Revision of the Tie Force and Alternate Path Approaches in the DOD Progressive Collapse Design Requirements

Abstract

The Department of Defense (DoD) Unified Facilities Criteria (UFC) 4-023-03 Design of Buildings to Resist Progressive Collapse was recently revised and a number of significant improvements were implemented, particularly in regards to the direct and indirect design approaches. Direct design explicitly considers progressive collapse during the design process and includes the Alternate Path method, in which the building bridges over a missing structural element, and, the Specific Local Resistance method, in which the building, or parts of the building, are designed for a specific load or threat. In indirect design, resistance to progressive collapse is incorporated implicitly through prescriptive requirements for strength and continuity, typically in the form of Tie Forces, which insure a minimum tensile strength in horizontal and vertical structural members. During the revision of UFC 4-023-03, the effectiveness of the indirect and direct design methods used in existing design requirements was evaluated and research was performed to improve these approaches. For indirect methods, tension membrane and catenary behaviors were used to develop improved Tie Force requirements. For the Alternate Path method, the linear and nonlinear analysis procedures were improved, through adaptation of the overall approach provided in ASCE 41-06 Seismic Rehabilitation of Existing Buildings . In addition, the load and dynamic increase factors were revised, to better account for inertial and nonlinear effects in linear static and nonlinear static models. The research, analyses, and improvements for the indirect and direct methods are reported in this paper.