EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS Earthquake Engng Struct. Dyn. 2009; 38:609–634 Published online 13 February 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/eqe.898 Modeling progressive collapse in reinforced concrete buildings using direct element removal Mohamed Talaat 1, ‡ and Khalid M. Mosalam 2, ∗, †, § 1 Simpson Gumpertz and Heger Inc., San Francisco, U.S.A. 2 University of California, 721 Davis Hall, Berkeley, CA 94720-1710, U.S.A. SUMMARY This paper presents a novel analytical formulation of an element removal algorithm based on dynamic equilibrium and the resulting transient change in system kinematics, by applying imposed accelerations instead of external forces at a node where an element was once connected. The algorithm is implemented into an open-source finite element code, numerically tested using a benchmark structural system with simplified element removal criteria, and able to capture the effect of uncertainty in member capacity. Realistic element removal criteria are introduced for mode-dependent gravity load collapse of seismically deficient and retrofitted reinforced concrete (RC) columns and unreinforced masonry (URM) infill walls. Two applications are conducted using structural systems of RC frames with URM infill walls. The first is a probabilistic study of a one-story model subjected to an ensemble of 14 ground motion recordings from similar neighboring sites during an earthquake event. The study produces empirical probability curves for partial and complete collapse conditioned on different hazard levels, and concludes that the intra-event variability is a major source of uncertainty affecting the outcome of progressive collapse simulations. The second application is a deterministic sensitivity study of progressive collapse response in a five-story structural model to uncertainty in live load, stiffness, damping, and seismic hazard level, subjected to one ground motion record. The analysis identifies the time at incipient collapse as an adequate sensitivity measure, and the uncertainty in ground motion intensity as the most important, followed by the stiffness of the URM infill wall. Copyright 2009 John Wiley & Sons, Ltd. Received 11 July 2008; Revised 1 November 2008; Accepted 5 January 2009 KEY WORDS: dynamic analysis; element removal; finite element; progressive collapse; reinforced concrete frame; unreinforced masonry infill ∗ Correspondence to: Khalid M. Mosalam, University of California, 721 Davis Hall, Berkeley, CA 94720-1710, U.S.A. † E-mail: mosalam@ce.berkeley.edu ‡ Senior Engineer. § Professor and Vice Chair. Contract/grant sponsor: Earthquake Engineering Research Centers Program; contract/grant number: EEC-9701568 Copyright 2009 John Wiley & Sons, Ltd.