Minimum flexural strength of columns for strong column–weak beam design of RC moment frames

Current seismic design codes allow a ductile inelastic response of moment frames but require the minimum flexural capacity of columns (or the minimum flexural strength ratio of column to beam: hereafter abbreviated to CBFSR) to prevent the yielding in the columns. The current requirement for CBFSR is founded on the equilibrium of elastic moments. However, the actual response of frames to earthquake motions can be influenced by the collapse mechanism in beams and columns. That is, the actual seismic demands in columns may significantly differ from the elastic moments, and consequently the current requirement for CBFSR may not work satisfactorily. Thus in this chapter, to investigate the CBFSR required to prevent column hinging, numerical analyses were conducted considering various design conditions. The results showed that CBFSR is affected by many conditions (such as the plastic rotation of elements, magnitude and vertical profile of earthquake load, gravity load, stiffness ratio, joint location, and frame height) and also depends on the story level. In particular, CBFSR needs to be defined individually for the columns above and below each joint, and the combined vertical distribution of CBFSRs over the entire stories was in the shape of a wine glass. Based on the numerical findings, a design method was suggested, which consists of two design equations: one for columns framing into a joint (considering the most unfavorable and most favorable combinations of design conditions), and the other for the bottom end of first-story columns (no beams). For verification, a regular frame and an irregular frame were designed according to the proposed method, and then nonlinear static and dynamic analyses were conducted. The evaluation showed that a safer and more cost-effective design can be achieved by using the proposed method, compared to the case of using 1.2 or 2.0 for CBFSR.