Progressive Damage Test of Hangers in Existing Retired Concrete-Filled Steel Tube Arch Bridges

Overview of the Study

This research conducts progressive damage testing on hangers (suspension cables) of existing retired concrete-filled steel tube (CFTST) arch bridges. The study provides critical insight into the damage tolerance and residual load-carrying capacity of these bridges when individual hangers are sequentially damaged or removed. For engineers involved in bridge assessment, maintenance, and rehabilitation, understanding the progressive failure mechanism is essential for developing safe intervention strategies and determining the remaining service life of existing structures.

Steel Tube and Cable Material Characteristics

The hangers in CFTST arch bridges are typically composed of high-strength steel strands or parallel wire cables, while the arch ribs are fabricated from steel tubes filled with concrete. The material properties of these components directly influence the damage propagation behavior observed during progressive testing.

The steel tube arch ribs undergo significant stress redistribution when hangers are removed. The weld joints at the arch rib splices and the cable anchorage connections become critical locations for stress concentration and potential failure initiation. The concrete fill provides additional stiffness and ductility to the arch rib, delaying the onset of local buckling in the steel tube.

Progressive Damage Sequence and Structural Response

The progressive damage test involves sequentially cutting or removing individual hangers, starting from the most critical location (typically near the arch crown or springing) and progressing outward. The structural response at each damage stage is monitored through strain gauges, displacement sensors, and load cells.

Damage Stage Analysis

The data reveals that the bridge can tolerate the loss of 1-2 hangers with minimal structural degradation, but the loss of 3 or more hangers in sequence leads to progressive stress escalation that approaches the yield point of the arch rib steel. This progressive failure mechanism is directly related to the structural redundancy provided by the multiple hanger system.

Weld and Connection Integrity Under Progressive Damage

From a welding and steel pipe engineering perspective, the progressive damage test reveals several critical observations about the integrity of welded connections:

• Anchorage weld integrity: The welds at the hanger anchorage points experience cyclic stress concentration as adjacent hangers are removed. Microcracks may initiate at the weld toe even before visible structural failure.
• Arch rib splice welds: The longitudinal splice welds in the arch rib experience increased bending stress as hangers are removed. The HAZ, being the weakest region of the weld joint, is the most susceptible to crack initiation.
• Tube-to-anchor plate welds: These full-penetration butt welds must maintain full cross-sectional integrity under the redistributed loads. Any lack of fusion or porosity in these welds represents a critical defect.
• Residual stress effects: The welding residual stresses in the arch rib splices interact with the operational stresses, potentially accelerating crack initiation under cyclic loading conditions.

NDT Requirements for Progressive Damage Assessment

Failure Mechanism and Damage Propagation

The ultimate failure of the bridge under progressive hanger damage typically follows a predictable sequence:

1. Initial damage: First hanger removal creates stress redistribution, with adjacent hangers experiencing 10-15% stress increase
2. Progressive stress escalation: Each subsequent hanger removal increases the stress in remaining hangers by 15-25%, approaching the ultimate tensile strength of the steel strands
3. Arch rib yielding: When the cumulative stress redistribution exceeds the arch rib capacity, the steel tube begins to yield locally, typically at the crown or springing regions
4. Concrete spalling: As the arch rib deforms, the concrete fill experiences tensile stresses that cause spalling, reducing the composite section properties
5. Weld crack initiation: Stress concentrations at weld toes in the arch rib splices initiate microcracks that propagate under continued loading
6. Catastrophic failure: When the arch rib loses sufficient cross-sectional capacity, the bridge collapses

The role of the concrete fill in delaying failure is significant. The confined concrete maintains composite action even after the steel tube has locally yielded, providing additional load-carrying capacity. However, once the concrete spalls, the loss of confinement leads to rapid degradation of the steel tube's buckling resistance.

Engineering Practice and Rehabilitation Implications

For engineers assessing and rehabilitating existing CFTST arch bridges, this progressive damage study provides the following guidance:

1. Hanger inspection protocol: All hangers should be inspected annually using visual examination, ultrasonic testing for internal corrosion, and pull-out tests for anchorage integrity
2. Redundancy assessment: Bridges with fewer than 8 hangers should be classified as having limited redundancy and require more frequent inspection intervals
3. Weld repair criteria: Any detected weld cracks in arch rib splices must be repaired using qualified hot-tap welding procedures with full NDT verification
4. Hanger replacement strategy: When hanger replacement is necessary, adjacent hangers should be temporarily reinforced to maintain structural continuity during the replacement operation
5. Load rating under damage: The bridge's load rating should be updated after any hanger damage event, with the rating reduced according to the number of damaged hangers

Study Insights and Conclusions

This progressive damage study provides invaluable data on the damage tolerance of CFTST arch bridges, confirming that these structures possess meaningful redundancy but also identifying the critical damage thresholds beyond which structural integrity is compromised. From a steel pipe and welding engineering standpoint, the key finding is that weld integrity at the arch rib splices and cable anchorages is the governing factor in determining the bridge's damage tolerance capacity. The concrete fill provides significant ductility enhancement, but once the steel tube yields and the concrete spalls, the structure enters a rapidly degrading phase. Engineers must therefore prioritize weld quality assurance during fabrication and implement comprehensive NDT programs during the service life to ensure that weld defects do not compromise the bridge's damage tolerance. The progressive failure mechanism observed in this study should be incorporated into bridge management systems to guide maintenance prioritization and rehabilitation planning.