Reinforced Concrete (RC) shear wall, is an effective primary earthquake resisting system due to strong stiffness and large shear-force resisting capacity. For a complex asymmetric wall, severe damage on a portion of the wall may directly affect the stiffness in other directions. Such a secondary damage mechanism is hard to capture. Hence, this study was devoted to determining a stiffness reduction index that can monitor current damage state of the wall system as a whole, and apply the unified damage index to decrease stiffness and strength on other directions. This study proposes an analytical framework at microscopic length scale that is based on a unit cell which consists of nonlinear steel spring, compression only gap, and concrete compression spring. For validation and applications, three U-shaped wall specimens available in literature (designed according to EC8) were modeled and simulated under cyclic lateral loading. These walls have the same dimensions and reinforcement except for the different loading directions. The present study concludes that the proposed unit cell model appears to be successful for predicting the stiffness reductions resulting from localized damages in different loading directions. The proposed unit cell-based framework seems to be a good starting point to consider secondary stiffness reductions for other complex non-rectangle walls such as L-, H- and T-shaped walls. This method may facilitate the fast determination of remaining stiffness of complex RC walls by using quick post-disaster observations.