PERFORMANCE BASED SEISMIC DESIGN OF COUPLED SHEAR WALLS

Abstract

One of the recent earthquakes in India on 26th January, 2001 caused considerable damage to a large number of RCC high-rise buildings (number of storey varies from 4 to 15) and tremendous loss of life. The reasons were: (a) most of the buildings had soft and weak ground storey that provided open space for parking; (b) poor quality of concrete in columns and (c) poor detailing of the structural design. This particular incident showed that designers and structural engineers should ensure to provide adequate earthquake resistant provisions with regard to planning, design and detailing in high-rise buildings to withstand the effect of an earthquake to minimize disaster. As an earthquake resistant system, the use of coupled shear walls is one of the potential options in comparison with moment resistant frame (MRF) and shear wall frame combination systems in RCC high-rise buildings. However, the Indian codes of practice governing the earthquake resistant design, such as IS 1893 (Part 1) (2002) and IS 4326 (1993) do not provide specific guidelines with regard to earthquake resistant design of coupled shear walls. On the other hand, IS 13920 (1993) gives credence to the coupled shear walls as an earthquake resistant option but it has incorporated very limited design guidelines of coupling beams that are inadequate for practical applications. Further, it is reasonably well established that it is uneconomical to design a structure considering its linear behavior during earthquake as is recognized by the Bureau of Indian Standards [IS 4326 (1993), IS 13920 (1993) and IS 1893 (Part 1) (2002)] currently in use. Hence an alternative design philosophy needs to be evolved in the n Indian context to consider the post-yield behavior wherein the damage state is evaluated through deformation considerations. In the present context, therefore, performance-based seismic design (PBSD) can be considered to offer significantly improved solutions as compared to the conventional design based on linear response spectrum analysis. On the basis of the forgoing an effort was made in this thesis to develop a comprehensive procedure for the design of coupled shear walls. In view of coupled shear walls being important earthquake resistant elements in high-rise buildings and lack of adequate guidelines in Indian Code of Practice, an effort was made in this thesis to make the point wise objective as below. 1. To study the behavior of coupling beams in coupled shear walls. 2. To determine how stable hysteresis with high-energy dissipation can be obtained in the coupling beams as well as at the base of walls. 3. To determine desirable failure/collapse mechanisms and to investigate how they can be incorporated in design. 4. To explore the design guidelines for obtaining best nonlinear behavior of coupled shear walls. 5. To determine the conditions for which NSP (Non-linear Static Procedure) is sufficient for design of coupled shear walls. 6. To investigate how adequate ductility and response reduction factor can be obtained in coupled shear walls. Accordingly, following methodologies were adopted to fulfill the above mentioned objectives. in 1. An analytical model of coupling beam was developed for considering different types of reinforcements. 2. A design technique was developed for PBSD of coupled shear walls. 3. Parametric study was carried out for finding out the limitation of the design technique. 4. A nonlinear static analysis was conducted for determining the ductility and response reduction factor of coupled shear walls. Since coupling beam primarily governs the behavior of coupled shear walls, the behavior of coupling beam was investigated. The geometry, rotations and moment capacities of coupling beams were reviewed from previous experimental and analytical studies. An analytical model of RCC coupling beam were developed to calculate the rotations for diagonally reinforced coupling beam and truss reinforced coupling beam. Next was to develop a design technique to obtain an adequate ductility of coupled shear walls considering its ideal seismic behavior (stable hysteresis with high earthquake energy dissipation). This technique was proposed to determine the dimensions of coupled shear walls under earthquake motion. It was developed based on the nonlinear static analysis with some assumptions regarding the behavior of coupled shear walls under earthquake motion. It was developed for the collapse prevention level. The design/capacity curve can be obtained with the help of this technique. It was applied to both fixed base and pinned base coupled shear walls. A case study was performed to validate this technique. The result obtained from design technique exhibits ideal seismic behavior of coupled shear walls and ductility was IV obtained more for pinned base in comparison with the fixed base condition. Parametric study was carried out to find the limitations along with remedial action of this technique. In this study, length of the coupling beam and number of storey was considered as variable parameters; whereas, other parameters were considered as constant. It was found that coupled shear walls with pinned base condition showed rigid body motion, which is expected as per behavior concerned. Pinned base condition showed better nonlinear behavior (adequate ductility with high earthquake energy dissipation) in comparison to the fixed base condition. Similarly, Coupled shear walls should be designed with an optimum level of structural height, coupling beam length to wall depth ratio, aspect ratio of the coupling beam and couplingbeam to wall stiffness ratio to obtain consistency between the behavior with respect to the wall rotation and earthquake energy dissipation. Nonlinear static analysis was considered to assess the proposed design technique. For this purpose, this study determined the response reduction factors for coupled shear walls at DBE and MCE levels, respectively. It was seen that response reduction factor with conventional reinforced coupling beam was found almost same as per the provision provided by CSA (1994) standard. Therefore, it was recemmended that proposed design technqiue can be considered to design the coupled shear walls under earthquake motion. Scope of further research was also included in this thesis.