Macroelement modeling of shallow foundations

The paper presents a new macroelement model for shallow foundations. The model is defined through a non-linear constitutive law written in terms of some generalized force and displacement parameters. The linear part of this constitutive law comes from the dynamic impedances of the foundation. The non-linear part comprises two mechanisms. One is due to the irreversible elastoplastic soil behavior: it is described with a bounding surface hypoplastic model, adapted for the description of the cyclic soil response. An original feature of the formulation is that the bounding surface is considered independently of the surface of ultimate loads of the system. The second mechanism is the detachment that can take place at the soil-footing interface (foundation uplift). It is totally reversible and non-dissipative and can thus be described by a phenomenological non-linear elastic model. The macroelement is qualitatively validated by application to soil-structure interaction analyses of simple real structures

Rapport du comité Information Scientifique et Technique

Rapport du Comité IST, Information Scientifique et Technique, remis le 19 mai 2008 au Directeur général de la recherche et de l’innovation et au Directeur général de l’enseignement supérieur, dont les recommandations encouragent à la définition d\u27une stratégie au niveau ministériel centrée sur deux pôles : les relations avec les éditeurs et les archives ouvertes

Rupture sismique des fondations par perte de capacité portante: Le cas des semelles circulaires

International audienceWithin the context of earthquake-resistant design of shallow foundations, the present study is concerned with the determination of the seismic bearing capacity of a circular footing resting on the surface of a heterogene-ous purely cohesive semi-infinite soil layer. In the first part of the paper, a database, containing case histories of civil engineering structures that sustained a foundation seismic bearing capacity failure, is briefly pre-sented, aiming at a better understanding of the studied phenomenon and offering a number of case studies useful for validation of theoretical computations. In the second part of the paper, the aforementioned problem is addressed using the kinematic approach of the Yield Design theory, thus establishing optimal upper bounds for the ultimate seismic loads supported by the soil-footing system. The results lead to the establishment of some very simple guidelines that extend the existing formulae for the seismic bearing capacity contained in the European norms (proposed for strip footings on homogeneous soils) to the case of circular footings and to that of heterogeneous cohesive soils

A Macro-Element for Dynamic Soil-Structure Interaction Analyses of Shallow Foundations

The scope of the paper is to present some aspects of the development of a "macro-element" for dynamic soil-structure interaction analyses of shallow foundations. Initially the concept of "macro-element" is introduced and is illustrated with the aid of a very simple example originating from structural engineering. Then the link is made with the modeling of the dynamic response of shallow foundations and the objectives and structure of such a tool are described with reference to the specific configuration of a circular footing resting on the surface of a heterogeneous purely cohesive soil. The principal features of the "macro-element" are then presented; the soil-structure interaction domain is reduced to a point that coincides with the center of the footing and all the (material and geometric) non-linearities are lumped at this point. A discussion on the most appropriate way to treat these non-linearities is undertaken based on experience gained with earlier works. It is suggested that the non-linearities be incorporated in the model within a unified formalism making use of the theory of multi-mechanism plasticity. Initial results concerning the definition of the ultimate surface for such a plasticity model, corresponding to the seismic bearing capacity of the foundation, are finally presented.Comment: 4th International Conference on Earthquake Geotechnical Engineering, Thessaloniki : Gr\`ece (2007

On the performance of non-conforming finite elements for the upper bound limit analysis of plates

This is a preprint version of an article accepted for publication in International Journal for Numerical Methods in Engineering Copyright © 2012 John Wiley & Sons, Ltd.International audienceIn this paper, the upper bound limit analysis of thin plates in bending is addressed using various types of triangular finite elements for the generation of velocity fields and second order cone programming (SOCP) for the minimization problem. Three different C1-discontinuous finite elements are considered : the quadratic 6 node Lagrange triangle (T6), an enhanced T6 element with a cubic bubble function at centroid (T6b) and the cubic Hermite triangle (H3). Through numerical examples involving Johansen and von Mises yield criteria, it is shown that cubic elements (H3) give far better results in terms of convergence rate and precision than fully conforming elements found in the literature, especially for problems involving clamped boundaries

Quelques facettes de l'œuvre de Barre Saint-Venant

Après sept ans passés aux Poudres et Salpêtres où il était entré à sa sortie de l’École polytechnique, Adhémar Barré de Saint-Venant fut admis en 1823, sans concours, à l’École des Ponts et Chaussées. Il y suivit avec assiduité les leçons de Navier, alors professeur suppléant puis professeur adjoint du cours de mécanique appliquée, dont l’enseignement novateur faisait largement appel à l’analyse, rompant avec celui d’Eisenmann, et dont il devint un fervent admirateur. Durant les années qu’il ..

Evidence of the Cost of the Production of Microcystins by Microcystis aeruginosa under Differing Light and Nitrate Environmental Conditions

The cyanobacterium Microcystis aeruginosa is known to proliferate in freshwater ecosystems and to produce microcystins. It is now well established that much of the variability of bloom toxicity is due to differences in the relative proportions of microcystin-producing and non-microcystin-producing cells in cyanobacterial populations. In an attempt to elucidate changes in their relative proportions during cyanobacterial blooms, we compared the fitness of the microcystin-producing M. aeruginosa PCC 7806 strain (WT) to that of its non-microcystin-producing mutant (MT). We investigated the effects of two light intensities and of limiting and non-limiting nitrate concentrations on the growth of these strains in monoculture and co-culture experiments. We also monitored various physiological parameters, and microcystin production by the WT strain. In monoculture experiments, no significant difference was found between the growth rates or physiological characteristics of the two strains during the exponential growth phase. In contrast, the MT strain was found to dominate the WT strain in co-culture experiments under favorable growth conditions. Moreover, we also found an increase in the growth rate of the MT strain and in the cellular MC content of the WT strain. Our findings suggest that differences in the fitness of these two strains under optimum growth conditions were attributable to the cost to microcystin-producing cells of producing microcystins, and to the putative existence of cooperation processes involving direct interactions between these strains

Quasistatic rheology and the origins of strain

Features of rheological laws applied to solid-like granular materials are recalled and confronted to microscopic approaches via discrete numerical simulations. We give examples of model systems with very similar equilibrium stress transport properties -- the much-studied force chains and force distribution -- but qualitatively different strain responses to stress increments. Results on the stability of elastoplastic contact networks lead to the definition of two different rheological regimes, according to whether a macroscopic fragility property (propensity to rearrange under arbitrary small stress increments in the thermodynamic limit) applies. Possible consequences are discussed.Comment: Published in special issue of "Comptes-Rendus Physique" on granular material

A numerical tool to integrate biophysical diversity of a large regulated river: hydrobiogeochemical bases. The case of the Garonne River (France)

This article presents the bases of a hydrobiogeochemical model of the Garonne River (southwest France) which has been developed to integrate physical and biological processes during summer low-water periods. The physical part of this model is composed of a one-dimensional unsteady hydrodynamic model, allowing the resolution of the Saint-Venant equations, and a transport model which simulates downstream changes in solute concentrations. Biogeochemical processes are considered through the definition of functional compartments which make up the channel bed. These different compartments are defined both by the organisms involved in the solute transformation processes and by the physical and hydraulic characteristics of their habitat. Integration of these functional compartments within the model required investigations at different scales. The scale at which biological processes take place ranges from millimetres to metres. The scale of a reach, at which organization of the functional compartments along the river can be linked to hydrodynamic and morphological characteristics, ranges from 500 m to several kilometres. The regional scale is that at which homogeneous reaches can be integrated. A feedback between numerical results and field experiments has allowed improvements to in situ measurement to increase modelling accuracy. For example, the model allows estimation of variables, such as fluxes, that are difficult to measure in situ. The developed model can integrate various functional compartments and their biogeochemical functioning. Two application examples, focused on dissolved inorganic nitrogen, are presented in order to illustrate the numerical tool functioning: integration of equations on nitrification processes in the water body, and integration of consumption/production terms on epilithic biofilm resulting from in situ experimental mean values. The model we have developed constitutes a promising analytical tool that will be able to integrate previous and future studies