Fatigue Reliability Assessment for Existing Bridges Considering Vehicle Speed and Road Surface Conditions

During the life cycle of a bridge, dynamic impacts due to random traffic loads and deteriorated road surface conditions can induce serious fatigue issues for bridge components. It is necessary, and more realistic than the deterministic approach, to use reliability methods and treat the input parameters as random variables for the vehicle-bridge dynamic system. This paper presents a framework of fatigue reliability assessment for existing bridges in lifetime serviceability considering the random effects of vehicle speed and road-roughness condition. Since each truck passage might generate multiple stress ranges, revised equivalent stress-range is introduced to include fatigue damage accumulations for one truck passage. Therefore, the two variables, i.e., the stress-range numbers and equivalent stress ranges per truck passage, are coalesced in the newly defined variable based on equivalent fatigue damage. The revised equivalent stress-range is obtained through a fully-computerized approach toward solving a coupled vehicle-bridge system, including a three-dimensional (3D) suspension vehicle model and a 3D dynamic bridge model. At each truck-pass-bridge analysis, deteriorations of the road-roughness condition are considered, and the vehicle speed and road surface profile are generated randomly. When the stress-range threshold is 3.45 Mpa (0.5 ksi) or below, lognormal distribution is proven a good model to describe the revised equivalent stress-range. In addition to the assumptions of other input random variables being normal or lognormal, fatigue reliability index and fatigue life for a target fatigue reliability index are predicted. The effects of the road surface condition, vehicle speed, and annual traffic increase rate on the fatigue reliability index and fatigue life are also discussed.