Finite element modeling and capacity analysis of post-tensioned steel frames against progressive collapse

Post-tensioned (PT) steel frames, in which beams are connected to columns through high-strength PT strands, have been successfully developed in the past decade as a novel earthquake-resistant structural system. Compared to conventional steel moment frames, a PT steel frame demonstrates superior seismic performance, notably the minimum damages in main structural components and the self-centering capability under a design basis earthquake. Despite the abundant research on the seismic behavior of PT steel frames, there is an apparent lack of study on their load-redistribution behavior upon the notional removal of critical load-bearing columns, a commonly used threat-independent local structural damage scenario that potentially triggers the progressive collapse of the column-removed frame. This paper presents a first-of-its-kind numerical investigation on the unique structural behavior of PT steel connections and frames in redistributing the unbalanced gravity loads due to column removal. High-fidelity finite element structural models are constructed and validated using the available experimental data in the literature. The capacity of PT steel frames subjected to a gradually increasing vertical displacement along the removed column line is systematically studied. It is found that, besides the resistance of energy-dissipating elements, beam arching action and strand catenary action are the major sources of structural capacity of a PT steel frame against progressive collapse. The corresponding failure modes are identified and the design implications are suggested.