Modern Construction Evaluated After World Trade Center Fire

Modern Construction Evaluated After World Trade Center Fire


Fire damage in office on 11th floor where fire at World Trade Center started.

New York Fire Department photo by R. Keller

The effects of modern construction practices on the fire safety of high-rise buildings are evaluated in a New York Board of Fire Underwriters report on a fire in one of the Twin Towers in the World Trade Center office building complex in New York City.

The fire that started shortly before midnight last February 13 on the 11th floor in the North Tower, or 1 World Trade Center, involved about 9000 square feet on the floor of origin and spread to telephone equipment closets on the 9th through the 19th floors. Fire came out vents in phone closet doors on the 12th and 13th floors and ignited office files. The phone closet fires were quickly extinguished, although the phone panels and wiring in three closets were destroyed and the equipment in the eight other closets was severely damaged.

Three alarms were struck for the fire in the North Tower, which with its twin South Tower shares the distinction of being the world’s tallest building—110 stories and 1350 feet—for the shortest time when the skyscraper title went to the 1454-foot Sears Building in Chicago. Half the contents of the offices of R. J. Saunders & Company in the southeast corner of the 11th floor were destroyed and the remainder was damaged.

How fire spread

The fire ignited phone cable in the plenum over the suspended ceiling and spread to the phone closets through the approximately 12 x 18inch non-fire-stopped openings in the floors of these closets. The cable had polyvinyl chloride insulation, the underwriters’ report noted.

The investigation of the fire and the report were done by W. Robert Powers, superintendent of the New York Board of Fire Underwriters.

In reviewing the fire record of the more than 2000 high-rise buildings, excluding residential occupancies, in New York City, it was found that there are about 170 fires in such buildings each year.

Records for the last six years show that in the 151 office buildings in New York that are fully sprinklered, there were 85 fires—from the basement to the 34th floor—which required sprinkler operation. Control was attained by the operation of one head in 64 of these fires, two heads in 11 fires and three heads in six fires. Three other fires were controlled by the operation of either four, five or six heads. A closed floor valve caused the single sprinkler system failure in these 85 fires.

In reference to construction tradeoffs for the installation of automatic sprinkler systems throughout buildings, the report declared, “The facts of life are that the trade-offs have already been taken in the modern design of buildings. The installation of sprinklers is the best way to return building protection to the level of the past.”

Among the recommendations made in the report of the New York Board of Fire Underwriters is the installation of sprinklers in stores and areas containing large amounts of combustible material or highly combustible material. The report pointed to the increased use of combustible furniture, partitions, carpeting and wall finishes in offices and mentioned in particular “highly flammable foamed polyurethane and foamed rubber cushioning” that can burn so rapidly that it is “a threat to people in the immediate or nearby areas.” The fire danger of large amounts of paper in offices and open-rack storage files also was mentioned.

Return air shafts in unsprinklered buildings, the report continued, should nave detectors on each floor to cause the air-conditioning system to discharge the return air and stop delivering fresh air to the fire area. If such a system is not feasible, then activation of the detector should shut down the air-conditioning system.

Insulation warning

A warning was given that polyurethane foam insulation on a wall can burn so fast that the flames will race ahead of the fusing of sprinkler heads and therefore, when polyurethane foam is used in walls, shafts or concealed spaces, it always should be enclosed by a noncombustible thermal barrier.

The report questioned the rationality of spending time and money to prove the fire resistance of a floor or wall construction and then permitting openings to be made that destroy the fire resistance. Fire stopping of the same fire resistance as that of the floor or wall should be specified for all openings made for pipes, cables, airconditioning ducts or other equipment, the report advised.

Also mentioned were the vertical shafts around columns or in the interior skin of high-rise buildings that allow fire to spread from one floor to another because the only barrier to fire in these voids is flammable plastic foam. It was recommended that these voids have the same protection as other shafts to maintain fire integrity between floors.

Burned insulation of telephone cable shows in this photo of 10th floor phone closet. Note rectangular opening in ceiling through which fire extended from 11th floor.

New York Fire Department photo by R. Keller

Fire-resistant components

The quest for light weight in building construction came in for criticism because, the report charged, components that have passed fire tests may have no reserve for unexpected conditions. In contrast to a concrete block wall that would still stand as a fire barrier long after its two-hour fire exposure, the report pointed out, “many modern assemblies would have disappeared only a few minutes after the duration of test fires.”

Sprayed fireproofing, it was noted, is subject to different conditions in the testing laboratory than in the field. The report explained that properly formulated material sprayed to the right thickness “may not adhere to the surface or may be knocked off as other building services are installed” with the result that “the expected fire resistance is not there when it is needed.”

The number of wires and cables needed to supply the large amount of electrical and communications equipment in office buildings present another fire problem. The report stated the need for fire stopping where wires or cables enter power or telephone closets. At the World Trade Center, telephone panels and wiring were completely burned out in closets on three floors, the report noted, and there was severe fire damage in eight other phone closets.

The fire hazard is “almost nonexistent,” the report stated, in conduits or underfloor raceways. Fire protection is needed, the report continued, under trench header ducts that are part of the cellular steel floor support to prevent the heating of wiring insulation to its ignition point and igniting carpeting over or next to floor plates and outlets on the floor above. The fluted steel floor forms in the Twin Towers have a 5-foot-wide sprayed fiber fire protection under the trench headers.

Hazardous conditions

The report described as “the worst and most hazardous condition” the installation of wires and cables with combustible insulation in plenums because this introduces combustibles into the air conditioning and necessitates openings to serve the floor above that do not have fittings or fire stopping equivalent to the fire resistance of the floor.

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Telephone cables, the report warned, “can carry fire through a wall even when conventional fire stopping is provided.” The mass of cables for communications equipment in many offices can “sustain a substantial fire,” the report added.

Tested fire stopping at shafts and walls was recommended for wiring installations.

Another hazard was seen in the telephone and communication equipment installed by private companies. The report warned that the equipment and wiring “contains considerable combustible plastics and could be a source of fire.” This equipment may be found in storage or other areas rather than in phone closets.

Low voltage not safe

New York City’s experience refutes the contention that phone and other communications wiring cannot create a hazard because of its low voltage and amperage, the report declared, adding that “a substantial number” of electrical origin fires have occurred in office building phone equipment.

The report recommended that communications wiring and equipment should comply with the National Electrical Code, as must power wiring.

Because finger-sensitive elevator call buttons “can be operated by heat, smoke, flames, and other conditions present during a fire,” the report urged that in unsprinklered buildings, smoke detectors be installed in elevator lobbies so that activation of a detector will send elevators directly to the ground floor. The report also called for the installation of “fireman switches” so that fire fighters can manually control elevators during a fire.

Occupancy during construction

Another area the report considered was the building in which construction is still under way while it is partially occupied by permanent tenants. The presence of hazards that will not exist after the completion of construction— such as floor openings for the installation of equipment, temporary elevator doors, wall and floor openings that have not been fire stopped, removal of protective coatings from structural steel, and the presence of combustible materials and packaging—was noted in the report.

Construction work, such as welding, cutting and soldering, can provide ignition sources along with careless smoking, the report asserted.

The tenants should have a “totally enclosed fire-safe construction with no unprotected openings,” the report stated. Fire partitions around tenant space should be complete even if temporary fire stopping has to be provided, and fire stairs, elevators, alarms and standpipes “must be in service to the level of the highest tenant.”

Heavy charring indicates fire intensity in 10th floor phone closet. Floor openings for phone cable allowed cable to burn from 8th through 19th floors.

New York Fire Department photo by R. Keller

Good points outweigh bad

As for the World Trade Center, the report pointed out that its “good points outweigh the bad.” It noted that the Port Authority of New York and New Jersey, owner of the center, had made recommendations to tenants about limiting the combustibility of furnishings brought into the buildings.

“If the tenants would support these recommendations, the chance of a serious fire would be small,” the report declared.

The vertical protection between windows and at the junction of the floor slab and exterior wall was characterized as “good.” The 4-inch concrete floors, poured on fluted steel forms, butt the spandrel steel girders, preventing any vertical flue. Plaster protects the interior face of the exterior steel columns, eliminating vertical flues at this point also. Vermiculite plaster was sprayed on the spandrel girders to provide vertical fire separation between windows.

Also approved were the noncombustible building components and the use of thermal insulation.

Smoke venting capability

Another good feature the report cited is the capability to vent smoke from a floor and to pressurize the core. When a fire occurs, the supply air fans for the air-conditioning system are shut down and the return air fans discharge smoke to the outside. At the same time, fresh air supply to the core area is continued, but the vents are closed. This action pressurizes the core area, which includes both the stairs and the elevator shafts, and keeps smoke from entering the exit corridors in the core area. This system, the report stated, worked well during the fire in the Saunders offices.

Automatic sprinklers were installed in basements, shops and special hazard areas during construction of the Twin Towers. Under New York City local law 5, high-rises’ large floor areas must be divided by one-hour fire partitions so that no single compartment is larger than 7500 square feet unless the entire floor is sprinklered. Compliance with this law, passed after the start of construction of the Twin Towers, is being carried out. Sprinklers are being installed for tenants who do not want their floors compartmentized.

Among other pluses for the Twin Towers, the report cited the once-amonth Sunday fire drills in which New York City fire companies participate along with Port Authority policemen on duty at the building, some of whom have an initial fire fighting assignment. The Twin Towers also have fire safety directors, floor wardens and fire command centers in compliance with local law 5.

The report concluded that “what is true of the Twin Towers is true of many, if not all, high-rise office buildings.”

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