Soil-Structure Interaction for Gravity Caissons in Bridge Seismic Design

Soil-structure interaction modeling techniques are presented for bridges supported on large gravity caissons at deep-water sites. The fundamental mode of vibration of these massive caisson foundations tends to be in the short period range, in contrast with the longer period superstructure response. This results in an overall system with complex behavior during an earthquake, which needs to be carefully considered in the analysis and design. Modeling techniques with varying levels of complexity are demonstrated based on project experience from three bridges in highly seismic regions including: Second Tacoma-Narrows Bridge (WA), Strait of Messina Bridge (Italy) and South Park Bridge (WA). Use of numerical model pushover analyses to develop nonlinear distributed soil springs is shown to help overcome several of the limitations of the more basic lumped linear stiffness matrix approach. The relative importance of wave scattering effects and the evaluations of earth pressure distributions considering seismic loading scenarios for assessment of required reinforcements are also addressed.