Equivalent viscous damping for steel eccentrically braced frame structures with buckling restraint braces

Steel eccentrically braced frame systems (EBF) with buckling restraint braces are extensively used as a lateral load resisting system due to their high-energy dissipation characterized by the inelastic shear deformation of the short links. In performance-based seismic design of such structural systems using the direct displacement-based design procedure (DDBD), equivalent viscous damping (EVD) coefficient is used to represent the energy dissipation capacity which has not been developed particularly for this structural system yet even though expressions to calculate EVD coefficient have been already developed for other major lateral load systems supporting DDBD procedure. Therefore, in this study, a new EVD expression was developed incorporating 25 predetermined single degree of freedom (SDOF) eccentrically braced frame (EBF) systems which were selected by varying the length and the section geometry of the link considering them as the dominant parameters that affect the energy dissipation of the system. Firstly, EVD coefficients were calculated using Jacobsen's area-based approach for quasi-static loading and they were corrected by considering the effects of earthquake characteristics using 13 spectrum compatible earthquake records. Finally, a new equation to calculate the equivalent viscous damping coefficient for eccentrically braced frame systems was formulated as a function of the ductility of equivalent SDOF system to the multi-degree of freedom (MDOF) systems.