The World Trade Center Complex

The World Trade Center Complex


The World Trade Center (WTC) complex was designed by Minoru Yamasaki and Associates of Troy, Michigan, and Finery Roth and Sons of New York. Site excavation began in August 1966, and construction of the towers started two years later. The first tenant moved into Tower 1 in December 1970, even though construction was not completed. In January 1972, the first tenant moved into l ow er 2. The official opening of the Vista Hotel in July 1981 marked the complex’s completion.

Hie foundations that support the WTC are truly gigantic. They are the “strong shoulders” that rest in bedrock 70 feet below ground, and they carry 1.25 million tons of building. In the area that contains the twin 110story towers, more than a million cubic yards of earth, rock, and subterranean miscellany were removed to make way for a basement 980 feet long, 510 feet wide, and 70 feet deep to provide for the PATH commuter rail station, mechanical equipment rooms, storage for tenants, and parking for 2,000 cars.

Prior to excavation, the entire eight-block area required underground walls down to and into the bedrock to withstand the external water and earth pressure and to prevent the undermining of adjacent buildings and streets. Conventional construction methods were considered before selection of the slurry trench method.

This technique had been used extensively in building construction in Europe and Canada but was new in the United States. The soil surrounding the site resulted in the decision to use this new approach.


First, excavating machinery carved out a three-foot-wide trench, the same width of the basement wall to come. As material was removed, it was steadily replaced by a slurry mixture of water and bentonite (a fluffy, expansive, gray clay that comes largely from Wyoming). The slurry has the consistency of pea soup and was constantly churned as the excavation rig deepened the trench. The mixture held back the ground water and maintained the sides of the trench without the need for shoring. The drilling and removal process continued down into the bedrock.

The Port Authority Command/Communications Center on the B-1 level, approximately 50 feet from the blast hole on that level, was damaged and communication with occupants in the towers was lost.

(Photo courtesy of Port Authority Risk Management.)

Next, preassembled seven-storvhigh cages of reinforced steel, weighing 25 tons, were lowered into the slurry. This formed the skeleton for the concrete that displaced the slurry. Then the concrete was placed at the bottom of the trench through a pipe with a big hopper at the top. Engineers call it a tremie, which is French for “hopper” or “funnel.” As the concrete wall, three feet thick, rose to ground level, the slurry was forced out on top. The same slurry then was piped to other trench segments. The final result, as the parts were joined, was a reinforced concrete underground cutoff wall around the site to be excavated.

The work was done in 22-foot-long segments, 152 in all, which allowed accommodation to the varying rates of building removal and utility line relocation, since the construction stages were interrelated. One depended on the other, just as new telephone vaults had to be provided in West Street (also by the slurry trench method) before the basement perimeter wall could be started, which in turn was a preliminary to foundation excavation and then the actual erection of buildings. In effect and in appearance as the excavation proceeded, this became a big bathtub—with water kept on the outside.

During excavation, the PATH railroad operated as usual, even though the old tubes under Fulton and Cortlandt streets became completely exposed as a result of the work. As digging proceeded, the tubes rested safely in protective cradles with deeprooted underpinning. Later on, new tracks were built into the new and bigger PATH terminal west of Greenwich Street.

As the “bathtub” excavation linelow ered and sections of the rectangular 3,100-foot concrete perimeter wall came into view, a system of rock anchors was installed. Holes were drilled diagonally downward and outward through the wall, through the surrounding earth, and into bedrock. Steel tendons were inserted, one end socketed in the rock and the other end anchored to the wall. The purpose was to brace the wall against exterior pressure and provide unimpeded working space inside the “bathtub.”

The excavated material was dumped into the Hudson River to create 23.5 acres of new land, extending six blocks along the waterfront from Cortlandt Street to below Rector Street and an average of 700 feet into the river. This landfill replaced abandoned piers and the old Central Railroad of New Jersey ferry slip at Liberty Street. The material was dumped behind a cellular steel cofferdam retaining structure. The new real estate became the property of the city of New’ York, at no cost to the city, and is now Battery Park City and the World Financial Center.

Gradually, as excavation proceeded, the “bathtub” area was dug out. The footings for the building foundations were carried into bedrock. From this base, six floors below ground level, the buildings began to rise. The basement floors braced the perimeter wall, and the rock anchors were no longer necessary.


In terms of their structural system, the twin tower buildings depart completely from other high-rise buildings. Conventional skyscrapers, since the 19th century, have been built with a skeleton of interior supporting columns that supports the structure. The buildings are not only physically striking—a pair of 209-foot-square tow ers with almost an acre of space on each of the 1 10 floors—but they are radically different in structural design, as the exterior wall is used as the loadbearing wall. The only interior columns are located in the core area, which contains the elevators.

The outer wall carries the buildings’ vertical loads and provides the entire resistance to wind. The wall consists of closely spaced vertical columns (21 columns 10 feet apart), tied together by massive horizontal spandrel beams that girdle the towers at every floor. On the inside of the structure, the floor sections consist of trusses spanning from the core to the outer wall. The towers are literally gigantic square box beams of enormous strength, designed to withstand 150-mph winds and sway up to three feet.


Elevator service within the towers was designed to maximize the use of floor space; each tower is divided into three zones, and each has its own lobby. Zone I s lobby is located on the concourse level, one level below the plaza, and permits access from the ground up to 44. Zone 2’s sky lobby (the transfer floors for other elevators serving upper floors) is 44 and services 44 to 77. Zone .Vs sky lobby is located on “’8 and serves 78 through 110. Express elevators (the fastest of their size ever built) are available from the lobby anil serve both the t » and 78 sky lobbies; each has a capacity of 55 people. The elevator system was designed to limit local runs to one-third of the building. Thus, three local elevators operate at different levels within a single shaft. Each tower has a total of 99 elevators.


The subgrade area of the WTC functions as the heart of the complex. In addition to the public and government parking areas, the subgrades house the refrigeration plant, which is three stories high; various types of telecommunications equipment; and office/locker space for the Port Authority and contract staff who monitor and maintain the complex’s systems.

Port Authority personnel are now assigned to vertical patrols of tower stairways, as part of the WTC enhancement package following the blast.

(Photo courtesy of Port Authority Risk Management.)

Electrical power and steam are supplied to the complex by Consolidated Edison of New York (Con Ed). Electrical power is distributed to the towers through eight feeders. The electrical systems are constructed so that service is not interrupted even though power loss may occur at any particular substation. A majority of the feeders are located in the ceiling or floors at the B-l and B-2 subgrade levels. These feeders are surrounded by concrete to their individual substation throughout the WTC complex and are connected to the subgrade power distribution center. From the power distribution center, service is provided through substations on floors B 1, 7, 41, 75, and 108. In the event of a service disruption, the complex has six primary backup generators located on the B-6 level. These backup generators have the ability to use either domestic water or river water for cooling in the event of a water main failure and supply interruption. Steam is distributed to all mechanical equipment rooms, located throughout the complex.

Natural gas is supplied to the complex but is in limited use, mainly in the Vista Hotel’s kitchens but also in restaurants located on the concourse of 5 WTC and the coffee tasting room at the New York Commodities Exchange in 4 WTC. No natural gas is used in the two towers.

Domestic water is fed into the complex at several locations from street level into the subgradc. From the subgrade, water is distributed throughout the complex and is used for restaurants, hotel rooms, the hotel swimming pool, cleaning, fountains, landscape maintenance, the sanitary system, refrigeration plant chillers “makeup water,” cooling emergency generators, and fire protection systems (reserve tanks, standpipes, and sprinklers). Conservation of the region’s precious water resources was the primary motivator in the design and construction of a system that uses Hudson River water for cooling the refrigeration plant chillers. This system involves pipes and pumps that transport the water from the Hudson under Battery Bark City, under West Street, and into and out of the B-5 refrigeration plant. This system enables the refrigeration plant to cool the complex without using potable water.

Emergency generators on the B-6 level. Cooling water lines for these units were ruptured by the blast, shutting down all emergency power.

(Photo courtesy of Port Authority Risk Management.)

Two systems often overlooked are sewage and storm water. Both are gravity systems in the WTC complex. All aboveground sewage lines lead into the municipal sewage line; however, below-grade lines feed into sewage ejectors, which pump up and into the city line. The storm water system leads into a 36-inch storm line, which eventually feeds into the return to the Hudson River from the B-5 refrigeration plant.


The WTC complex has the following fire safety features:

Smoke detection T he WTC complex is protected by smoke-detection equipment that operates as freestanding units or in conjunction with other systems. The smoke-detection equipment, for example, has been built into the heating, ventilation, and air-conditioning systems, which automatically trigger alarms indicating the presence of smoke. T he complex has more than 3,000 smoke detectors.

lireak-glass emergency alarms. An emergency call box system is installed on each floor and, when activated, provides immediate two-way voice communications with the Eire Command Center, which is started 2-1 hours a day.

Smoke purge systems. A smoke purge system built into the WTC tower’s ventilation system is activated in the event of fire. The system draw s smoke out of tenant areas and at the same time pumps 100 percent fresh air into the public corridors. This system is designed to assist individuals evacuating an area as well as those responding to the scene.

Emergency fire exit stairwell svs* tern. Each tower has three independent emergency fire exit stairwells. T hese stairs are designated A. B. and C. Stairwells A and C run from the 1 10th floor down to the lobbv mezzanine, which provides access to the plaza. Stairwell B runs from the 100th floor down to the B-6 level of each tower. These stairwells provide three widely separated evacuation alternatives in the event of fire and give firefighters safe passage upward to fight fires. Each of these fire stairways has a standpipe, walls of fire-resistant construction, fire doors, and lighting fed by the emergency generators. Similar fire exit stairwells also are installed in the other buildings in the complex.

Standpipe system and fire pumps. The fire standpipe system in the WTC is an arrangement of piping, valves, pumps, tanks, and hose stations. Its function is to provide a reliable means of applying water streams on a fire in the shortest possible time. The hose stations are situated so that any area of a floor can be reached by a stream of water from a hose station in that area.

The source of water for the fire standpipe system in the WTC is the domestic water supply system. The water is transported to all buildings and subgrade areas in the WTC by two eight-inch loopmains (loops). Also in the system are manually controlled fire pumps and reserve tanks. The function of the reserve tanks is to provide water tor the hose stations during the initial period of firefighting, before the fire pumps can be started.

The WTC has two fire standpipe eight-inch water distribution loops. One loop is in the ceiling of the concourse level and the other is in the ceiling of the B-l level. The first loop supplies water to the risers in 5 WTC and 4 WTC and their sublevels. The other loop serves 1 WTC and 2 WTC, the hotel, the Customs House, and the subgrades. The loops are connected at two points—on the truck dock of the Customs House at the north central portion of the WTC and in the southern portion of the WTC.

Fire protection enhancements include batterypowered backup lighting. (Photo

courtesy of Port Authority Risk Management.)

Fire safety directors now are stationed in lobbies 1 and 2 and sky lobbies 44 and 78 for each tower.

(Photo courtesy of Port Authority Risk Management.)

Vertical risers emanating from the loops earnwater up into the towers and the other buildings in the WTC and down into the sublevels and subgrades. Risers connected to the loop on the concourse level carry water up into 5 WTC and 4 WTC and down into their sublevels.

In addition to the piping in the standpipe system, there are seven water storage tanks. The storage tanks supply water to the hose stations during the initial period of firefighting before the pumps can be started. The pumps are required to supply the additional water needed to fight larger fires.

There are 1 5 fire pumps in the fire protection system in the WTC. Twelve are centrifugal pumps and three are jockey pumps. Eight of the centrifugal pumps are for the standpipe system (located on floors B 1,7, 41, and 75 in each tower), and four are for the sprinklers (located on floors B 1 and 108 in each tower). The three jockey pumps are for the sprinkler system—one on the B-l level and one in each tower on floor 108 (for pressure maintenance).

Sprinkler system. The design of the sprinkler system in both towers is similar. Generally, the direction of water flow in the risers is downward. There are three separate risers in each tower, with each riser serving different groups of floors.

The risers are designated A, B. and C. Riser A supplies the uppermost floors, 99 through 110; riser B supplies floors 32 through 98; and riser C supplies floors 1 through 31.

The risers pass through the floors in the service closets on each tenanted floor and deviate from plumb vertical only as required by changes in building construction at various elevations.

Two different capacity holding tanks—10,000 gallon and 5,000 gallon—are used in the same sprinkler system. The 10,000-gallon tanks supply the sprinkler system exclusively and are located on the 110th floor; the 5,000-gallon tanks serve both the sprinkler and fire standpipe systems and are located on the 110th and 42nd floors.

The sprinkler tanks supply water to inlets of risers A and B and are automatically refilled by the domestic water system via float-control valves. The fire standpipe tanks supply water to risers A, B, and C and normally are also refilled by the domestic water system. Riser C in 1 WTC is crossconnected with riser C in 2 WTC via a four-inch pipe from the Siamese connections. The function of the crossconnections is to provide FDNY with the capability to supply water to the C risers in both towers and control their pressures.

The sprinkler centrifugal pump in each tower on the 108th floor has a dual purpose. Its main function is to increase the pressure of the water to the sprinklers on the floors closest to the holding tanks (floors 107 to 99) to that required for operating the sprinklers. However, the pump also is used to supply water to some of the fire hose racks (FHRs) on the 1 10th floor.

As the downward distance from the holding tanks increases (floors 98 and below), the pressure (head) of the water in the risers increases to the point where the additional pump pressure is not required.

The rest of the complex—namely, the subgrade levels, 5 WTC, and 4 WTC—is part of a second sprinkler system completely separate from the one in the towers. This second system consists of a large loopmain with risers going down into the subgrades and sublevels and rising up in 5 WTC and 4 WTC.

The function of the fire pumps and the jockey pump is to maintain the water pressure in the system at 155 psi.

Of the three sprinkler fire pumps (two centrifugal and one jockey) on the B-l level, one centrifugal pump and the jockey pump are located in a pump room under 1 WTC; the other centrifugal pump is located in a pump room under 2 WTC.

Fire hose and extinguishers. Oneand-a-half-inch fire hose stations and fire extinguishers are located inside the emergency fire exit stairwells. The WTC Port Authority Police also have pre-positioned firefighting equipment carts in the sky lobbies of the towers. These carts are equipped with three kinds of fire extinguishers (water, carbon dioxide, and dry chemical), fire hose and nozzles, selfcontained breathing apparatus, turnout gear, entry tools, rcsuscitators, and first-aid equipment.

Emergency power supply. The WTC complex is provided with emergency power, when needed, by six l,20()-kw emergency generators. The emergency generators are located on the B-6 level and are checked on a routine basis to ensure they will operate properly when needed. Phis equipment provides an emergency power supply for communications equipment, elevators, emergency lighting in corridors and stairwells, and fire pumps.

El era tor recall and firefighter service. An automatic elevator “override” system, detector-activated, commands all elevators serving or affected by a fire area to immediately return to the ground or their sky lobby. The elevators then are operated manually under the direction of the FDNY officer-in-charge.

Public address system. Public address speakers are installed in the corridors to provide tenants and visitors with instructions and updated information in the event of an emergency. This system is operated from the Operations Control Center, w hich is staffed 24 hours a day.

WTC fire brigade. The Port Authority Police at the WTC have been trained as firefighters and respond to any location within the complex to investigate smoke detector activations, smoke reports, or reports of actual fires. They work hand-in-hand with FDNY to investigate reported conditions and take the appropriate actions for correction.

Fire safety floor wardens. On each floor of the complex, individuals have volunteered to be fire safety floor wardens and are provided with specific training to fully understand their responsibilities during fire drills and actual fire emergencies. Mandatory fire drills are scheduled biannually. WTC fire drills are more detailed than those required by the city of New York.

FDNY Siamese connections. The WTC complex has 24 Siamese connections, located at street level, for use for FDNY apparatus. Water is taken from the street main, enters the fire department apparatus, and then is pumped under high pressure into the piping system to supply sprinklers and the standpipe system. Each of these Siamese connections serves various portions of the complex and is identified.

Flammability standards. The risk management group of the PA’s Treasury Department assists WTC-based PA staff by providing technical advice on the flammability of furnishings prior to tenants moving in or updating work areas. In addition to office furniture being evaluated, displays, carpeting. draperies, and decorations also must be tested to meet specific PA flammability standards prior to installation.


The bombing resulted in the following:

  • six fatalities (four PA employees, one hotel employee, one parking lot patron);
  • 1,042 injuries;
  • collapsed subgrade concrete slabs;
  • approximately 2,500 tons of debris covering the refrigeration plant;
  • severe smoke and soot condition throughout the towers;
  • four of seven of the electrical feeders supplying the complex fault-
  • ing and, due to safety concerns, the remaining three feeders being secured 1 ¼ hours after the explosion (one additional feeder was “down” for routine maintenance—there were eight total feeders into the complex);
  • loss of the PA Police Desk;
  • loss of the Operations Control Center, fire alarm system, and elevator two-way communications system (from elevator to Operations Control Center);
  • loss of automatic smoke-detection capability;
A view of the crater from the B-1 to B-5 levels. Note the steel column. Horizontal supports were destroyed in some cases, but the columns did not give.

(Photo courtesy of Port Authority Risk Management.)

Temporary column supports are installed in the restoration process.Column bracing at left of photo is destroyed.

(Photo by FDNY Photo Unit.)

  • loss of automatic sprinkler protection, primarily in subgrade areas;
  • loss of standpipe system, primarily in subgrade areas;
  • severed pipes creating a flooding condition of approximately two million gallons of water on the B-6 level;
  • elevators getting stuck between floors, trapping almost 500 passengers;
  • broken windows on the plaza and lobby levels and in the towers;
  • fire stairwell doors/jambs forced open and damaged;
  • the largest FDNY response in history;
  • a massive personnel and equipment response from NYPD and NYC*EMS (first mutual-aid request— first Code Zebra involving 500 or more injuries)—174 ambulances, 12 volunteer ambulances, 49 commercial ambulances, and 69 units;
  • loss and damage to hundreds of government and privately owned vehicles in the parking levels; and
  • massive temporary relocation of tenants to other sites.


The bombing of the WTC was the worst terrorist attack ever to occur in the continental United States. The criminal investigation that began that afternoon required personnel from federal, state, and local law enforcement at all levels to work together for an extended period. Mobile command posts from these agencies were positioned on West Street in front of the Vista Hotel and 1 World Trade Center, and their operations were coordinated from this common location. The criminal investigation required law enforcement personnel to work in conjunction with civilian staff of almost every conceivable discipline. The media also were extremely interested in the hour-to-hour progress that was to be reported and were provided with frequent updates by PA media relations.

Concurrent with the criminal investigation were the following almost herculean tasks:

  • stabilizing the subgrade blast zone;
  • pumping out almost two million gallons of water from the B-6 level;
  • removing debris;
  • coordinating all activities at the site;
  • relocating police, operations, and maintenance offices that had occupied subgrade office space;
  • restoring and recertifying elevator service;
  • isolating and restoring the fire protection system in the blast zone;
  • cleaning smokeand soot-damaged areas;
  • removing and relocating vehicles that had sustained damage;
  • removing and releasing undamaged vehicles from the parking lots;
  • providing meals for law enforcement, fire, construction, and other staff working at the site;
  • reconstructing collapsed and destroyed slab areas;
  • determining the repairs needed to the B-5 refrigeration plant;
  • designing and implementing an access control system to allow authorized staff to gain access to the blast site; and
  • dealing with the media and providing escorts to individuals needing access.

As the rescue operations were ongoing, representatives from all the agencies began to assemble in the Vista ballroom. The groups then were repositioned into what had been the “big kitchen,” located on the main concourse near the PATH escalators. Activity within the big kitchen only could be described as amazing. Tables were being constructed out of 4 X 8 plywood sheets and 2 x 4s, new telephone cables were being strung from the ceiling, new phones were being installed, chairs were being brought in, ceiling lighting was being upgraded, new power lines were being installed, supplies were being assembled on the concourse, and communications equipment was being installed. This level of activity and teamwork among people who did not know one another was never before witnessed during peacetime. The big kitchen became the focal point and the nerve center for the recovery and restoration activities. Twice a day, briefings were held to assess the progress made and determine the steps to be taken next.

PA engineering staff, assisted by consulting engineers (some of whom had been involved in the original construction), determined that the columns that were no longer braced by horizontal concrete slabs should be braced as soon as possible. Immediately, Karl Koch Engineering began welding box beams, six inches square by 35 feet in length, onto the columns horizontally and diagonally for support. A total of 200 beams were used to provide support. Hie criminal investigation then moved into high gear once the additional support was in place. Blast debris w as removed from the B-5 refrigeration plant area by two routes—through the plaza by crane and out on PATH railroad flatcars. At the same time, Turner Construction Company was pumping out the almost two million gallons of water that had collected in the B-5 anti B-6 levels at the same time.

The WTC mechanical staff began evaluating the condition of the refrigeration plant on the B-5 level as soon as they could safely gain access. They determined that the plant had sustained serious enough damage and recommended that backup units be obtained for the summer season Temporary backup units were installed on Liberty and Vesey streets, and structures were built around them to limit noise to the surrounding area. These temporary chillers allowed the B-5 plant to be repaired and recertified w ithout a reduction in cooling capacity.

A major task that had to be undertaken was restoring the fire protection system. Major sections of the below-grade standpipe and sprinkler system were damaged and had to be repaired. The damaged and destroyed sections of the standpipe and sprinkler systems were isolated by a crew of Ogden mechanical employees w ho worked around the clock. A team of 20 Hatzel & Buehler electricians tested the 3,000 smoke detectors, working 24 hours a day for one week, to verify that all were functioning properly. Under normal conditions, such a detail would have taken up to six months to complete. Simultaneously, another team of electricians reconnected many hundreds of feet of severed wire that led to the fire alarm system. While the towers were closed and fire protection systems were being brought back on line, FDNY connected pumpers to Siamese connections to allow 4, 5, and 6 World Trade Center and the concourse to be open for business the Monday morning following the blast.

The dense smoke from vehicles burning in the subgrade parking lots traveled up the towers through the breached elevator shafts. The PA made arrangements with M. F. Hank/The Restoration Company (TRC) to supervise the cleaning of interior space damaged by smoke and soot. TRC coordinated the activities of Ogden, Modern. Nelson, and Altus contract cleaners. A total of 3,000 workers were used—1,000 per shift. They accomplished the cleaning tasks in seven days for Tower 2 and 10 days for lower 1. lower 2 reopened March 18 with the return of New York Governor Mario Cuomo’s office, lower 1 reopened March 29, when employees of the Brown & Wood law firm returned to their offices.

The bombing of the WTC forced people from many diverse backgrounds to work together as a team as they never had before. Their resolve and determination to get the job done was inspirational to all who have witnessed their work.


The bombing at the WTC truly was a “worst-case scenario” in which the primary and backup systems were disrupted. The following enhancements subsequently made to the WTC have made the complex one of the safest in New York City.

  • 1,600 emergency batterypowered lighting units have been installed in exit stairwells, elevator lobbies, and all elevator cabs.
  • Secondary backup generators are available in case primary and backup generators fail.
  • A New Jersey Public Service Electric & Gas utility feeder cable will be routed through PATH tunnels to provide an additional backup should New’ York electrical service be interrupted.
  • Phosphorescent signs have been installed to guide the way to floor entry doors in fire stairwells.
  • Phosphorescent tape-paint has been applied to stair threads, handrails, and the perimeters of doorways
  • in the fire stairwells.
  • Vertical patrols (personnel responsible for checking obstructions, safety hazards, and systems throughout stairwells and corridors) have been created.
  • Six satellite communications control stations, staffed by deputy fire safety directors, are in operation.
  • Evacuation chairs to assist mobility-restricted people are available.
  • New Radiax cable and antenna have been installed so that FDNY can use its radios in the towers.
  • Cellular phones have been issued to each fire floor warden. The phones are to be used as a backup should the public address system fail.
  • Delta Barriers have been installed to prevent unauthorized vehicles from gaining access into the subgrades.
  • An access-control ID card system and a visitor check-in system have been implemented.
  • A battery backup system to the elevator car position indicator will enable passengers to see which floor they’re on if the elevator gets stuck in an express shaft.
  • The elevator emergency call button will be tied into a battery power supply on top of the elevator car; thus, in the event of a power loss, WTC personnel can communicate with trapped elevator passengers.*

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