Keywords: seismic response, massive foundation, numerical model, bridges.

The design of modern bridges and overpasses, and the seismic retrofit of existing ones, has moved towards performance based evaluations. The dynamic response of vehicular overpasses with massive foundations built in highly populated earthquake prone regions is studied, to assess the potential of massive foundations to be a technically sound means of reducing the structural response during major earthquakes. The study consists of numerical simulations using three-dimensional finite element models. Two typical supports for a major 23 km long vehicular overpass currently under construction in the north east part of Mexico City valley were considered in this research. This zone is characterized by the presence of stiff soils comprised by dense and very dense silty sands and sandy silts, randomly intercalated by stiff clays layers of variable thickness. The overpass is comprised of an upper deck resting on top of central and support beams that are structurally tied to the columns, which, in turn, are monolithically attached to a rectangular foundation. The upper part of this foundation consists of a reinforced 1.7 m mat covered with a concrete filling. This mat rests on top of a reinforced 0.25 m concrete slab, which, in turn, is underlain by a 3.6 m by 4.6 m massive un-reinforced concrete block, of variable depth. The mat foundation is connected to four 0.8 m diameter, cast-in-place, concrete piles. Lateral dynamic stiffness and damping were computed for all cases in order to evaluate the effect of the massive foundation. Sets of three-dimensional finite element models were developed and the response of the systems was evaluated for a typical seismic scenario such as that prevailing at the zone, assuming an M_w 8.7 earthquake generated in the Pacific subduction zone, considering a hypothetical epicenter located 423 km away from the sites studied. A synthetically derived strong ground motion was obtained from a deterministic seismic hazard analysis. Several response spectra of the most representative events recorded in three seismological stations located near by the project, on very stiff soil were normalized with respect to the peak ground acceleration. To assess the seismic performance of the overpass-foundation-soil system, the response was obtained at the upper beam, at the top of the raft foundation, and at the free field. Important attenuations of about 23% to 36% at the upper deck spectral accelerations, and of 17% to 21% in displacements were achieved with massive soil improvement, for the case analyzed. Thus, the massive foundations seem to be a convenient alternative to reduce the overall structural seismic response.

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