Impact Coefficient of Railway CFST Tied-Arch Bridge Under Vehicle-Bridge Coupled Vibration

Literature Overview

The paper by Lin Pengzhen and Wang Yaping, published in Vibration and Shock (Vol. 40, No. 6, 2021), investigates the impact coefficient of a 136-meter span concrete-filled steel tubular (CFST) tied-arch bridge on a Chinese high-speed railway line. The study employs a vehicle-bridge coupled vibration analysis method and considers pile-soil interaction effects. Using probabilistic hypothesis testing, the authors analyzed two vehicle weights, 30 sets of track irregularities, and 126 speed combinations to evaluate impact coefficients on the tie beam, hanger rods, and arch ribs. The results indicate that the arch rib experiences the highest impact coefficient, followed by the tie beam, with hangers exhibiting the lowest values. Under a 95% guarantee rate, the impact coefficients for the tie beam, hanger, and arch rib are approximately 1.05, 1.04, and 1.08 respectively, all exceeding current code values.

Core Technical Findings and Implications for Steel Pipe Engineering

From a steel pipe manufacturing and structural integrity standpoint, the finding that impact coefficients exceed code values is of significant concern. In CFST arch bridges, the arch ribs are typically fabricated from large-diameter steel tubes, often in the range of 1000 mm to 1500 mm in outer diameter with wall thicknesses of 20 mm to 40 mm, conforming to standards such as GB/T 14976 or SY/T 5037 for seamless and welded pipes respectively. The steel grade commonly employed is Q345 or Q420, with yield strengths of 345 MPa and 420 MPa respectively. When the impact coefficient reaches 1.08, the dynamic amplification directly translates into increased cyclic stress ranges on the pipe walls and their welded joints.

The following table summarizes the key impact coefficient data reported in the study:

The arch rib, being the primary load-bearing element, is the most critical component from a fatigue and fracture mechanics perspective. The arch rib is typically assembled from prefabricated steel tube segments joined by butt-welded connections. The welding process for such large-diameter thick-walled tubes commonly involves multi-pass submerged arc welding (SAW) or gas metal arc welding (GMAW) with flux-cored wire, followed by post-weld heat treatment (PWHT) to relieve residual stresses. The heat-affected zone (HAZ) of these welds, particularly for Q345 and Q420 grades, is susceptible to microstructural coarsening and potential softening under repeated dynamic loading.

Welding Quality and Residual Stress Considerations

The dynamic impact loading characterized by the elevated impact coefficients has direct implications for welding quality control. In CFST arch ribs, the longitudinal and circumferential welds are critical stress concentration sites. The residual stresses from welding, which can reach values approaching the yield strength of the base metal, interact with the dynamic stresses from train passage. This superposition effect can accelerate fatigue crack initiation, particularly at the weld toe or in the HAZ.

For engineering practice, the following quality control measures are recommended:

• Pre-weld inspection using magnetic particle testing (MT) or ultrasonic testing (UT) to ensure freedom from surface and subsurface defects.
• Post-weld PWHT at temperatures between 580°C and 620°C for a minimum of 1 hour per 25 mm of wall thickness to reduce residual stresses.
• Post-PWHT hardness survey of the HAZ to confirm hardness values remain below 350 HV, in accordance with ASME B31.3 Section 331.
• Full radiographic testing (RT) of butt welds, with acceptance criteria per GB/T 3323 or ISO 5817 Level B.

The study's finding that impact coefficients vary significantly among different structural components underscores the need for component-specific fatigue assessment rather than a uniform safety factor applied to the entire structure.

Study Insights and Engineering Implications

This research provides valuable quantitative data that should inform the design and fabrication specifications of CFST arch bridges on high-speed railways. From a pipe fabrication standpoint, the elevated dynamic amplification necessitates stricter welding quality standards and potentially higher material grades for arch rib segments. The probability distribution of impact coefficients following the Type I extreme value distribution suggests that rare but severe loading events are possible, which must be accounted for in the fatigue life assessment of welded connections. Engineers involved in the procurement and fabrication of steel tubes for such bridges should ensure that mill certificates confirm impact toughness values (Charpy V-notch at the specified service temperature) and that weld procedures are qualified for the anticipated cyclic stress ranges. The recommendation to apply an impact coefficient amplification factor in design is prudent and should be reflected in the fabrication quality assurance plan, particularly regarding weld defect acceptance criteria and non-destructive testing coverage.