Based on a site of a large span continuous rigid frame bridge in Shanghai

the influence of hydrodynamic pressures on ground motion is discussed. The input ground motion has been obtained by using the response spectrum of code

one-dimension model and two-dimension model. It shows that hydrodynamic pressures can reduced the vertical acceleration response significantly

while it has little effect on horizontal acceleration response. The ground motion calculated by two-dimension model which consider the topography is quite different from that of other two models. The predominant periods of input ground motion calculated by two-dimension model exceeds the flat section of response spectrum of code. Finally

the influence range of valley topography is discussed

which shows that the influence region in the direction normal to the river is no more than 5 times of soil layer thickness or 140 times of deepness of the river valley. Any structures in this region should take river valley influence into consideration.