Spatial variation of earthquake ground motion and development of bridge fragility curves

This dissertation deals with the study of the effect of spatial variability of seismic ground motion on the response of reinforced concrete highway bridges. The phenomenon of spatial variability of earthquake ground motion has been well recognized after the recent earthquakes in Northridge, California and Kobe, Japan. It is believed that the collapse of some multi-span bridges during these two earthquakes might be attributed to the significant difference in the seismic excitation at different supports of the structure. Recent studies have demonstrated that differential or asynchronous support ground motions can be highly detrimental for multi-supported long span structures. It should be pointed out that current design practice does not take into account the effect of spatial variation of ground motion. One of the main objectives of this dissertation is to study and quantify the effect of spatial variation of ground motion on the seismic response of reinforced concrete highway bridges. The validity of the assumption of identical support ground motion commonly used in design practice is evaluated. Conclusions of major importance for the seismic behavior of these bridges are established. These conclusions indicate that the earthquake response of highway bridges can increase significantly when spatial variation of seismic ground motion is taken into account. The peak response values can easily double in cases where the supports of the bridge are located on different local soil conditions. The assumption of identical support ground motion is found to be inaccurate and generally unconservative.; Another major objective of this dissertation is to develop analytical fragility curves for reinforced concrete bridges subjected to spatially varying seismic ground motions. A fragility curve for a particular bridge provides the probability of exceeding a prescribed level of damage (e.g. light, medium, severe or collapse) for a wide range of ground motion intensities. Such fragility curves are extremely important for estimating the overall risk to civil infrastructure from potential earthquakes. They make it possible to estimate the expected level of damage for a large class of bridges, determine the ones that are most vulnerable, and take appropriate measures to mitigate damage by strengthening the weakest ones. In general, there is limited funding available for retrofitting existing bridges and it is quite important to be able to recognize the ones at the highest level of risk from a future earthquake. To the knowledge of the author, this is the first time that fragility curves have been established for bridges taking into account the effect of spatial variability of seismic ground motion. These curves will be extremely useful for Federal and State emergency management agencies and insurance companies wishing to estimate the overall loss after a scenario earthquake.