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How the Towers Kept Standing

Why the Towers Fell

How the Towers Fell

Protecting Skyscraper Occupants


When the Twin Towers Fell

One month after the attack on the World Trade Center, M.I.T. structural engineers offer their take on how and why the towers came down.

by Steven Ashley
Image: NOAA

BEFORE SEPTEMBER 11, 2001, the twin towers of the World Trade Center seemed a permanent part of the Manhattan skyline.

When New York City's giant World Trade Center towers plunged to earth following successive suicide terrorist attacks on September 11th, the world was confronted with one of most shocking—and sickening—sights of modern times. The mechanisms by which these huge and seemingly solid edifices suddenly collapsed, snuffing out the lives of thousands, was the subject of a preliminary postmortem conducted last week in Cambridge, Mass. A panel of Boston area-based civil and structural engineers convened to discuss the fate of the superskyscrapers, struck by hijacked passenger planes, in front of an overflow audience on the campus of the Massachusetts Institute of Technology. Their starkly sobering analyses highlighted the vulnerabilities of ultra-tall buildings to fire and pointed out steps that could be taken to lessen them.

After first describing the highly redundant structural system that kept the 110-story twin towers standing for decades despite hurricane-force winds and a terrorist truck bomb, the engineers then delineated how that system was breached and finally overcome on that fateful day when America was attacked. The main culprits in bringing the famously lofty buildings down, they concluded, were the two intensely hot infernos that erupted when tens of thousands of gallons of aviation fuel spilled from the doomed airliners. Once high temperatures weakened the towers' supporting steel structures, it was only a matter of time until the mass of the stories above initiated a rapid-sequence "pancaking" phenomena in which floor after floor was instantly crushed and then sent into near free fall to the ground below. Significantly, the panel stated that any mitigating reinforcements and redundancies added to these buildings could have only delayed the inevitable failure, though they would have bought more time for the evacuation of the occupants. No existing or foreseeable economically viable skyscraper structure, they agreed, could have withstood this kind of cruel onslaught. Clearly, prevention is the best defense against this kind of assault.

"Though the twin towers were not much taller than their famous uptown predecessor, the Empire State Building, the World Trade Center rose during the late 1960s, a new era of construction characterized by rapidly erected, lightweight steel structures rather than heavy masonry walls," explained Robert Fowler, senior engineer at the structural engineering firm of McNamara and Salvia. Fowler was then a junior member of the WTC's engineering firm of record, Worthington, Skilling, Helle & Jackson, later renamed Skilling Helle Christiansen Robertson. "As the Trade Center was so much lighter in comparison to earlier designs, it was a watershed building in the history of skyscrapers," he added. Leslie E. Robertson, then the project manager, was the engineer most responsible for the superskyscraper's design, Fowler noted. He is currently principal partner at Leslie E. Robertson Associates, the current structural consultants to the WTC. The late Seattle-based architect Minoru Yamasaki designed the World Trade Center.

How the Towers Kept Standing

As with all large buildings, the main structural engineering design criteria for the facility's 1,362-foot-tall south tower and 1,368-foot-tall north tower centered on two things: ensuring resistance to the gigantic gravity loads of the buildings themselves as well as to sideways or lateral forces caused by high winds and earthquakes, which can generate huge overturning forces at the bases. The former condition, Fowler explained, depends on specifying strong vertical columns that can efficiently transmit the mass of the building to the ground. The latter consideration concerns not only structural integrity but also "requires developing an acceptable comfort level for the occupants" by avoiding too much swaying. Opposition to lateral motion is controlled by "the design's structural mass [weight], the stiffness of its lateral members and the degree of structural damping employed," Fowler said.

"Though the WTC towers stood over 1,360 feet above the street level, the structures' bases were actually set 70 feet into the ground, and one had a 100-foot-tall antenna atop it, so with 205-foot widths, they had a lot of [exterior] area facing the wind," the engineer stated. He calculated that the approximate maximum wind shear force that a single face needed to withstand to be somewhere around 11,000,000 pounds. The gravity loads (weight) produced by the towers at their bases were on the order of 500,000 tons, Fowler said.

To handle these immense forces, the engineers "designed the World Trade Center essentially as a large beam section," explained another panel member, Robert McNamara, president of the engineering firm McNamara and Salvia. Called structural tubes in the business, each twin tower was strongly framed in structural steel. The frame comprised inner and outer rectangular box tubes consisting of closely spaced steel box columns connected by steel spandrel members or truss beams that supported 40,000-square-foot cross-braced floors, each nearly an acre in area, the empaneled engineers said. This configuration created a complete exterior tube around the building and a center tube down the middle.

The 90-foot-long central core, formed of massive vertical steel columns that held most of the building's weight, contained elevator shafts, stairways and utility spaces, they said. The core's columns were thicker toward the base to support huge accumulated gravity loads. The outer perimeter tube, a tight prefabricated latticework with 61 14-inch steel box columns (spaced 39 inches on center) on each building face, provided all the bracing resistance against lateral and twisting forces from wind and seismic action. This exterior grid served as a moment frame, providing a large moment arm (of torque) against overturning and deflection forces. The outer tube bore part of the gravity-induced downward load as well as, they noted.

The huge inner and outer rectangular tubes "needed to be protected to maintain their structural integrity, so the floors acted as reinforcing diaphragms or bulkheads [the term used in shipbuilding]," said panel member Jerome Connor, professor of civil and environmental engineering at M.I.T. The office floors, which each comprised a 35- to 60-foot clear span from the core to the exterior grid, were panelized structural members supported by open web joists with steel decks above them, he said. The horizontal truss struts, bolted and welded to the exterior grid and the core column structures, included viscoelastic stringers that provided increased damping to help make the structure less lively in the wind, according to Connor. Each steel floor deck was covered with four inches of concrete. "With almost an acre of area for each floor and figuring about 100 pounds per square foot of area," he estimated that "each floor system weighed about 3,200,000 pounds."

Why the Towers Fell

With all of its structural redundancies, "the World Trade Center was probably one of the more resistant tall building structures," McNamara said, adding that "nowadays, they just don't build them as tough as the World Trade Center." His statement is bolstered by the fact that the support structures of both twin towers withstood the initial hits of the two kamikaze airliners despite the breaching of many levels of framing. After the deletion of key structural members from about the 90th to 96th floors on the north face of the north tower, One WTC, and from about the 75th to the 84th floors of the south, east and north faces of the south tower, Two WTC, the buildings' skeletons found alternative paths to take the loads. Each impact and following explosion imparted first a large local lateral force and then an omnidirectional force to the structures, together causing massive initial damage to the columns and floor systems at the elevation of the crash.

Despite shocks and explosions estimated to be equivalent to that of the 1995 truck bombing of the Alfred P. Murrah Federal Building in Oklahoma City (about 400 tons of TNT), the towers remained upright. "The buildings displayed a tremendous capacity to stand there despite the damage to a major portion of the gravity system, and for an hour or so they did stand there," McNamara said. "The lateral truss systems redistributed the load when other critical members were lost. It's a testament to the system that they lasted so long."

Newspapers and TV newscasts reported that the twin towers had been designed to withstand a collision with a Boeing 707. The events of September 11th show that this was indeed the case. "However, the World Trade Center was never designed for the massive explosions nor the intense jet fuel fires that came next—a key design omission," stated Eduardo Kausel, another M.I.T. professor of civil and environmental engineering and panel member. The towers collapsed only after the kerosene fuel fire compromised the integrity of their structural tubes: One WTC lasted for 105 minutes, whereas Two WTC remained standing for 47 minutes. "It was designed for the type of fire you'd expect in an office building—paper, desks, drapes," McNamara said. The aviation fuel fires that broke out burned at a much hotter temperature than the typical contents of an office. "At about 800 degrees Fahrenheit structural steel starts to lose its strength; at 1,500 degrees F, all bets are off as steel members become significantly weakened," he explained.

Some have raised questions about the degree of fire protection available to guard the structural steel. According to press reports, the original asbestos cementitious fireproofing applied to the steel framework of the north tower and the lower 30 stories of the south were removed after the 1993 terrorist truck bombing.

Others have pointed out the possibility that the aviation fuel fires burned sufficiently hot to melt and ignite the airliners' aluminum airframe structures. Aluminum, a pyrophoric metal, could have added to the conflagrations. Hot molten aluminum, suggests one well-informed correspondent, could have seeped down into the floor systems, doing significant damage. "Aluminum melts into burning 'goblet puddles' that would pool around depressions, [such as] beam joints, service openings in the floor, stair wells and so forth...The goblets are white hot, burning at an estimated 1800 degrees Celsius. At this temperature, the water of hydration in the concrete is vaporized and consumed by the aluminum. This evolves hydrogen gas that burns. Aluminum burning in concrete produces a calcium oxide/silicate slag covered by a white aluminum oxide ash, all of which serve to insulate and contain the aluminum puddle. This keeps the metal hot and burning. If you look at pictures of Iraqi aircraft destroyed in their concrete shelters [during the Persian Gulf war], you will notice a deep imprint of the burned aircraft on the concrete floor.

Though the Boeing 767s airliners that hit the towers were somewhat larger than the Boeing 707 (maximum takeoff weights: 395,000 pounds versus 336,000 pounds) the structures were designed to resist, the planes carried a similarly sized fuel load as the older model—about 24,000 gallons versus 23,000 gallons, according to Kausel. "Most certainly," he continued, "no building has or will resist this kind of fire." The sprinkler system, which was probably compromised, would have been are useless against this kind of fire, he said, adding, "The World Trade Center towers performed admirably; they stood long enough for the majority of the people to be successfully evacuated."

Kausel also reported that he had made estimates of the amount of energy generated during the collapse of each tower. "The gravitational energy of a building is like water backed up behind a dam," he explained. When released, the accumulated potential energy is converted to kinetic energy. With a mass of about 500,000 tons (5 x 108 kilograms), a height of about 1,350 ft. (411 meters), and the acceleration of gravity at 9.8 meters per second 2, he came up with a potential energy total of 1019 ergs (1012 Joules or 278 Megawatt-hours). "That's about 1 percent of the energy released by a small atomic bomb," he noted.

The M.I.T. professor added that about 30 percent of the collapse energy was expended rupturing the materials of the building, while the rest was converted into the kinetic energy of the falling mass. The huge gray dust clouds that covered lower Manhattan after the collapse were probably formed when the concrete floors were pulverized in the fall and then jetted into the surrounding neighborhood. "Of the kinetic energy impacting the ground, only 0.1 percent was converted to seismic energy," he stated. "Each event created a (modest-sized) magnitude 2 earthquake, as monitored at Columbia University's Lamont-Doherty Observatory, which is located about 30 kilometers away from New York City." Kausel concluded that the "the largest share of the kinetic energy was converted to heat, material rupture and deformation of the ground below."

Despite the expert panel's preliminary musings on the failure mechanisms responsible for the twin towers' fall, the definitive cause has yet to be determined. Reportedly, the National Science Foundation has funded eight research projects to probe the WTC catastrophe. The American Society of Civil Engineers is sponsoring several studies of the site. Meanwhile the Structural Engineering Institute of the American Society of Structural Engineers has established an investigative team to analyze the disaster and learn from the failure. W. Gene Corley, senior vice president of the Construction Technology Laboratory in Skokie, Ill., is said to be heading the ASSE study team through its initial phase of data gathering, and then William Baker, a structural engineer at the Chicago-based firm of Skidmore Owings & Merrill in Chicago, will lead the following analysis phase. The Structural Engineering Institute is to partner with the American Institute of Steel Construction, the National Fire Protection Association and the Society of Fire Protection Engineers. The Federal Emergency Management Agency has been invited to join as well.

How the Towers Fell

Given the lack of firm conclusions regarding how the collapses occurred, the M.I.T. panel participants asked their audience to consider various theories they put forth. In general, it was agreed that as the structure warped and weakened at the top of each tower, the frame, along with the concrete slabs, furniture, file cabinets and other materials, became an enormous consolidated weight that eventually crushed the lower portions of the structure below. The details of how the frame members failed remain under contention.

Professor Connor's theory focused on weaknesses in how the vertical and horizontal structural members were tied together. During construction, he explained, each prefabricated floor system was lifted into place by a crane and "supported at the ends like a hinge, where they were bolted and welded to the inner and outer framing tubes" so that part of the gravity load went through the core and the other part through the exterior structure. "The floor trusses sat on beams and were tied down so the core was locked to the exterior," he said. "It was an unusual system and very lightweight. If you lose the connection between them, however, you lose the ability to carry the floor loads and allow the floors to slide back and forth under stress. If a damaged floor system were to fall, it would break the end connections in the lower floors and down and down the floors would go."

"In my theory, the hot fire weakened the supporting joint connection," Connor continued. "When it broke, one end of a floor fell, damaging the floor system underneath, while simultaneously tugging (pulling) the vertical members to which it was still attached toward the center of the building and down." This phenomenon started a parasitic process that accelerated until total failure and the structure fell in on itself, he said.

Eduardo Kausel proposed an alternative failure explanation that he acknowledged was independently developed by Zdenek Bazant, a professor at Northwestern University. "I believe that the intense heat softened or melted the structural elements—floor trusses and columns—so that they became like chewing gum, and that was enough to trigger the collapse," he said. "The floor trusses are likely to have been the first to sag and fail. As soon as the upper floors became unsupported, debris from the failed floor systems rained down onto the floors below, which eventually gave way, starting an unstoppable sequence. The dynamic forces are so large that the downward motion becomes unstoppable."

Via two simple models, Kausel was able to determine that the fall of the upper building portion down onto a single floor must have caused dynamic forces exceeding the buildings’ design loads by at least an order of magnitude. He also performed some computer simulations that indicate the building material fell almost unrestricted at nearly the speed of free-falling objects. "The towers' resistive systems played no role. Otherwise the elapsed time of the fall would have been extended," he noted. As it was, the debris took about nine seconds to reach the ground from the top.

"It's difficult to judge which of these failure mechanisms occurred first; probably all occurred and interacted," said panel member Oral Buyukozturk, professor of civil and environmental engineering at M.I.T.. "The prolonged effect of high heat is likely to have led to the buckling of the columns, collapse of the floors, as well as to the shearing of the floors upon the failure the joints." He noted that videotapes of the catastrophe showed some tilting of the top portion of the south tower before it collapsed. "This indicates the buckling of one building face while the adjacent face was bending [placed into tension]." After that, the upper portions of the tower are shown disintegrating, with "a dynamic effect and amplification process" following that led to a progressive collapse—"a kind of pancaking or deck of cards effect"—down to ground zero, Buyukozturk stated.

Kausel addressed the oft-asked question of why the towers did not tip over like a falling tree. "A tree is solid, whereas building is mostly air or empty space; only about 10 percent is solid material. Since there is no solid stump underneath to force it to the side, the building cannot tip over. It could only collapse upon itself." Robert McNamara said his failure mechanism theory "focuses on the connections that hold the structure together," but he cautioned that "we really need to wait for a detailed investigation, before we decide if we have to up the code ratings for these connections in signature structures."

Protecting Skyscraper Occupants

The expert panel then turned its attention to changes for future tall structures in the wake of what has been learned. Though the recent "disaster couldn't be envisioned as a design scenario in the 1970s, it means we have to change the way we design and construct tall buildings in the future," Buyukozturk said.

Existing skyscrapers should probably be retrofitted with some additional safety measures, but the professors say that it doesn't make sense economically—and aesthetically—to protect them all physically from similar catastrophes. "Retrofitting is very expensive and is therefore usually done only for monumental buildings," Connor said.

"There will never be a building that won't fall," Kausel noted. "The best we can do is to ensure that it will stand long enough for all the people to escape." Back when the WTC was built, no one seems to have anticipated the need to evacuate an entire large building at once. To do so successfully means boosting a building's structural redundancy—the provision of additional means to assist system function. Panel members discussed providing improved fire protection for the structural elements, alternative load paths to stand in for damaged structures and fixing diaphragm floor beams more strongly to vertical members. Also mentioned was the idea of installing blast-resistant, energy-absorbing materials such as concrete-encased steel exterior columns and/or cavities (reinforced concrete cores) in future large structures that could help them survive or at least promote failure in certain slower, less deleterious sequences.

One audience attendee, a West Coast-based structural engineer who did not give his name, created a provocative moment when he claimed that it would cost about 10 percent more than the original building cost to install floor joint reinforcements for greater redundancy. "According to our analysis, it could add several more hours to the evacuation period," he stated. "If each tower cost about $1 billion to build, then an extra hundred million dollars could have saved most of the occupants. Though it's horrible to contemplate," he continued, "a human life is valued for insurance purposes at about a million dollars apiece, so this helps put the extra investment into perspective. After all, the World Trade Center was retrofitted with 10,000 viscoelastic dampers to reduce its swaying, so safety improvements can't be ignored. Building clients have to become more demanding, even if the probabilities of a repeat disaster are very slim...."

The panel also considered the need to improve the effectiveness of building safety systems. Kausel pointed out problems with the twin towers' emergency communications systems ("just when coordination was most critical, the people didn't know what to do"), the emergency illumination system and protection against smoke ("the great killer in building fires is smoke inhalation"). He also suggested that more effort should be expended to create "alternative escape routes, so evacuees aren't faced with a wall of smoke. If two stairwells are close together," he noted, "one explosion could block them both." Other ideas floated included installing better fire-suppression systems, using the aqueous film-forming foams employed in aviation fires, and creating protected access ways for firefighters. There was also mention of the need to harden stairwells and egress pathways, and perhaps develop "deployable evacuation systems" for building occupants. Beyond robotic stairway evacuation devices, deployable systems might include escape tubes deployed out windows, exterior people-lowering machines, flying platforms or even parachutes."

One audience member asked the assembled experts whether a reinforced concrete skyscraper such as the current height record-holder, the 452-meter Petronas Towers in Kuala Lumpur, Malaysia, would have better resisted a collision with a fuel-filled airliner. Their response indicated that a concrete structure would have probably lasted for a couple of more hours than did the steel World Trade Center towers. Robert McNamara stated that many of the more recent superskyscrapers were constructed of reinforced concrete mainly because of the high cost of steel in Asia. He also mentioned that the Petronas Towers contain "safe refuge floors" to allow building occupants to reach fresh air during fires. McNamara said that this concept was now somewhat discredited, as similar refuges in the WTC would not have ultimately saved anyone.

A lively discussion then ensued about whether the terrorist pilots knew where to hit the buildings for maximum effect. McNamara opined that the position of impact seems significant. "They hit them at just the right place—about two thirds to three quarters of the way up. The earlier [truck bomb] attack showed that the explosion at bottom had little effect and that it's much easier to collapse a building from the top than the bottom. If they had hit the very top of the building, the fire damage wouldn't have had such a catastrophic effect. At the bottom, the columns are much heavier and stronger and so they would have taken a much larger load." Connor offered that one would "need graduate-level engineering training to choose the prime target location."

In the aftermath of the World Trade Center disaster, questions arose whether superskyscrapers should be built in the future. Clearly, these top engineers would reply in the definite affirmative. Inevitably, they said, new tall towers will rise.