Impact factor formulas used for traditional rail bridges (AREMA) or highway bridges (AASHTO) cannot be assumed for high speed rail application. Trains crossing through a wide range of speeds are likely to have a case which drives a forced vibration that resonates with the dynamic behavior of the bridge. Because these bridges are excited for a short time, the transient response has shown to be significant, making simplified use of steady-state modal analysis inadequate. Several high speed rail bridges of various configurations in the California area were studied. Impact factors were computed based on the ratio of dynamic response to the corresponding static case. To compute the full dynamic transient response, a series of time-history analyses were performed on a finite element model. This study considered different train sets at speeds ranging from 90 to 250 mph. In addition to analysis of moving loads, vehicle-bridge interaction was considered by utilizing a series of moving mass-spring-dampers with multiple degrees of freedom and stepping those elements across the bridge. For the responses considered, it was shown that in most cases the critical impact factors were considerably larger than those calculated using AREMA or AASHTO equations. However, the inclusion of vehicle-structure interaction can help in reducing these large impact factors.
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