Static equivalency in progressive collapse alternate path analysis: Reducing conservatism while retaining structural integrity

The existing General Services Administration (GSA) and Department of Defense Unified Facilities Criteria (UFC) guidelines make use of the alternate path approach for evaluation of a structural system to determine susceptibility to progressive collapse. The alternate path approach presumes that one critical or key member, typically a column, is damaged and rendered incapable of supporting load. The remaining structure must be able to span across this lost member. The existing procedures incorporate material nonlinearity through allowable plastic deformations or through the use of a modified static capacity to incorporate plasticity. The procedures also permit an analyst to evaluate the response of a structure either statically or dynamically. Dynamic inertial effects can be considered directly through the equations of motion inherent in a dynamic analysis or considered indirectly through the modification of dead and live loads in a static analysis. Both the GSA and UFC procedures recommend a static "multiplier" of 2.0 to account for these inertial effects. The analysis presented in this paper illustrates that this multiplier may be conservative, resulting in structural designs less efficient than may be otherwise achievable. A dynamic multiplier of 1.5 better captures the dynamic effects when a static analysis is performed, and will result in more economical designs.