Three-dimensional seismic environment effects on bridge fragility

Bridges built on soft soils, such as highly compressible plastic clays, present a larger seismic vulnerability due to the low shear strength, low stiffness, and small damping increment with shear strain, which can lead to large amplifications of the seismic movements. These factors are accentuated when dealing with three dimensional seismic environments that can preclude both foundation and structure failures, and which in turn, can result in the loss of total or partial functionality, and interruption of the transport network. Therefore, the design of urban bridges built on soft soils in densely populated cities requires the use of numerical models capable of simulating both the structure and soil conditions properly to establish the probability of reaching or exceeding a given damage state in a seismic event. This paper presents a numerical study of these effects on an urban bridge built on the typical soft soils found in Mexico City. The evaluations were carried out considering both intraplate and interface fault events expressed in uniform hazard spectra for several return periods (i.e., 125, 187, 250, 362, 475, 1475, and 2475 years), assuming two-dimensional and three-dimensional seismic environments. To assess the critical supports probability of reaching or exceeding a given damage state, nonlinear response history analyses were conducted, and site-specific fragility curves were derived numerically. The seismic response analyses were carried out by series of three-dimensional numerical models developed with the software FLAC3D, accounting for the effect of both, soil conditions and ground motion characteristics on the soil-structure system. To develop the fragility curves, the damage index was defined in terms of the relative displacements in the columns. Based on the results gathered in here, it was clearly established the detrimental effects of three-dimensional seismic environments in bridges, and the important increase of the probability of damage of 40 %, with respect to the performance expected for two-dimensional seismic loading. • Fragility curves for urban overpasses in a 3D seismic environment are defined. • Vertical seismic motion raises bridge damage probability up to 40 %. • Highest failure probability occurs in the bridge's transverse direction. • Bridge seismic vulnerability must account for 3D seismic environments.