In this paper, shear forces attracted by shear walls according to the Simplified Method for Seismic Analysis (SMSA) allowed by Mexican Codes (NTCM-2004 2004) are compared with respect to those obtained with rigorous 3D analysis. The impact of shear deformations in the 3D distribution of the forces absorbed by these walls is assessed for different wall aspect ratios (H/L). Based on extensive parametric studies, effective shear area factors (F AE) originally proposed in the SMSA are modified to improve the estimates of shear forces using this method. New F AE are proposed for three different performance levels for the structure: (a) elastic response, (b) completely nonlinear (cracked) response of all walls along the building height and, (c) partially nonlinear (cracked) response along the height.