The problem of soil structure interaction in the seismic analysis and design of long-span bridges in seismically active regions is potentially important to highway facilities design. A quite general nonlinear dynamic soil structure interaction analysis methodology is developed to estimate seismic response characteristics of cable-stayed bridges tower, in which the effect of the soil flexibility underneath the foundation and its associated geometric and hysteretic damping are included. Due to rocking response of a spread foundation under a strong near-field ground motion that may result in the foundation uplift and the yield of the underlying soils, both of strain-dependent material nonlinearity and geometrical nonlinearity by base mat uplift are considered. The soil parameters values that are considered in the design process are used to observe the effect of soil nonlinearities upon the tower-pier seismic response. Numerical results indicate that force response quantities of the physical sub-structure stiffness tower model can be reduced by considering soil foundation interaction and different soil nonlinearities. The soil yielding below the foundation and uplift at the interface have significantly contribution to foundation rocking response. The soil bearing stress beneath the footing base dramatically increases due to footing base uplift. The predominant contribution to the vertical response at footing base comes from the massive foundation rocking rather than from the vertical excitation. The spectrum amplitude at tower top contains only the dominant flexible super-structure frequency and all other frequencies, which present in the massive and rigid sub-structure base level, have been essentially filtered out.
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