Parametric Analysis of Stability Bearing Capacity of Steel Tube Concrete Truss Arch Bridges

Literature Overview

The paper by Xie Weiwei, Yang Lufeng, Wang Jianjun, Zheng Jian, and Tan Qiuhong (2018), published in Journal of China and Foreign Highways, presents a systematic parametric analysis of the stability bearing capacity of steel tube concrete (CFST) truss arch bridges. The authors developed an efficient linear elastic iteration method based on the homogeneous generalized yield function of CFST members and an elastic modulus adjustment strategy. This method was validated against model test results and then used to investigate the influence of geometric and material parameters on the stability capacity. The study is particularly relevant to the design of large-span CFST truss arch bridges, where stability is often the governing design criterion.

Core Technical Points

The linear elastic iteration method developed in this study offers a computationally efficient alternative to full nonlinear finite element analysis for stability assessment. The method iteratively adjusts the elastic modulus of CFST members based on their stress state, approaching the nonlinear response through successive elastic analyses. This approach significantly reduces computational cost while maintaining acceptable accuracy for stability analysis.

The rise-to-span ratio emerges as the most influential geometric parameter. A higher rise-to-span ratio increases the stability capacity by reducing the bending moment in the arch ribs and improving the overall structural efficiency. However, increasing the rise also increases the structural weight and foundation requirements, so an optimal range of 0.20 to 0.25 is recommended.

The chord-to-web stiffness ratio is another critical parameter. In a CFST truss arch, the chord members (arch ribs) and web members (diagonals and horizontal members) must have properly matched stiffness to ensure efficient load distribution. If the chord stiffness is too high relative to the web stiffness, the web members may buckle prematurely. Conversely, if the web stiffness is too high, the chords may experience excessive bending. The study recommends a systematic approach to stiffness matching based on the specific geometry and loading conditions.

The material strength parameters (steel and concrete) have a moderate to minor influence on stability capacity. This is because stability is primarily governed by geometric properties and the structural system rather than material strength alone. However, higher material strengths can increase the ultimate load before material yielding, which indirectly affects the stability capacity through second-order effects.

Process and Standards Analysis

The fabrication of CFST truss arch bridges involves complex welding operations for both the steel tube members and the connection nodes. The stability of the structure depends critically on the quality of these welds, as any weld defects can reduce the effective stiffness and trigger premature buckling.

For the steel tube fabrication of truss arch members:

• Seamless tubes should comply with GB/T 8163 (for structural applications) or ASTM A53 Grade B, with yield strength of at least 245 MPa.
• HFW welded tubes should comply with SY/T 5037 or ASTM A53, with longitudinal welds inspected by eddy current testing (ECT).
• LSAW welded tubes should comply with GB/T 9711 or API 5L, with longitudinal welds inspected by ultrasonic testing (UT).
• All tubes should undergo hydrostatic testing per GB/T 24160 at 1.5 times the maximum allowable stress.

For the connection nodes of the truss arch:

• Butt-weld connections should achieve full penetration with a reinforcement height of 1.5-3.0 mm per side.
• Fillet welds should have a leg length of at least 6 mm for members with thickness up to 16 mm.
• All welds should be inspected by visual testing (VT) and magnetic particle testing (MT) per ISO 17637 and ISO 17638.
• Critical welds at the arch crown and springing should undergo ultrasonic testing (UT) per ISO 17640.

The concrete filling process for truss arch members requires special attention:

• Concrete should be placed from the bottom up to ensure complete filling without voids.
• Vibrators should be used with a frequency of 50-100 Hz and amplitude of 0.5-1.0 mm.
• Concrete strength should be verified by cube testing at 7 and 28 days per GB/T 50081.
• For long members, concrete should be placed in segments with overlap zones to prevent cold joints.

Integration with Engineering Practice

CFST truss arch bridges are increasingly used for medium to large spans (50-200 m) due to their high efficiency, aesthetic appeal, and constructability. The stability analysis presented in this study has direct implications for the design of such bridges.

Practical design considerations include:

1. The rise-to-span ratio should be selected within the recommended range of 0.20 to 0.25, considering both stability and economic factors. A rise-to-span ratio of 0.22 is often a good compromise for most applications.
2. The steel tube grade should be selected based on the required strength and stiffness, with Q345 being the most common choice for economic reasons. Higher grades (Q390, Q420) may be justified for long spans or heavy loading conditions.
3. The steel ratio should be optimized to balance stability capacity and cost. A steel ratio of 0.08-0.12 is typical for truss arch applications.
4. The chord-to-web stiffness ratio should be carefully designed to ensure proper load distribution. A systematic parametric study should be conducted during the preliminary design phase to determine the optimal stiffness ratio.
5. Second-order effects (P-delta effects) should be included in the stability analysis, as they can significantly reduce the stability capacity for slender truss arches.

Key Questions and Reflections

The linear elastic iteration method developed in this study is an efficient tool for stability analysis, but its applicability has limitations. The method assumes that the material response can be adequately represented by an equivalent elastic modulus, which may not capture the full nonlinear behavior of CFST members under combined axial and bending loads. For highly nonlinear problems, such as those involving local buckling of the steel tube or concrete crushing, a full nonlinear finite element analysis may be necessary.

The study focuses on geometric and material parameters but does not address the influence of initial imperfections on stability capacity. In practice, fabrication tolerances, weld defects, and construction deviations introduce initial imperfections that can significantly reduce the stability capacity. The sensitivity of the stability capacity to initial imperfections should be investigated further.

From a welding perspective, the quality of the longitudinal welds in the steel tubes directly affects the stability capacity. Any reduction in weld integrity (such as lack of fusion, porosity, or cracks) reduces the effective cross-sectional area and stiffness, which can trigger premature buckling. A comprehensive weld quality control program is essential for ensuring the stability capacity predicted by the analysis.

Study Insights and Implications

This research provides valuable parametric guidance for the design of CFST truss arch bridges. The identification of the rise-to-span ratio and chord-to-web stiffness ratio as the most influential parameters offers practical design recommendations. The linear elastic iteration method is a useful tool for preliminary stability assessment, but engineers should be aware of its limitations and use full nonlinear analysis for critical designs. The study also highlights the importance of stiffness matching in truss structures, which is often overlooked in practice. Engineers should incorporate these findings into their design process to ensure safe and efficient CFST truss arch bridges.