شبیه‌سازی عددی و آزمایشگاهی ساز وکار شاخه‌ای در سطوح ترک‌خورده بتن مسلح تحت بارگذاری چرخه‌ای

Abstract: Evaluation and analysis of RC structures performance without using proper constitutive models doesn't have acceptable accuracy due to the complicated nature of shear transfer mechanism across cracks. Cracks and interfaces have been recognized as important planes in global behavior of RC structures. In fact, response of structures, failure modes and even the capacity of RC elements can be affected by stress transfer mechanism across cracks. On the other hand, understanding and expressing the different kinds of stress transfer mechanisms in finite element-based analyses is important. Aggregate interlock and dowel action are the two main shear transfer mechanisms across RC cracks and stress-induced interfaces. Cyclic nature of earthquake excitation increases cracks opening and causes major reduction in the contribution of aggregate interlock mechanism. This makes dowel action as the main mechanism resisting against applied deformation hence it is important to assess the behavior of crossing bars under cyclic loading. During the past years, extensive experimental and analytical investigations have been carried out. Many researchers have predicted the load-carrying capacity of a dowel within a limit-analysis method considering the simultaneous formation of plastic hinge in the embedded bar and a localized crushed zone in subgrade concrete. Some results showed that a localized plastic hinge is just a rough approximation. Therefore, beam on elastic foundation analogy (BEF) has been known as the most common approach to simulate dowel action mechanism. In spite of its shortcomings, the BEF analogy has been recognized as a suitable approach to simulate concrete and reinforcement interaction across cracks and different types of connections in RC structures. In this paper, dowel action mechanism has been examined analytically and experimentally and in order to simulate the shear transfer by dowel bar under different loading conditions, the beam on elastic foundation analogy has been generalized by proposing the elasto-plastic relation for foundation springs. The subgrade stiffness is the most relevant parameter to capture the global behavior of embedded dowel bars hence by adopting the proper formulation, the final loading stage as well as the initial stage can be described. Local crushing and high inelasticity of surrounding concrete near the interface is simulated by gradual changes in the spring stiffness due to increasing bar shear displacements. On the other hand, the BEF is developed to beam on inelastic foundation (BIF) by proposing an appropriate relation for spring stiffness. The suggested equation can specify the stiffness changes in consistent and simple manner during loading path. In order to have a better insight into cyclic response of crossing bars and to determine the relative parameters, some beam-type specimens under pure cyclic and repeating shear loading have been tested. Dowel action mechanism has a considerable nonlinear response under reversed cyclic loading path. The source of nonlinearity should be sought in the plasticity of dowel bars and fracturing of the surrounding concrete. The amount of applied shear displacements as well as the direction of loading and also the number of loading cycles can lead to nonlinear response. To extend the formula to unloading and reloading cases, spring deformation is divided into two components, the elastic and the residual plastic deformations. Some efforts have been devoted to assume direct proportionality between the maximum and the plastic deformation and kept constant regardless of loading history. Experimental results show thatthe maximum deformation changes due to increasing deformation of bar and cannot be assumed a constant value for it. It seems that determining the plastic bar displacement can improve the stiffness of unloading and reloading diagrams. So, the results of the experimental program and the available experimental results has been categorized for cyclic and repeating loading in order to obtain a reasonable relation between the maximum and the plastic deformation. The plastic displacement of dowel bar in each loading step is suggested by statistical analysis of collected experimental data. The effect of cyclic loading is thought to be a degradation of surrounding concrete represe-nted by inelastic springs. Therefore, improving the spring stiffness-deformation relation can capture the global behavior properly. Stiffness-deformation relation for cyclic loading has been suggested based on the available experimental results. A systematic experimental verification has been carried out to examine the reliability of the proposed model. The closed form equations lead to the considerable reduction in computational efforts. The results confirmed the accuracy of the new approach.