C V R Murty

A quantitative approach is required to ensure desired global ductile behaviour with at least a guaranteed level of deformation capacity of buildings. This paper quantifies the required column-to-beam strength ratio, and insufficiency of the current code prescribed values of strength ratio to achieve a ductile mechanism. Column-to-beam strength ratio is varied by varying column size and reinforcement in the columns; beam size and reinforcement are kept constant. Nonlinear static pushover analyses of designed RC buildings with different column-to-beam strength ratios show different collapse mechanisms, lateral strengths and energy dissipation characteristics. Column-to-beam strength ratio alone does not result in the desired global ductile behaviour and do not guarantee the desired deformability. Further, the required column-to-beam strength ratio is same, depending on the design, for different hazard levels (reflected by the design seismic coefficient). An appropriate value of minimum column-to-beam strength ratio to achieve a ductile mechanism is presented along with pointers to ensure large deformability of buildings.

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.