C V R Murty

A simple hand calculation based method is presented to develop axial flexure design P-M interaction curves of rectangular reinforced concrete sections. The proposed method uses basic principles of mechanics satisfying compatibility of strains, equilibrium of forces and constitutive relations of constituent materials. Simple step-wise calculation is enough to develop the interaction curve; it does not require any iterations. Accuracy of the method is demonstrated by comparing the results of two RC sections with the interaction curves of the sections obtained using SP:16. Step-wise calculations are presented of the two RC sections to illustrate the use of this method for generating design P-M interaction curves.

The paper reviews seismic behaviour and performance of reinforced concrete (RC) deep vertical members, particularly bridge piers and structural walls. The provisions of the relevant Indian codes of practice, concerning shear strength and shear demand in these members, are reviewed in light of the provisions of international codes of practice. The deficiencies are identified in the seismic shear design philosophy, prescribed by Indian codes.

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.

Based on the review of seismic shear design provisions of Indian codes for reinforced concrete (RC) deep vertical members (i.e., bridge piers and structural walls) in a companion paper, basic provisions are identified for shear resistance of RC sections. Draft provisions are presented for the consideration of the relevant Indian design code committees on bridge and building. The best seismic design practices seem to suggest that these provisions will be beneficial for the seismic performance of deep RC vertical members.