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One approach to evaluate progressive collapse of structures is to study the effects of instantaneous removal of a load-bearing element such as a column. In this paper, using experimental and analytical results, potential progressive collapse of an actual 10-story reinforced concrete (RC) structure following the explosion of an exterior column is evaluated. Development of Vierendeel action is identified as the dominant mechanism in redistribution of loads in this structure. The concrete modulus of rupture is identified as an important parameter in limiting the maximum recorded vertical deformation of the system to only 0.25 in. (6.4 mm). The changes in the directions of bending moments in the vicinity of the removed column and their effects such as potential reinforcing bar pullout (bond failure) are studied. Potential failure modes and their consequences are studied. Some shortcomings of integrity requirements in current codes are pointed out and effects of beam reinforcement detail on the development of catenary action are discussed.
Keywords: brittle failure; dynamic structural analysis; integrity requirements; load redistribution; progressive collapse.
INTRODUCTION
Progressive collapse has been an important issue in structural failure since a partial collapse of the Ronan Point apartment building in 1968. The attack on the Murrah Federal building in 1995 and terrorist attacks on September 11, 2001, started a second wave of attention on structural failure and better understanding of progressive collapse. Progressive collapse is defined as the spread of an initial local failure from element to element eventually resulting in collapse of an entire structure or a disproportionately large part of it.1 The initial cause of the local failure can be man-made (explosions) or natural (earthquakes). From an analytical point of view, progressive collapse occurs when a structure has its load pattern or boundary conditions changed such that some structural elements are loaded beyond their capacity and fail.2 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.
Following the approaches proposed by Ellingwood and Leyendecker,3 ASCE/SEI 71 defines two general methods for structural design of buildings to mitigate damage due to progressive collapse: indirect and direct design methods. Breen4 has shown that improved structural integrity is obtained by provision of integral ties...