Improving Progressive Collapse Performance of Steel Moment-Resisting Frames Through X-Bracing Slack Cables

The intended purpose of this study is the steel moment-resisting frames strengthening through cable elements to improve the progressive collapse and seismic performance of the structure simultaneously. To achieve this goal, X-bracing slack cables were applied to all surrounding bays of the top story of the model with variable parameters, including different cable sizes and distinct amounts of slackness, to reach the most appropriate cable characteristics for one column removal or even two columns loss scenario. Increasing the number of paths to redistribute the loads developed due to initial local damage leads to improvement of the progressive collapse response in the existing or new structures. The progressive collapse and seismic performance of the non-strengthened and strengthened model structures were assessed using the nonlinear dynamic alternate path method recommended in the Unified Facilities Criteria guidelines and the pushover analysis of ASCE 41, respectively. Finally, two diagrams were extracted to find a relation between the size and slackness of the cables with the performance level of the structure. Obtained diagrams show that the proposed strengthening scheme can improve the progressive collapse resistance of the steel moment-resisting frames to the desired performance level corresponding to the selected slackness and minimum breaking load of cable. Pushover analysis results showed that the introduced strengthening scheme led to a more appropriate or at least unchanged seismic response. It should be considered in the design phase of columns and cables that adding cables leads to compression enhancement of columns after activation of cable elements.