On July 28, 1945, a B-25 bomber accidentally struck the north face of the 79th floor of the Empire State Building, then the tallest building in the world. The plane ripped an 18-foot-wide 2 20-foot-high hole in the outer wall of the building. The force of the impact sheared the wings from the plane and propelled one of the two engines through the building and out the opposite wall. It then struck the 12th-story roof of a building across 32nd Street, where it ignited a destructive fire. The other engine fell into an elevator shaft and down to the subcellar. The fuel emptied from the ruptured tanks, ignited, and killed most of the 13 ensuing casualties, including the three-man flight crew.

The 35,000-pound gross weight of the plane and its 1,000 gallons of gasoline were traveling at some 275 mph when they struck the building floor, almost head-on. The right engine of the plane passed on one side of a structural column; the fuselage and the second engine passed to the other. The building column spacing was 19 feet on center in both directions; the overall wingspan of the plane was 67 feet. The mass of the building floor, together with the rigid riveted steel frame, had been designed to withstand a considerably greater wind load and easily overcame the momentum of the plane. The building was repaired and remains in service to this day.1

On May 20, 1946, another (smaller) military aircraft lost in fog struck the 58th floor of 40 Wall Street, killing the five crew members and passengers. There were no other casualties in the comparatively empty building and neighborhood. Once again, the building, of construction similar to that of the Empire State Building, suffered no major damage. The structure was repaired and remains in service to this day. (1, 29-30)

On the morning of September 11, 2001, two jet aircraft struck each of the twin towers of the World Trade Center (WTC) in New York. The result was the worst building disaster in history, with nearly 3,000 fatalities and an as yet undetermined toll of injuries and collateral casualties. In total, 10 major buildings partially or totally collapsed. Approximately 30 million square feet of commercial space were rendered unusable.

Why the vast difference in results? BUILDING ROBUSTNESS!

The ability of a building to withstand an event is called “robustness,” defined as “the ability of an engineered structure or system that enables it to survive a potentially damaging incident or extreme accident without disproportionate loss of function.”

According to the original structural engineer of the WTC, the buildings were designed to withstand the accidental impact of a Boeing 707 jet aircraft with a maximum gross weight of 336,000 pounds and a reduced fuel load of approximately 10,000 pounds traveling at a pre-landing speed in the range of 250 mph. The actual aircraft that struck the WTC towers were of the Boeing 767-200ER, with an approximate gross weight of 395,000 pounds and an estimated fuel load of 80,000 pounds flying at speeds of up to 590 mph.

It is somewhat prophetic that at a convention of the Structural Engineers Association of California in October 1973, R. J. M. Sutherland, of Harris and Sutherland, London, England, stated:

What would the effect be of a Boeing 747 colliding with a tower block, and what difference would the structure make? U Statistically, the chances of such an impact may be low, but surely it must happen soon U. We conveniently ignore such possibilities until something dramatic happens … Just wait until the loss in the building is comparable with or disproportionately greater than [that] in the plane, and then the shouting will begin U. I believe that we should start thinking of the possibility now. What is more, rather than concentrating on the statistical side of the risk, it might be useful to study the possible variation in extent of damage and the degree to which this could be re-duced at a reasonable cost.

The two WTC 110-story buildings (the North Tower and the South Tower) suffered structural damage from the impact, which, combined with the ensuing fires, resulted in the complete collapse of the buildings. The WTC buildings were of a unique structural system known as “tube tower,” with stiff exterior walls and columns spaced only three feet apart, connected by deep spandrel beams to minimize horizontal deflection. As each building was struck, there was extensive local damage to the unique exterior structure, the central structural core, and several floors directly impacted by the aircraft.

The initial plane strike in each case removed as much as 20 percent of the exterior columns and portions of several floors, together with an undetermined amount of the structural core.
In the case of the South Tower, the strike included the corner of the building, a particularly devastating blow to the structure. Despite this damage, the exterior columns, which had been designed to resist lateral loads from wind and earthquake, had excess structural capacity to resist vertical gravity loads and appeared to have at least temporarily bridged over the void and redistributed the load.

This was followed by a partial explosion of the fuel from the ruptured aircraft tanks as a fuel-air bomb (a nonideal explosive consuming as much as 25 to 40 percent of the fuel) in a fireball extending as much as 200 feet beyond the face of the building and igniting the building contents on several floors. The remainder of the unconsumed fuel drained through elevator and utility shafts to lower floors and spread the fire to the contents of these floors. The initial shock and explosion severed the building sprinklers, dislodged fireproofing, and blocked many of the fire stairs.


Larry Griffis, senior principal of Walter P. Moore and Associates, a Houston-based structural engineer, believes that the following happened: “The inferno ultimately destroyed the fireproofing, and the steel framing was heated to a temperature at which it lost its steel properties. Columns buckled, initiating progressive collapse in which one floor banged down on the floor below, with the increasing weight of higher floors becoming an impact load.”4
Another accepted view is that the fuel-air explosion, the loss of lateral bracing from the destroyed floors, and the impacts of collapsing floors were minor elements in the final collapse. The catastrophe was largely caused by the impact of the plane’s strike, which destroyed a large number of the exterior columns and also destroyed or damaged a significant number of columns in the central core, Additionally, the heat of the internal fire caused a reduction in the strength of the vertical steel members below the design/working strength required to sustain the building load. The remaining central core building columns failed by crushing in followed by progressive collapse of the entire building.

The exact mechanisms of the failures may never be fully known.

A number of studies have been or are being carried out in an attempt to define the causes of the collapse and to identify the lessons to be learned from this tragedy so that we can mitigate, if not prevent, such a high-consequence act in the future. FEMA AND ASCE STUDY
The Federal Emergency Management Agency (FEMA) Building Performance Study (BPS)/Structural Engineering Institute of the American Society of Civil Engineers (ASCE) report, issued in May 2002, states that these heat-induced additional stresses into the damaged building frames weakened the frames, causing the collapse of both structures.

It further states that preliminary analyses suggest that “absent other severe loading events such as earthquake or windstorm, the buildings could have remained standing in their damaged states until subjected to some significant additional load. However, the structures were subjected to a second, simultaneous severe loading event in the form of the fires caused by the aircraft impacts.” The FEMA/ASCE study “did not reveal any specific structural features that would be regarded as substandard; and, in fact, many structural and fire protection features of the design and construction were found to be superior to the minimum construction requirements.”

The study team noted that the following design features made it possible for the buildings to remain standing as long as they did and allowed evacuation of most building occupants:
robustness and redundancy of the steel-framing system, adequate egress stairways that were well marked and lighted, and conscientious implementation of emergency exiting training programs for building tenants.

The following design factors may have played a role in allowing the buildings to collapse as they did and prohibiting victims at and above the impact floors to safely exit, according to the team:
the steel floor truss system present in the building, robustness, and redundancy;
the impact resistance of enclosures around egress paths;
resistance of passive fire protection to blasts and impacts; and
grouping of emergency stairways in the central building core instead of being disbursed throughout the structure.

Although the team did not recommend that these items be evaluated for detailed performance, they are not regarded as design deficiencies or as features prohibited in building codes.
The U.S. House of Representatives, on July 12, 2002, voted broad investigative powers to the National Institute of Standards and Technology (NIST) to analyze building failures. It is anticipated that NIST will undertake a fuller investigation of the WTC collapse, with a budget estimated at $16 million.

The New York Times, in reporting this legislation, commented: “After six months, (FEMA/ASCE) investigators concluded that they were unable to provide a detailed analysis of how well the buildings performed. As a result, the investigators could not say if the buildings had specific weaknesses. That, in turn, meant that investigators could not say whether buildings across the country might be vulnerable, as well.”6 EXPECTATION PERSPECTIVES
•The Council on Tall Buildings and the Urban Habitat (CTBUH) formed a Task Force on all Buildings: The Future, chaired by Ron Klemencic, president of Skilling Ward Magnuson Barkshire, one of the two structural engineering firms derived from the original WTC design team. Klemincic says that he and his colleagues were amazed that the towers stood for as long as they did.

At a meeting of the task force in Chicago on October 15, 2001, Jon Magnuson of the firm said the following:

The fact that the WTC towers stood fooled many of the people in the profession, certainly most people in the public and most public policy makers, about what our expectations should be when an airplane hits a building. I can say without exaggeration, 99 percent of all buildings would collapse immediately if hit by a 767 …. Buildings should not and cannot be designed for airplane attack. It’s a problem, really, about airplane security …. If you take a random building in the country, the chance of its being hit by a meteorite is higher than being hit by an act of terrorism.

Another speaker at this meeting commented that the United States had suffered six (actually 10) major bombing incidents within the past nine years (each with casualty counts in the triple- to quadruple-digit range and fatality counts in the double- to quadruple-digit range).
(Magnuson, at a National Workshop on Prevention of Progressive Collapse, in Rosemont, Illinois, July 10-12, 2002, stated, “We really don’t need solutions looking for a problem.”)
Other speakers at the CTBUH meeting commented on the following: egress from and evacuation of the buildings, evacuating the handicapped, the potential use of elevators for evacuation, the capability of sprinklers to handle fires of this size, and the performance of the building fireproofing.

A structural engineer noted the problems of locating a big crane over a sidewalk designed to conform to a New York City rule of 600 pounds per square foot capacity. (Unfortunately, most New York City structural sidewalks were designed for 300 pounds per square foot. In 1970, the rules were changed to 600 pounds per square foot. Many of the sidewalks in Manhattan were designed to maintain only half that load.

Another speaker, Thomas Fridstein, of Tishman Speyer Properties, a major property owner in New York, stated: “From a real estate point of view, the 100-story building has long been impractical.”

The American Institute for Steel Construction, the Associated General Contractors, and the Georgia Tech College of Architecture has each formed task groups to investigate the subject. The British Institution of Structural Engineers has formed an international Working Group on Safety in Tall Buildings (and Buildings of Large Occupancy) to investigate more broadly the effects of tall and large buildings on the public, the environment, and the economy. This report includes not only the long British experience with terrorism but also fire and natural hazards. The report was released in late July 2002. The reports are chilling but practical and state in part: “It has to be assumed that there may be more severe and different extreme events in tall/large buildings than have occurred to date.” (“Safety in Tall Buildings and Other Buildings with Large Occupancy,” Institution of Structural Engineers, London UK—a discussion of this report will be furnished in the near future.)


The New York Times has been aggressive in its investigation of the WTC catastrophe. On December 13, 2001, it published an article quoting a fire protection expert, Professor Frederick W. Mowrer of the University of Maryland, as saying that the fireproofing applied [at the WTC] had peeled off or [had] been inadequately applied. Said Mowrer: “It seems as if the fireproofing was not up to what it should have been.” Mowrer based his statement on an archive of 1,200 photographs taken during inspections below the 78th floors in both towers from 1986 through June 2000 by Roger G. Morse, a consultant from Troy, New York. Morse was employed by United States Mineral Products during lawsuits over the asbestos content of the fireproofing.10
The New York Times has published a large number of investigatory articles through July 7, 2002, when it criticized the Fire and Police Disaster Response Plans used in the 9-11 catastrophe.

Whereas few buildings will face a bombing attack, almost all large buildings will be the location for a major fire in their useful life. No major high-rise building has ever collapsed from fire. The WTC was the location for such a fire in 1975; however, the building survived with minor damage and was repaired and returned to service.

The WTC was also the object of a bomb placed adjacent to a core column of one of the towers at a parking garage level in 1993. The bomb, of approximately 1,200 pounds (TNT equivalent), caused seven deaths and substantial damage. Although it was not publicized at the time, the bomb that destroyed the lateral bracing of at least one column for a length of 60 feet could have caused considerably more damage or even a partial collapse of the building had the building been subjected to other loads, such as a strong windstorm. This was one of the emergency repairs addressed after that attack. The Port Authority restricted parking in the WTC following this incident to avoid a recurrence.


Beyond the structural questions, several issues seem rather apparent. The first is that of firefighting. The WTC depended on sprinklers and fireproofing to prevent the rapid growth and spread of fires in expectation of the prompt response of firefighters. The evidence is that the rupture of supply standpipes had made the sprinkler system inoperable. There are further claims that the sprinklers would not supply adequate water for a fire of this size. This deficiency possibly could be overcome in tall buildings by providing a second remote standpipe, looping of individual floor systems to permit two-way flow or supply of flow on alternate floors from opposite sides of the building, and installing valving to permit the closure of the systems’ breached portions. Despite the failure of the sprinkler systems and the above comments regarding damage to fireproofing, review of the photos of the fires appear to show that the spread of the fire was not progressively from floor to floor, because of the system’s failure, but was probably caused by ignition from the aircraft fuel.

In a sketch accompanying the July 7 article in The New York Times , the highest reported floor reached by a firefighter was the 65th floor of the North Tower (although a fire marshal may have made it as high as the 78th floor of the South Tower). Several firefighters on the 20th floor of the North Tower reported chest pains. All of these firefighters were attempting to reach the fire scene by climbing the fire stairs in full turnout gear and equipment since the elevators were unusable.

Historically, fire ladder companies were established to fight fires in tall buildings, but today’s structures of 20, 40, 60, or 100 stories far outstrip their capabilities. (More than 20 years ago, after a series of fires in buildings under construction, New York City passed a law requiring that temporary elevators be installed in all buildings higher than six stories during construction, resulting in the adding of temporary elevator cars with a capacity of 30 passengers on the outside of new buildings. Unfortunately, it takes days, even weeks, to install each elevator.)
Helicopters were considered. Attempts to use helicopters to evacuate a burning building in Brazil some years ago were unsuccessful because of the updrafts of the large fire.

Another possibility is adding the window-washing staging found on most new tall buildings to the emergency electrical system and using this device for access and evacuation.

Another suggestion is to add a small, full-height, continuous-belt, powered firefighters lift inside a highly fire resistant enclosure. This would be supplemented by dedicated and sealed caches of firefighting and rescue equipment at a number of points throughout the tall building.


At the “Street Architecture for Security” workshop, organized by the ASCE’s Architectural Engineering Institute in New York in May 1996, and the “Terrorism and Sensitive Facilities” workshop in Washington in November 1998, Douglas Karpiloff, director of life safety and security for the WTC, stated that a computer simulation for evacuation of the WTC indicated that complete evacuation would require one hour and 50 minutes. This was based on an assumption of phased evacuation. Evacuation of handicapped persons would be accomplished by building elevators under fire department control. Because of the height and size and the complexity of the facility, no full-scale fire drill/evacuation was ever carried out. As we know, the elevators could not be used for this purpose. In neither case did the towers remain standing long enough to permit the planned evacuation.

The New York Times July 7 article discusses a number of cases in which firefighters and others attempted unsuccessfully, and with fatal results to both parties, to assist in evacuating the handicapped down the existing stairs. Although it may be politically incorrect to say so, it is time to review the intent of the ADA legislation and to consider the obligations, moral if not legal, it places on legislators, owners, designers, and—most of all—the rescuers, not only in bombings but in all building disasters.

Further, it seems that required lighting (automatically actuated supplementary battery power) and smoke prevention of fire stairs (positive pressurization as is required under many current building codes) should become mandatory. There should also be better exit planning and a more fire- and shock-resistive enclosure of fire stairs and elevators than the currently allowed gypsum board enclosure.

Despite all of the above, forewarned by the February 1993 bombing, the WTC was one of the best-prepared facilities in the country to handle an emergency, but it was not ready for the strike of two Boeing 767 aircraft.

According to the FEMA/ASCE report, the AirBus A-380 will be flying by 2006. It is more than three times the gross weight of the Boeing 767 and has a fuel load of 82,000 gallons. It would be criminally negligent if we did not learn the lessons so expensively taught by the WTC events and apply them to existing and future buildings.


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