A new bridge that will replace the existing Woodrow Wilson Bridge is under construction over the Potomac River in Washington, DC. The river piers are supported on cylindrical steel piles of 72, 54 and 48 inches in diameter. The longest piles (at the bascule piers) are approximately 200 ft long. Outside the river, nearly all piers are supported on 24-inch square prestressed concrete piles. Although the seismicity of the region is modest, seismic issues were thoroughly addressed in the design to ensure acceptable performance during a 500 and a 2,500-year earthquake. The results from 3-D dynamic soil-foundation interaction analyses presented demonstrate the effect of site conditions and pile foundations on the design seismic motions. In the presence of scour, pile foundations in the river were found to move significantly more than the free field, especially for motions with periods close to the fundamental period of the soil-pile system. Conversely, for foundations outside the river, pile cap motions were found to be smaller than free-field motions. Furthermore, strong kinematic pile bending due to passage of seismic waves was observed at deep elevations below the surface, close to interfaces with large stiffness difference between adjacent soil layers.
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