A simplified design procedure for seismic retrofitting of RC frames with setbacks using eccentric steel braces having vertical shear link elements

The main aim of this study is to derive a simple design procedure to determine the required sizes of vertical shear links and eccentric steel braces for the seismic retrofitting of an existing reinforced concrete frame structure with in-elevation irregularities due to setbacks designed for gravity load only. The proposed procedure distributes the vertical shear links and eccentric steel braces over the height of the structure to dissipate seismic energy away from the main structural members without any numerical iterative strategy. In fact, the capacity curve through the extension of the improved upper-bound pushover analysis method, adapted to the frame structures with setbacks, and the capacity spectrum method are the only required input parameters. A six-story RC frame structure with a towered type of setback at the second story is used in a numerical investigation to assess the effectiveness of the proposed design procedure. The structure has to be retrofitted to withstand the seismic demand imposed by the current Algerian seismic design code in a high-risk area. Nonlinear time-history analyses of original non-retrofitted and retrofitted frames are carried out considering a set of seven artificially generated records for obtaining the mean value of structural responses, which corresponds to the specified seismic demand. The results show that the proposed simplified design procedure is effective in significantly reducing both global and local seismic demand parameters and in avoiding structural instability due to the formation of a column-hinging mechanism, which has occurred at the second story where the setback is located.