On the influence of the steel-concrete bond model for the simulation of reinforced concrete structures using damage mechanics

International audienceWhen dealing with the simulation of reinforced concrete structures, the bond model between steel and concrete can become a key point if crack properties are studied. The interface between both materials is indeed partly responsible for stress transfer and consequently for the crack spacing and openings. In the context of finite element simulations using damage mechanics for concrete, the influence of the relation between steel and concrete is evaluated by comparing two solutions a perfect relation (same displacement at the interface) and a recently developed bond model (sliding allowed). The approaches are compared on a reinforced concrete tie, a bending beam and a shearing wall. The interest of including a fine description of the steel-concrete bond rather than a simple perfect relation between materials, regarding the simulation of local properties (crack openings especially) depends on the type of applications (loadings) and on the expected crack pattern (and/or distribution of steel)

A new modeling Strategy for the behavior of walls under dynamic loading

International audienceA new simplified modelling strategy to simulate the non‐linear behaviour of reinforced concrete shear walls under dynamic loading is presented. The equivalent reinforced concrete (ERC) model is derived from the framework method and uses lattice meshes for concrete and reinforcement bars and uniaxial constitutive laws based on continuum damage mechanics and plasticity. Results show the capacity of the model to analyse structures having different slenderness and boundary conditions. For low reinforcement ratios however, results are sensitive to the angle formed by the diagonals of the concrete lattice and the horizontal bars. The method is compared with the shear multi‐layered beam model that uses Timoshenko multi‐layered 2D beam elements and biaxial constitutive laws. Comparisons for both models with experimental results of two research programs (one organized by NUPEC and the other by COGEMA and EDF) are provided. ERC is a simplified method that intends to save computer time and allows parametrical studies

The equivalent reinforced concrete model for simulating the behavior of shear walls under dynamic loading

A new simplified modeling strategy for simulating the non-linear behavior of reinforced concrete structures submitted to severe dynamic shear is presented. The Equivalent Reinforced Concrete model (ERC) uses lattice meshes for concrete and reinforcement bars and uniaxial constitutive laws based on the principles of continuum damage mechanics and plasticity. Verification is provided through comparisons with the results of the NUPEC experimental program. ERC is a simplified method that intends to save computer time and to allow for parametrical studies. The proposed lattice model is promising and could be extended to 3D calculations or to simulate the behavior of plastic zones

From historical site to art museum: Xu Bing's Tobacco Projects Durham and Virginia

Examining the development of Xu Bing's Tobacco Project from its first to its third iteration, I argue that in addition to changes in the work itself, the installation's changing display spaces have influenced scholarly and critical interpretation of its core message. In published responses to this project, scholars have extracted a variety of meanings from the displays. They have moved from interpreting Xu's work as a passionate stand against the tobacco industry, framed by his father's untimely death from lung cancer, to an ambiguous installation that explores both positive economic benefits and negative health effects of tobacco. I argue that changes to this discourse were heightened as critics and scholars responded to Tobacco Project's interaction with the established narratives of the spaces in which the project was displayed. To focus my study, I concentrate on the differences between the Tobacco Museum and Duke Homestead in Durham, North Carolina, where Xu first installed a portion of his work, and the Virginia Museum of fine arts in Richmond, where Xu displayed his third project. Using the field of critical museum studies as a guide, I explore the differences between a museum dedicated to a positive tobacco history and a public fine arts museum that may have affected Tobacco Project's own narratives. I conclude that space has played an inextricable role in the presentation and reception of Xu Bing's Tobacco Project

A 3D beam element analysis for R/C structural walls

To analyse the real 3D functioning of a structure under seismic loading the dialogue between tests and numerical simulations is needed. Within the framework of the TMR-ICONS research program, dynamic and cyclic tests on U-shaped shear walls have been performed at CEA Saclay and JRC Ispra respectively. More recently, for the French program ìCAMUS 2000î, shaking table tests have been performed on reinforced concrete structural walls. In order to simulate these tests, 3D multi-fiber beam elements are used. Comparison with the experimental results shows the well matching and the limitations of the approach

Characterization of the axon initial segment (AIS) of motor neurons and identification of a para-AIS and a juxtapara-AIS, organized by protein 4.1B

<p>Abstract</p> <p>Background</p> <p>The axon initial segment (AIS) plays a crucial role: it is the site where neurons initiate their electrical outputs. Its composition in terms of voltage-gated sodium (Nav) and voltage-gated potassium (Kv) channels, as well as its length and localization determine the neuron's spiking properties. Some neurons are able to modulate their AIS length or distance from the soma in order to adapt their excitability properties to their activity level. It is therefore crucial to characterize all these parameters and determine where the myelin sheath begins in order to assess a neuron's excitability properties and ability to display such plasticity mechanisms. If the myelin sheath starts immediately after the AIS, another question then arises as to how would the axon be organized at its first myelin attachment site; since AISs are different from nodes of Ranvier, would this particular axonal region resemble a hemi-node of Ranvier?</p> <p>Results</p> <p>We have characterized the AIS of mouse somatic motor neurons. In addition to constant determinants of excitability properties, we found heterogeneities, in terms of AIS localization and Nav composition. We also identified in all α motor neurons a hemi-node-type organization, with a contactin-associated protein (Caspr)⁺paranode-type, as well as a Caspr2⁺and Kv1⁺juxtaparanode-type compartment, referred to as a para-AIS and a juxtapara (JXP)-AIS, adjacent to the AIS, where the myelin sheath begins. We found that Kv1 channels appear in the AIS, para-AIS and JXP-AIS concomitantly with myelination and are progressively excluded from the para-AIS. Their expression in the AIS and JXP-AIS is independent from transient axonal glycoprotein-1 (TAG-1)/Caspr2, in contrast to juxtaparanodes, and independent from PSD-93. Data from mice lacking the cytoskeletal linker protein 4.1B show that this protein is necessary to form the Caspr⁺para-AIS barrier, ensuring the compartmentalization of Kv1 channels and the segregation of the AIS, para-AIS and JXP-AIS.</p> <p>Conclusions</p> <p>α Motor neurons have heterogeneous AISs, which underlie different spiking properties. However, they all have a para-AIS and a JXP-AIS contiguous to their AIS, where the myelin sheath begins, which might limit some AIS plasticity. Protein 4.1B plays a key role in ensuring the proper molecular compartmentalization of this hemi-node-type region.</p

Poutre multifibre Timoshenko pour la modélisation de structures en béton armé: Théorie et applications numériques

International audienceLes équations d'un élément poutre 3D multifibre Timoshenko et son utilisation pour la modélisation de structures en béton armé sont ici présentées. L'originalité de l'élément est qu'il a deux nœuds et des fonctions d'interpolation d'ordre supérieur pour éviter les problèmes lies au blocage par le cisaillement. Des exemples numériques comparés avec des résultats expérimentaux montrent la pertinence de l'approche

Modélisation simplifiée pour l'endommagement des structures en béton arme sous sollicitations sévères

La simulation du comportement non-linéaire des voiles en béton armé soumis à des sollicitations sismiques est un problème prioritaire pour la communauté parasismique nationale et internationale. Les séismes récents à Kobe (Japon), Izmit (Turquie) et Athènes (Grèce) ont prouvé encore une fois le rôle primordial que tels éléments jouent pour la sécurité des ouvrages. Recevant la plus grande partie de l'effort sismique, les voiles conditionnent le comportement des structures à murs. Il est donc important de trouver des méthodes numériques adéquates pour simuler le comportement de différents types de voiles souvent rencontrés dans les bâtiments, les installations nucléaires etc. L'équipe « Modélisation des ouvrages sous sollicitations extrêmes » du Laboratoire de Mécanique et de Technologie (LMT) a depuis des années privilégié la recherche vers la mise au point de méthodes simplifiées, fiables et rapides pour le calcul du comportement non linéaire des structures. Pour des voiles ordinaires-élancement supérieur à 1.0-une stratégie de modélisation a été déjà proposée (Mazars et al 1999, Ragueneau 1999). La méthode consiste à utiliser des éléments poutres multicouches de type Bernoulli, capables de simuler le comportement des voiles, dominés par la flexion. La loi constitutive du béton est basée sur la mécanique de l'endommagement. Pour des problèmes de murs d'élancement voisin de 1.0 où le cisaillement est prépondérant, une approche nommée 1.5D a été développée (Dubé 1997). Dans ce cas, des poutres de type Timoshenko sont utilisées et le cisaillement est pris en compte par une contribution non-linéaire des contraintes de cisaillement dans la section. La méthode a été appliquée avec succès pour la modélisation de la maquette NUPEC (élancement 0.7)