Catenary CFT Single-Tube Arch Failure Test Research

Literature Overview

This paper by Liu Zengwu et al. (2023), published in the Journal of Rail and Transportation Engineering, presents a 1:16 scale model test on catenary steel tube concrete (CFT) arches, motivated by the Washiwo Bridge with a 95 m clear span. The study investigates failure modes under single-point loading at the crown and quarter-span positions, and extends parametric analysis using ANSYS beam elements to explore the influence of loading patterns, arch axis coefficients, rise-to-span ratios, and steel ratios on ultimate bearing capacity.

Core Technical Content and Key Findings

The test arches had a span of 5.938 m, a rise-to-span ratio of 1/3.5, and an arch axis coefficient of 1.5. Two loading conditions were examined: crown loading and quarter-span (L/4) loading. The results reveal that both loading conditions produce a four-hinge failure mode, which is characteristic of plastic mechanism formation in arch structures. Importantly, although large external deformations occurred at the loading points, no local buckling of the steel tube was observed, indicating that the confining effect of the steel tube on the core concrete effectively prevents local instability up to the ultimate load stage.

A critical observation is that the steel tube begins to confine the core concrete at approximately 55% of the ultimate load. This confinement activation occurs earlier than in parabolic arches, suggesting that the catenary geometry provides a more efficient load path that engages the composite action of the CFT section at a lower load level. The L/4 loading condition produced larger deformations and strains compared to crown loading, identifying quarter-span loading as the more critical design scenario.

Parametric Analysis Results

Implications for Steel Pipe Selection and Fabrication

From a steel pipe manufacturing and welding perspective, the findings have several practical implications. The absence of local buckling in the test arches confirms that the steel tube wall thickness and material grade selected for the 1:16 model were adequate to prevent local instability. In full-scale applications, this translates to requirements for adequate wall thickness-to-diameter ratios and appropriate material grades (typically Q345 or Q355 per GB/T 1591) for the arch rib steel tubes.

The four-hinge failure mode indicates that plastic deformation concentrates at discrete locations. For welded CFT arches, this means that weld quality at critical sections—particularly at the crown, quarter-span, and haunch regions—must be rigorously controlled. Any weld defects, such as incomplete fusion, lack of penetration, or weld metal cracks, could initiate premature failure at these critical plastic hinge locations.

The early activation of confinement at 55% of ultimate load in catenary arches compared to parabolic arches has implications for the design of the steel tube itself. The steel tube must be capable of withstanding significant compressive and hoop stresses during the confinement phase. For seamless steel tubes, this requires adequate tensile strength and elongation to accommodate the plastic deformation of the core concrete without premature rupture. For welded steel tubes, the weld zone must exhibit comparable ductility to the base metal.

Quality Control Considerations

Given the critical role of the steel tube in CFT arch structures, the following quality control measures are recommended:

1. Ultrasonic testing (UT) of the steel tube along its entire length to detect internal defects such as laminations, seams, and inclusions.
2. Hydrostatic testing per GB/T 241 to ensure weld integrity for welded pipes.
3. Visual and magnetic particle inspection (MT) of all welds in the arch rib assembly.
4. Mechanical property verification of the steel tube material, including yield strength, tensile strength, and elongation at fracture.

Study Insights and Reflections

This research contributes valuable experimental data to the CFT arch bearing capacity database, which remains relatively sparse in the literature. The finding that catenary arches engage confinement earlier than parabolic arches is particularly noteworthy for design engineers, as it suggests that catenary geometry may offer superior composite efficiency. However, the parametric analysis using beam elements has limitations in capturing local buckling and weld effects. Future work should incorporate shell or solid elements to more accurately represent the steel tube behavior and weld zones. From a manufacturing standpoint, the emphasis on preventing local buckling reinforces the importance of producing steel tubes with uniform wall thickness, consistent material properties, and defect-free welds throughout the entire length.