Keywords: progressive collapse, framed structures, stiffness, ductility.

Progressive collapse of building structures is relatively rare but potentially catastrophic. Events that can trigger progressive collapse are diverse and may be natural or manmade. Therefore, a multifaceted approach that includes various disciplines involved in the design process may be necessary. The extent to which a structure must be protected against progressive collapse requires public input and is not left entirely for design professionals. The extent of the protection is sometimes determined based on designation of the desired levels of protections such in the United States Department of Defense (DoD) guideline entitled "UFC 4-023-03" [1]. The general field of security engineering is the framework in which decisions at the design stage are made as to the extent of the building protection.

This paper explores certain structural engineering solutions that may be considered during the design of new structures to mitigate progressive collapse in steel and concrete framed structures. These solutions are hazard-independent as implied in most building codes and design standards for civil engineering structures.

Structural stiffness is the primary property necessary to resist the initiation of damage while ductility is crucial for post-damage mitigation. Stiffening is effective for small magnitudes of forces or deformations that can trigger progressive collapse. However, once collapse is initiated, it is structural ductility that can save life.

The current practice for structural stiffening in progressive collapse designs employs the alternate path (AP) method. Exterior or corner panels of the framed structure may require different stiffening measures compared to interior members. For interior and exterior panels, supporting beams may be designed and stiffened to extend the increased span resulting from the notional removal of a vertical member. Beams supporting corner panels are more difficult to stiffen when the scenario of corner removal is considered. This is because long cantilevers are likely to result from the notional removal of corner columns. Such columns may be stiffened with steel jackets designed for a judicially selected force. In addition or alternatively, steel bracing present in corner bays may be incorporated in the load path.

Structural ductility is important to achieve safe post-damage behavior of the structural system. Enhanced structural ductility may be sufficient for protection of buildings where low level of protection is desired based on risk or cost considerations. However, many design standards such as ACI318 have structural ductility and integrity provisions embedded in the prescriptive code, such as continuity of reinforcement bars. For reinforced concrete structures, special detailing of beams and column to enhance rotation capacity has been considered equally appropriate for progressive collapse mitigation. The design of joints, especially for steel structures, is especially important for improved rotation capacity.

1

"Unified Facilities Criteria (UFC): Design of Buildings to Resist Progressive Collapse - UFC 4-023-03", United States Department of Defense, Washington, D.C., USA, 2005.

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