An improved chord measurement method for determining track irregularity thresholds on long-span high-speed railway bridges

Current static acceptance methods for track irregularities on high-speed long-span bridges primarily focus on middle-to-long wavelengths using a single-chord-length chord measurement method (CMM). This study aims to enhance the existing static acceptance methods and criteria for track irregularities on long-span bridges to ensure good running safety and ride comfort. First, a train-track-bridge coupled dynamics model is developed and validated, incorporating carbody flexibility to accurately predict ride comfort indices. Subsequently, the track irregularity evaluation method for long-span bridges was refined based on the existing CMM by integrating composite chord lengths and static bridge deformation. In this improved method, multiple chord lengths are simultaneously adopted to evaluate full-wavelength random track irregularities, while the static bridge deformation serves as an independent evaluative indicator which is excluded from the chord-measured deviation (CMD) of track irregularities. On this basis, using a 300-m main span bridge as a case study, extensive simulations were conducted with the developed coupled dynamics model to ascertain the limit values of CMD. Results indicate that, for this long-span bridge, an 80-m and a 5-m composite chord configuration is recommended, with lateral and vertical limits set at 8 mm and 1.7 mm, and 9 mm and 1.8 mm, respectively. Additionally, the vertical limit for the 80-m CMD should be reduced to 7 mm when bridge deformation exceeds 60 mm downward or 80 mm upward. This study provides a scientific basis for static acceptance standards of high-speed ballastless track on long-span bridges.