Structural Response of Cable Failure in Concrete-Filled Steel Tube Arch Bridges Based on Real Bridge Monitoring

Overview of the Study

This research investigates the structural response of concrete-filled steel tube (CFTST) arch bridges when the tie cables (hangers) experience fracture, using data from real bridge monitoring systems. The study is significant because tie cable failure represents one of the most critical failure modes in tied-arch bridge configurations, where the tie cable acts as the horizontal thrust collector for the arch ribs. When a cable fractures, the entire load path redistributes, potentially leading to catastrophic collapse if the remaining structural components cannot accommodate the redistributed forces.

Key Technical Points on Steel Tube and Welding Aspects

From the perspective of steel pipe and welding engineering, several critical aspects deserve attention. The arch ribs in these bridges are typically fabricated from high-strength steel tubes, often in the range of Q345 to Q460 grade according to Chinese standards, or equivalent ASTM A500 Grade C/D and ASTM A572 Grade 50/65 per American standards. The concrete-filled configuration provides composite action where the steel tube confines the concrete core, enhancing ductility and post-yield deformation capacity.

The welding joints at the arch rib splices are critical stress concentration points. Longitudinal submerged arc welding (LSAW) or submerged arc welding (SAW) processes are commonly employed for the fabrication of arch rib segments. The heat-affected zone (HAZ) microstructure must be carefully controlled to avoid brittle transformation, particularly in the lower-shelf temperature range where impact toughness degrades. For cables under high tensile stress, any welding repair must follow strict qualification procedures per NB/T 47014 or ISO 9606-1.

Structural Response Analysis

The monitoring data reveals that upon tie cable fracture, the arch ribs experience significant additional bending moments and axial force redistribution. The concrete-filled steel tube configuration provides a degree of damage tolerance because the composite section maintains load-bearing capacity even when the outer steel tube yields locally. However, the weld connections at the arch springings and the cable anchorage zones become the governing failure locations.

Defect Analysis and Failure Modes

• Weld crack initiation at the anchorage plate weld toes due to cyclic loading combined with the sudden load redistribution
• Steel tube local buckling in regions of high compressive stress concentration
• Concrete cover spalling at the arch rib crown due to increased bending moment
• Cable anchorage block shear failure when the remaining cables are overloaded beyond design capacity

The progressive damage sequence observed in the monitoring data shows that initial cable fracture triggers a chain reaction where adjacent cables experience increased stress, potentially leading to sequential failures. This progressive failure mechanism is directly influenced by the structural integrity of the weld connections and the ductility of the steel tube arch ribs.

Engineering Practice Implications

For engineers involved in the design and maintenance of CFTST arch bridges, this study underscores the importance of weld quality assurance at critical connections. The weld qualification procedures must account for the specific loading conditions that occur during cable failure scenarios. Non-destructive testing (NDT) programs should include phased array ultrasonic testing (PAUT) and time-of-flight diffraction (TOFD) at all welded joints in the arch rib and cable anchorage zones, given the criticality of these locations.

Study Insights and Conclusions

The real bridge monitoring approach provides invaluable data that validates analytical models and highlights the importance of considering damage scenarios in the design phase. The steel tube material properties, welding quality, and composite action between steel and concrete all play integral roles in determining the bridge's damage tolerance. Engineers should advocate for higher weld quality acceptance criteria and more comprehensive NDT coverage at cable anchorage connections, recognizing that these are the first links in the progressive failure chain.