Eccentric Compressive Mechanical Properties of Bolted-Welded Joints for Large-Span Steel Tube Concrete Arch Bridges

Literature Overview

The research by Yu Peng, Yun Weijing, Zheng Jialian, and colleagues, published in the China Civil Engineering Journal in 2023, introduces and investigates a novel bolted-welded joint connection for large-span steel tube concrete (STC) arch bridge ribs. This study was supported by the Guangxi Innovation-Driven Major Special Project, the Guangxi Key Research and Development Program, and the Guangxi University Overseas High-Level Talent Recruitment Program. The work addresses a pressing practical problem: as arch bridge spans and construction elevations increase, the welding of arch rib joints becomes increasingly difficult, and traditional welding-only connections cannot simultaneously satisfy safety and economic requirements.

Core Technical Points

The proposed bolted-welded joint combines external flange bolted connections with steel tube welding. This hybrid connection approach offers several distinct advantages over traditional all-welded connections. First, the bolted flange connection provides immediate load transfer capacity upon assembly, which significantly improves construction safety during the erection phase. Second, the welding is performed after the bolted connection has established the joint geometry, which improves weld accessibility and reduces distortion. Third, and perhaps most importantly, this approach eliminates the need to cut the flange plates at the joint, which is a common practice in traditional connections. Cutting flange plates at joints causes stress redistribution in the arch rib, which can lead to unexpected stress concentrations and potential structural issues.

The experimental program included scaled model tests of STC column joints under eccentric compression with different connection types and eccentricity ratios. The tests examined failure modes, load-displacement curves, cross-sectional strain distributions, and lateral deflections. Based on the test results, a finite element model was developed and validated, which was then used to investigate the influence of steel tube strength, core concrete strength, steel ratio, and eccentricity ratio on the mechanical performance of the bolted-welded joint.

Failure Mode and Load-Displacement Characteristics

The bolted-welded connection significantly improves joint stiffness and stability, effectively preventing bulging deformation at the joint region. Under small eccentricity conditions, the STC bolted-welded joint demonstrates superior load-bearing capacity compared to the welded-only joint, which is consistent with the small eccentric compression design philosophy of arch bridge ribs. This finding is particularly significant because arch bridge ribs typically operate under small eccentric compression, where the axial force dominates and the bending moment is relatively small.

The load-displacement curves of the bolted-welded joints exhibit a more linear elastic response compared to welded-only joints, indicating that the bolted connection provides additional stiffness that delays the onset of plastic deformation. The cross-sectional strain distribution reveals a more uniform stress state in the bolted-welded joints, as the bolted connection helps distribute the load more evenly across the joint interface. The lateral deflection of the bolted-welded joints is also reduced, which is beneficial for maintaining the geometric alignment of the arch rib during construction and service.

Parametric Study Insights

The finite element parametric study reveals several important trends. The steel tube strength has a direct influence on the joint load-bearing capacity, with higher steel grades providing greater capacity but potentially reduced ductility. The core concrete strength also positively affects the joint capacity, as the concrete contributes to the compressive resistance through both direct bearing and confinement of the steel tube. The steel ratio, defined as the ratio of steel cross-sectional area to total cross-sectional area, is a critical design parameter that balances the cost-effectiveness of the steel tube with the structural performance of the joint.

The eccentricity ratio, defined as the ratio of the eccentricity distance to the section depth, is the most influential parameter on the joint behavior. As the eccentricity ratio increases, the joint transitions from compression-dominated to bending-dominated behavior, and the load-bearing capacity decreases significantly. For the bolted-welded joints, the transition from small to large eccentricity occurs at a higher eccentricity ratio compared to welded-only joints, indicating that the bolted connection provides additional resistance to bending-induced deformation.

Welding and Assembly Process Considerations

From a welding engineering perspective, the bolted-welded joint concept offers several process advantages. The bolted flange connection establishes the joint geometry before welding, which reduces the risk of weld distortion and misalignment. This is particularly important for large-diameter steel tubes used in arch bridge ribs, where the weld length is substantial and the thermal distortion can be significant. The welding sequence should be carefully planned to minimize residual stresses, with symmetric welding patterns used to balance the thermal input on opposite sides of the joint.

The weld between the steel tube and the flange plate is a fillet weld or a partial-penetration groove weld, depending on the design requirements. The weld between the two steel tube halves is a full-penetration butt weld. The bolted connection provides a mechanical stop that prevents relative movement during welding, which improves weld quality and reduces the need for post-weld straightening. However, the presence of the flange plate and bolts near the weld zone may complicate the welding access, and careful planning of the welding sequence is necessary to ensure full weld coverage.

Integration with Engineering Practice

In the context of large-span arch bridge construction, the bolted-welded joint concept addresses several practical challenges. The construction of arch bridge ribs at high elevations requires rapid assembly and reliable load transfer during the erection phase. The bolted connection provides immediate structural integrity, allowing the arch rib to bear its own weight and construction loads before the welding is completed. This significantly reduces construction time and improves construction safety, which is critical for projects in remote or high-altitude locations.

The elimination of flange plate cutting at joints is a significant practical advantage. In traditional all-welded connections, the flange plates must be cut to allow the steel tubes to be brought together for welding. This cutting process creates stress concentrations and requires careful machining to ensure fit-up accuracy. The bolted-welded joint avoids this issue entirely, as the flange plates remain intact and the bolts provide the initial connection. This approach also simplifies the fabrication process, as the flange plates can be manufactured separately from the steel tubes and assembled in the field.

For quality control, the bolted-welded joint requires inspection of both the bolted connection and the welded connection. The bolted connection should be inspected for proper torque values and bolt hole alignment. The welded connection should be inspected by ultrasonic testing (UT) or radiographic testing (RT) to detect internal defects. The interface between the bolted flange and the steel tube should be inspected for full contact and absence of gaps, which can be verified by dye penetrant testing (PT) or magnetic particle testing (MT).

Key Questions and Reflections

Several important considerations arise from this study. First, the long-term performance of the bolted-welded joint under fatigue loading is a critical concern. The bolted connection may experience cyclic slip at the flange interface, which could lead to bolt loosening and joint degradation over time. Second, the thermal compatibility of the bolted and welded connections must be considered, as the different materials and connection types may have different thermal expansion behaviors under temperature variations. Third, the seismic performance of the bolted-welded joint requires investigation, as the bolted connection may behave differently from a welded connection under cyclic lateral loading.

From a design code perspective, the bolted-welded joint concept requires the development of specific design provisions. Current codes primarily address all-welded or all-bolted connections, and the hybrid bolted-welded connection falls into a regulatory gap. The design provisions should address the load-sharing mechanism between the bolted and welded connections, the interaction effects between the two connection types, and the appropriate safety factors for each connection component.

Study Insights and Implications

This study presents a practical and innovative solution to the challenges of arch bridge rib joint construction. The bolted-welded joint concept is well-suited for large-span arch bridges where construction efficiency and safety are paramount. The experimental and numerical results demonstrate that the bolted-welded joint provides superior stiffness and stability compared to traditional welded-only joints, particularly under small eccentric compression conditions that are typical of arch bridge ribs.

For practicing engineers, the key takeaway is that the bolted-welded joint concept should be considered for large-span arch bridge projects, particularly those with challenging construction conditions. The design of the bolted-welded joint requires careful consideration of the load-sharing mechanism, the welding sequence, and the quality control protocols. Future research should focus on the fatigue and seismic performance of the bolted-welded joint, as well as the development of design code provisions that specifically address this hybrid connection type. The successful application of this concept in actual bridge projects would provide valuable practical data to validate and refine the theoretical models presented in this study.