Number of stress cycles for fatigue design of simply-supported steel I-girder bridges considering the dynamic effect of vehicle loading

• Propose a method to determine number of stress cycles for fatigue design.
• Dynamic vehicle loading greatly affects the equivalent number of stress cycles.
• Propose expressions for calculating number of stress cycles.
• Number of stress cycles used in LRFD code may be underestimated.

The number of stress cycles (NSC) specified in the AASHTO LRFD bridge design specifications for the fatigue design of steel bridges is evaluated in this paper. A new approach for determining the reasonable number of stress cycles for the fatigue design (NSC_FD) of simply-supported steel I-girder bridges is proposed which takes the dynamic effect of vehicle loading into account. A three-dimensional vehicle–bridge coupled model is developed to simulate the interaction between the bridge and vehicle, in which both the bridge and fatigue load models are adopted from the LRFD code. The equivalent number of stress cycles (ENSC), which is calculated based on the fatigue damage accumulation from the dynamic stress time history due to each truck passage, is used for the fatigue analysis of steel girders. Numerical simulations are conducted to study the influence of three important parameters, including the road surface condition (RSC), bridge span length and vehicle speed, on the ENSC of simply-supported steel I-girder bridges. Results show that the RSC has a great impact on the ENSC. By considering the cumulative fatigue damage caused by each truck passage under different RSCs and the deterioration process of the RSC during its whole life cycle, simple and reasonable expressions are proposed for calculating the NSC_FD of simply-supported steel I-girder bridges under the given traffic and environmental condition.