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Capacité portante des fondations superficielles dans la ville de Mexico pendant le séisme de Michoacan de 1985
No full textLe séisme de Michoacan de 1985 a fourni une opportunité unique pour tester la validité des concepts théoriques développés pour l'évaluation de la capacité portante des fondations superficielles, filantes, reposant sur un sol argileux. Pendant ce séisme, de nombreux bâtiments ont subi, de façon inattendue, des ruptures de fondations. Deux d'entre eux, pour lesquels les informations disponibles étaient de qualité, ont fait l'objet d'analyses dynamiques détaillées. Ces analyses ont montré que les méthodes actuelles d'évaluation de la capacité portante des fondations, basées sur la théorie du calcul à la rupture, étaient satisfaisantes et permettaient d'obtenir des évaluations raisonnables des déplacements sismiques liés à la rupture de la fondation, pour peu que tous les paramètres entrant dans cette évaluation soient correctement pris en considération
Seismic bearing capacity of shallow foundations in Mexico city during the 1985 Michoacan earthquake
No full textDuring the 1985 Michoacan-Guerrero earthquake, a significant number of foundation problems were reportedin Mexico City. Large permanent movements and, in some cases, actual bearing capacity failures affectedfloating piles foundations as well as shallow foundations. These observations were the motivation of anextensive research. The objective was to understand the behavior of foundations during earthquakes, to derivenew bearing capacity formulae with due account of the soil inertia forces and to propose modifications toMexico City seismic code. On the one hand, the bearing capacity of strip shallow foundations has been derived within the framework of the yield design theory in which four independent loading parameters are applied to the foundation: the vertical force, the shear force, the overturning moment and the inertia forces within the soil mass. Simple analytical formulae were established which reflect the influence of the four parameters. Moreover, two simplified methodologies, based on different assumptions, were derived to compute the foundationpermanent displacements. On the other hand, information on buildings that experienced foundation failures was gathered and their seismic responses during the 1985 earthquake was computed with finite element analyses. These analyses yielded the applied seismic forces which were used together with the theoretical results to evaluate the seismic behavior of their foundations. It appears that the effect of the soil inertia forces can be disregarded, provided the foundation exhibits an adequate safety with respect to static loads, and that permanent displacements cannot be tolerated because the foundation behavior becomes unstable
Microwave surface impedance of a. -(BEDT-TTF)zCu(NCS)z, where BEDT-TTF is his(ethylenedithio)tetrathiafulvalene: Evidence for unconventional superconductivity
No full textInternational audienceHigh-sensitivity microwave-impedance measurements performed on the quasi-two-dimensional organic superconductor ~-(BEDT-TTF)2Cu(NCS)2 give sound evidence for an unconventional type of super- conductivity. The penetration depth lambda( T) is in excellent agreement with recent muon-spin-relaxation measurements which are consistent with anisotropic pairings with lines of nodes in the energy gap. The real part of the conductivity shows a peaked structure which is free of coherence factor effects and results from a competition between two temperature dependences, the penetration depth and the inelasticrelaxation time of the normal flui
DYNAMIC PROPERTIES OF COMMENSURATE AND INCOMMENSURATE SPIN DENSITY WAVES AS PROBED BY laC NMR
No full textInternational audienceWe have measured 13C NMR lineshape and nuclear spin lattice relaxation in the spin-densitywave (SDW) state of (TMTTF)2Br and (TMTSF)2PF6. The incommensurability of the SDW in (TMTSF)2PF6 is confirmed, and we show that the phason mode is dominant in the relaxation. However, we found that (TMTTF)2 Br is commensurate and estimated that the SDW amplitude is small.No phason contribution to the relaxation is observed in this compound. We discuss some implications for theory
Possible observation of a Luttinger liquid behaviour from photoemission spectroscopy of one dimensional organic conductors
No full textInternational audienc
