C V R Murty

IS 456:2000 and IS 13920:2002 distinguish columns from walls with both geometrical and empirical definitions that result in a sudden drop in the requirements of minimum longitudinal reinforcing steel at 0.4 cross-sectional aspect ratio or 2.5 length-to- width ratio. Designer wishing to offer economical designs would be tempted to use this ambiguity leading to unsafe designs. Therefore, to make structures safe and to check the lacunae in the codes, this paper argues in favour of having a gradual transition in the reinforcement requirement as the crosssectional aspect ratio changes from small to large. In light of international practice, the paper critically reviews IS codes' columns and walls reinforcement-detailing provisions and suggests possible improvements.

Current code IS 13920:2002 for ductile detailing of concrete structures assumes that moment capacity of a RC structural wall with boundary elements is the sum of moment capacity of the web portion of the wall and that due to the couple using axial capacity of the boundary elements and lever arm between them. This assumption leads to gross over-estimation of design moment capacity of the wall. In this paper, provisions are reviewed and improvements suggested eliminating this deficiency in the code provisions. A nonlinear method is suggested based on principles of mechanics for estimating the combined Pu-Mu strength envelope, considering the combined contribution of the web and boundary elements of the wall. Using this, a numerical study was performed of moment capacity of RC structural walls (both with and without boundary elements) to demonstrate that superposition principle is not acceptable in the design of RC structural walls with boundary elements.

IS 456:2000 and IS 13920:2008 do not prohibit failure of RC structural walls by concrete in compression reaching the limiting strains. But, IS 13920:2008 (Annex A), which follows guidelines set by IS 456:2000, does not discuss the failure of RC walls under pure or almost pure compression of concrete in the P-M interaction curve. But, designers need the entire P-M diagram to be able to better understand the designs being proposed by them. In the determination of the ultimate moment of resistance (Ma), even when steel reaches the limiting strain first, secondary compression failure (εc=0.0035) is used as a basis (as has been customarily done in shallow RC beams and slabs) for arriving at the expressions for Mu. This is inadmissible in deep RC members, like structural walls. Tins paper presents expressions for Mu of RC walls, (1) based on basic principles of mechanics, namely equilibrium of the section, compatibility of strains and constitutive laws, (2) for all possible positions of neutral axis, and (3) without assuming secondary compression failure of RC members. Also, effect of concrete confinement by transverse reinforcement on P-M curve is presented; comparisons are presented of current method (unconfined) and proposed methods (unconfined and confined) of arriving at Mu and MΩ of RC structural walls.