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In this paper, straight beams with parallel fibres, homogeneous, i.e. made of a single material characterized by its elastic modulus E and its shear modulus G are considered. The beam is related to an Oxyz direct orthonormal system of axis, the axis Ox being the longitudinal axis of the beam. Therefore, the section is in the Oyz plane. It is defined by a medium contour  endowed with a curvilinear abscissa s associated with a function representing the thickness(es). The outline  may or may not have branches as well as one or more closed cells.

Saint-Rkmy-lis-Chevreuse R~SUM.!?. Dans cette itude, le modile de comporternent cyclique du be'ton est bas6 sur la thiorie de I'endornrnagement isotrope, sur la micanique des milieux continus ei la thermodynamique des processus irre'versibles. Ce modile tient compte des diformations re'siduelles et de la perte de raideur due a la fissuration. h e ame'lioration irnportante pour l'application du modsle a it6 de dijinir une mithode d'identifcation des paramitres ci partir des donne'es physiques obtenues dans les essais classiques de contr6le du biton et d'une expression analytique de la loi de comportement d'usage courant. Des essais de validation ont e'te' rialise's sur des e'prouvettes cylindriques sous charges cycliques de compression et sur des poteaux courts armb sous compression et traction cycliques alterne'es. La comparaison apporte des rksultats encourageants pour l'analyse des structures en be'ton anne' sous charges cycliques.ABSTRACT. In this study, the model for the concrete behavior is based on the theov of damage, on the mechanics of continuum media and the thermodynamics of irreversible processes. The model takes into account the inelastic strains and the loss of sti' ess due to cracking. An important improvement for the application of the model has been made in the method of identification of parameters; it is based on physical data obtained from concrete classical tests and from an analytical expression of a constitutive law of common use. Vaiidation tests have been carried out on standard cylinders subjected to repeated compressive loads and on short reinforced columns under compression-tension alternate cycles. The comparison between the model and tests gives promizing results for analysis of reinforced concrete structures under cyclic loading.