Seismic Assessment of Existing Steel Chevron Braced Frames

The seismic response of existing, nonductile 3- and 10-story steel chevron braced frames is examined to identify potential deficiencies. The structures are located on the west coast of Canada. They were designed in accordance with the 1980 Canadian building code, without any specific seismic ductility considerations for braces, brace connections, or floor beams. The frame beams are light because they carry low tributary gravity loading. The evaluation was performed using the procedures of the Canadian building code and American standard. Response spectrum analysis and nonlinear time-history analysis (NLTHA) methods were conducted. Different numerical models were used for NLTHA to verify the effect of the nonlinear response of different components on seismic response. The evaluation using the Canadian building code required strengthening of all braces in both structures, whereas the braces were found to be acceptable by the American standard’s linear dynamic procedure (LDP). However, the American standard’s nonlinear dynamic procedure (NDP) revealed high inelastic demand in the upper and lower levels of the 10-story frame, in excess of the brace deformation capacities. From both LDP and NDP, beams are expected to fail by strong axis buckling prior to brace buckling, a behavior confirmed by detailed finite-element analysis. Linear dynamic procedure showed that columns are also deficient. Columns could not be assessed using NDP because of beam buckling occurring first in the analyses. For the 10-story frame with retrofitted beams, NDP showed the possibility of column buckling.