Effective flexural stiffness for linear seismic analysis of concrete walls

A trilinear bending moment - curvature model is used to develop a general method for determining the effective flexural stiffness of concrete walls for use in linear seismic analvsis. The method accounts for the initial section stiffness EcIg prior to cracking and the retention of initial stiffness after cracking due to axial compression from gravity loads closing cracks. Accounting for the initial stiffness is particularly important for typical high-rise concrete walls, which have significant compression and low percentages of vertical reinforcement. The effective flexural stiffness is determined from the slope of the elastic portion of an equivalent elastic-plastic load-deflection curve that has the same area under the curve as the actual nonlinear relationship. The general method requires information about the vertical reinforcement and strength of the wall. Simplified expressions are proposed to estimate the effective flexural stiffness based only on the axial compression near the base of the wall. Separate expressions for upper- and lower-bound effective stiffness establish the range of possible values, and this is less than the variation of recommendations in the literature on how much to reduce the flexural stiffness of concrete walls to account for cracking.