Application of elastic cantilever column to limit story drift concentration of braced two story frames

Concentrically braced frames are generally required to be stronger than other types of steel frame with the same period according to current codes. This is expressed by the smaller response modification factor R factor in USA codes, and the larger D_s in the Japanese codes (which is inversely proportional to R). Some possible perceptions which result in braced frames being designed to be stronger than other frame types are that: i) a story mechanism could occur in these frames more readily, ii) the hysteretic energy dissipated is less (thereby increasing the displacement demand), and iii) ductility capacities of buckling braces under reversed cyclic displacements may be small. However, if buckling restrained braces, with large deformation capacities and hysteretic energy dissipation, are used and if large story drift concentrations can be prevented over the story height, then these problems are mitigated. This paper describes the effect of continuous cantilever columns on the seismic response of two story frames with concentric buckling restrained braces. The main seismic resistant element is the braced frame. After the braces yield, the column prevents large soft drift concentrations. Equations for estimating the column fixed base moment demand and drift concentration factor are developed. Recommendations for column stiffness and strength, to keep the frame elastic for a specified drift concentration, are then proposed. Finally, dynamic analyses are performed and the column moment demand and equations for the drift concentration factor are developed.