Seismic response of bridges with massive foundations

The dynamic response of vehicular overpasses with massive foundations built in highly populated earthquake prone regions is studied to assess the massive foundation potential of being a technically sound mean to reduce the structural response during major earthquakes. The study consists on numerical simulations using 3-D finite element models. Two typical supports of a major 23 km long vehicular overpass recently built 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. Initially, a conventional raft foundation structurally connected to four precast closed-end concrete piles was considered. Then, the potential beneficial effect of a massive foundation of variable depth was assessed. Sets of 3-D 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 a potential 8.1 M_w seismic event, and for a hypothetical 8.7 M_w extreme event. Important attenuations of about 39% to 48% at the upper deck spectral accelerations and of 21% to 30% in maximum lateral foundation displacements were achieved with the massive soil improvement for the cases analyzed. Thus, the massive foundations seem to be a convenient alternative to reduce the overall structural seismic response.