Effect of Spatial Variation of Ground Motion on Highway Structures

Keywords: Ground motion. Spatial variability.  Long span bridges.  Highway bridges.  Wave passage effect.  Incoherence effect.  Local soil conditions.   Monte Carlo methods.  Artificial time histories.  Nonstationary processes.  Design recommendations

Abstract: A methodology is  developed to generate spatially varying ground motion time histories along the length of a long, multi-span bridge at its supports.  Spatial variability of seismic ground motion can be mainly attributed to the differences in arrival times of seismic waves at different locations (the wave passage effect); loss of coherence of seismic waves due to multiple reflections refraction as they propagate through the highly inhomogeneous soil medium (incoherence effect); and change in the amplitude and frequency content of seismic ground motion due to different local soil conditions, (local soil effect).  Our methodology reflects all three effects.  Representative bridges were analyzed using two cases of ground motions at bridge supports: 1 ) input motions identical at all supports; 2) different input motions at each pier along the length of the bridge reflecting wave passage, incoherence and local soils effects.  A sensitivity analysis was carried out as a function of various parameters controlling spatial variability.  Based on study results, two guidelines were proposed: one for  highway bridges less 1,500 feet with all supports on the same local soil conditions, and another for bridges more than 1, 500 feet long or of any length with supports on different local soil conditions.