Experimental Investigation of Load-Dependent Fatigue Behaviour in Orthotropic Steel Decks under Moving Vehicles

Orthotropic steel decks (OSDs) are commonly used in long-span bridges due to their reduced self-weight and high load-carrying efficiency. Despite these advantages, fatigue remains a major durability concern because OSDs are subjected to millions of stress cycles from daily traffic loading. While past experimental studies often apply a single wheel load level, real traffic contains vehicles with a broad range of axle loads, which can significantly influence fatigue behaviour. Therefore, a better understanding of load-dependent fatigue response is required to support improved bridge design and maintenance decisions. This paper presents an experimental investigation into the effects of different axle load magnitudes on the fatigue performance of OSDs. A scaled OSD specimen was designed based on common bridge configurations and tested under several wheel load levels selected in accordance with Australian bridge standards. Strain gauges and linear potentiometers were positioned around fatigue-sensitive locations to measure variations in strain range, local deformation, and fatigue response as the applied axle load increased. The results showed that heavier axle loads produce considerably larger stress fluctuations at welded joints, particularly at rib-to-deck and rib-to-diaphragm connections, which are known to be critical fatigue hotspots. Progressive stiffness reduction was also observed as loading severity increased, indicating early damage development under higher load demands. These findings enhance understanding of the role of heavy vehicles in accelerating fatigue damage in OSD bridges and provide practical insight for traffic load management, fatigue assessment, and lifecycle maintenance planning.