Effect of axial restraint on the behaviour of coupling beams in coupled wall structures

Abstract

Coupling beams are used in high-rise buildings to connect shear walls that possess a large capacity to dissipate energy in order to resist seismic events trough inelastic action. These beams are subjected to high shear loads and are very important for the stiffness, strength and ductility of the building. Therefore, the design of those beams is very significant to avoid partial or complete collapse of the structure. Experimental studies conducted in the past showed that coupling beams tend to get longer when they failed in shear. As a result, significant axial forces and axial deformations develop in the coupling beams which affect their behaviour. Although the phenomenon is well known, it is under- studied. Only few experiments take into account the effect of restraint by adjacent walls and record the axial forces generated due to the constraining effect. The goal of this thesis is to study the behaviour of coupling beams axially restrained by stiff ad- jacent shear walls. Therefore, two main objectives were identified. First, it is targeted to evaluate the magnitude of axial compression forces on the coupling beams due to the restraint. Then, the second objective of this work is to study the effect of these axial compression forces on the behaviour of coupling beams in coupled shear walls in terms of strength and ductility. In order to study the behaviour of these beams, three different ways of modelling were used. The first and the simplest one is strut-and-tie models. However, because of the many parameters that influence the shear behaviour, strut-and-tie models are not always able to predict the right behaviour of these members. For that reason, non-linear finite element modelling of coupled wall structures and individual coupling beams was mainly used. The advantage of this complex method is that it precisely predicts the shear behaviour and shear strength of reinforced concrete coupling beams. Numerical mod- els are powerful tools to simulate samples with a great realism but they consist in high-dimensional non-linear differential equations that lead to time-consuming simulations. Therefore, in order to have another means of comparison, a kinematic model proposed by Mihaylov et al. (2015) was also used. This type of model, based on the three-parameter kinematic theory (3PKT), is more mathematical; it is a fast and efficient way to capture an approximation of the shear strength of members made of conventional concrete through equilibrium equations, compatibility and stress-strain relationships. The later employs only three different degrees of freedom compared to the previous method which used a very large number. Once the models were validated and matched in terms of results with values found from experi- mental studies, the last contribution of this work was to learn more about the constraining effect by performing a parametric study. The results of this study shows that compression forces generated by axial restraint are not insignificant. Also, it was observed that compression forces generated in coupling beams improve the ultimate shear resistance of the beams but decrease the ductility of the later. In coupled wall structures, axial compression forces are generally greater at the lower storeys rather than at the upper ones. For future investigations on this topic, additional experimental tests should be conducted in order to draw more conclusions.