hypotheses are consistent with all evidence currently
held by NIST, including photographs and videos, eyewitness
accounts, and emergency communications records. However, the
hypotheses released today still may be revised for the investigation
team’s final report, scheduled for release as a draft
document for public comment in December 2004 or January 2005.
The leading
collapse hypothesis for each tower is as follows:
WTC
1
- Aircraft
impact damaged the perimeter columns, mainly on the north
face, resulting in redistribution of column loads, mostly
to the adjacent perimeter columns and to a lesser extent,
the core columns.
- After
breaching the building’s perimeter, the aircraft continued
to penetrate into the building, damaging floor framing,
core columns and fireproofing. Loads on the damaged columns
were redistributed to other intact core and perimeter columns
mostly via the floor systems and to a lesser extent, via
the hat truss (the steel structure that supported the antenna
atop the towers and was connected to the core and perimeter
columns).
- The
subsequent fires, influenced by the impact-damaged condition
of the fireproofing:
- Softened
and buckled the core columns and caused them to shorten,
resulting in a downward displacement of the core relative
to the perimeter. This led to the floors (1) pulling
the perimeter columns inward, and (2) transferring vertical
loads to the perimeter columns; and
-
Softened the perimeter columns on the south face and
also caused perimeter column loads to increase significantly
due to restrained thermal expansion.
- Due
to the combined effects of heating on the core and perimeter
columns, the south perimeter wall bowed inward and highly
stressed sections buckled.
- The
section of the building above the impact zone began tilting
to the south as the bowed south perimeter columns buckled.
The instability rapidly progressed horizontally across the
entire south face and then across the adjacent east and
west faces.
- The
change in potential energy due to the downward movement
of the building mass above the buckled columns exceeded
the strain energy that could be absorbed by the structure.
Global collapse then ensued.
WTC
2
- Aircraft
impact damaged the perimeter columns, mainly on the south
face, resulting in redistribution of column loads, mostly
to the adjacent perimeter columns and to a lesser extent,
the core columns.
- After
breaching the building’s perimeter, the aircraft continued
to penetrate into the building, damaging floor framing,
core columns and fireproofing. Loads on the damaged columns
were redistributed to other intact core and perimeter columns
mostly via the floor systems and to a lesser extent, via
the hat truss.
- The
subsequent fires, influenced by the impact-damaged condition
of the fireproofing:
-
Caused significant sagging of the floors on the east
side that induced the floors to pull the perimeter columns
inward on the east face;
-
Softened and buckled the core columns on the east side
and caused them to shorten, which transferred significant
additional load to the perimeter columns on the east
face primarily through the floor system and to a lesser
extent, the hat truss; and
-
Softened some of the perimeter columns that were exposed
to high temperatures toward the northern half of the
east face.
- Due
to the additional loads on the perimeter columns on the
east face and the inward pulling of those columns, the east
perimeter wall bowed inward and highly stressed sections
buckled.
- The
section of the building above the impact zone began tilting
to the east and south as both the east perimeter columns
and the impact-damaged south perimeter columns buckled.
The instability rapidly progressed horizontally across both
faces and across the north face.
- The
change in potential energy due to the downward movement
of the building mass above the buckled columns exceeded
the strain energy that could be absorbed by the structure.
Global collapse then ensued.
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