Progressive Collapse of Reinforced Concrete Structures: A Multihazard Perspective

Through accident or act of terrorism, structures may be subject to conditions that could lead to progressive collapse. Redistribution of loads following an imposed local damage to a structure depends on strength, continuity, redundancy, and deformation and energy dissipation capacities of the structure. For reinforced concrete frame structures, these characteristics depend on the seismic and wind design loads to a great extent. In this paper, using the response of multi-degree-of-freedom and equivalent single-degree-of-freedom systems, it is demonstrated that the vulnerability of frame structures against progressive collapse caused by man-made hazards depends heavily on their resistance to natural hazards. It is also shown that following the loss of a column and in spite of satisfying the current structural integrity requirements, premature beam bottom bar fracture can occur. Such bar fracture can be avoided if the minimum beam bottom continuous bars are set equal to the minimum flexural reinforcement.

Keywords: bar fracture; brittle failure; integrity requirements; load redistribution; progressive collapse.

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INTRODUCTION

Progressive collapse is defined as the spread of an initial local failure from element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it.1 Recent terror attacks demonstrated that most casualties are due to building collapse rather than the initial explosion or impact. That is, progressive collapse increases the likelihood of mass casualties. Precautions can be taken in the design of structures to confine the effects of local failure and resist progressive collapse. Breen2 and Ellingwood and Leyendecker3 have made a distinction between direct and indirect design. Indirect design incorporates implicit consideration of resistance to progressive collapse through the provisions of minimum levels of strength, continuity, and ductility. Direct design incorporates explicit consideration of resistance to progressive collapse through two methods. One is the alternative path method in which local failure is allowed to occur but seeks to provide alternative load paths so that the damage is absorbed and major collapse is averted. The other method is the specific local resistance method that seeks to provide strength to resist failure. Whereas direct design is used in the design provisions specifically developed for progressive collapse analysis of structures,4,5 general building codes and standards1,6 use indirect design by increasing the overall integrity of structures.

ACI 318-056 requirements for structural integrity are to improve the redundancy and ductility in structures, which are primarily based on providing some continuous reinforcement in beams and floor systems to bridge a damaged support. Following the loss of a column, the gravity load carried by such an element needs to be redistributed to the neighboring elements. The redistribution depends on the strength, deformation capacities, and energy dissipation capacities of the affected elements. Whereas resistance to progressive collapse is primarily an issue of gravity load-carrying capacity, the design of elements also depends on the demands from other loads such as seismic and wind actions. In other words, if the elements resisting progressive collapse had larger capacities due to more severe wind and seismic effects used in design, they would have a higher likelihood of confining the initial local damage and preventing progressive collapse.

The effects of seismic detailing on progressive resistance of structures are discussed by Breen.2 More recent publications7-9 have also discussed relationships between seismic design/rehabilitation and progressive collapse resistance. Corley8 and Hayes et al.9 have examined the effects of alternative seismic design and strengthening, respectively, of the Murrah Federal Building on its collapse. They have pointed out positive impacts of seismic resistance on progressive collapse resistance. The main objective of this paper is to evaluate the effects of levels of design lateral loads on progressive collapse resistance of frame structures following initial local failure.

The initial local failure can be due to different sources. For instance, in the case of an explosion, the air-blast shock wave is the primary damage mechanism. Damage due to the airblast may be divided into direct air-blast effects and progressive collapse. Direct air-blast effects are damage caused by the high-intensity pressure that induces localized failure to nearby buildings. Such localized failure depends on the size of the explosion, its distance to the building, and the building characteristics. Structures may confine the damage to the initially affected zone, otherwise the collapse may propagate. This paper evaluates such potential collapse propagation and the direct air-blast effects are not studied.

RESEARCH SIGNIFICANCE

In this paper, the level of design lateral loads is identified as an important factor in limiting the vertical deformation and resisting progressive collapse of reinforced concrete frame structures following the loss of a column. Relating potential progressive collapse to the design loads can be used to identify the existing structures that are more susceptible to progressive collapse based on their levels of wind and seismic design loads (site locations). Furthermore, a change to the ACI integrity requirements is discussed, which can enhance the progressive collapse resistance of structures designed for low levels of lateral loads.