In This Issue
September 1, 1998 Volume 28 Issue 3
Volume 28, Number 3 - Fall 1998

Mitigating Terrorist Hazards

Tuesday, September 1, 1998

Author: Eugene Sevin and Richard G. Little

The engineering community has a critical role to play in finding and promoting rational, balanced solutions to terrorist violence.

In 1995, the National Research Council published Protecting Buildings From Bomb Damage: Transfer of Blast-Effects Mitigation Technologies from Military to Civilian Applications (National Research Council, 1995). Of the several findings reached by the study committee that prepared the report, two appear particularly germane today. The first, "Attacks against civilian buildings pose an unquantifiable but real threat to the people of the United States," has been borne out by events over the past 3 years. The second, "Blast-hardening technologies and design principles developed for military purposes are generally relevant for civilian design practice," has spawned an ambitious research and testing program within the U.S. government. This effort is being led by the Department of Defense (DOD) and is intended to identify design and construction practices as well as improved materials that would produce more durable construction to protect its forces from terrorist attack. However, the extension of this existing body of knowledge of blast-effects on structures, appropriately refined and applied to commercial design and construction practice, also could save civilian lives and reduce property damage.

In the aftermath of the embassy bombings in Africa, the most recent in a series of terrorist attacks against U.S. government facilities or symbols of the United States stretching back almost 20 years, the engineering community can legitimately ask itself whether technology, properly deployed, could have played a role in saving lives, reducing the number and severity of injuries, and protecting property from damage. And, after five devastating bombings in the United States or against U.S. facilities in other countries in as many years, as well as a heightened awareness in both government and industry of the need to protect the nation's critical infrastructure against a wide range of potential terrorist threats, the engineering community is finally coming to recognize both the scope of the problem and the domain of the solution.

Although designing and building structures to withstand the effects of explosive devices has been a topic of active interest and research within the defense community for many years, these efforts have focused mostly on structural and systems survivability, leading to heavily reinforced bunker-type or underground construction where human safety is less a direct design consideration. However, the government also makes extensive use of commercial facilities to house military troops and civilian personnel. Because of the high casualties experienced in the Oklahoma City and Saudi Arabia (Khobar Towers) bombings, force protection - ensuring the safety of personnel within all types of structures - is now considered by the U.S. military to be a critical mission parameter. Addressing the threat of terrorist bombings of commercial buildings requires a solid understanding of how structures are affected by explosions and the ensuing role that the structure plays in both causing and mitigating injuries and death.

The primary objectives of any efforts to improve the blast resistance of commercial buildings should be to minimize the number and severity of casualties sustained in the initial blast; limit the subsequent response of the building; and improve the chances of successful rescue and recovery of the survivors. Given the many possible consequences of a bomb attack against a building, a thorough and detailed knowledge of how people are injured and killed in a bombing is an important step to achieving this objective.

A fuller understanding of blast-related injuries could guide the development of procedures for rapid response and rescue, facilitate planning for emergency medical treatment, and help set priorities for building-related research aimed at minimizing casualties.

Data from a growing number of incidents continues to extend the knowledge of blast-related injuries that has been developed within DOD. For example, the Oklahoma City Health Department conducted a detailed epidemiological study of the bombing of the Alfred P. Murrah Federal Building (Mallonee et al., 1996). The study concluded that the primary cause of death in the tragedy was related to building collapse, and most nonfatal injuries were caused by blast-generated debris, mainly glass fragments. (In the milliseconds following an explosion, much of the glass in a building is transformed into fragments and shards and propelled into the building at high velocity or sucked out of the structure during the subsequent vacuum phase. Many blast-related deaths and injuries are attributable to the body of the victim being penetrated by these missiles.)

Reducing Collapse, Blast Debris Critical
In the case of the Khobar Towers bombing in 1996, the structure did not collapse, and a smaller percentage of the occupants received fatal injuries compared with the Oklahoma City bombing. At Khobar Towers, both fatal and nonfatal injuries were attributed to blast-induced debris, with glass fragments again a prominent cause of nonfatal injury. Although these two incidents do not constitute a comprehensive data set, observations from other terrorist acts and accidental explosions offer compelling evidence that high fatality rates are strongly correlated with collapse of an occupied building and that glass fragments are a leading cause of nonfatal injury. This suggests two critical challenges for the structural engineer: ensure against the progressive collapse of the structure and minimize the quantity and hazards of broken glass and other blast-induced debris. These are active areas of research both here and abroad (e.g., Britain, Israel).

Progressive collapse occurs when the loss of load-bearing capacity (for example, through the destruction of one or more columns, or of load-bearing walls) results in localized structural failure which leads to further loss of support and, ultimately, collapse of all or part of the structure. Redundancy in the design can provide multiple load paths to the ground so that if one or more load-bearing elements are compromised, sufficient capacity remains to support the structure. Better continuity in structural joints between beams, columns, and floor slabs by means of increased reinforcement is one means of ensuring redundant load paths. (See Levy and Salvadori, 1992, for an overview of the phenomena of progressive collapse.) In California, concrete bridge piers have been wrapped with carbon-fiber materials to increase their strength and improve their performance in an earthquake. Applying such composite materials to building columns and slabs in the form of wraps or blankets can also increase the confinement strength of concrete members that are subject to blast loading.

Although there are ways to reduce the tendency of a building to undergo progressive collapse, there is no uniform, straightforward solution to this problem, because our current knowledge of the mechanisms of progressive collapse is incomplete. Following the collapse of an apartment building in Great Britain as the result of a gas explosion in 1968, there was considerable interest in progressive collapse. Although some advances were made in the 1970s, research funding waned in the absence of continuing public concern. Further increasing our understanding of progressive collapse will require physical testing of structures at full and partial scales, coupled with advanced computer modeling.

Research on Glazing Materials
Modern buildings typically contain several tons of glass in the form of windows, curtain walls, and skylights. As noted previously, following an explosion, much of this glass in transformed into hazardous projectiles. An obvious solution to this problem, greatly reducing the size and number of windows, has been implemented by the U.S. State Department in several embassy applications and found to be aesthetically wanting. A more proactive approach is to develop glazing materials that meet aesthetic and functional design objectives but do not contribute to the explosion-induced projectile hazard, either by controlling the nature of the projectile patterns or limiting their range and dispersion patterns.

Glazing materials for security applications are available in many forms. There are several types of glass (e.g., tempered, annealed, laminated), protective window films, and glass substitutes such as polycarbonates. All of these have different performance characteristics under blast loading. This is an active field of research with potentially large payoffs. Recent tests of glazing materials by the Defense Special Weapons Agency (DSWA) and others indicate that suitable glazing materials are being developed that will permit the design of open, attractive structures that reduce the risk to the building's occupants and neighbors. Fiber composite materials also show promise for retrofit applications. When window replacement is not feasible, materials such as Kevlar can be woven into blast curtains and drapes to limit the dispersion of blast-generated debris.

As the 1995 sarin gas attack in the Tokyo subway system revealed, the use of weapons of mass destruction (WMD), such as chemical and biological agents, by terrorist groups is cause for increasing concern. Although acting in a fundamentally different manner than a bomb, such agents place building populations at considerable risk and require the design of appropriate intervention strategies and systems. These systems could incorporate sophisticated sensor, microprocessor, and control technologies coupled with ultra-high-efficiency filtration devices to react instantly to an attack and automatically initiate an appropriate response. The capability to implement this type of intervention strategy is resident in what is termed "intelligent building technologies" that are available today to monitor and control building systems and equipment. Unfortunately, if the nation's reaction to the bombing threat is any guide, public demand to consider deployment of such technology will await an actual attack utilizing WMD.

Among the earliest practitioners of protective construction were military engineers who were concerned with the design and construction of fortifications on one hand and their destruction and defeat on the other. Vitruvius, the noted Roman architect and engineer who practiced in the first century B.C., included in The Ten Books on Architecture (Morgan, 1960) a discussion of siege engines and how they could be both employed and defeated. Several hundred years earlier, Sun Tzu in The Art of War (Griffith, 1971) had discussed the siege of cities and how that might be physically accomplished. In fact, the earliest formalized training in engineering was provided at schools such as the Ecole Polytechnique, British Royal Military Academy, and West Point, which were established in the 18th and 19th centuries with the goal of training military engineers as a core part of their mission. Much more recently, the United States sponsored a comprehensive program of research to increase the blast resistance of military structures, largely in response to the nuclear threats of the Cold War. During this period, work was also begun on developing design procedures for structures subject to accidental explosion and attacks from conventional weapons. Although these activities were driven by military needs and concerns, they provided a technical basis to address the threat of terrorist attacks against commercial structures.

The current concern with protecting buildings and those they shelter may be viewed as a logical extension of basic military engineering principles. In this light, it is useful to think of a strategy to prevent, mitigate, and respond to future attacks, if and when they occur, in terms of the integration of four fundamental security design objectives: (1) denying the means of attack; (2) maintaining safe separation of attackers and targets through good planning and architectural practice; (3) providing strong, resilient construction to protect personnel and other key building assets; and (4) facilitating rescue and recovery operations in the event an attack occurs.

The first line of defense is denying access to explosives and detecting and apprehending potential perpetrators before they can act. This is primarily a law enforcement and security function and encompasses a broad range of activities such as explosive-detection devices, research to determine the feasibility of rendering inert common chemicals used in explosives such as ammonium nitrate fertilizer, and tagging explosive materials so that their source may be traced more easily.

The second and third objectives will challenge the engineer to work closely with other professionals such as architects, landscape architects, and security specialists to ensure the attractive integration of site and structure in a manner that minimizes the opportunity for attackers to approach or enter a building. This approach uses such features as landscaped berms that function as blast barriers and traffic controls and bollards disguised as planter boxes that prevent vehicular access. The building itself may have a range of blast-resistant features such as additional steel reinforcement, composite fiber wraps, and high-performance glazing materials. The structure's electrical and utility systems may be placed in protected raceways, critical facilities or operations housed in specially hardened areas or underground, and primary and backup systems located in different parts of the building.

Engineering Role in Rescue and Recovery
It is difficult if not impossible to prevent destructive acts by persons unconcerned with their own safety or survival. Therefore, engineering design also plays a key role in facilitating rapid rescue and recovery of victims in the aftermath of an attack. The speed with which rescue personnel can safely enter and secure a damaged building can reduce the loss of life, mitigate injuries, prevent further damage to the structure, and help restore the building to productive use. These efforts will be aided by computerized building plans, structural analysis programs, and damage assessment models - all tools requiring the active involvement of the engineering professions.

The engineer is also in an excellent position to frame the discussion of the cost-benefit tradeoffs that occur in the risk management process. For example, the enormous cost of implementing safety strategies for the U.S. nuclear weapons program was acceptable given the potential consequences of failure. It was, in essence, risk avoidance regardless of cost.

This level of surety is not practical or appropriate for commercial buildings and civilian infrastructure. An initial risk management strategy might reasonably assume that risk should be avoided only up to the point where the costs incurred are less than the cost of failure multiplied by the probability of the failure occurring. The analysis can treat life-safety issues differently from property damage but the principles are the same. The values that the general public places on various safety and security upgrades can be determined through survey and interview techniques that are more appropriately in the realm of the social scientist.

Addressing Risk through Design
Because people are notoriously unwilling or unable to state specifically what level of risk they will accept, the engineering community does this by default in the design process (or by providing passive restraints like airbags in automobiles). But this is not an issue for the engineer alone to solve. It needs to be elevated to a high level of public discourse. This is particularly true for the private sector. Protecting Buildings From Bomb Damage found that financial considerations were a serious barrier to the deployment of blast-resistant practices in commercial structures.

The NRC has been advising the U.S. government on issues of blast-effects mitigation and physical security for over 50 years. Protecting Buildings From Bomb Damage was supported by DSWA, which in 1996 was directed by Congress to apply its extensive knowledge of the effects of high explosives and WMD to counterterrorism activities within the United States. In response to this directive, DSWA, through the DOD Technical Support Working Group (TSWG), has initiated a comprehensive research and assessment program to address a number of interrelated issues, including threat analysis methods for federal buildings, blast response of structural components and nonstructural systems, and the value of computer-based tools for design and consequence assessment and management.

More recently, the President's Commission on Critical Infrastructure Protection proposed a national strategy for protecting and assuring critical infrastructures from physical and cyber threats. For the purposes of the commission's work, critical infrastructures are systems whose incapacity or destruction would have a debilitating impact on the defense or economic security of the nation. They include telecommunications, electrical power systems, gas and oil, banking and finance, transportation, water supply systems, and government and emergency services.

The commission's final report (President's Commission on Critical Infrastructure Protection, 1997) identified electronic, or cyber, attacks as a major challenge that will confront the nation in the 21st century. While primarily in the realm of computer technology and electronic security, the systems (and the people who operate them) on which we depend so critically also have physical components that must be protected. Despite the commission's emphasis on cyber attack, recent experience and the considered opinion of antiterrorism experts suggest that attacks against buildings using conventional explosives will, in all likelihood, continue to be the primary tactic of terrorists for the foreseeable future.

Implementation of the commission's recommendations is under way. The Clinton administration has set forth its policy for addressing terrorist acts against the built environment in two recent Presidential Decision Directives, PDD-62 (Combating Terrorism) and PDD-63 (Protecting America's Critical Infrastructures). The former document highlights the growing threat of unconventional attacks against the United States and details a new and more systematic approach to fighting terrorism by bringing a program management approach to U.S. counterterrorism efforts. PDD-63 calls for a national effort to assure the security of the increasingly vulnerable and interconnected infrastructures of the United States. The directive requires immediate federal government action, including risk assessment and planning to reduce exposure to attack. It stresses the importance of cooperation between the government and the private sector by linking designated agencies with private-sector representatives.

The administration's policy for protecting critical infrastructures also calls for the National Academy of Sciences and the National Academy of Engineering to consider establishing a roundtable for bringing together federal, state, and local officials with industry and academic leaders to develop national strategies for enhancing infrastructure security (The White House, 1998).

Separately, DSWA has asked NRC to establish a standing committee to assist the agency develop a blast-effects research agenda and provide recommendations on priority activities. The committee will also recommend appropriate mechanisms for transferring the results of such research to civilian government agencies and commercial engineering and architectural practice. Providing this information to the private sector is viewed as a key step in ensuring that technological advances are incorporated into the building stock. The committee will also provide a forum for enhancing interaction and information sharing among other federal government agencies, state and local governments, and professional organizations and societies. This effort will provide an intellectual basis for the debate of such public policy questions as the level of security protection for different types of buildings (e.g., government vs. private) and whether to consider retrofitting existing buildings for blast effects.

In an open, democratic society, there is inevitable friction between national security needs and personal freedom. In the built environment, these conflicts often manifest themselves in the design and accessibility of our public buildings. Unfortunately, the state of the world as we approach the 21st century requires that a prudent balance be struck between free and open access on the one hand, and security-driven fortresses on the other. Ongoing research is aimed at finding technical solutions to combat terrorism. How and where these technologies are deployed is an issue for a broader debate. At the conclusion of their 1992 book, Why Buildings Fall Down, NAE members Matthys Levy and Mario Salvadori posed a question with perhaps unintended relevance to the threat of terrorism: "Will progress in the field of structures reduce the number of failures?"

Although more robust construction in and of itself will not eliminate the consequences of terrorist attacks on commercial buildings, the engineering community has a valuable role to play in finding and promoting rational, balanced solutions to what remains an unbounded threat. Ultimately, the goal should be to develop and disseminate knowledge that will enable the construction of a generation of buildings that are more robust and safer but still aesthetically appealing.

Griffith, S. B. (trans.) 1971. Sun Tzu, The Art of War. New York: Oxford University Press.

Levy, M. P., and M. Salvadori. 1992. Why Buildings Fall Down. New York: W.W. Norton and Company.

Mallonee, S., S. Shariat, G. Stennies, R. Waxweiler, D. Hogan, and F. Jordan. 1996. Physical Injuries and Fatalities Resulting From the Oklahoma City Bombing. Journal of the American Medical Association 5:382-387.

Morgan, M. H. (trans.). 1960. Vitruvius, The Ten Books on Architecture. New York: Dover Publications.

National Research Council. 1995. Protecting Buildings From Bomb Damage: Transfer of Blast-Effects Mitigation Technologies from Military to Civilian Applications. Washington, D.C.: National Academy Press.

The President's Commission on Critical Infrastructure Protection (PCCIP). 1997. Critical Foundations: Protecting America's Infrastructures. Washington, D.C.: PCCIP.

The White House. 1998. White Paper. The Clinton Administration's Policy on Critical Infrastructure Protection: Presidential Decision Directive 63. Washington, D.C.

About the Author:Eugene Sevin is a member of the National Academy of Engineering and former science advisor to the Department of Defense. He chaired the committee that produced Protecting Buildings From Bomb Damage: Transfer of Blast-Effects Mitigation Technologies from Military to Civilian Applications. Richard G. Little is director of the Division of Infrastructure and the Board on Infrastructure and the Constructed Environment of the National Research Council.