Stiffness reduction model of vertically divided reinforced concrete structural walls under lateral loading

Abstract

When conducting extension work on existing structures, the existing structural walls resist increased lateral loads. Consequently, there is a concentration of load on the shear walls with large stiffness due to the increased lateral forces. As the existing walls have already been constructed, structural analysis should be performed based on the given stiffness without arbitrary adjustments. This necessitates retrofit of multiple walls and peripheral components due to the amplified internal forces, leading to an exponential increase in retrofit locations. However, reducing the load acting on the existing walls by decreasing their stiffness might potentially overestimate the safety of the walls. To prevent this, a method is proposed in this study to reduce the stiffness by vertically dividing the walls, allowing for a redistribution of loads by decreasing the stiffness of the existing walls. Therefore, this research proposes a vertical division method to reduce the stiffness of existing walls, enabling the redistribution of loads to address this concern. The purpose of experimental study is to investigate the effects of vertical division of reinforced concrete shear walls on wall stiffness and strength. 10 full-scale specimens were fabricated and reversed cyclic load tests were performed. The study found that both strength and stiffness decreased due to vertical division, with the rate of stiffness reduction being higher than the rate of strength reduction. The flexural rigidity decreased by 30-50% due to vertical division. It is expected that safety can be ensured by reducing the load acting on the wall due to the redistribution of the load caused by the vertical division. Structural analysis was performed for an actual building, and it was confirmed that the applied moment decreased as the wall flexural rigidity decreased. To analyze the stiffness reduction mechanism in structural walls due to vertical division, finite element analysis was conducted using DIANA FEA. The FEA accurately predicted the strength; however, assessing member stiffness between the foundation and walls was challenging due to the complex bond-slip behavior of reinforcement bars, leading to an overestimation of initial stiffness. By isolating the flexural and shear rigidity of the walls through finite element analysis and comparing it with experimental results, precise predictions were achieved. Consequently, stress distributions in horizontal and vertical rebars, as well as compressive stress distributions within the walls, were examined to derive the stiffness reduction mechanism. Based on deformation distributions of vertical rebars, assumptions for yield curvature and yield moment were proposed for sectional analysis to compare and validate against the analytical results, resulting in accurate predictions of yield moments. Based on experimental and analytical outcomes, a stiffness reduction model for vertically partitioned shear walls was proposed. This model demonstrated relatively accurate predictions compared to experimental data. Leveraging these findings, it is anticipated that by reducing the stiffness of existing walls, redistributing lateral loads, and without additional reinforcement, the safety of the original walls can be ensured.