An overview of progressive collapse in structural systems

Abstract

It has become evident recently that abnormal loads need to be considered in the design of structures so that progressive collapse can be prevented. Building collapses such as the Ronan Point, Alfred P. Murrah, and World Trade Center have shown the catastrophic nature of progressive collapse and with an increasing trend towards more terrorist action in the future, it is clear structural design must include progressive collapse mitigation. The most critical abnormal loadings that have potential to cause progressive failure are blast and impact. These loads are impulsive and dynamic in nature with the potential to induce destructive forces, and to further complicate matters is the random nature of occurrence which makes it difficult to predict adequate levels of design. Much research has been conducted over the past several decades, but to this day very little standardized language has been published to help designers create progressive collapse resistant structures. What is known is that robust structures can be built economically by following a general design philosophy of redundancy, ductility, and overall structural integrity. Reinforced concrete structures are especially well suited for resisting progressive collapse by specifying steel reinforcement detailing such as continuous top and bottom reinforcement, close spacing of stirrups, strategic locations of splices, continuous reinforcement through joints, and designing slabs for two-way action. Steel structures have good ductility, but connection detailing is usually the weakest point and requires special design, such as the use of the SidePlate (tm) connection.

(cont.) Regardless of the type of material used, the design should strive for a uniform, regular layout of the structural system with limited span lengths and close spacing of beams and columns. Perimeter defense systems should be employed as this decreases the threat of an abnormal loading. Since there has been little consideration of extreme loadings, existing structures may be inadequate and require retrofit. Although more difficult, it is possible to achieve improved progressive collapse resistance through the use of externally applied retrofits, such as concrete encasement or the application of composite polymer materials.