A parametric study on the shear behavior of reinforced concrete shear walls in tall buildings

Since the beginning of 20th century, due to the an increase in demand and advancements in the construction technology, number of tall building increased significantly. As the height of tall buildings increased, various structural systems have been proposed and used to fulfill safety and comfort requirements. Most of these structural systems used reinforced concrete shear walls for improving lateral stiffness and resistance against wind and earthquake loads. In this study, the focus is seismic behavior of reinforced concrete shear walls of tall buildings. There are two main considerations for seismic design of reinforced concrete shear walls in tall buildings; design for flexure and shear. Flexural design is considered to be well-understood in engineering community. On the other hand, design for shear can be quite arduous due to complicated nonlinear and dynamic shear behavior of reinforced concrete shear walls. To achieve better shear design, shear behavior of reinforced concrete shear walls needs to be better understood. In this study, the effects of two main shear design parameters on the behaviour of reinforced concrete shear walls are investigated. These parameters are wall thickness and shear reinforcement. For this purpose, a tall building designed and constructed in İstanbul, Turkey is considered as a testbed. The subject building is 163.4 m tall and has a plan area of 28 m by 34 m. Structural system of building comprises of two symmetrical C-shaped reinforced concrete core shear walls, steel coupling beams connecting them, composite perimeter columns, reinforced concrete band beams at the perimeters of the floors and 26 cm thick flat slab. The thickness of the shear walls are 70 cm between base and Level 15, 60 cm between Level 16 and Level 30 and 50 cm between Level 31 and Level 43. Original building is simplified to avoid unnecessary complexities. In the first part of the study, the effect of the shear wall thickness on the general design and shear behavior is investigated through linear structural analysis. Two control models are established by reducing the shear wall thickness. The first model has shear wall thickness of 55 cm, 45 cm and 35 cm, while the second model has thicknesses of 40 cm, 30 cm and 30 cm. Various linear response spectrum analysis are conducted and axial force ratio to axial capacity of wall, top displacements, base shear, story drifts, required shear reinforcement and story forces are compared between original model and control models. Results of the first part of the study reveals that thicker shear walls do not necessarily translate intro better design. Thicker shear walls improve the rigidity and deflection of structure. Increased rigidity, however, increases the seismic loads on the structure significantly. Therefore, there is circular interdependency between the thickness of the shear walls and seismic loads. In overall, it can be said that there is and optimum level of shear wall thickness that results acceptable level of stiffness and seismic loads. As long as it ensures ductility and displacement demands, thinner shear walls leads to a more efficient and economic structural system. In the second part of the study, effect of shear reinforcement in shear walls on the structural performance is investigated through nonlinear-time history analysis. In this part of the study, only the structural system with 55 cm, 45 cm and 35 cm thick shear walls is investigated due to time limitations. Selected system is re-design per Turkish Seismic Code (DBYBHY, 2007). The resulting shear reinforcement is designated as Ash.These design with shear reinforcement Ash is modeled in Perform3D and considered to be the original model. In order to find out effects of shear reinforcement in shear walls, four different control models are derived from the original model by changing only the shear reinforcement ratios to %25, %50, %75 and %125 of Ash. In nonlinear model, inelastic shear model of Perform3D is used to represent nonlinear shear behavior of shear walls. Among there are two well-documented nonlinear shear model available in the literature: ASCE 41-13 and FEMA 356. In this study, ASCE 41-13 shear model with slight modification is used instead of FEMA 356 to consider cracking of walls under shear loading. In ASCE 41-13 model, cracking strength is defined as %60 of yield strength. But most of the previous studies asserted that shear cracking is expected to occur at around shear strength of concrete. Therefore, in this study, shear strength of the nonlinear shear model at cracking is determined from shear strength of concrete. Further plastic hinges for beams and columns and fiber sections for shear walls are used to model other nonlinear behavior. In the second part of the study nonlinear-time history analysis results show that, shear behavior of the shear walls is highly affected by the aspect ratio, which can be defined as the ratio of the height of the wall to the length of the wall. It is observed that shear walls with smaller aspect ratios attract more seismic loads due to rigidity. Since the design philosophy of the Turkish Seismic Code is based on the requirement that shear walls shall not experience shear yielding, amount of reinforcement that will lead yielding is investigated for different walls. It is found that shear walls with smaller aspect ratio yield earlier than the shear walls with larger aspect ratios, i.e. larger aspect ratio walls have more reserved capacity. In spite of fact that all shear walls are designed according to same code requirement basis, shear walls with different aspect ratios achieve different inelastic shear deformations and demand/capacity ratios. While shear walls with aspect ratio or 25.14 remains elastic region in shear behavior at all control model, shear walls with aspect ratio of 10.2 is already at edge of nonlinear shear deformation. Further, models with shear walls with larger shear reinforcement, produce larger seismic loads, due to loss of rigidity is less. Therefore, models with larger shear reinforcement experience larger inelastic deformations at structural elements other than shear walls while limited inelastic deformations are occured at concrete core shear walls. Also, interstory drifts in models with larger shear reinforcement slightly increases due to larger seismic loads at upper stories; as interstory drifts reached excessive points due to low capacity from decreased shear reinforcement of shear walls. On the other hand, interaction between flexure and shear behavior of shear walls is area of interest of this study. Eventhough a full interaction model of shear walls is not used in this study, it is investigated that whether inelastic flexural deformation effects shear capacity of shear walls. Previous studies asserts that in reinforced concrete elements, increasing inelastic flexural deformations decreases the shear capacity of element. Obviously inelastic flexural deformation of shear walls does not jeopardize shear capacity of walls. As a recommendation for further research, effects of width of shear wall on seismic performance of the structure should be investigated with more width and shear reinforcement ratio options using nonlinear-time history analysis. With more results, parameters that can lead to an optimum solution can be revealed. Also relation between aspect ratio of walls and shear yielding within the scope of design procedure of binding codes needs to be researched.