Effects Of Ground Motion Spatial Variability On The Response Of Long-Span Structures

Long-span structures are generally important facilities, e.g. long-span bridges, gymnasiums, dams, or nuclear power plants. Therefore, their aseismatic capabilities are highly relevant to public safety and so in the last twenty years much research has gone into establishing practical seismic analysis and design methods for them. Interest in this problem stems from earthquake engineering for the complex nature of the earth's crust, which causes earthquake motions to vary along the length of structures, coupled with the increasing application of long-span structures. The variations in the ground motion arise mainly from three sources: the "wave passage effect" due to the difference in the arrival times of waves at support points;the "incoherence effect" due to reflections and refractions of seismic waves through the soil during their propagation;and the "site-response effect" due to the differences in local soil conditions at the support point. Therefore, a rigorous seismic analysis of spatially extended structures should account for the spatial variability of the ground motion.In this paper, the effects of ground motion spatial variability on the response of long-span structures are investigated based on the project of the new "Earthquake-Resistant Code for Highway Bridges". The main works include:1) A comprehensive investigation of the variation of the random seismic responses due to the wave passage effect is performed based on the pseudo excitation method (PEM). For a single-degree-of-freedom with two supports, the power spectrum density functions of the internal forces are derived, and means of maximum response values are investigated due to different apparent wave velocities. In addition, the influences of the responses of the quasi-static and the relative dynamic components are also discussed. It is pointed out that the internal forces vary significantly with the variation of the apparent wave velocities, especially for structures with larger span. The relative dynamic component has important effects on the dynamic responses of structures with lower natural frequencies. For structures with larger natural frequencies, the quasi-static component has important influence on the seismic responses. Moreover, random seismic responses of a suspension bridge are analyzed with different apparent wave velocities. It is also shown that the wave passage effect has great influence on the seismic responses.2) Based on the pseudo excitation method (PEM), a random vibration methodology is formulated for the seismic analysis of multi-supported structures subjected to spatiallyvarying ground motions. The ground motion spatial variability consists of the wave passage, incoherence and site response effects. Random seismic responses of two example structures due to these effects are extensively investigated. It is shown that all these effects have significant influence on the seismic response of the structure, depending heavily on the fundamental natural frequency of the structure. Moreover, the relative significances of the quasi-static and dynamic components of the response are also studied. The contribution of the quasi-static displacement components is independent of the fundamental natural frequency of the structure while those of the dynamic components and of the coupling of the dynamic and the quasi-static displacement components depend heavily on the fundamental frequency of structures.