Seismic design of braced frame columns with and without replaceable brace modules

In concentrically braced frames (CBFs), the braces are intended to dissipate seismic energy while the beams and columns remain elastic according to capacity design. However, past studies have shown that column yielding is possible with current seismic design. In this study, nonlinear response history analyses are performed to examine the effects of column design on braced frame performance. Two connections are considered for joining the braces to the other elements: a typical connection with a gusset plate, and a novel connection in which bolts are used instead of field welding in an effort to confine damage to replaceable brace modules (RBMs). Three special concentrically braced frames of different heights are modelled, and their seismic performance is discussed in terms of column demands at two intensity levels, as well as collapse capacity. The results show that, when columns are designed considering only the axial force demands from a capacity design procedure, column yielding is frequent and the columns are not stiff enough to avoid a soft-storey mechanism, leading to an unacceptable collapse capacity. The eccentricity in the beam-column connections with RBMs increases the column moment demands only when the axial forces are not maximized, leading to no increase in combined column demands. Modifications are proposed for considering moments in design to ensure that columns can resist the seismically induced axial and flexural demands, so as to pass the collapse capacity criterion of FEMA P695. The improved column design approaches are also shown to reduce the residual drifts, facilitating post-earthquake repairs.