Development and Application of Linear and Non-Linear Static Approaches in UFC 4-023-03

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). Although both documents incorporate some of the same approaches, there are notable differences in the application of these procedures. 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). It is important that the design requirements incorporate appropriate dynamic and nonlinear factors such that the linear static and nonlinear static designs are more representative of the actual nonlinear and dynamic response of the structure.