In the past decades, there have been cases where buildings around the world have experienced partial or total collapse under extreme abnormal loading conditions. Although building collapse is a rare event, it may result in significant catastrophes and property loss when it occurs. In particular, the collapse of the World Trade Center in 2001 following the terrorist attacks has triggered increased interest in blast and progressive collapse resistant building design. The upgrading of steel frames building has thus become an important topic for researchers. In this study, progressive collapse response was investigated using the Alternate Path Method (APM) recommended in the United States' General Service Administration (GSA 2003) and Department of Defense (DoD 2005) guidelines. Nonlinear dynamic analysis for progressive collapse is done as a precise tool for evaluation of the progressive collapse potential of building structures. The studied models were 18 typical floors, with 6 bays in the longitudinal direction, 3 bays in the transverse direction, where six cases of removed column scenarios were conducted. The objectives of this study are to 1) investigate the effect of three retrofit strategies (by increasing strength and/or stiffness of the beams) on the response of steel frames for model of span 6m (reference model) subjected to GSA and DoD loads by studying three parameters (the chord rotation, tie forces and displacement ductility demand), 2) investigate the effect of variation of bay span on the three performance parameters by studying three models of different spans of 5.0, 7.5 and 9.0 m subjected to GSA load, and 3) propose equations for predicting the effect of retrofit strategy and bay span on the three performance indicators
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