Experimental and Numerical Assessment of Thermal Stresses and Fatigue in Ballasted CWR Bridges Using Field Measurements

Introduction With increasing global temperatures and more frequent thermal extremes, continuous welded rail (CWR) systems over bridges may experience heightened risks of failure due to temperature-induced stress fluctuations in the rail. Objectives This study aims to quantify Rail-structure interaction (RSI)-induced thermal stress amplification, evaluate the effective allowable fatigue stress range, and develop field-based assessment parameters for long multi-span CWR viaducts. Methodology Focussing on India’s first instrumented CWR viaduct, this study presents an investigation of thermal stress amplification and effective allowable fatigue stress range in an existing ballasted steel–concrete composite CWR bridge, comprising multiple short spans and elastomeric bearings. A comprehensive field monitoring program was conducted to record real-time rail strains, bridge displacements, and temperature profiles. Results Results revealed that thermal stresses in CWR for such bridges may get amplified by more than 200% over the assumed limits due to RSI. These additional compressive and tensile stresses were observed to exceed national code limits by as much as 124% and 172%, and UIC 774–3R limits by 83% and 127%, respectively. Moreover, an equivalent thermal expansion coefficient $$\alpha _{\textrm{eq}}$$ of about $$5.54 \times 10^{-6}$$ /°C was computed to account for differential solar heating and composite action. A revised effective allowable fatigue stress range incorporating actual thermal rail stresses was found to reduce the conventional fatigue range of 199 MPa for UIC 60 rail by 38–43%, thereby narrowing the safety margin towards finite-life territory under in-service train loads. Conclusion This study thus provides a field-validated methodology for estimating thermal rail stresses, equivalent thermal expansion coefficients, and allowable fatigue stress range in composite steel–concrete ballasted CWR bridges. The results are directly transferable to bridges with similar geometry and boundary conditions, especially in climates with severe thermal fluctuations and aging infrastructure.