Superposition principle invalid in IS 13920 design of slender RC walls with boundary elements

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.