C V R Murty

Capacity design precludes brittle actions thereby maximising energy dissipation capacity of moment resisting frame buildings through flexural yielding in beams before possible yielding in columns during strong earthquake shaking. The flexural strength of columns is required to be more than that of beams framing into it. Seismic design codes stipulate guidelines for design and detailing of members to achieve desired ductile behaviour of buildings. This paper examines seismic behaviour of RC moment frame buildings in seismic Zone V and IV designed as per the column-to-beam strength ratio (CBSR) requirements of the draft IS 13920, and its adequacy along with the detailing requirements. The CBSR of 1.4 specified in the draft code is not sufficient for buildings in seismic Zone V.

A simple approach is presented to derive limiting strain-based bilinear overstrength moment-curvature response curves of reinforced concrete sections. These curves are obtained by simple hand calculations proposed in the paper. Implications of use of these idealized curves are examined on estimating seismic behaviour of RC frame buildings by nonlinear static pushover analyses. The use of idealized moment-curvature curves and its properties, as proposed, offers a consistent and non- empirical approach compatible with actual section properties.