Progressive Collapse Resistance
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An Engineer's Guide to: Concrete Buildings and Progressive Collapse Resistance

Progressive collapse is defined as a situation where local failure of a primary structural component(s) leads to the collapse of adjoining members, which in turn leads to additional collapse. Hence, the extent of total damage is disproportionate to the original cause. Another way of describing progressive collapse is a chain reaction or propagation of failures following damage to a relatively small portion of a structure.

Regardless of the definition, blast loading or other unforeseen events can cause progressive collapse due to damage of some key element(s) which can either make the structure unstable or trigger the failure of the main portions of the gravity structural system. Blast generally results in a high-amplitude impulse loading which lasts for a very short period of time and produces high pressure loading. The loading in many situations is local in the sense that only those elements closest to the blast may be directly impact-ed. Elements far from the blast site may experience little or no direct impact due to sharp attenuation (dissipation) of blast energy with distance. The forces experienced by structural components depend on the size, geometry and proximity of the explosion. Because all of these parameters can vary, it is not easy to accurately predict the force level that a particular structure could experience as a result of an unexpected blast.

Large amounts of explosives at short distances from the structure can cause excessive pressure forces, which cannot be accommodated in the design of an ordinary structure. Thus it becomes imperative to put in place other measures such as perimeter control and standoff distances to reduce the possibility of a blast at close proximity to the structure.

The response of reinforced concrete under blast loading is different from its response to typical static and dynamic loads because of the very short duration and extreme pressure loading caused by blast. The stiffness and strength of reinforced concrete is likely to increase with the higher rate of loading experienced under blast conditions. This, in turn, increases the strength of reinforced concrete members and translates into higher resistance. On the other hand, the high rate of loading expected during blasts may also reduce the deformation capacity and the fracture energy of reinforced concrete significantly. This translates to a reduction of ductility of reinforced concrete in blast loading situations, a property generally mandated by most codes and standards to preserve the integrity of a structure.

To achieve targeted integrity during blast, the redundancy of the gravity load carrying structural system takes center stage in tackling the issue of progressive collapse. This is not explicitly addressed in mainstream building codes. However, ASCE 7-02 and ACI 318 imply a desired alternate load path in the event one or more beams and/or columns of a building fail as a result of a blast. The structure should be able to remain stable by redistributing the gravity loads to other members and subsequently to the foundation through an alternate load path, while keeping building damage somewhat proportional to the initial failure.

The inherent mass and stiffness characteristics of reinforced concrete offer distinct advantages over other building materials such as steel and timber under blast loading. Reinforced concrete structures are better able to resist the overall shock due to local disintegration caused by the blast. There is more information on blast resistance of reinforced concrete than for any other material. Reinforced concrete structures have been studied and researched in much detail by governmental, public and military agencies for decades. These aspects give reinforced concrete advantage over other materials for blast type of loading. Most of U.S. embassies, governmental buildings, and public facilities have been entrusted to reinforced concrete.

With the tragic events of September 11, 2001, preceded by the bombing of the Alfred P. Murrah Federal Building in Oklahoma City, it became evident that certain buildings will need to be designed to address the threat of explosions. Most structural engineers would not expect the World Trade Center Towers to survive the extraordinary events on September 11 that included fire on several floors combined with the loss of fire suppression water. It is, however, likely that certain owners and insurers of buildings will be interested in seeing more provisions in the building codes for design against the threat of terrorism. The available provisions currently in the codes, standards, and procedures used for design of tall buildings are under close scrutiny. It is imperative that new buildings which may be subject to terrorist attack be designed to provide anti-terrorism and force protection features that protect and ensure the safety of its occupants.

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