Coupling beams are structural elements used to connect two or more shear walls. The most common material used in the construction of coupling beam is reinforced concrete. The use of coupling beams along with shear walls require them to resist large shear forces, while possessing sufficient ductility to dissipate the energy produced due to the lateral loads. This study has been undertaken to produce a computational model to replicate the behavior of conventionally reinforced coupling beams subjected to cyclic loading. The model is developed in the finite element analysis software ABAQUS. The concrete damaged plasticity model was used to simulate the behavior of concrete. A calibration model using a cantilever beam was produced to generate key parameters in the model that are later adapted into modeling of two coupling beams with aspect ratios: 1.5 and 3.6. The geometrical, material, and loading values are adapted from experimental specimens reported in the literature, and the experimental results are then used to validate the computational models. The results like evolution of damage parameter and crack propagation from this study are intended to provide guidance on finite element modeling of conventionally reinforced concrete coupling beams under cyclic lateral loading.

In reinforced concrete (RC) structures, adequate bond between the reinforcement and concrete is required to achieve a true composite system, in which reinforcing steel carries tensile stress, once concrete cracks, and concrete and reinforcing steel carry compression. Determining bond strength and required development length for shear transfer between concrete and reinforcement is an ongoing research subject in the field of reinforced concrete with advances in the concrete and reinforcement materials requiring continuous experimental efforts. Finite element analysis (FEA) provides opportunities to explore structural behavior of RC structures beyond the limitations of experimental testing. However, there is a paucity of research studies employing FEA to investigate the reinforcement-concrete bond-zone behavior and related failure mechanism. Instead, most FEA-based research associated with RC bond has centered on developing a bond (or interface) constitutive model for use in FEA that, by itself, can characterize bond-zone behavior, typically represented by the bond stress-slip displacement relationship. This class of bond models is useful for simulating the global behavior of RC structures but is limited in its ability to simulate local bond resistance for geometries and material properties that differ substantially from those used to calibrate the model. To fill this gap in research, this study proposes a finite element (FE) modeling approach that can simulate local bond-zone behavior in reinforced concrete. The proposed FE model is developed in a physics-based way such that it represents the detailed geometry of the bond-zone, including ribs on the deformed reinforcement, and force transfer mechanisms at the concrete-reinforcement interface. The explicit representation of the bond-zone enables simulation of the local concrete compression due to bearing of ribs against concrete and subsequent hoop tension in the concrete. This causes bond failure either due to local concrete crushing (leading to reinforcement pullout) or global concrete splitting. Accordingly, special attention is given to the selection and calibration of a concrete model to reproduce robust nonlinear response. The power of the proposed modeling approach is its ability to predict bond failure and damage patterns, based only on the physical and material properties of the bond area. Thus, the successful implementation and application of this approach enables the use of FEA simulation to support the development of new design specifications for bond zones that include new and improved materials.

Les techniques de renforcement de structures en béton armé (BA) par collage de polymères renforcés de fibres (PRF) trouvent un important champ d'applications dans le renforcement des poteaux en BA. Le chemisage par PRF confine le noyau du poteau et permet d'augmenter sa résistance et sa ductilité. Bien que de nombreux travaux expérimentaux aient été consacrés à l'étude de l'effet de confinement du PRF sur le comportement des poteaux en BA, la réalisation d'une simulation réaliste de la réponse structurelle de tels éléments présente de nombreuses difficultés liées aux modèles de comportement peu appropriés à reproduire précisément la réponse mécanique du béton confiné. Dans cette recherche, un modèle de comportement élasto-plastique endommageable est développé pour reproduire la réponse mécanique du béton sollicité suivant un chemin triaxial de contraintes. Ce modèle prend en compte différents mécanismes de comportement du béton tels que les déformations irréversibles, l'endommagement dû à la microfissuration, la sensibilité au confinement et les caractéristiques de dilatation. Un processus d'identification des paramètres du modèle est proposé sur la base d'essais classiques. La validation de ce modèle est ensuite démontrée en comparant des résultats de simulations à des données expérimentales de la littérature sur des bétons confinés activement puis des bétons confinés par des PRF présentant une large gamme de rigidité. Le modèle proposé est également comparé à différentes modélisations de la littérature. Les capacités du modèle sont illustrées et analysées sur des applications tridimensionnelles de poteaux en BA de taille réelle, non confinés et confinés par PRF. / For the past two decades, externally bonded Fiber Reinforced Polymers (FRP) has gained much popularity for seismic rehabilitation of reinforced concrete (RC) columns. In this technique, FRP wrap installed on the surface of a column acts as lateral confinement and enhance the strength and deformation capacity of the concrete element. Although many experimental works have been devoted to the study of confining effect of FRP on the behavior of RC columns, the numerical simulation of FRP-jacketed RC columns remains a challenging issue due to the lack of appropriate constitutive model for confined concrete. In this study, a damage plastic model is developed to predict the behavior of concrete under triaxial stress states. The proposed model takes into account different material behavior such as irreversible strain, damage due to microcracking, confinement sensitivity and dilation characteristic. A straightforward identification process of all model’s parameters is then presented. The identification process is applied to different normal strength concrete. The validity of the model is then demonstrated through confrontation of experimental data with simulations considering active confined concrete and FRP confined concrete with a wide range of confinement stiffness. The proposed constitutive model is also compared with other models from the literature and the distinguishing features of this new model are discussed. Furthermore, the capacity of the model in the three-dimensional finite element analysis of full-scale RC columns is demonstrate and discussed.

Reinforced concrete frames, infilled with brick/concrete block masonry, are the most common type of structures found in multi-storeyed constructions, especially in developing countries. Usually, the infill walls are considered as non-structural elements even though they alter the lateral stiffness and strength of the frame significantly. Approximately 80% of the structural cost from earthquakes is attributable to damage of infill walls and to consequent damages of doors, windows and other installations. Despite the broad application and economical significance, the infill walls are not included in the analysis because of the design complexity and lack of suitable theory. But in seismic areas, ignoring the infill-frame interaction is not safe because the change in the stiffness and the consequent change in seismic demand of the composite structural system is not negligible. The relevant experimental findings shows a considerable reduction in the response of infilled frames under reverse cyclic loading. This behaviour is caused by the rapid degradation of stiffness, strength, and low energy dissipation capacity resulting from the brittle and sudden damage of the unreinforced masonry infill walls. Though various national/international codes of practice have incorporated some of the research outcomes as design guidelines, there is a need and scope for further refinement. In the initial part of this work, a numerical modelling and linear elastic analysis of masonry infilled RC frames has been done. A multi-storey multi-bay frame infilled with masonry panels, is considered for the study. Both macro modelling and micro modelling strategies are adopted. Seismic loading is considered and an equivalent static analysis as suggested in IS 1893, 2002 is done. The results show that the stiffness of the composite structure is increased due to the obvious confinement effects of infill panels on the bounding frame. A parametric study is conducted to investigate the influence of size and location of openings, presence/absence of infill panels in a particular storey and elevation irregularity in terms of floor height. The results show that the interaction of infill panel changes the seismic response of the composite structure significantly. Presence of openings further changes the seismic behaviour. Increase in openings increases the natural period and introduce newer failure mechanisms. Absence of infill in a particular storey (an elevation irregularity) makes it drift more compared to adjacent storeys. Since the structural irregularities influence the seismic behaviour of a building considerably, we should be cautious while construction and renovation of such buildings in order to take the advantage of increased strength and stiffness obtained by the presence of infill walls. A nonlinear dynamic analysis of masonry infilled RC frames is presented next. Material non linearity is considered for the finite element modelling of both masonry and concrete. Concrete damage plasticity model is employed to capture the degradation in stiffness under reverse cyclic loading. A parametric study by varying the same parameters as considered in the linear analysis is conducted. It is seen that the fundamental period calculation of infilled frames by conventional empirical formulae needs to be revisited for a better understanding of the real seismic behaviour of the infilled frames. Enhancement in the lateral stiffness due to the presence of infill panel attracts larger force and causes damage to the composite system during seismic loading. Elevation irregularities included absence of infill panels in a particular storey. Soft storey shows a tendency for the adjacent columns to fail in shear, due to the large drift compared to other storeys. The interstorey drift ratios of soft storeys are found to be larger than the limiting values. However this model could not capture the separation at the interfaces and related failure mechanisms. To improve the nonlinear model, a contact surface at the interface is considered for a qualitative analysis. A one bay one storey infilled frame is selected. The material characteristics were kept the same as those used in the nonlinear model. Contact surface at the interface was given hard contact property with pressure-overclosure relations and suitable values of friction at the interface. This model could simulate the compressive diagonal strut formation and the switching of this compressive strut to the opposite diagonal under reverse cyclic loading. It showed an indication of corner crushing and diagonal cracking failure modes. The frame with central opening showed stress accumulation near the corners of opening. Next, the micro modelling strategy for masonry suggested by Lourenco is studied. This interface element can be used at the masonry panel-concrete frame interface as well as at the expanded masonry block to block interface. Cap plasticity model (modified Drucker – Prager model for geological materials) can be used to describe the behaviour of masonry (in terms of interface cracking, slipping, shearing) under earthquake loading. The blocks can be defined as elastic material with a potential crack at the centre. However, further experimental investigation is needed to calibrate this model. It is required to make use of the beneficial effects and improve upon the ill-effects of the presence of infills. To conclude, infill panels are inevitable for functional aspects such as division of space and envelope for the building. Using the lateral stiffness, strength contribution and energy dissipation capacity, use of infill panels is proposed to be a wiser solution for reducing the seismic vulnerability of multi-storey buildings.

Implementing pre-stressing cables is a viable option aiming at controlling deformation and cracking of concrete flat slabs in serviceability limit state. The pre-stressing cables also contribute to punching shear capacity of the slab when they are located in vicinity of the column. The positive influence of pre-stressing cables on punching capacity of the concrete slabs is mainly due to the vertical component of inclined cables, compressive in-plane stresses and counter acting bending moments near the support region. The method presented in Eurocode 2 to determine the punching capacity of the pre-stressed concrete flat slabs considers the in-plane compressive stresses but totally neglects the effect of counter acting moments. The effect of vertical forces introduced by inclined cables is only considered when they are within the distance 2d from the face of the column. This area is called basic control area in the Eurocode 2. In this master thesis nonlinear finite element analysis is carried out to study the effect of pre-stressing cables on punching shear capacity of concrete slabs respecting the distance of cables from the face of the column. To attain this objective, the concrete damage plasticity model is implemented to model the concrete. The results indicate that until the distance of 6d from the face of the column the contribution of pre-stressing cables in punching shear capacity of slabs is significant. Furthermore, comparing the numerical results with the punching shear capacity of slabs predicted by Eurocode 2 reveals that Eurocode tremendously underestimates the punching shear capacity when the cables are located outside the basic control area.

The ability to predict fatigue crack growth rates is essential in safety critical systems. The discovery of fatigue crack closure in 1970 caused a flourish of research in attempts to simulate this behaviour, which crucially affects crack growth rates. Historically, crack tip plasticity models have been based on one-dimensional rays of plasticity emanating from the crack tip, either co-linear with the crack (for the case of plane stress), or at a chosen angle in the plane of analysis (for plane strain). In this thesis, one such model for plane stress, developed to predict fatigue crack closure, has been refined. It is applied to a study of the relationship between the apparent stress intensity range (easily calculated using linear elastic fracture mechanics), and the true stress intensity range, which includes the effects of plasticity induced fatigue crack closure. Results are presented for all load cases for a finite crack in an infinite plane, and a method is demonstrated which allows the calculation of the true stress intensity range for a growing crack, based only on the apparent stress intensity range for a static crack. Although the yield criterion is satisfied along the plastic ray, these one-dimensional plasticity models violate the yield criterion in the area immediately surrounding the plasticity ray. An area plasticity model is therefore required in order to model the plasticity more accurately. This thesis develops such a model by distributing dislocations over an area. Use of the model reveals that current methods for incremental plasticity algorithms using distributed dislocations produce an over-constrained system, due to misleading assumptions concerning the normality condition. A method is presented which allows the system an extra degree of freedom; this requires the introduction of a parameter, derived using the Prandtl-Reuss flow rule, which relates the magnitude of slip on complementary shear planes. The method is applied to two problems, confirming its validity.

Les simulations numériques des fissures fragiles par les modèles d’endommagement à gradient deviennent main- tenant très répandues. Les résultats théoriques et numériques montrent que dans le cadre de l’existence d’une pre-fissure la propagation suit le critère de Griffith. Alors que pour le problème à une dimension la nucléation de la fissure se fait à la contrainte critique, cette dernière propriété dimensionne le paramètre de longueur interne.Dans ce travail, on s’attarde sur le phénomène de nucléation de fissures pour les géométries communément rencontrées et qui ne présentent pas de solutions analytiques. On montre que pour une entaille en U- et V- l’initiation de la fissure varie continument entre la solution prédite par la contrainte critique et celle par la ténacité du matériau. Une série de vérifications et de validations sur diffèrent matériaux est réalisée pour les deux géométries considérées. On s’intéresse ensuite à un défaut elliptique dans un domaine infini ou très élancé pour illustrer la capacité du modèle à prendre en compte les effets d’échelles des matériaux et des structures.Dans un deuxième temps, ce modèle est étendu à la fracturation hydraulique. Une première phase de vérification du modèle est effectuée en stimulant une pré-fissure seule par l’injection d’une quantité donnée de fluide. Ensuite on étudie la simulation d’un réseau parallèle de fissures. Les résultats obtenus montrent qu’il a qu’une seule fissure qui se propage et que ce type de configuration minimise mieux l’énergie la propagation d’un réseau de fractures. Le dernier exemple se concentre sur la stabilité des fissures dans le cadre d’une expérience d’éclatement à pression imposée pour l’industrie pétrolière. Cette expérience d’éclatement de la roche est réalisée en laboratoire afin de simuler les conditions de confinement retrouvées lors des forages.La dernière partie de ce travail se concentre sur la rupture ductile en couplant le modèle à champ de phase avec les modèles de plasticité parfaite. Grâce à l’approche variationnelle du problème on décrit l’implantation numérique retenue pour le calcul parallèle. Les simulations réalisées montrent que pour une géométrie légèrement entaillée la phénoménologie des fissures ductiles comme par exemple la nucléation et la propagation sont en concordances avec ceux reportées dans la littérature. / Phase-field models, sometimes referred to as gradient damage, are widely used methods for the numerical simulation of crack propagation in brittle materials. Theoretical results and numerical evidences show that they can predict the propagation of a pre-existing crack according to Griffith’s criterion. For a one- dimensional problem, it has been shown that they can predict nucleation upon a critical stress, provided that the regularization parameter is identified with the material’s internal characteristic length.In this work, we draw on numerical simulations to study crack nucleation in commonly encountered geometries for which closed-form solutions are not available. We use U- and V-notches to show that the nucleation load varies smoothly from the one predicted by a strength criterion to the one of a toughness criterion when the strength of the stress concentration or singularity varies. We present validation and verification of numerical simulations for both types of geometries. We consider the problem of an elliptic cavity in an infinite or elongated domain to show that variational phase field models properly account for structural and material size effects.In a second movement, this model is extended to hydraulic fracturing. We present a validation of the model by simulating a single fracture in a large domain subject to a control amount of fluid. Then we study an infinite network of pressurized parallel cracks. Results show that the stimulation of a single fracture is the best energy minimizer compared to multi-fracking case. The last example focuses on fracturing stability regimes using linear elastic fracture mechanics for pressure driven fractures in an experimental geometry used in petroleum industry which replicates a situation encountered downhole with a borehole called burst experiment.The last part of this work focuses on ductile fracture by coupling phase-field models with perfect plasticity. Based on the variational structure of the problem we give a numerical implementation of the coupled model for parallel computing. Simulation results of a mild notch specimens are in agreement with the phenomenology of ductile fracture such that nucleation and propagation commonly reported in the literature.