Designing Terrorist-Resistant Buildings
BY TOD RITTENHOUSE
Terrorist bombings, until recently, primarily were thought to occur in the Middle East, South America, England, and Ireland. Even after the World Trade Center Bombing in February 1993, most observers believed terrorist attacks in the United States would be a one-time event. Headlines describing a radical fundamentalist group striking a major city landmark did not cause much concern for the rest of the country. In fact, only major coastal cities seemed concerned enough to prepare for terrorist attacks. On April 19, 1995, the other 90 percent of the U.S. population was forced into a major reality check when one of its own people struck in the heartland. A former U.S. soldier and reported paramilitary activist allegedly detonated the largest terrorist bomb on U.S. soil, killing 169 people and wounding countless others.
While the probability of becoming the victim of a terrorist attack has probably changed a little, it still remains infinitely low and the cost of such an attack continues to skyrocket. According to The Sentinel (Vol. I, No. 3, Third Quarter 1993), a publication of the Industrial Risk Insurers, explosion has the highest average dollar loss of all hazardous events. Therefore, another cost factor entering today`s construction and building operation economy is blast mitigation costs. Building owners and developers, led by the U.S. government, are taking a closer look at incorporating measures to alleviate the effects of a terrorist attack on their buildings. Military installations for years have been designed to resist conventional weapons or blast attacks, with the single focus toward sustaining the structure for the purpose of maintaining the mission. For commercial buildings, the single most important design consideration is to construct buildings to save lives in the event of a terrorist attack. There is absolutely no concern for saving the structure other than to save the people.
Many articles have been written addressing the cost and appearance of designing military-like installations for civilian use. In addition, designing commercial buildings as military installations is very impractical, as ordinary people do not want to work in bunker-like buildings. Commercial buildings serve a very different purpose and therefore must be designed to a different standard. Accepting that fact, commercial buildings should be designed to sustain a certain amount of attack, meaning they are designed to allow for limited localized damage–not total failure–to permit rescue teams to evacuate the victims. Our goals as structural design professionals are twofold: to design safer buildings that will not fail when attacked and to help rescue professionals gain entrance to a damaged building to tend to the survivors. This can only be done if the building has been properly constructed, which is where the structural engineer and blast consultants enter the picture.
DEFINING THE THREAT
When considering protection for a building, the building owners and architects must work with structural engineers and blast consultants to determine which threats they are trying to protect against. In recent months we have witnessed bombing attacks, small arms ballistic attacks, chemical and biological attacks, forceful entry and hostage attacks, and even a mortar attack. As for the bombings, the terrorist threats range from the large truck bomb to the mid-size car bomb to the small package or letter bomb. For these assaults, the source can originate either external or internal to the structure. No matter what size the bomb, there will always be some localized damage and some unavoidable deaths. However, by limiting the localized failure, we can prevent triggering a catastrophic structural failure. Optimally, blast mitigation provisions for a new office building should be addressed in the early stages of the design. However, existing buildings can be upgraded to behave better when attacked.
Some secondary design considerations should be included whether considering a new structure or a security renovation. First, the building`s operational control areas and utility feeds should be protected from direct attack to lessen the negative effects of the blast. As was witnessed in the World Trade Center, one bomb disabled the primary utility feeds and their backup systems. In an unrelated accidental event at Newark International Airport, the prime and backup power lines were severed by a single foundation pile that was being driven for a new garage. Fortunately, in this case, there were no emergencies. However, accidental or terroristic, these events not only disrupted the operations of the facility but also impaired the rescue operations. Therefore, it is important to design operational redundancies to survive all kinds of attack.
PROTECTIVE DESIGNING AND RESCUE OPERATIONS
Protection for a commercial building, which comes in active and passive forms, will impact the damage sustained by the building and the rescue efforts of the emergency workers. The first mode of protection is to create a “keep-out zone” that ensures a minimum guaranteed distance between the explosion and the target structure. This keep-out zone is achieved by placing at the site perimeter bollards, planters, fountains, and other barriers that cannot be compromised by ramming with a vehicle. While this is the most effective measure to lessen the effect of a terrorist attack, it also can work against rescue teams since the barriers could deter access to the rescue and firefighting vehicles. In most urban settings, the typical setback distance from the street to the building facade is 10 to 25 feet, which does not pose any access problems for emergency vehicles. However, when designing prestigious buildings, including landmark office towers, hospitals, and museums, the setback is often increased to create a grand public space. These setbacks could create keep-out distances of more than 100 feet with barriers to guarantee protection but which also could limit emergency vehicle access. Details that may provide operational bollards or fences should be included in the design to allow emergency access. If plaza or monumental stairs were used, some secondary access must be incorporated to similarly allow entry. During the emergency response group`s preplanning exercise, consideration must be given to gaining access to buildings with large terrorist keep-out zones.
Several methods that enhance the building`s structural performance will directly benefit the emergency response team. As the building cannot be designed to be bombproof, the key is to limit the acceptable damage to a confined area. The question remains, How extensive and how widespread is this localized or “acceptable” damage?
Acceptable damage is a relative term, with the spectrum of damage ranging from a few broken windows to regional slab failure to confined structural frame failure. In any case, the ultimate goal is to prevent widespread structural failure or progressive collapse. The damage at the World Trade Center was extensive but somewhat localized, relative to the massive twin structures. Unfortunately, the damage to the Alfred P. Murrah Federal Building in Oklahoma City, which was very extensive, could have been limited had the design included provisions to mitigate progressive collapse.
Simple structural modifications include designing redundancies into the structure to carry additional loads imposed after a bomb attack. These provisions include properly detailing beams, girders, and columns to carry damaged slabs or columns. In the Oklahoma City Bombing, the banded beam floor system used did not have any redundancy or backup support system for slab or band beam failure. By properly designing and detailing the building`s structural elements, the slab may fail locally, but the entire floor will not collapse. Similarly, columns are designed to carry additional loads such that if one column is severely damaged to the point where it cannot properly function, the load will be distributed to the neighboring columns.
Other subtle changes now being incorporated into the design of some office buildings involve the proper designing and detailing of transfer girders and beam/column connections. The use of transfer girders can significantly increase the damage caused by the blast. As witnessed in Oklahoma City, transfer girders along the perimeter eliminated several columns at the base of the building by redistributing the column loads. While this is a typical design practice, under blast loading, the loss of one of the lower support columns can effectively damage three columns. Thus, the localized damage zone becomes significantly greater and can propagate into a catastrophic failure. However, transfer girders still can be used very effectively as long as they are properly designed and detailed to transfer all loads if their support columns are damaged. In addition, all beam and beam/column connections also must be properly detailed to resist blast loads, which can be upward or downward force.
These changes all attempt to enhance the structure`s response to the severe dynamic loading by adding strength to resist the blast, ductility to absorb the energy, and redundancy to reduce the chance of progressive collapse. Many of these steps incorporate some of the recommendations made in the “Structural Engineering Guidelines for New Embassy Office Buildings,” developed by Weidlinger Associates for the Department of State-Office of Foreign Buildings Operations after the attacks on American embassies. It remains today the only design guideline approved for government design or construction.
The last major structural consideration includes the construction of the exterior facade. Second only to the impact the standoff distance has on the effects of the blast, the facade remains the occupant`s last form of true protection. Not only does the building`s skin protect the occupants from the weather, but it also limits the amount of blast that can actually enter the workspace. The facade is built of two elements–the structural skin or wall section and the window or glazing. By constructing the exterior wall of a more durable material such as reinforced cast-in-place concrete instead of block walls or curtain walls, at least for the lower floors, the building will be able to resist or curtail certain blast loads, resulting in significantly less bodily injury and building damage. In addition, a properly designed concrete wall will aid in preventing progressive collapse, as the wall will assist in carrying the load of a damaged column.
While most blast engineers would like to eliminate windows, they remain an essential element in building design. Windows, unfortunately, are the weak link in the facade design, as they typically will fail before any other element. The two keys to protecting the workspace are attempting to prevent the windows from failing and then ensuring that the windows fail properly if overloaded. While a great number of human injuries are related to flying glass shards, it is not the only significant source of injury, though usually a more visible one. The other visible cause of injury is falling debris. One of the less visible causes of human injuries is blast pressure, which can rupture the ear drum, collapse the lung, or even crush the skull as a result of the blast wave`s getting into the workspace. These injuries, which begin at pressures near 15 pounds per square inch (psi), can be reduced if the level of blast pressures entering the space is curtailed. The amount of blast that enters the space is directly proportional to the amount of openings on the facade of a structure.
In embassies, the first type of civilian office building designed to resist blast events, fenestration (openings in the facade) is limited to 15 percent. While this helps in the protective design, it does not provide the proper lighting or open feeling that is required in modern office buildings; therefore, the fenestration requirements have been reluctantly increased for commercial buildings.
The second design aspect for windows is to ensure that they fail properly if overloaded. Windows can be designed not to fail for the small to mid-sized opening described above, provided that the loading is limited. However, standard annealed glass used in window sections for both homes and office buildings behaves quite poorly in blast loading and failure. Not only is the peak allowable pressure less than two psi, but this type of glazing breaks into sharp shards. It is these “knives and daggers” that cause most of the injuries so vividly seen in the news clips.
Several other types of glazing are available for building design, including thermally tempered glazing (TTG) and polycarbonate glazing, also known as bullet-resistant glass. TTG is commonly used in special areas where glass breakage and bodily injury are likely to occur; the most common examples are the side and rear windows of automobiles. This glazing, which can be designed for loads up to 30 and 40 psi, breaks into rocksalt pieces and would cause significantly fewer injuries to people occupying the building under attack. The polycarbonate glazing also performs quite well under blast loading but does not experience the same failure mode as the TTG. Instead of breaking into small pieces, it remains a single unit as it dislodges from its support frame. The most common use of polycarbonate glazing is the front windshield of an automobile. When damaged, it develops spider web cracks but does not break into small shards. The downside of this mode of failure is that the glass unit can become a large flying object, causing additional injury and damage. Other window treatments include adding a mylar film to the glass, which will reduce the shards but can discolor and be easily vandalized, reducing its effectiveness. Furthermore, the failure mechanism is similar to the bulletproof glass, which creates large flying obstacles. Blast curtains, which collect the glass fragments, also can be used but reduce the light and view that the windows are trying to provide in the first place.
While these glazing window solutions appear to function quite well, do not expect to see them installed in too many office buildings. Some drawbacks in these high-performance glazing systems include cost and high maintenance. Not only are the windows themselves very costly, but to fail properly, their support and attachment systems must be properly designed, which further increases the cost.
With the cost for installing these windows reaching several million dollars, the protective design money would be better spent keeping the structure standing rather than keeping the windows in place. This is because the anticipated blast pressures from a car or truck bomb far exceed the allowable pressures any window system can resist. As a point of reference, the blast pressures felt on the facade in the Oklahoma City Bombing were on the order of 4,000 psi–100 times higher than the design pressures described above. The growing sentiment is, Why spend all that money installing blast-resistant windows that are going to fail under nominal blast loads when you can put the money into preserving the structure to facilitate rescuing the people?
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The biggest structural engineering lesson learned from the Oklahoma City Bombing is that the building`s response to the dynamic loading of a terrorist bomb must be improved. The single most important improvement would be to mitigate catastrophic failure or progressive collapse by improving the structure`s ductility and incorporating some structural redundancy. This measure alone, regardless of glazing or facade improvements or maximizing the standoff distance, will significantly reduce the number of fatalities. When more than 80 percent of the deaths are caused by the structure`s falling on top of occupants who otherwise would have survived the blast, construction money would be most prudently spent to properly design, reinforce, and detail the building to improve its response to explosions. While the localized damaged zone will not be in pristine condition, it will remain safe enough to facilitate the rescue of potential victims. n
TOD RITTENHOUSE, a principal with Weidlinger Associates Consulting Engineers in New York City, has 11 years of experience designing conventional buildings and hardened structures to resist nuclear, conventional, and terrorist explosions. He has degrees in physics from Fordham University and civil engineering from Columbia University. He is a blast consultant to the Department of State, Office of Foreign Buildings Operations for embassy design, as well as the General Services Administration and the Department of Defense. He is a member of the ASCE Task Committee for the Mitigation of Effects of Terrorism.