Stability Analysis of Steel-Concrete Composite Tied-Arch Bridge with 254m Span

Literature Overview and Scope

This paper by Huang Yun, Zhang Qinghua, Ye Huawen, and Cui Chuang from Southwest Jiaotong University, published in Bridge Construction (2014, Vol. 44, No. 4, pp. 50-56), presents a three-dimensional nonlinear finite element analysis of a 254-meter-span lower-deck steel tube concrete (CFST) tied-arch bridge. The study employs ANSYS to simulate both construction and operational phases, utilizing a fiber element model that accounts for the confining effect of the steel tube through radial stress gradient considerations on the core concrete. The authors focus on stability assessment during the critical concrete casting stage of the arch rib chord tubes and the final operational phase. This work is highly relevant to pipe engineering practitioners because the arch rib chords are essentially large-diameter steel tubes whose structural integrity, weld quality, and geometric precision directly govern the overall stability of the bridge system.

Core Technical Viewpoints and Key Findings

The central conclusion of this study is that the tied-arch bridge possesses a high stability safety margin during both construction and operational phases. However, the authors identify material nonlinearity as the most significant factor influencing structural stability, far more than geometric imperfections, live load effects, or the asynchronism of concrete casting in the twin arch rib tubes. This finding carries profound implications for steel pipe manufacturing: if material nonlinearity dominates stability behavior, then the mechanical properties of the steel tube material, particularly its yield strength uniformity, strain hardening behavior, and the quality of weld joints within the arch rib chords, become critical design parameters. A weld defect or a localized material degradation in the arch chord tube could trigger nonlinear material behavior at lower loads than anticipated, potentially compromising the stability margin.

The use of a fiber element model incorporating the confining effect represents a sophisticated approach to capturing the composite action between the steel tube and the enclosed concrete. From a pipe engineering perspective, this confining effect depends critically on the uniformity of the steel tube wall thickness, the geometric regularity of the tube cross-section, and the quality of the longitudinal and circumferential welds. Any ovality, wall thickness variation, or weld-induced residual stress distribution in the arch rib tubes would alter the radial stress gradient and thereby modify the confining pressure on the core concrete.

Steel Pipe and Welding Implications

The arch rib chords of a 254-meter tied-arch bridge typically employ large-diameter steel tubes, often in the range of 600 to 1200 mm in diameter with wall thicknesses of 12 to 25 mm, fabricated either as seamless tubes or as longitudinal submerged-arc welded (LSAW) pipes conforming to standards such as GB/T 9711 or API 5L. The concrete casting process into these tubes introduces complex thermal and mechanical interactions at the steel-concrete interface.

The asynchronism of concrete casting in the twin arch rib tubes, while reported to have minimal stability impact in this study, raises practical concerns about differential thermal effects during casting. When concrete is placed into one tube while the adjacent tube remains unfilled, the filled tube experiences thermal expansion and self-weight loading that the unfilled tube does not. This creates asymmetric loading on the bridge's structural system. For the steel pipe manufacturer, this means that the tubes must have sufficient geometric tolerance to accommodate these differential deformations without inducing local buckling or excessive weld stress.

Material Nonlinearity and Weld Metallurgy

The finding that material nonlinearity significantly affects stability warrants detailed discussion from a welding metallurgy standpoint. In the heat-affected zone (HAZ) of LSAW welds, the microstructure can vary from fine-grained regions near the fusion line to coarser-grained regions in the thermally affected but un-melted base metal. This microstructural gradient creates a corresponding gradient in yield strength, ductility, and strain-hardening characteristics. Under the compressive and bending loads experienced during concrete casting and operational phases, these HAZ regions may reach their yield point before the base metal, initiating nonlinear material behavior at lower global loads.

The welding process parameters must therefore be carefully controlled to minimize the extent of the coarse-grained HAZ. For LSAW welding of thick-walled arch rib tubes, preheating temperatures of 80-120 degrees Celsius are typically specified for carbon equivalent values above 0.45, with interpass temperatures maintained at 100-150 degrees Celsius to limit hydrogen-induced cracking and excessive grain growth. Post-weld heat treatment may be required for tubes exceeding 40 mm wall thickness to relieve residual stresses and homogenize the HAZ microstructure.

Engineering Practice and Quality Assurance

In engineering practice, the stability analysis findings should be translated into a comprehensive quality assurance program for the steel pipe supply chain. The following inspection and testing regime is recommended for arch rib tubes in large-span CFST bridges:

1. Raw material verification: mill certificates for chemical composition, mechanical properties, and non-metallic inclusions (per ASTM A262 or GB/T 1978)
2. Geometric inspection: 100% dimensional survey including diameter, ovality, wall thickness at multiple stations, and straightness
3. Weld inspection: 100% radiographic testing of longitudinal and circumferential welds, supplemented by ultrasonic testing for detection of laminar and volumetric defects
4. Surface inspection: magnetic particle or penetrant testing of all weld surfaces and heat-affected zones
5. Hydrostatic testing: all tubes subjected to hydrostatic pressure testing at 1.5 times the design pressure, with leak detection
6. Mechanical property verification: coupon testing from heat-matched material, with Charpy V-notch impact testing at service temperature
7. Residual stress measurement: ultrasonic or hole-drilling method on representative tubes to verify stress levels after welding and any post-weld treatment

Key Questions and Reflections

The paper raises several questions that deserve further investigation from the pipe engineering perspective. First, the fiber element model assumes a uniform confining effect around the tube circumference, but in reality, the concrete casting process creates a vertical gradient in confining pressure, with the lower portion of the tube experiencing higher pressure due to the hydrostatic head of the wet concrete. Second, the study does not appear to address the effect of weld-induced residual stresses on the buckling behavior of the arch rib tubes. Longitudinal residual stresses from LSAW welding can reduce the effective cross-sectional area and alter the buckling mode shape. Third, the stability analysis focuses on global buckling but does not explicitly consider local buckling of the tube wall, which could be triggered by the radial pressure from the concrete casting process, particularly at locations of geometric imperfection or weld defects.

Study Insights and Implications for Pipe Engineering

This study reinforces the critical importance of steel pipe quality in large-span CFST bridge structures. The dominance of material nonlinearity in stability behavior means that the steel pipe manufacturer must ensure exceptional uniformity of material properties throughout the tube length. Variations in yield strength, elongation, and strain-hardening behavior along the tube can create weak links that initiate nonlinear behavior prematurely. From a welding standpoint, the HAZ must be carefully managed through appropriate welding procedure specifications, including controlled heat input, proper preheating, and post-weld treatment where necessary. The practical implication is that the steel pipe supply chain for CFST bridge arch ribs must be managed with the same rigor as pipelines for critical service applications, with full traceability, comprehensive non-destructive testing, and mechanical property verification.

The confining effect analysis in this paper also highlights the importance of tube geometric regularity. Any deviation from the nominal circular cross-section, whether due to rolling tolerance, welding distortion, or transport damage, will alter the confining pressure distribution and potentially reduce the composite action between steel and concrete. Pipe manufacturers must implement strict geometric tolerance control, including in-process monitoring of tube diameter and ovality during the welding and expansion operations, and final dimensional survey before shipment.