Numerical Study of the Seismic Response of an Urban Overpass Support System

A strategic urban overpass is to be built in the so-called transition and hill zones in Mexico City. The subsoil conditions at these zones typically consist on soft to stiff clay and medium to dense sand deposits, randomly interbedded by loose sand lenses, and underlain by rock formations that may outcrop in some areas. Several critical supports of this overpass are going to be instrumented with accelerometers, inclinometers and extensometers to assess their seismic performance during future earthquakes and to generate a database to calibrate soil-structure-interaction numerical models. This paper presents the seismic performance evaluation of one of these supports. The support foundation is a 3.6 by 4.6 m mat, structurally connected to four cast-in-place 0.80 m diameter piles. A finite elements model of the soil-foundation-structure system was developed. Initially, the model was calibrated analyzing the seismic response that an instrumented bridge support exhibited during the June 15th, 1999 Tehuacan (Mw=7) Earthquake. This bridge is located also within the surroundings of Mexico City, but at the lake zone, where highly compressible clays are found. The computed response was compared with the measured response in the free field, box foundation, and structure. Once the model prediction capabilities were established, the seismic response of the critical support of the urban overpass was evaluated for the design earthquake in terms of transfer functions and displacement time histories