Journal of Performance of Constructed Facilities

This paper presents some findings of the FEMA and SEI/ASCE sponsored studies of structural performance of New York’s World Trade Center (WTC) following the attacks of September 11, 2001, and the Murrah Building following the April 19, 1995, Oklahoma City bombing. The WTC collapses were caused not by aircraft impact alone but by the combination of impact and the resulting fire that weakened structural members and connections. On the other hand, the Murrah Building collapsed as a direct result of the blast. Although these studies call for further research in a number of areas, this report summarizes some of the lessons learned.

Following the collapse of the World Trade Center towers in September 2001, there has been heightened interest among building owners and government entities in evaluating the progressive collapse potential of existing buildings and in designing new buildings to resist this type of collapse. The General Services Administration and Department of Defense have issued general guidelines for evaluating a building’s progressive collapse potential. However, little detailed information is available to enable engineers to confidently perform a systematic progressive collapse analysis satisfying these guidelines. In this paper, we present four successively more sophisticated analysis procedures for evaluating the progressive collapse hazard: linear-elastic static; nonlinear static; linear-elastic dynamic; and nonlinear dynamic. We discuss the advantages and disadvantages of each method. We conclude that the most effective analysis procedure for progressive collapse evaluation incorporates the advantageous parts of all four procedures by systematically applying increasingly comprehensive analysis procedures to confirm that the possibility of progressive collapse is high.

Explosive attacks on buildings create a rapid release of energy in the form of a shock wave. Window glass is often the weakest part of a building, breaking at low pressures compared to other building components. Explosion-related injuries caused by glass breakage include eardrum damage, lung collapse, and penetration- or laceration-type injuries from airborne glass fragments which may lead to fatalities. As glass breakage may extend for miles for a large external explosion, reducing glass hazard is an important aspect of blast-mitigation for buildings. Blast-mitigating window products are in their infancy in the United States. Categories of available window retrofit solutions include: window film technologies, mullion and frame upgrades, catch systems, window replacement systems, installation of secondary windows, and/or a combination of these technologies. Installation of daylight film is the least costly but provides the lowest level of protection, while replacing windows is the most costly with the greatest gain in blast protection. Selection of the best window retrofit solution is usually based on a number of design constraints including blast criteria, building constraints—which can be broken down into type of building, maintenance and operability requirements, interior finishes, energy conservation requirements, lifecycle—and finally the retrofit cost.

In recent years, many buildings around the world have had their windows protected or strengthened to resist the effects of explosions in an attempt to reduce the level of casualties associated with terrorist bomb attacks. While the windows form the most vulnerable parts of a building, occasionally it becomes necessary to strengthen the walls as well—particularly in older and weaker historical structures or where the blast loads are high. This paper highlights a number of different techniques available to the structural engineer and security specialist that can be employed to make existing masonry walls stronger and more capable of safely resisting the effects of explosions.

The most widely used terrorist tactic is the improvised explosive device, which can range in size from hand-held to truck-size. Most casualties and injuries sustained in such an attack are not caused by the blast itself, but rather by the disintegration and fragmentation of walls, the shattering of windows, and by nonsecured objects propelled at high velocities by the blast. Since 1995, the Air Force Research Laboratory at Tyndall Air Force Base has investigated methods of retrofitting wall structures to better resist blast loads from external explosions. This paper summarizes results from recent tests that involve an innovative use of a sprayed-on polymer to increase blast resistance of unreinforced concrete masonry walls. Test methodology, retrofit materials considered, material properties, mechanisms of effectiveness, and research challenges are discussed.

Under blast loading, nuclear containment structures are subjected to cyclic flexural, axial, and shear forces. Less attention has been paid to modeling the cyclic behavior of reinforced concrete elements in which shear deformations are significant, such as in nuclear containments. Research has demonstrated that the strength of concrete in the principal compression direction is softened by tension in the lateral direction. This interaction has been considered for monotonic loading for many years. To consider this interaction for cyclic loading, the material laws recently derived by Mansour et al. in 2001 for the cyclic softened membrane model (CSMM) can be used. Both fire and explosion effects resulting from blast loading can also be incorporated into the constitutive models of concrete and reinforcing bars. This paper presents a method to implement the CSMM model, so that it can be used to simulate the performance of nuclear containments for security by the design communities.