ABSTRACT
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The purpose of this paper is to evaluate the impact factor (IM) specified in the AASHTO LRFD bridge design specifications for fatigue design and to propose a method for determining reasonable impact factors for the fatigue design of steel I-girder bridges that can more rationally consider the effect of the deterioration of the road surface condition (RSC) during its whole life cycle. The impact factor for stress range (IMsSR) is studied based on numerical simulations and the whole life cycle of the road wearing course is considered. A three-dimensional vehicle-bridge coupled model is developed to simulate the interaction between the bridge and vehicle. The bridge models and the fatigue truck model in the AASHTO LRFD code are adopted. The deterioration process of the RSC under the given traffic and environment condition is investigated to obtain the number of truck passages and the time taken for the RSC to deteriorate from one class to the next, for instance, from the “very good” class to the “good” class. The effects of three important parameters, including the bridge span length, vehicle speed, and road surface condition, on impact factors for the fatigue design of steel I-girder bridges are investigated. Results show that the RSC has a greater impact on the IMSR than on the traditional IM and the IMSR is greater than the traditional IM calculated using the maximum stress. By considering the cumulative fatigue damage caused by the passage of each truck under different RSCs and the deterioration process of the RSC during its whole life cycle, simple and reasonable expressions are proposed for the impact factors for fatigue design of steel I-girder bridges under the given traffic and environmental condition.
