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Proceedings from the MCEER Workshop on Lessons from the World Trade Center Terrorist Attack:
Management of Complex Civil Emergencies and Terrorism-Resistant Civil Engineering Design

held at the City University of New York, New York City     |   June 24-25, 2002



Background and Objectives

Multi-hazard approaches have long been proposed as a viable and cost effective means to achieve a greater level of protection of the infrastructure. But which hazards could be combined to achieve benefits in an economical manner? Floods and earthquakes have little in common in terms of their engineering solutions and approaches, and simultaneous protection against both hazards essentially costs the same as the sum of the costs for mitigating each hazard individually. Wind engineering and earthquake engineering are closer, but there are still major significant and conceptual differences in engineering solutions for these two hazards. However, earthquakes and the blast forces from an exploding bomb can both push the structural and non-structural elements of a building to their ultimate, near-collapse limit state. Therefore, the design tools and strategies to enhance building performance in blast or earthquake events are somewhat similar (even if not totally identical). As such, the communities concerned with earthquake-protection and protection from terrorist bombings are today at a critical and strategic juncture, and presented with an opportunity to take a major step toward the implementation of multi-hazard protection for buildings and critical infrastructure. This can only be achieved successfully, however, if the two communities start to work together.

The objective of this workshop was to bring together leaders from the earthquake engineering community, the blast engineering community, the social sciences, and the emergency response community, to identify possible linkages between earthquake-protection and terrorism-protection issues, and how groups focusing on these two different problems could potentially work together. The following conclusions and recommendations provide ideas on how to work toward this objective.

It is important to note that, for purposes of this workshop, the terrorist attacks considered were limited to bombings. Terrorist attacks using biological, chemical, or radiological weapons were beyond the scope of this workshop.

Differences and Possible Synergy in Blast and Earthquake Effects

Significant differences exist between the effects of earthquakes and blasts on a building or infrastructure component.

Earthquake sources are below ground, and transfer energy to infrastructure through ground shaking. These vibrations affect the entire structural system globally, and the source of excitation can last up to a minute, with input frequencies of 1 to 10 hertz. Damage to non-structural components ensues as a consequence of excessive structural behavior. Emergency response activities have to simultaneously address a wide geographical area, as well as rescue and recovery needs and priorities of multiple communities across the impacted region.

To date, terrorist bombing attacks have either been above ground, detonated outside of a building, or in some instances inside a building. In both cases, the effects are localized. Blast forces last a few milliseconds and consist mainly of a strong main pressure wave followed by a slightly longer but less intense reversed pressure. The shock waves travel rapidly from one point of a building to another. Blast forces can directly produce damage to non-structural components, although failure of structural elements can further compound this damage. Disaster response activities are generally confined to the immediate area where the blast has occurred.

Earthquake-resistant design alone does not inherently confer a sizeable measure of blast-resistant design, nor does blast-resistant design automatically provide earthquake-resistance. However, it is conceivable that new types of systems could be developed holistically taking into account both threats, resulting in greater effectiveness than if both hazards were simply considered independently and sequentially using conventional systems. Research is required to better specify the characteristics of such new and innovative systems.

Shared Engineering Objectives

In spite of the differences outlined above, the key engineering objective of preventing catastrophic failure and collapse is common for both earthquake and blast engineers. Similar analysis and design approaches can be used to achieve this objective, such as increased ductility of the structural systems and redundant load paths, to name a few, even though applications may vary in their details. Both the earthquake engineering and blast-resistant design communities have many tools that can be exchanged and shared, and more interaction and cross-pollination of ideas would be helpful. Research would be needed to assess how the many existing engineering analysis methods, design philosophies, structural concepts and retrofit strategies could also be best modified to address both hazards.

Design of Commercial (Non-Mission Critical) Buildings

While blast-resistant design is likely to become a standard requirement for most government buildings and mission-critical infrastructure, it appears that little incentive exists to add such mandatory provisions for other buildings and infrastructure. Some argue that this in fact may not be necessary or desirable. However, building codes should include effective requirements to prevent progressive collapse; to the extent such measures are threat-independent, they can provide protection from multiple hazards. For some types of construction, the cost of protecting against collapse could be relatively small in comparison to potential losses of life and property.

A major difficulty in designing against progressive collapse is the definition of the initial triggering design condition, or the initial tolerable levels of loss of structural elements for which progressive collapse should be prevented. Deliberate airplane collisions are not believed to be a likely design condition for such buildings. Instead, measures to prevent the high jacking of airplanes are more cost-effective by many orders of magnitude than efforts to make the entire infrastructure resistant to such attacks.

Engineers are generally confident that they can satisfactorily execute designs that meet any limit state or specified circumstance, provided that the loads are adequately defined. However, technical design guidance and results from military research and testing would be of great assistance if made more widely available to those with a desire to voluntarily incorporate blast-resistant features into their designs. Although some may argue that such actions could potentially provide terrorists with specific information on design levels, thereby guiding their destructive actions, the benefits of making existing knowledge available to the design community in a secure manner outweigh the potential risks, especially if it can lead to the design and construction of more resilient structures.

Societal Issues

An argument can be made that the collapse of the World Trade Center twin towers were not disproportionate to the damage that caused them. Similarly, the collapse of the Murrah Building in Oklahoma City may not have been disproportionate to the effects of 5,000 pounds of high explosive detonated less than 20 feet from its key structural elements. However, there is a need for broader discussion with all stakeholders and societal actors to assess whether it is desirable to prevent such disastrous outcomes at all costs, whether a balance exists between risks and costs, and, if so, where lies this desired balance-point. As is the case with hazards of all types, the challenge is to better determine what the public and key stakeholder groups consider acceptable levels of risk and how much they are willing to pay to protect themselves against the range of existing and emerging threats. It is quite possible that without a political driving force and leadership, such questions will not be addressed and few incentives will be provided to enhance resilience against terrorist attacks beyond what currently exists (i.e., for mission-critical facilities).

Likewise, measures to improve mitigation, preparedness, response, and recovery require cooperation and trust between government agencies at all levels and between the public and private sectors. Achieving higher levels of protection is not the responsibility of government alone. As demonstrated following 9/11, timely and truthful communication on plans and actions is critical; people need to know why actions are being taken and what the impact will be on their operations. Consequences need to be identified and planned for, and plans must be exercised. Ongoing collaboration breeds trust, and the importance of broad collaboration among various governmental levels and between government, the private sector, and the public cannot be overemphasized.


Structural performance in major fires is relatively uncharted territory. There is a need for extensive focused research on the behavior of different structural systems, both to improve the building stock and to provide better guidance to emergency responders.

Infrastructure Systems

New York City was able to restore critical services and initiate early recovery activities relatively quickly after the terrorist attacks of September 11, 2001 because of the inherent redundancy of many of its physical and institutional infrastructures. Many of the service providers in New York (e.g., Consolidated Edison, Verizon, MTA) possessed sufficient excess capacity in people, equipment, and other resources to provide an effective and relatively rapid restoration of services. Less robust systems, or infrastructure systems in less highly resilient cities, would likely not have fared as well.

Infrastructure systems in major urban areas are inherently interconnected and vulnerable to complex system failures. We do not fully understand all of the vulnerabilities that can cause losses to proliferate and hinder recovery efforts.

Real-Time Sensing Technology

Technology was helpful in managing response, restoration, and early recovery activities following 9/11, and the use of accrued real-time sensing technology has the potential to yield clear benefits. First responders in particular need real-time data on damaged buildings, which when coupled with assessment tools and decision-support systems, can assist in making informed choices about the feasibility of rescue operations and the safety of emergency personnel, as well as supporting other post-disaster response activities. Opportunities exist to incorporate advanced sensing technologies into emergency response activities in a manner that would enhance the safety and survivability of first responders.

Recommendations for Future Research, Knowledge Transfer, and System Development

The following recommendations flow directly from workshop presentations and discussion sessions:

Finally, although the attacks of September 11 have focused attention on the severity of the terrorist threat, the engineering and emergency management communities need to continue to focus on the entire range of hazards that threaten the built environment, including natural, technological, and human-induced hazards. With that goal in mind, multi-hazard loss-reduction approaches should receive emphasis in both research and implementation efforts. There is a need to develop overarching approaches focusing on broad improvements that enhance the overall resilience of the civil infrastructure and society more generally, rather than on strategies for coping with specific threats to the built and societal environment, however severe those threats may appear to be. Approaches and strategies that focus on addressing both the earthquake hazards and the blast forces from an exploding bomb are promising, as they both focus on prevention of collapse for structural and structural elements at their extreme limit states.

The events of 9/11 have opened a window of opportunity for the entire disaster loss-reduction community to engage key societal decision makers and convey to them the need to formulate and enact appropriate strategies for preventing extreme events whenever possible, and for limiting damage and responding effectively when such events do occur. The recommendations from this workshop make a useful contribution to such efforts by suggesting how such strategies should be developed.

George Lee

Director, Multidisciplinary Center for Earthquake Engineering Research
University at Buffalo

Michel Bruneau

Deputy Director, Multidisciplinary Center for Earthquake Engineering Research
University at Buffalo

Richard Little

Director, Board on Infrastructure and the Constructed Environment
National Research Council

Kathleen Tierney

Director, Disaster Research Center
University of Delaware

Rae Zimmerman

Professor and Director, Institute for Civil Infrastructure Systems
New York University



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