WORLD TRADE CENTER EXPLOSION AND FIRE
NEW YORK, NY
FEBRUARY 26, 1993 

http://www.nfpa.org/Research/summaries/world_trade_center.html

The following is the Executive Summary from the report. The full
report can be ordered, for a fee, from NFPA's Charles S. Morgan
Library at library@nfpa.org.

SUMMARY

On Friday, February 26, 1993, a violent explosion ripped through a
parking garage in the sub-basement levels of New York's World Trade
Center complex, immediately killing six people. The explosion caused
extensive damage in several basement levels and resulted in an intense
fire that quickly distributed thick, black smoke to the upper levels of
many of the complex's seven buildings, causing a massive evacuation.
The explosion also disabled much of the fire protection systems within
the complex, including the fire alarm communication system for the "Twin
Towers," leaving evacuating occupants without emergency instructions
during their prolonged escape. Many of the approximately 150,000
occupants within the complex were confronted with dense smoke on
occupied floors and then in stairways as they began reaching the lower
levels of the buildings. Further, the smoke-filled stairways were
immersed in darkness when both the normal and emergency electrical
power were interrupted. More than 1,000 people were injured during
the evacuation of the complex, most suffering from the effects of smoke
exposure. Unlike most fatal fire scenarios, however, there were no
fatalities attributed to the effects of smoke despite the severe exposure to
products of combustion and the lengthy evacuation time.

The World Trade Center complex includes seven high-rise buildings, a
shopping mall, and several levels of underground parking. The two
largest high-rise buildings are 110 stories tall and are commonly referred
to as the "Twin Towers." In addition to these high-rise buildings, there is
a 47-story high-rise building, a 22-story high-rise building (Vista Hotel),
two 9-story high-rise buildings, and one 8-story high-rise building.
Excluding the hotel, most of the occupied space within the buildings is
dedicated for office use. All of the buildings, except for the 47-story
high-rise building, are constructed over a plaza area that contains the
60-store shopping mall, four underground levels of public parking, and
two utility levels. In addition to the large number of people working
there, the complex also serves as a commuter rail station, a connecting
point for thousands of commuters into and out of the city and attracts
thousands of daily visitors.

A van containing explosives was parked on the B-2 level of the
underground parking garage in a position adjacent to one of the towers
and under the Vista Hotel. At 12:18 p.m., the explosives were detonated
causing varying degrees of physical damage on all six basement levels. In
the immediate area of the explosion, the floor slabs for two basement
levels collapsed onto vital electrical, communications, and domestic
water systems equipment for the complex. Further, masonry fire walls
and fire doors separating the buildings within the complex were voided
by the force of the explosion. The explosion also penetrated vertically
into a first-floor public assembly area of the Vista Hotel and shattered
several glass partitions that separated the hotel from the lobby area of
one of the Center's towers. This penetration of the structures enabled
dense, black, super-heated smoke from the explosive materials and the
ensuing fire to quickly fill the lobby area and move into numerous
elevator shafts.

At the same time, the smoke, greatly accelerated by the extreme
temperatures and pressures of the explosion, was forced horizontally
throughout the basement area and entered additional elevator shafts
through doors and walls damaged by the explosion. Once in the elevator
shafts, the smoke spread vertically and began to fill the occupied areas of
the two towers, the Vista Hotel, and another high-rise building in the
complex.

The degree of smoke spread into the buildings varied according to the
extent of damage in the basement areas and according to the individual
building's distance from the explosion site. The most severely affected
buildings were the twin towers and the Vista Hotel. In time, smoke also
spread into a fourth building within the complex; however, all seven
buildings in the complex were evacuated. The initial speed of the smoke
spread was influenced most by the explosion, and forces associated with
the "stack effect," a natural, ever-present condition in high-rise buildings.
The spread of smoke to the upper levels of buildings by the "stack effect"
is common in high-rise fires.

The explosion also interrupted most of the normal electrical circuits in the
complex. In response to the power outage, the electrical demand was
transferred automatically to some of the emergency circuits for affected
areas in the towers, and diesel-powered emergency generators started
automatically in order to supply power to the emergency systems.
Shortly after starting, however, the emergency generator engines
overheated and shut down because components of their cooling system
were damaged by the explosion. Approximately 1 hour and 15 minutes
into the incident, all remaining electrical power to the affected buildings
within the complex was shut down.

Once the detonation and subsequent ignition of adjacent combustibles
occurred, the explosive forces and natural air movements quickly
distributed products of combustion to occupied areas through horizontal
and vertical openings, and soon total evacuation of the complex became
necessary. Many occupants entered stairways and were confronted by
the smoke. Later, due to the electrical power failure, the occupants
experienced total darkness in the stairways and resorted to other means
of illuminating their exit paths. The unexpected mass evacuation further
influenced adequate performance of the exiting system since the
stairways soon filled with people who were waiting for an opportunity to
enter stairways and held doors open, thus allowing smoke from the
respective floors to migrate into the stairways.

The loss of the normal electrical service and of the emergency generators
also affected the standpipe and sprinkler systems for most of the
buildings. The primary water supply for the standpipe systems and some
of the sprinkler systems was municipal water mains and electric fire
pumps. The primary water supply for the sprinkler systems in the towers
was gravity tanks which were not affected. With the loss of electrical
power, the primary water supply was limited to that provided by the
normal pressures in the water distribution system. Furthermore, the loss
of electrical power to domestic water pumps limited the capability of the
sprinkler systems in the towers to that water in the gravity tanks.
Fortunately, the fire did not propagate from the basement levels and thus
did not challenge the performance of the remaining fire protection
features.

Successful occupant response during a fire emergency in this complex is
dependent upon a transfer of information from emergency personnel in
the operations control center. After initial assessment of a reported fire,
for example, selective evacuation of occupant floors would be initiated as
determined by the control room operator or by arriving fire suppression
officers. In this incident, however, the control center was destroyed by
the explosion, leaving occupants without vital fundamental information
from emergency responders. As a result, the occupants' response to the
fire was uncoordinated, underscoring the necessity for all building
occupants to understand and be trained in proper fire safety procedures.
A detailed human behavior study of the occupants' actions in this massive
emergency is currently being undertaken by the NFPA.

The New York City Fire Department responded to the explosion and
fire at the World Trade Center with 16 alarms, involving hundreds of fire
fighters at the height of activities. This commitment represented
approximately 45 percent of the New York City Fire Department
on-duty resources and was the largest single response in the history of
the New York City Fire Department. Several fire crews were committed
to the suppression of cars and other combustible materials burning in the
basement. Though the vast majority of the fire fighters, however, were
committed to time-consuming tasks of search and rescue in all areas of
the seven high-rise buildings and assisting in the care of escaping
occupants.

During the explosion and fire and the prolonged evacuation, six people
died and 1042 people were injured, though there was a real potential for
many more deaths and severe injuries. Many of the injured occupants
suffered from smoke inhalation but, remarkably, none died from the
exposure. This occurred primarily because there was a limited amount of
combustibles that were initially ignited, and because of the basement
floor collapse, there was limited fire spread to adjacent materials.
Because of the limited burning there was a significant dilution of the
products of combustion as they moved through this massive building
complex. Had there been a more continuous burning or less dilution of
the smoke, it is likely that the loss of life in this tragedy would have been
far greater than the six casualties who were directly exposed to the force
of the explosion.

The facts of the World Trade Center incident and the lessons learned
should serve as an impetus for the fire safety community to re-examine
the current design philosophies and future directions for high-rise fire
technology. Prior to the incident there was no attempt to treat
"mega-high-rises" such as the World Trade Center differently from other
high-rise buildings with regard to reliability of fire protection system
design. However, since such mega-high-rises can simultaneously expose
tens of thousands of people to life-threatening conditions from a "single
event," more reliable performance of fire protection components may be
appropriate. Further warranting consideration is where the structures
contain critical, private enterprises or government agencies, as was the
case in the World Trade Center.

Although fire protection designers normally would include a "single event"
scenario in their performance criteria of fire protection systems, this
single event at the World Trade Center complex has initiated an active
discussion as to what a "single event" scenario should include. The
NFPA believes fire protection designers and code officials should
perhaps broaden their responsibilities to include security issues or other
such subtle changes in our society in achieving dependable and reliable
fire protection system performance for such critical occupancies.

The complexity of this incident also demonstrates the importance of
compliance with fundamental fire protection requirements for high-rise
buildings. The growing inventory of high-rise buildings in this country
should be reviewed to ensure that minimum levels of fire protection are
present. Further, recent fatal high-rise building fires have demonstrated
the importance of inspection and maintenance of these systems. One of
the more recent high-rise fires resulted in three fire fighters losing their
lives, further underscoring the importance of fire safety requirements for
these buildings. Finally, owners, insurers, security, fire protection
professionals, and code developers may want to re-examine fire
protection designs in high-rise structures, especially where those
structures accommodate large numbers of people and the economic
impact is great, to ensure that the protection systems achieve a high
degree of reliability during a "single event" occurrence. The NFPA's Life
Safety CodeŽ addresses occupant safety in both new and existing
buildings; an important consideration in fundamental fire safety principles
in the built environment.