The 14 th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China EXPERIMENTAL INVESTIGATION OF PROGRESSIVE COLLAPSE OF STEEL FRAMES UNDER MULTI-HAZARD EXTREME LOADING Antonis Tsitos 1 , Gilberto Mosqueda 2 , André Filiatrault 3 and Andrei M. Reinhorn 4 1 Ph.D. Candidate, Dept. of Civil, Structural & Environmental Engineering, University at Buffalo-The State University of New York, Buffalo, NY, USA - Email: atsitos@eng.buffalo.edu 2 Assistant Professor, 3,4 Professor, Dept. of Civil, Structural & Environmental Engineering, University at Buffalo-The State University of New York, Buffalo, NY, USA ABSTRACT : Experiments were performed to evaluate the progressive collapse resistance of steel buildings considering multi-hazard extreme loading. Two 1/3 scale three-story, two-bay steel frames, a special moment resisting frame and a post-tensioned energy dissipating frame designed and previously tested for seismic performance on a shaking table, were adapted for quasi static collapse testing. The experiments simulated the structural response after the sudden failure of a column. The objective of the tests was to evaluate the effectiveness of earthquake resistant design details in enhancing the resistance to progressive collapse. In these tests, an effort was made to document the load resisting mechanism, the sequence of damage in the frames and correlate observed damage with changes in the resisting strength. The experimental results demonstrate significant vertical displacement capacity for both frame systems and the capacity to redistribute loads after the failure of a single column. However, the steel moment frame appears to have significantly more ductile response by maintaining its yield strength up to the point of first connection fracture. The vertical load carrying capacity of the special moment frame appears to be dependent on the rotation capacity of plastic hinges before buckling and fractures occur. The vertical load carrying capacity of the post-tensioned energy dissipating frame depends heavily on the performance and ultimate strength of the tendons used for connecting the beams to the columns. The PTED frame lost a significant amount of strength after failure of one of the tendon strands. KEYWORDS: Experimental study, progressive collapse, steel frames, post-tensioned frames, multi-hazard loading. 1. INTRODUCTION Since the collapse of the Ronan Point Towers in London 1968 and more recently spurred by the September 11 2001 events at the World Trade Center in New York City, the structural engineering community has focused significant efforts towards better understanding of the phenomena of progressive collapse in building structures. The ultimate goal is to establish rational and reliable methods for the assessment and the enhancement of structural resistance to extreme accidental events. Although design methodologies (such as the alternative path method and the tie force method) and analysis procedures to enhance resistance to progressive collapse are proposed in guideline documents issued by the U.S. General Services Administration (2003) and the Department of Defense (2005), there is a scarcity of experimental data to support the numerical modeling of building structures under extreme loads, particularly to the point of failure. Several numerical studies have been published investigating the progressive collapse of buildings (Kaewkulchai and Williamson, 2004; Bazant and Verdure, 2007) and the adequacy of commercially available structural analysis software to perform collapse analyses (Marjanishvili and Agnew, 2006). The dynamics of impact of collapsing failed elements has also been investigated by Kaewkulchai and Williamson (2006). In a different approach to capture the dynamics of impact and other phenomena involved in a progressive collapse, Sivaselvan and Reinhorn (2006) proposed a method using a Mixed Lagrangian Formulation (MLF). In most cases, it is attempted to predict the global response of the building using simplifying assumptions without explicitly considering detailed but important phenomena, such as axial-flexure-shear force interaction in the beams, joint failure, local buckling and panel zone deformation (for steel structures). To address this need, Khandelwal et al. (2008) and Bao et al. (2008) proposed macro-models of beam-column subassemblies that capture the key local and global response