pport structures. Bridges with varying cross section and different types of support are considered irregular. The degree of seismic activity is based on the acceleration coefficient at the bridge site. The acceleration coefficient is a dimensionless constant used to describe groundmotion.(2) Bridges with an acceleration coefficient greater that 0.19 are considered to bein an area of high seismic activity. This coefficient, along with whether the bridge isclassified as essential or not, are used to assign the bridge a seismic performance category(SPC).(2)Based on the SPC and the number of spans, one of two different analysis methods arechosen to calculate the loading on the bridge due to earthquake forces. The two methodsof analysis used are:* Single-mode spectral analysis* Multimode spectral analysisIn general, regular bridges in area of low seismic activity utilize the less involvedsingle-mode analysis method, while irregular bridges in high seismic risk locations requirethe multimode spectral analysis. The former can be performed using conventional handcalculation method, but the latter demands more rigorous computer-aided solutions.The single-mode spectral analysis method assumes loading in basic transverse andlongitudinal directions as illustrated in figure 2.2(4) The multimode approach, however, isrequired because of the irregular bridge geometry which necessitates analysis to determinethe effects of coupling in three coordinate directions for each vibration mode. For regularmultispan bridges the single-mode spectral analysis method for calculating equivalentstatic earthquake loading is shown below.The single-mode spectral analysis procedure uses the same method for calculating bothlongitudinal and transverse earthquake loading. This method utilizes the principle ofvirtual displacements to develop a mode shape model of the bridge. An arbitrary, uniformstatic loading po is applied to the length of the structure to provide an...
