Key
Findings of NIST’s June 2004 Progress Report on the Federal
Building and Fire Safety Investigation of the World Trade Center
Disaster
The findings address four objectives. These are:
- To
determine (a) why and how the WTC 1 and WTC 2 collapsed following
the initial
impact of the aircraft, and (b) why and how
the 47-story WTC 7 collapsed;
- To
determine why the loss of life and injuries were so low or
so high depending
on location, including technical aspects
of fire protection, occupant behavior, evacuation and emergency
response;
- To
determine the procedures and practices which were used in
the design, construction,
operation and maintenance of
the WTC buildings; and
- To
identify, as specifically as possible, areas in national
building and fire
codes, standards and practices that warrant revision.
The following must be considered when reviewing the interim findings:
- Buildings
are not specifically designed to withstand the impact of
fuel-laden commercial airliners. While documents
from the Port Authority of New York and New Jersey (PANYNJ)
indicate that the impact of a Boeing 707 flying at 600 miles
per hour, possibly
crashing into the 80th floor, was analyzed during the design
of the WTC towers in February/March 1964, the effect of subsequent
fires was not considered. Building codes do not require building
designs to consider aircraft impact.
- Buildings
are not designed for fire protection and evacuation under
the magnitude
and scale of conditions similar to those caused
by the terrorist attacks of Sept. 11, 2001.
- The
load conditions induced by aircraft impact and the extensive
fires on Sept. 11,
2001, which triggered the collapse
of the WTC towers, fall outside the norm of design loads considered
in building codes.
- Prior
evacuation and emergency response experience in major events
did not include
the total collapse of tall buildings
such as the WTC towers and WTC 7 that were occupied and in
everyday use; instead, that experience suggested that major
tall building
fires result in burnout conditions, not global building collapse.
- The
PANYNJ was created as an interstate entity, under a clause
of the U.S. Constitution permitting compacts between states,
and is not bound by the authority of any local, state or federal
jurisdiction, including local building and fire codes. The
PANYNJ’s
long-standing stated policy is to meet, and where appropriate,
exceed the requirements of local building and fire codes.
Collapse
of the WTC Towers – Working Hypothesis
NIST is interested
in determining how and why WTC 1 stood nearly twice as long as
WTC 2 before collapsing (103 minutes versus 56
minutes), even though they were hit by virtually identical aircraft.
In addition, NIST is interested in determining what factors related
to normal building and fire safety considerations not unique to
the terrorist attacks of Sept. 11, 2001, if any, could have delayed
or prevented the collapse of the WTC towers.
The NIST investigation
team has formulated the following chronological sequence of major
events leading to the eventual collapse of
the towers:
- Aircraft
impact damaged the perimeter columns, causing redistribution
of column
loads to adjacent perimeter columns and
to the core columns via the hat truss (the steel structure
that supported the antenna atop the towers and was connected
to the
core and perimeter columns).
- After
breaching the building’s
exterior, the aircraft continued to penetrate into the buildings,
damaging core columns
with redistribution of column loads to other intact core and
perimeter columns via the hat truss and floor systems.
- The
subsequent fires, influenced by the post-impact condition
of the fireproofing,
weakened columns and floor systems (including
those that had been damaged by aircraft impact), triggered
additional local failures that ultimately led to column instability.
- Final
column instability resulted when redistributing loads could
not be accommodated
any further.
Among the factors
relevant to the condition and collapse of the WTC towers – and currently under analysis – were:
- The
innovative structural system at the time they were built,
incorporating many new and unusual features, including:
|
a
composite floor system, using open-web bar joist elements,
and |
|
the use
of wind tunnel testing to estimate lateral wind loads in
the design; |
- The
relative roles of the aircraft impacts and subsequent
fires;
- The
post-impact condition of the fireproofing on the
floor systems;
and
- The
qualities and properties of the structural steel used.
Following are
key points related to each of the four relevant factors:
Innovative Structural System
- The
fire protection of a truss-supported floor system by directly
applying spray-on
fireproofing was innovative and not
consistent with prevailing practice at the time of construction.
- The
fireproofing thickness (specified to meet a 2-hour fire endurance
rating)
was 1/2 inch at construction and was upgraded
on some floors to 1-1/2 inches prior to Sept. 11, 2001.
- Unrelated
to the WTC buildings, a model code evaluation system service
recommended
in June 2001 a minimum thickness of
2 inches for a similar floor system to achieve the 2-hour fire
rating.
- The three-to-four-fold
difference (between 1/2 inch and 2 inches) in specifying the
fireproofing thickness to meet the required
fire rating is extraordinarily large and confirms the lack of technical
basis in selecting a thickness.
- While
the building designers recognized the benefits of conducting
a full-scale
fire endurance test to determine the
required fireproofing thickness, no such tests were conducted
on the floor system used in the WTC towers (NIST will be conducting
this test later this summer).
- If
a “structural frame” approach
(considering that the floor truss was connected to the interior
and perimeter
columns, essentially forming a single structural unit) had
been used, the needed fire rating would likely have been 3
hours, as
it was for the perimeter columns alone.
- NIST
computer simulations indicate that flames in a given location
lasted about
20 minutes before spreading to adjacent,
yet unburned combustibles, and that this spread was generally
continuous because of the even distribution of combustibles
throughout the
floors and the lack of interior partitions.
- The
results of two sets of wind tunnel tests on the WTC towers
conducted by independent laboratories in 2002 and provided
to NIST show large differences – as much as 40 percent – in
resultant forces on the structures. Additionally, the wind
loads estimated from these tests are about 20-60 percent higher
than
those apparently used in the original design of the WTC towers.
- Wind
load capability is a key factor in determining the overall
strength of a tall
building and important in determining
its ability to withstand not only winds but also its reserve
capacity to withstand unanticipated events such as a major
fire or impact
damage.
- NIST
is conducting an independent analysis to establish the baseline
performance
of the WTC towers under the original design
wind loads and will compare those wind load estimates with
the then-prevailing code requirements.
Relative Roles of Aircraft Impact and Fires
- The
two WTC towers withstood the initial impact of virtually
identical aircraft
(Boeing 767 200ER) during the terrorist attacks
of Sept. 11, 2001. The robustness of the perimeter structural
system and the large dimensional size of the WTC towers helped
the buildings
withstand the aircraft impact.
- Following
impact, the WTC towers displayed and withstood vibrational
forces that
were as much as half the levels (in extreme
wind conditions) for which the buildings were designed.
- Preliminary
aircraft impact damage analysis indicates that the impact
of a fuel filled wing section resulted in extensive
damage to the exterior wall panel, including complete failure
of the perimeter columns.
- Fires
played a major role in further reducing the structural capacity
of the
buildings, initiating collapse. While aircraft
impact damage did not, by itself, initiate building collapse,
it contributed greatly to the subsequent fires by:
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compromising
the sprinkler and water supply systems; |
|
dispersing
jet fuel and igniting building contents over large areas; |
|
creating
large accumulations of combustible matter containing aircraft
and building contents; |
|
increasing
the air supply into the damaged buildings that permitted
significantly higher energy release rates than would normally
be seen in ventilation-limited building fires, allowing the
fires to spread rapidly within and between floors; and; |
|
damaging
ceilings that enabled “unabated” heat transport
over the floor to ceiling partition walls and to structural
components. |
- The
jet fuel, which ignited the fires, was mostly consumed within
the first
few minutes after impact. The fires that burned
for almost the entire time that the buildings remained standing
were due mainly to burning building contents and, to a lesser extent,
aircraft contents, not jet fuel.
- The
typical WTC office workstation furnishings were able to
sustain intense
fires for at least an hour on a given WTC floor.
Role of Fireproofing Conditions
- Most
of the floor systems in WTC 1 impacted by the aircraft crash
and fires
had upgraded or thicker (1-1/2 inches) fireproofing
while most of the affected floors in WTC 2 had the original
(1/2-inch) thickness.
- The
response of a structural component to fire is sensitive to
variability in fireproofing thickness along its length.
- As
applied – both in the original spraying and
in later upgrades – the fireproofing was found to be
thermally equivalent to uniform thicknesses that were greater
than the specified
minimums required by the building owner.
- It
was found that the acceleration of a structural component
would have to
be about 100-150 times the acceleration due to gravity
to dislodge 1-inch-thick fireproofing similar to that used
in the WTC towers. NIST is currently conducting analytical
studies to
estimate the magnitude of accelerations of the structural components
due to aircraft impact. This will help identify those regions
where fireproofing may have been dislodged.
Analysis of Recovered WTC Steel
- The
collection of 236 pieces of steel in NIST’s
possession is adequate for analyzing the quality and properties
of the steel for the investigation, emphasizing regions of
impact and fire damage. Pieces of all specified grades of steel
(for the
exterior panels, core columns and steel trusses in the floor
systems) were acquired.
- Analysis
of the recovered steel indicates that each of the structural
components
of the WTC towers had the grade specified
in the design drawings.
- Metallography
and mechanical property tests indicate that the strength
and quality of the steel was as specified, typical
of the era and likely met all qualifying test requirements.
- The
room-temperature strength of the steel used in the towers
met the relevant standards
and, in many instances, exceeded
the requirements by 5-10 percent.
- Analysis
is ongoing of the performance of the steel components under
impact and fire conditions up to the starting point of the
total building collapse.
Collapse
of WTC 7 – Working
Hypothesis
NIST is interested in determining why and how the 47-story WTC
7 building, a more typical tall building, collapsed even though
it was not directly hit by an aircraft.
The NIST investigation team has formulated the following chronological
sequence of major events leading to the eventual collapse of WTC
7:
- An
initial local failure at the lower floors (below Floor 13)
of the building
due to fire and/or debris induced structural
damage of a critical column (the initiating event), which supported
a large span floor bay with an area of about 2,000 square feet.
- Vertical
progression of the initial local failure up to the east penthouse,
as large
floor bays were unable to redistribute
the loads, bringing down the interior structure below the east
penthouse.
- Horizontal
progression of the failure across the lower floors (in the
region of Floors 5 and 7, that were much thicker
than the rest of the floors), triggered by damage due to the
vertical failure, resulting in the disproportionate collapse
of the entire
structure.
The working hypothesis is consistent with all evidence currently
held by NIST, including photographs and videos, eyewitness accounts
and emergency communication records.
Based on a
review of the fuel system for emergency power in WTC 7, Floor
5 – which did not have any exterior windows and
contained the only pressurized fuel distribution system on the
south, west and north floor areas – is considered a possible
fire initiation location, subject to further data and/or analysis
that improve knowledge of fire conditions in this area.
Evacuation and Emergency Response
NIST is interested in determining what factors related to normal
building and fire safety considerations, if any, could have saved
additional WTC occupant lives on Sept. 11, 2001, or could have
minimized the loss of life among the first responders.
Evacuation
Based on information and data gathered during the first-person
interviews of WTC surviving occupants, the following was learned:
- It
is estimated that 17,400 occupants (± 1,200)
were present in the WTC towers on the morning of Sept. 11,
2001. The initial population of each tower was similar.
- About
6 percent of the surviving occupants reported a pre-existing
limitation
to their mobility. These limitations included
obesity, heart condition, needing assistance to walk, pregnancy,
asthma, being elderly, chronic condition, recent surgery or
injury, and other.
- About
7 percent of the surviving occupants reported having special
knowledge
about the building. These included fire safety
staff, floor wardens, searchers, building maintenance and security
staff.
- Two-thirds
of surviving occupants reported having participated in a
fire drill in the 12 months prior to Sept. 11, 2001, while
17 percent reported that they received no training during that
same period. Of those participating in fire drills, 93 percent
were instructed about the location of the nearest stairwell.
Overall, slightly over half of the survivors, however, had
never used a
stairwell at the WTC prior to Sept. 11, 2001.
- Approximately
87 percent of the WTC tower occupants, including more than
99 percent of those below the floors of impact,
were able to evacuate successfully.
- Rough
estimates indicate that about 20 percent or more of those
who were in the
WTC towers and lost their lives may have
been alive in the buildings just prior to their collapse.
- Overall,
about 7,900 survivors evacuated WTC 2 in 73 minutes (i.e.
from the
instant the WTC 1 was struck by aircraft
until WTC 2 collapsed) while about 7,500 survivors evacuated
WTC 1 in 103 minutes. Thus, the overall evacuation rate in
WTC 2 (108
survivors per minute) was about 50 percent faster than that
in WTC 1 (73 survivors per minute). Functioning elevators allowed
many survivors to evacuate WTC 2 prior to aircraft impact.
Most
of the elevators in WTC 1 were not functioning, and survivors
could only use the stairways.
- After
the first airplane struck WTC 1 and before the second airplane
struck WTC
2, the survivors in WTC 2 were twice
as likely as those in WTC 1 to have already exited the building
(41 percent versus 21 percent). The rate of evacuation completion
in WTC 2 was twice the rate in WTC 1 during that same period.
- Soon
after the airplane struck WTC 2 until about 20 minutes before
each
building collapsed, the survivors in WTC 2 and WTC
1 had exited at about the same rate (the prior evacuation rate
of WTC 1).
- During
the last 20 minutes before each building collapsed, the evacuation
rate in both buildings had slowed to about one-fifth
the immediately prior evacuation rate. This suggests that for
those seeking and able to reach and use undamaged exits and
stairways,
the egress capacity (number and width of exits and stairways)
was adequate to accommodate survivors.
Based on use
of existing egress models and actual evacuation time on Sept.
11, 2001, it
is estimated that a full capacity evacuation
of each WTC tower with 25,000 people – three times the number
present on Sept. 11, 2001 – would have required about 4 hours.
To achieve a significantly faster total evacuation at full capacity
would have required increases in egress capacity (number and width
of exits and stairways).
Emergency Communication Systems
The analysis of the emergency responder communication tapes from
Sept. 11, 2001, indicates that:
- After
the first aircraft struck WTC 1, there was an approximate factor
of 5 peak
increase in traffic level over the normal level
of emergency responder radio communications, followed by an approximate
factor of 3 steady increase in the level of subsequent traffic.
- A
surge in communications traffic volume made it more difficult
to handle
the flow of communications and delivery of
information. Roughly a third to a half of the radio messages
transmitted during these radio traffic surge conditions were
not complete messages
or understandable.
- The
Fire Department of New York’s
(FDNY) citywide high-rise Channel 7 (PAPD Channel 30) radio
repeater at the WTC
site was operating.
- New
York Police Department (NYPD) aviation unit personnel reported
critical information
about the impending collapse of the
WTC towers several minutes prior to their collapse. No evidence
has been found to suggest that the information was further
communicated to all emergency responders at the scene.
Command and Control
Based on face-to-face
interviews, NIST has determined that first responders – including key incident commanders – did
not have adequate information (voice, video and data) on, nor an
overall perspective of, the conditions in the WTC buildings and
what was happening elsewhere at the WTC site. Interagency information
sharing was inadequate.
Active Fire Protection Systems
Investigation of the design, capabilities and performance of the
active fire protection systems in the WTC towers and WTC 7 indicates
that:
- The
smoke management systems in the WTC towers were not activated
during the fires
on Sept. 11, 2001, likely due to damage
inflicted by the aircraft impacts.
- HVAC
(heating, ventilation, and air-conditioning) ductwork was
a major path
for vertical smoke spread in the buildings.
- Computer
modeling shows that stair pressurization systems would have
provided minimal
resistance to the passage of smoke
in WTC 1 and WTC 2 had they been installed on Sept. 11, 2001.
- The
fire alarm system in WTC 7 sent only one signal (at 10:00:52
a.m. shortly after the collapse of WTC 2) to the monitoring
company indicating a fire condition. The signal did not contain
any specific information about the location of the fire within
the building. Since the system was placed on TEST for a period
of 8 h beginning at 6:47:03 a.m. on September 11, 2001, alarm
signals
would not have been shown on the operator’s display;
instead, they would have to be recorded into the history file.
- The
resistance to failure of the fire alarm system communications
paths between
the fire command station and occupied WTC tower floors
could have been enhanced if fiber optic communications cable
had been used instead of copper lines.
- Although
the fire sprinkler system was damaged by aircraft impact, the
water supply riser system lacked redundancy and there
existed the potential for single point failure of the water supply
connection on each floor.
Procedures and Practices
NIST seeks to determine the building and fire safety procedures
and practices that were used over the life of the WTC buildings
and how well those procedures and practices conformed to accepted
national building and fire safety practices, standards and codes.
Applicable Building Codes
The Port Authority of New York and New Jersey (PANYNJ) adopted
the provisions of the proposed 1968 edition of the New York City
Building Code, more than three years before it went into effect.
The 1968 edition allowed the PANYNJ to take economic advantage
of less restrictive provisions compared with the 1938 edition that
was in effect when design began for the WTC towers in 1962. The
1968 code:
- Eliminated
a “fire tower” (a
smoke-free stairwell) as a required means of egress;
- Reduced
the number of required stairwells from 6 to 3, and the size
of doors
leading to the stairs from 44 inches to 36
inches;
- Reduced
the fire rating of the shaft walls in the building core from
3 hours
to 2 hours;
- Changed
partition loads from 20 pounds per square foot to one based
on weight of
partitions per unit length (that reduced
such loads for many buildings including the WTC buildings);
and
- Permitted
a 1-hour reduction in fire rating for all structural components
(columns from 4 hours to 3 hours and floor framing members
from 3 hours to 2 hours).
The New York City Department of Buildings reviewed the WTC tower
drawings in 1968 and provided comments to the PANYNJ concerning
the plans in relation to the 1938 NYC building code. The architect-of-record
submitted responses to those comments to the PANYNJ, noting how
the drawings conformed to the 1968 NYC building code.
Standards, Codes and Regulations
NIST has reviewed past and current standards, codes and regulations
relevant to assessing the procedures and practices used in the
design, construction, operation and maintenance of the WTC buildings.
Based on that review, the following issues merit further consideration:
- Code
provisions with detailed procedures to analyze and evaluate
data from fire
resistance tests of other building components
and assemblies to qualify an untested building element.
- Code
provisions that require the conduct of a fire resistance
test if adequate
data do not exist from other building components
and assemblies to qualify an untested building element.
- Regulations
that would adopt code provisions using the “structural
frame” approach to fire resistance ratings that requires
structural members – other than columns – that
are essential to the stability of the building as a whole to
be fire
protected to the same rating as columns.
- Code
provisions that ensure that structural connections are provided
the same
degree of fire protection as the more restrictive
protection of the connected elements.
- Code
provisions and standards to establish whether the minimum
mechanical and
durability related properties of spray-applied
fire resistive materials (SFRM) are sufficient to ensure acceptable
in-service performance in buildings. While minimum bond strength
requirements exist, there are no requirements for such materials
to withstand typical shock, impact, vibration or abrasion effects
over the life of a building.
- Rigorous
field application and inspection provisions and regulatory
requirements to assure that the as-built condition
of the passive fire protection, such as SFRM, conforms to conditions
found in fire resistance tests of building components and assemblies.
- Rigorous
provisions and regulatory requirements for in-service inspections
of passive
fire protection during the life of the building.
- Early
installation of sprinklers in existing buildings, not as
an option in lieu
of compartmentation (office space separated
by internal walls).
- Standards
and code provisions that provide minimum structural integrity
to
protect the means of egress (stairwells and elevator
shafts) in the building core which are critical to life safety.
- Standards
and code provisions to install fire-protected elevators and
permit their use for routine emergency access by
first responders or as a secondary method (after stairwells)
for emergency evacuation of building occupants.
- Explicit
standards and code provisions for structural integrity that
mitigate progressive
collapse.
- Standards
and code provisions for conducting wind tunnel tests and
for the
methods used in practice to estimate design wind
loads from test results.
- Regulatory
requirements for retention of documents related to the design,
construction, operation, maintenance and modifications
of buildings, including retention off-site.
Fire Safety and Egress Design Methods
Performance-based methods that explicitly define the design objectives
and specific design-basis fire hazards or evacuation events are
better suited to risk analysis than traditional prescriptive methods
of deriving code provisions and standards. Historical fire loss
data suggest that prescriptive methods have considerable built-in
conservatism to adequately protect building occupants. Performance-based
methods enable appropriate protection to be provided where it is
needed.
The increasing use of performance-based methods, as an alternative
to prescriptive design, in fire safety and egress design, raises
the following issues that merit further consideration:
- Considering
fire as a design condition in structural design, including
evaluation of the fire performance of the structure as a whole
system. This is already done with other hazards such as wind
and
earthquakes.
- Detailing
procedures to select appropriate design-basis fire scenarios
for performance-based design of the sprinkler
system,
compartmentation
and passive protection of the structure.
- Validating
and verifying tools for use in performance-based design practice
to analyze the dynamics of building fires and their
effects on the structural system that would allow engineers
to evaluate structural performance under alternative fire scenarios
and fire protection strategies.
- Developing
the technical basis to establish whether the construction
classification and fire rating requirements are risk-consistent.
Specifically, it is not apparent how the current height and
area
tables in building codes consider the technical basis for the
progressively increasing risk to an occupant on the upper floors
of tall buildings
that are much greater than 200 ft in height. The maximum fire
rating in current codes applies to any building more than about
12 stories
in height.
- While
sprinklers improve safety in most common building fires and
prevent them
from becoming large fires, the technical
basis is not available to establish the sprinkler trade-off
in current codes which allows a lower fire rating to be used
for structural
components in spinklered buildings.
- Designing
egress systems to achieve a target performance (e.g., evacuation
rate
or time) for a given occupant population
by adequately considering travel distance, remoteness requirements,
and human factors (such as occupant size, stairwell environmental
conditions, visibility and congestion).
Building Practices
The PANYNJ entered into agreements with the NYC Department of
Buildings (DOB) in the 1990s with regard to conformance of its
buildings to the NYC Building Code. However, the PANYNJ did not
yield jurisdictional authority for regulatory and enforcement oversight
to the DOB. The PANYNJ was created as an interstate entity and
is not bound by the authority of any local, state or federal jurisdiction.
The architect is responsible for specifying the fire protection
in current building practice. The structural engineer is not
required to evaluate and certify that the passive fire protection
is adequate to protect the structural system. In accordance with
established practice, the structural engineer was not responsible
for the passive fire protection in the design of the WTC tower
structures.
In addition,
there is no requirement to involve a fire protection engineer
in the design
and evaluation of a building’s fire
protection system. In some cases, architects retain fire protection
engineers to assist with the fire protection design for a building.
There are few academic degree programs or continuing education
programs that qualify engineers (or architects) to evaluate the
fire performance of structures. The current state-of-practice is
not sufficiently advanced for engineers to routinely analyze the
performance of a whole structural system under a prescribed design-basis
fire scenario.
Approach to Recommendations
NIST does not
set building codes and standards but provides technical support
to the private
sector and other government agencies in
the development of U.S. building and fire practices, standards
and codes. NIST recommendations are given serious consideration
by private sector organizations that develop national standards
and model codes – which provide minimum requirements for
public welfare and safety.
The NIST building and fire safety investigation of the WTC disaster
has not yet formulated recommendations. However, in doing so, NIST
will consider the following:
- Findings
from the first three independent investigation objectives
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.
- Whether
the risk is in all buildings or limited to certain building
types (such as height and area or structural system),
buildings that contain specific design features, iconic/signature
buildings or buildings that house critical functions.
Date created:
6/18/2004
Last updated: 6/18/2004
Contact: inquiries@nist.gov
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