3D discrete element modeling of concrete: study of the rolling resistance effects on the macroscopic constitutive behavior

The Discrete Element Method (DEM) is appropriate for modeling granular materials [14] but also cohesive materials as concrete when submitted to a severe loading such an impact leading to fractures or fragmentation in the continuum [1, 5, 6, 8]. Contrarily to granular materials, the macroscopic constitutive behavior of a cohesive material is not directly linked to contact interactions between the rigid Discrete Elements (DE) and interaction laws are then defined between DE surrounding each DE. Spherical DE are used because the contact detection is easy to implement and the computation time is reduced in comparison with the use of 3D DE with a more complex shape. The element size is variable and the assembly is disordered to prevent preferential cleavage planes. The purpose of this paper is to highlight the influence of DE rotations on the macroscopic non-linear quasi-static behavior of concrete. Classically, the interactions between DE are modeled by spring-like interactions based on displacements and rotation velocities of DE are only controlled by tangential forces perpendicular to the line linking the two sphere centroids. The disadvantage of this modeling with only spring-like interactions based on displacements is that excessive rolling occurs under shear, therefore the macroscopic behavior of concrete is too brittle. To overcome this problem a non linear Moment Transfer Law (MTL) is introduced to add a rolling resistance to elements. This solution has no influence on the calculation cost and allows a more accurate macroscopic representation of concrete behavior. The identification process of material parameters is given and simulations of tests performed on concrete samples are shown

Multi–scale modelling of timber–frame structures under seismic loads

This paper introduces a versatile hysteretic constitutive law, developed for various joints with steel fasteners commonly used in timber structures (nails, screws, staples, 3D connectors of bracket type, punched plates). Compared to previous models available in literature, the proposed one improves numerical robustness and represents a step forward by taking into account the damaging process of joints with metal fasteners. Experimental tests carried out on joints are used for calibration purpose, and quasi–static and dynamic tests performed on shear walls allow validating the proposed Finite Element model. Finally, the development of a computationally efficient simplified FE model of timber–frame structures for shear walls is described, with emphasis on its validation and its use at the scale of a complete structure

Multi-scale analysis of timber framed structures filled with earth and stones

This paper deals with the seismic analysis of timber framed houses filled by stones and earth mortar using a multi-scale approach going from the cell to the wall and then to the house. At the scale of the elementary cells, experimental results allow fitting the parameters of a new versatile hysteretic law presented herein through the definition of a macro-element. Then, at the scale of wall, the numerical simulations are able to predict its behavior under quasi-static cyclic loading and is compared to experimental results allowing validating the macro-element model

Using damage mechanics to model a four story RC framed structure submitted to earthquake loading

A simplified model is proposed to simulate the nonlinear behavior of a four-story full-scalereinforced concrete framed structure subjected to severe dynamic loading. The structure has been testedpseudodynamically in the European Laboratory for Structural Assessment (ELSA) at the Joint ResearchCenter of the European Commission. The proposed model uses 2D multi layered Bernoulli beam elementsand uniaxial constitutive laws based on damage mechanics and plasticity. Comparison with theexperimental results shows the efficiency of the approach

Mesoscopic scale modeling of concrete under triaxial loading using X-ray tomographic images

This paper focuses on the discrete modeling of triaxial behaviour of concrete. The originality of this work comes from two points. The first one concerns the predictive feature of the model developed for simulating the response of concrete specimens; the behaviour of mortar, rock, and their interaction being identified a priori or by means of experimental tests on the mortar and the rock. The second originality relates to the construction method of the discrete element assembly based on the 3D segmentation of tomographic images. Such a method allows modeling of concrete at the mesoscopic scale with an internal structure similar to the one of the concrete tested experimentally. The comparisons between numerical and experimental results show the model is capable to reproduce the triaxial behavior of concrete for confining pressure varying from 0 to 650 MPa

Damage mechanics of interfacial media: Basic aspects, identification and application to delamination

International audienceThis chapter presents the development of a model to bridge between damage mechanics and delamination by including all the damage mechanisms in delamination analysis. For this, a damage meso-modeling that includes both inner layer damage mechanisms and interracial ones is used. Delamination often appears as the result of interactions among different damage mechanisms, such as fiber-breaking, transverse microcracking, and debonding of the adjacent layers themselves. At the meso-level, the laminate is described as a stacking sequence of inelastic and damageable homogeneous layers throughout the thickness and of damageable interlaminar interfaces. One limitation of the meso-modeling is that the fracture of the material is described by means of only two types of macrocracks: (1) delamination cracks within the interfaces and (2) cracks, orthogonal to the laminate, with each cracked layer being completely cracked in its thickness. The ideas and framework that govern the interface damage modeling are similar to those which are used for deriving the layer damage modeling

Experimental characterization of interstitial pore pressure in concrete under high confinement

Le but de cette étude est de mesurer la pression interstitielle du béton sous haut niveau de confinement. Lorsque les structures de béton sont soumises à un chargement de type impact ou explosion, le matériau subit de très fortes contraintes triaxiales qui sont fortement influencées par le taux de saturation. Afin de mesurer la pression interstitielle d’eau à l’intérieur du béton sous fort chargement triaxial, un nouveau dispositif expérimental a été développé. Des éprouvettes de 14 cm de longueur et 7 cm de diamètre sont testées à l’aide de la presse GIGA. Dans le nouveau concept, la longueur de l’échantillon est réduite à 8 cm et une enclume de mesure de la pression interstitielle de longueur 6 cm a est incorporée. Des micro-trous sont percés sur sa face supérieure de sorte que la pression de l’eau libre dans l’échantillon puisse ainsi être transmise dans la cavité durant l’essai. Un capteur cylindrique équipé d’une jauge est placé dans la cavité permet de mesurer cette pression. Les résultats préliminaires d’un essai hydrostatique effectué sur un béton saturé indiquent que la pression interstitielle a augmenté jusqu’à 200 MPa pour 300 MPa de confinement.This study focuses on measuring the pore pressure of concrete under high confinement. When concrete structures are subjected to an impact loading, material exhibits high triaxial compressive stresses which are highly influenced by the saturation ratio. In order to measure the interstitial water pore pressure inside the concrete under mechanical loading, a new experimental device was developed. Usually, specimens of 14cm in length and 7cm in diameter are tested using the GIGA press. In the new concept, the specimen length is reduced to 8cm and a pore pressure measurement cell of 6 cm in length is incorporated. The cell has micro-holes on its upper face, so that, when the specimen is loaded water pressure is transmitted from concrete to the cell where a cylindrical sensor equipped with a gage is placed. Preliminary results indicate that concrete pore pressure increases up to 200 MPa during an hydrostatic test at 300 MPa of confinement

Prédiction de la pénétration dans une dalle en béton d'un missile rigide par la méthode aux éléments discrets

Un modèle numérique 3D utilisant la méthode des éléments discrets a été développé pour prédire la profondeur de pénétration d'un missile impactant une dalle en béton armé. Le modèle a été calibré sur un essai d'impact de référence impliquant un missile à nez plat. Une fois le modèle calibré, des simulations numériques ont été réalisés en faisant varier seulement la forme du nez du missile. Les résultats numériques sont comparés avec les résultats expérimentaux réalisés par CEA-EDF et la loi de prédiction de Li. La bonne prédiction de la profondeur de pénétration par le modèle numérique est confirmée par les observations issues des essais expérimentaux

Simulation d'un impact localisé sur un ouvrage en béton armé par une approche couplée continue/discontinue

La simulation des structures par éléments discrets est bien adaptée aux problèmes dynamiques mettant en jeu de la fragmentation mais elle est difficile à mettre en œuvre sur des structures de grande taille. Couplées à une méthode aux éléments finis, cette méthode devient performante pour simuler des ouvrages en béton armé soumis à des impacts autant à l'échelle locale de l'impact qu'au niveau global de la structure. Les méthodes de couplage proposées permettent de supprimer les réflexions d'onde dues aux variations de taille de la discrétisation. Le caractère prédictif du modèle est obtenu par une démarche d'identification des paramètres du modèle discret. La méthode couplée est utilisée en 3D sur l'impact rocheux d'une dalle en béton