Shape Optimization Method for Steel Tube Concrete Arch Rib Hoisting Process

Literature Overview

This paper by Yuan Haiqing, Fan Jianfeng, and Fan Xiaochun from Wuhan University of Technology, published in the Journal of Wuhan University of Technology (Vol. 24, No. 2, 2002, pp. 32-35), addresses a critical construction engineering challenge: the geometric shape control of steel tube concrete (CFST) arch ribs during the hoisting process. The study is based on the Nanlidu Grand Bridge project, a landmark steel tube concrete arch bridge in China. The authors propose a two-stage arch rib shape optimization method that simulates the entire hoisting process, providing a practical framework for achieving the design line shape after erection.

Core Technical Approach

The fundamental challenge in CFST arch bridge construction is that the arch rib, being a steel tube structure, undergoes significant elastic deformation during the hoisting and erection process. The steel tube's high stiffness-to-weight ratio means that while it is efficient structurally, the temporary loading conditions during hoisting introduce deviations from the target geometry. The two-stage optimization method proposed in this paper breaks the problem into two distinct phases:

Stage One: Initial Line Shape Determination

The first stage involves calculating the pre-deformed shape that the arch rib segments must possess before hoisting, accounting for self-weight deformation under the temporary support conditions. Key considerations include:

• The elastic modulus and moment of inertia of the steel tube sections along the arch rib
• The span length, rise-span ratio, and segment joint locations
• The temporary support configuration and its stiffness contribution
• The sequential loading pattern as each segment is lifted and connected

Stage Two: Post-Erection Line Shape Adjustment

The second stage deals with the correction of any residual deviations after the arch rib is fully erected and the temporary supports are removed. This involves:

• Applying corrective forces or adjusting segment joints to bring the as-built geometry within tolerance
• Monitoring the convergence of the arch rib shape through iterative adjustments
• Validating the final geometry against the design line shape using surveying data

Process Analysis and Engineering Practice

From a steel pipe manufacturing and welding perspective, the shape optimization during hoisting has direct implications for the fabrication quality of the arch rib segments. The steel tube segments must be manufactured with high dimensional accuracy, and the weld joints between segments must maintain geometric continuity. Key observations from this study include:

1. Weld Joint Quality Impact on Shape: The residual stresses and distortions introduced during longitudinal and circumferential welding of the steel tube segments directly affect the initial line shape. Longitudinal submerged-arc welding (LSAW) or UOE forming processes must be controlled to minimize angular distortion, which can accumulate over multiple segments.
2. Segment Joint Design: The bolted or welded joints between segments act as semi-rigid connections during the hoisting process. The rotational stiffness of these joints influences the deformation pattern and must be accurately modeled in the optimization analysis.
3. Temporary Support Interaction: The interaction between the arch rib and temporary supports (such as cable-stayed temporary towers or falsework) introduces complex boundary conditions. The stiffness of these supports should be calibrated through load testing before the optimization calculations are finalized.
4. Temperature Effects: During hoisting, temperature variations cause thermal expansion and contraction of the steel tube, which can introduce additional geometric deviations. The optimization method should account for temperature-induced deformation, particularly for long-span bridges where the cumulative effect can be significant.

Key Technical Insights

The two-stage optimization method represents a systematic approach that bridges theoretical analysis and construction practice. The first stage is essentially a pre-deformation calculation problem, while the second stage is a real-time adjustment problem that requires close monitoring and feedback control. The study demonstrates that without proper shape optimization, the as-built arch rib may deviate from the design line shape by amounts that compromise structural performance, particularly in terms of the internal force distribution and the subsequent concrete filling operation.

The practical significance of this work extends beyond the specific bridge project. For any steel tube concrete arch bridge construction, the shape optimization during hoisting should be treated as an integral part of the construction engineering design, not as an afterthought. The steel pipe fabrication shop must be involved early in the optimization process to ensure that the manufacturing tolerances are compatible with the required pre-deformed shapes.

Implications for Steel Pipe Manufacturing and Welding

For steel pipe manufacturers supplying arch rib segments, the following quality control measures are recommended:

• Dimensional tolerance control: The outer diameter, wall thickness, and length of each segment must be controlled within tight tolerances to ensure that the pre-deformed shape calculations are accurate.
• Weld distortion control: Post-weld straightening or correction processes should be applied to minimize angular and longitudinal distortions that would affect the arch rib line shape.
• Segment identification and marking: Each segment should be clearly marked with its position in the arch rib sequence and its required pre-deformation parameters to prevent installation errors.
• Weld residual stress management: Post-weld heat treatment or mechanical stress relief should be considered for critical segments to reduce the risk of delayed distortion during hoisting.

Study Reflection

This paper, published in 2002, was among the early systematic studies on arch rib shape optimization for steel tube concrete arch bridges in China. The methodology it presents has since been refined and applied to numerous subsequent projects. The fundamental principle of simulating the hoisting process and pre-deforming the structure remains valid, though modern computational tools and real-time monitoring systems have enhanced the implementation significantly. The study reinforces the importance of integrating structural engineering analysis with construction engineering practice, and highlights the role of steel pipe manufacturing quality in achieving the intended structural geometry.