ABSTRACT
In the past, some structures designed to withstand normal loading conditions were able to resist abnormal loads. This was due, in part, to the inherent strength and continuity of most traditional forms of engineered construction. Recent developments in the efficient use of building materials,
1 INTRODUCTION
Lessons learned from recent earthquakes evidenced the unsatisfactory behavior of many existing structures because of insufficient lateral capacity and limited ductility. Furthermore, many structures may have little reserve capacity to accommodate abnormal loading conditions that have very low frequency of occurrence, but extraordinary consequences resulting from sudden changes to the building’s geometry and load-path. On the other side, changes in design and construction practices over the past several decades have lessened inherent robustness in certain modern structural systems, making them vulnerable to such events. In these structures the local failure of a primary structural component may lead to the additional collapse of adjoining members. In such cases the sudden loss of a critical load-bearing element initiates a chain reaction of structural element failures, eventually resulting in partial or full collapse of the structure. In other words, the structure has its loading pattern or boundary conditions changed and the residual structure is forced to seek alternative load paths in order to redistribute the loads applied to it. As a result other elements may fail causing further load redistribution. The process will continue until the structure can find equilibrium either by shedding load as by-products of elements failing or by finding stable alternative load paths. This progressive collapse, that is the spread of an initial local failure from element to element, may be produced not only by wind and seismic loads, but also by abnormal loads (faulty construction practice, foundation
innovative framing systems, and refinements in analysis techniques have resulted in structures with a considerably smaller margin of safety. Such structures may have little reserve capacity to accommodate abnormal loading conditions. As a consequence, it is unsafe to assume that a structure designed for normal conditions can withstand abnormal or accidental load conditions. Furthermore, improved building practices and design procedures to control the likelihood of progressive collapse now are receiving heightened interest by engineers, architects, and standards organizations in the aftermath of the tragedy of September 11, 2001.
