Alternate Path Method in Progressive Collapse Analysis

As a result of the increasing number of terrorist attacks registered against American facilities in the United States or abroad, United States government agencies continue to improve the design of their buildings to make them safer and less vulnerable to terrorist attacks. One of the factors typically considered in designing safer buildings and structures, is their ability to prevent total collapse after the loss of load-carrying components (Progressive Collapse) resulting from a terrorist attack. The consequences of not having a building capable of reducing the potential for progressive collapse could be catastrophic, as it was the case of the Oklahoma City bombing in 1995 where 42% of the Alfred P. Murrah Federal Building was destroyed by progressive collapse and only 4% by the explosion or blast. This attack claimed 168 lives and left over 800 injured. Over the last 10 years, two United States government agencies have developed guidelines for the design of their structures to resist progressive collapse: (1) The General Services Administration, "Progressive Collapse Analysis and Design Guidelines," (GSA Guidelines) and (2) The Department of Defense Unified Facilities Criteria 4-023-03 "Design of Buildings to Resist Progressive Collapse" (UFC 4-023-03). Within both approaches, the main direct design procedure is the Alternate Path (AP) method, in which a structure is analyzed for collapse potential after the removal of a column or section of wall. Different analytical procedures may be used, including Linear Static (LS), Nonlinear Static (NLS), and Nonlinear Dynamic (NLD). Typically, NLD procedures give better and more accurate results, but are more complicated and expensive. As a result, designers often choose static procedures, which tend to be simpler, requiring less labor. As progressive collapse is a dynamic and nonlinear event, the load cases for the static procedures require the use of factors to account for inertial and nonlinear effects, similar to the approach used in ASCE Standard 41 "Seismic Rehabilitation of Existing Buildings" (ASCE 41). A number of inconsistencies have been indentified in the way the existing guidelines applied dynamic and non-linear load factors to their static approaches. As part of an existing effort to update the existing guidelines, this study used SAP2000 to perform several AP analyses on a variety of Reinforced Concrete and Steel Moment Frame buildings to investigate the magnitude and variation of the dynamic and non-linear load increase factors. The study concluded that the factors in the existing guidelines tend to yield overly conservative results, which often translate into expensive design and retrofits. This study indentified new load increase factors and proposes a new approach to utilize these factors when performing AP analyses for Progressive Collapse. 

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Alternate Path Method in Progressive Collapse Analysis 
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