Structural System Design of Large-Span Double-Continuous Steel Tube Concrete Arch Bridges

Overview and Engineering Context

The study by Zhou Qian, Feng Pengcheng, Hu Lifei, Zhou Wuzhao, and Xu Jiantao (2024) focuses on the structural system design of a large-span double-continuous steel tube concrete (STC) arch bridge, specifically the Shuangbao Special Bridge on the Chongqing-Yuxiang Expressway with a main span of 2 × 405 m. Using MIDAS Civil finite element modelling, the authors evaluated the structural response under self-weight, traffic loading, and foundation settlement for various constraint configurations at the central pier and different central pier stiffness ratios. The investigation culminated in a recommended structural scheme with quantified justification.

This work carries significant relevance to pipe engineering because the arch ribs are constructed from large-diameter steel tubes filled with concrete, typically in the range of 1500–2000 mm outer diameter with wall thicknesses of 20–35 mm. The fabrication, welding, and quality control of these large-diameter tubes directly influence the structural performance and long-term durability of the bridge.

Structural System Comparison and Findings

The authors compared four structural schemes differing in the connection type at the central pier: fully rigid (double rigid), fully hinged (double hinged), and two semi-rigid variants. The analysis considered three load cases: self-weight, moving vehicle loads, and differential foundation settlement.

The double rigid connection scheme exhibited symmetric arch rib deformation under self-weight, favourable force distribution, and the simplest construction approach. The central pier stiffness sensitivity analysis revealed that increasing the pier stiffness from 0.8EI to 2.0EI produced negligible improvements in mechanical performance: maximum tensile stress decreased by only 0.33% and maximum compressive stress by 0.47%. This led to the recommendation of adopting the double rigid connection with a central pier stiffness of 0.8EI, which satisfied all strength, stiffness, and stability requirements per applicable codes.

Implications for Steel Tube Fabrication and Welding

The arch ribs of large-span STC bridges are typically fabricated as large-diameter longitudinal submerged-arc welded (LSAW) pipes or as plate-fabricated tubes with multiple longitudinal and circumferential welds. For a tube of approximately 1800 mm diameter and 25 mm wall thickness, the manufacturing process involves plate cutting, edge preparation (beveling), longitudinal SAW welding, and circumferential SAW or FCAW welding. The welding procedure specification (WPS) must account for the thick section thickness, which introduces significant cooling rate effects and potential for martensitic transformation in the heat-affected zone (HAZ).

Post-weld heat treatment (PWHT) is often required for thick-walled tubes to relieve residual stresses and prevent hydrogen-induced cracking. The residual stress distribution, particularly the circumferential tensile stress in the HAZ, can interact with the compressive hoop stress induced by concrete filling, potentially reducing the effective confinement pressure. Quality control should include full-penetration ultrasonic testing (UT) in accordance with EN 12668 or ASME V, with acceptance criteria aligned to the applicable bridge code.

Concrete filling of large-diameter arch tubes is a critical construction phase. The filling process must ensure complete, void-free concrete placement, which is challenging for horizontal or near-horizontal tube segments. Vibration-assisted pumping, segment-by-segment filling with controlled flow rates, and periodic inspection through access holes are standard practices. The bond between the steel tube inner surface and the concrete is enhanced by surface treatment such as shot-blasting or the use of shear studs welded to the tube interior.

Design Recommendations and Practical Considerations

The finding that central pier stiffness beyond 0.8EI provides negligible benefit is practically significant. It suggests that over-designing the central pier section is wasteful and may introduce unnecessary construction complexity. Engineers should focus instead on ensuring the integrity of the rigid connection details, which include the moment-resisting welds or bolted connections between the arch rib and the pier cap. These connections are subject to combined axial, shear, and moment loads, requiring careful detailing and rigorous inspection.

From a welding perspective, the rigid connection at the central pier often involves butt welds between the arch rib tube end and a thick steel plate or transition piece. The weld geometry, groove preparation, and welding sequence must be optimised to minimise angular distortion, which could lead to misalignment of the arch rib axis and subsequent eccentric loading. Pre-weld fit-up inspection, welder qualification testing, and post-weld dimensional checks are essential quality control measures.

The study also implicitly highlights the importance of foundation settlement analysis. Differential settlement induces secondary moments in the arch ribs, which are superimposed on the primary bending moments from self-weight and traffic. For long-span arch bridges, even small settlement values can produce significant additional stresses. The recommended 0.8EI pier stiffness provides sufficient flexibility to accommodate moderate settlement without generating excessive secondary forces, making the structural system robust against foundation uncertainties.

This research demonstrates that rational structural system selection, supported by comprehensive finite element analysis, can lead to simpler, more economical, and more constructable bridge designs without compromising safety or performance. The emphasis on quantified sensitivity analysis rather than qualitative assessment is a methodological strength that should be adopted in similar large-span STC bridge projects.