RC shear walls are one of the lateral load resisting systems in concrete structures that are widely used due to their proper seismic performance. In many cases, shear walls contain openings due to architectural purposes, including doors and windows or crossing with mechanical and electrical installations. Openings in walls significantly influence their behavior such as changing their force transfer mechanism, reducing their strength, stiffness and ductility level. Design by strut-and-tie models (STM) is an emerging method that simultaneously considers shear forces, bending and torsional moments, providing appropriate and optimum design for structures. These models, replace the structure with a simple truss, although several models can be considered for more complex structures. According to the effects of strut-and-tie model on the behavior of a structure, it is very impressive to find the best truss in accordance with the principal internal structural stress. Boundary elements of shear walls are parts of the walls edges, or edges of the openings that significantly strengthen longitudinal and transverse reinforcements. Boundary elements significantly affect on the strength and ductility of shear walls. Thus, considering the boundary elements is an important step to improve the behavior of shear walls. The objective of the present research is to study the strut-and-tie models in 5 shear walls with openings and investigate effects of boundary element of shear wall in STM. Two strut-and-tie models with their associated boundary elements are considered to investigate effects of boundary elements on the behavior of shear wall with openings; a strut-and-tie that the designer considers and the other model which can be achieved from topology optimization. In order to realize the performance of strut-and-tie models, shear walls are designed according to the conventional method in ACI 318-08 and are compared with the strut-and-tie method. For better compression of design methods, the same amount of reinforcement is used. Total volume of transverse and longitudinal reinforcement used for the wall designed by STM method is identical with total volume of transverse and longitudinal reinforcement used for the wall designed by conventional method of ACI 318-08. To study the behavior of RC shear wall, FEM nonlinear program is used. Comparative result of the FEM and experimental behavior of RC shear wall is used and it can be concluded that the FEM results using damage plasticity and tension stiffening effects are in close agreement with the experimental ones. It can be seen from the results that shear wall designed using strut-and-tie models shows better behavior than the wall designed with ACI 318-08. The results also show that boundary elements in shear wall have improved the final capacity and ductility of shear walls. The wall ductility is affected by two important components; transverse reinforcement in boundary elements and longitudinal reinforcements at wall edges. The former improves ductility due to confinement although the latter might decrease it. Implementation of boundary elements in the proposed STM method leads to higher strength and ductility in comparison with ACI 318-08 approach.

Key words

Strut-and-tie (STM), RC Shear wall, Boundary elements of shear wall, Topology optimization.