Design of steel structures for blast-related progressive collapse resistance

Structural steel framing is an excellent system for providing building structures the ability to arrest collapse in the event of extreme damage to one or more vertical load carrying elements. The most commonly employed strategy to provide progressive collapse resistance is to employ moment-resisting framing at each floor level so as to redistribute loads away from failed elements to alternative load paths. Design criteria commonly employed for this purpose typically rely on the flexural action of the framing to redistribute loads and account for limited member ductility and overstrength using elastic analyses to approximate true inelastic behavior. More efficient design solutions can be obtained by relying on the development of catenary behavior in the framing elements. However, in order to reliably provide this behavior, steel framing connections must be capable of resisting large tensile demands simultaneously applied with large inelastic flexural deformations. Moment connections prequalified for use in seismic service are presumed capable of providing acceptable performance, however, research is needed to identify confirm that these connection technologies are capable of reliable service under these conditions. In addition, some refinement of current simplified analysis methods is needed. 42 INTRODUCTION Many government agencies and some private building owners today require that new buildings be designed and existing buildings evaluated and upgraded to provide ability to resist the effects of potential blasts and other incidents that could cause extreme local damage. While it may be possible to design buildings to resist such attacks without severe damage, the loading effects associated with these hazards are so intense that design measures necessary to provide such performance would result in both unacceptably high costs as well as impose unacceptable limitations on the architectural design of such buildings. Fortunately, the probability that any single building will actually be subjected to such hazards is quite low. As a result, a performance-based approach to design has evolved. The most common performance goals are to permit severe and even extreme damage should blasts or other similar incidents affect a structure, but avoid massive loss of life. These goals are similar, though not identical to the performance goals inherent in design to resist the effects of severe earthquakes, and indeed, some federal guidelines for designing blast resistant structures draw heavily on material contained in performance-based earthquake-resistant design guidelines. While there are many similarities between earthquake-resistant design and blast-resistant design, there are also important differences. Blast-resistant design typically focuses on several strategies including, provision of adequate standoff to prevent a large weapon from effectively being brought to bear on a structure, provision of access control, to limit the likelihood that weapons will be brought inside a structure; design of exterior cladding and glazing systems to avoid the generation of glazing projectiles in occupied spaces as a result of specified blast impulsive pressures, and configuration and design of structural systems such that loss of one or more vertical load carrying elements will result at most, in only limited, localized collapse of the structure. Although blast pressures can be several orders of magnitude larger than typical wind loading pressures for which buildings are designed, the duration of these impulsive loads is so short that they are typically not capable of generating sufficient lateral response in structures to trigger lateral instability and global collapse. Steel structures with complete lateral force-resisting systems capable of resisting typical wind and seismic loads specified by the building codes for design will generally be able to resist credible blast loads without creation of lateral instability and collapse. However, explosive charges detonated in close proximity to structural elements can cause extreme local damage including complete loss of load carrying capacity in individual columns, girders and slabs. Consequently, structural design of steel structures for blast resistance is typically focused on design of vulnerable elements, such as columns, with sufficient toughness to avoid loss of load carrying capacity when exposed to a small charge and provision of structural systems that are capable of limiting or arresting collapse induced by extreme local damage to such elements and avoiding initiation of progressive collapse. Steel building systems are ideally suited to this application. The toughness of structural steel as a material, and the relative ease of designing steel structures such that they have adequate redundancy, strength and ductility to redistribute loads and arrest collapse facilitate the design of collapse-resistant steel structures. However, effective design strategies that will provide collapse resistance at low cost and with minimal architectural impact are urgently needed as is research necessary to demonstrate the effectiveness of technologies employed to provide the desired collapse resistance. This paper explores these issues. DESIGN STRATEGIES Typical design strategies for collapse resistant buildings involve removal of one or more vertical load carrying elements and demonstrating that not more than specified portions of the building will be subject to collapse upon such occurrence. The element removal could occur as a result of any of several loading events including blast, vehicle impact, fire, or similar incidents. Regardless, the design strategy can be traced to lessons learned from observation of the blast-induced collapse of the Alfred P. Murrah Building in Oklahoma City. As illustrated in Figure 1 (Partin 1995) extreme damage to columns at the first story of the building, led to progressive collapse of much of the structure (Figure 2).