A WARPING MULTIFIBRE BEAM MODEL TO ASSESS THE SEISMIC VULNERABILITY OF EXISTING REINFORCED CONCRETE STRUCTURES

Abstract eng:
To determine the seismic vulnerability of existing reinforced concrete structures, computations at the structural scale accounting for material non-linearities are needed. The multifibre finite element method was proved to be efficient to simulate the behaviour of slender structural elements subject to normal stresses. However, the response of the model in case of large shear stresses lacks in accuracy. Indeed, the plane cross section assumption used to compute the strains is not precise enough to convey the effect of shear. This problem is addressed in the present work by introducing additional warping degrees of freedom, which can come from both transverse shear and torsion. The Mu model, a 3D damage model accounting for the unilateral effect, is used as a constitutive law. The warping degrees of freedom are computed along with the global beam degrees of freedom by satisfying the structure equilibrium through an implicit solution procedure. The enhanced element is first validated using an analytical model for a beam subject to transverse shear. Then the linear elastic behaviour of a beam subject to both shear and torsion is successfully compared to the results of a 3D simulation. To investigate the effect of warping coupled with material nonlinearities, the experimental behaviour of beams in torsion is simulated. Material parameters are calibrated using one of the experimental specimen by a genetic algorithm. Then, the torque-twist behaviour of beams with various cross section shapes was predicted. Taking warping into account leads to an average improvement of 90% of the torsional stiffness prediction, and 70% of the peak prediction. It also leads to a better assessment of damage in the cross-section. The enhanced method is eventually used to compute the behaviour of a full structure subject to a seismic loading. The results of this study show the effect of warping on the structural modes, and thus on the nonlinear dynamic response of the structure. The model enhancement leads to an additional computation cost. To increase the efficiency of the modelling strategy, the additional warping degrees of freedom should be activated only when necessary. Parametric studies should be further conducted, to quantify the importance of including warping in the model depending on the structure features.

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