Stress Optimization in Completed State of Large Span CFST Arch Bridges

Literature Overview

The paper by Tu Guangya and Yuan Hang (2020), published in China Journal of Highway and Transport, addresses a critical practical challenge in the construction of large-span concrete-filled steel tubular (CFST) arch bridges: optimizing the stress state of the main arch ribs at the completed bridge stage. The study focuses on the Da Xiao Jing Grand Bridge in Guizhou Province, China, and proposes a novel construction sequencing strategy involving the delayed removal of stay cables (tie-back ropes). The core innovation lies in maintaining the stay cables after arch ring closure, allowing them to remain in place until the internal concrete reaches its design strength, at which point the stay cables are removed and the steel tube and concrete share subsequent loads jointly.

Core Technical Viewpoint and Construction Sequencing Strategy

Traditional construction methods for CFST arch bridges typically involve closing the arch ring and then immediately removing the stay cables before concrete infilling. This approach results in the steel tube bearing significant bending moments during the concrete pouring phase, leading to elevated stress levels in the final completed state. The proposed method fundamentally changes this sequence by retaining the stay cables through the concrete infilling process. The engineering rationale is straightforward: the stay cables continue to provide external support to the arch rib during the vulnerable period of concrete curing, thereby reducing the bending demand on the steel tube. Once the concrete reaches its design strength, the composite action between steel and concrete becomes fully effective, and the stay cables can be safely removed without inducing excessive stress redistribution.

The finite element analysis conducted on the Da Xiao Jing Grand Bridge demonstrates that under the proposed sequencing, the stress in both the upper and lower chord steel tubes decreases compared to the conventional approach, while the stress in the internal concrete increases slightly. Importantly, the stay cable force values show minimal variation before and after concrete infilling, and the tensile forces remain within allowable limits. This finding is significant because it confirms that the stay cables do not become overstressed during the extended service period, ensuring structural safety throughout the construction phase.

Engineering Practice Implications and Process Analysis

From a steel pipe manufacturing and welding perspective, this study has direct implications for the fabrication and quality control of arch rib steel tubes. The optimization strategy reduces peak stresses in the steel tubes during construction, which translates into reduced risk of local buckling, weld cracking, and fatigue damage in the arch rib connections. For engineers involved in the fabrication of large-diameter steel tubes for bridge applications, this means that the design stress level can potentially be reduced, allowing for more economical section selection without compromising structural safety.

The following table summarizes the key comparative findings:

Key Reflections and Independent Thinking

This study raises important questions about the interplay between construction sequencing and final structural performance. In my experience with large-diameter steel pipe fabrication for bridge applications, the welding quality of arch rib segments is often the controlling factor in long-term structural reliability. By reducing peak stresses during construction, the proposed method effectively lowers the fatigue damage accumulation in welds and weld heat-affected zones (HAZ), which is particularly beneficial for the longitudinal and circumferential welds in arch rib steel tubes. The slight increase in concrete stress is a reasonable trade-off, as concrete typically has a much higher compressive capacity margin than steel in tension or bending.

One critical consideration that the study does not fully address is the practical feasibility of maintaining stay cables over extended periods. In real construction scenarios, stay cables are subject to environmental degradation, including corrosion, UV exposure, and thermal cycling. The engineering team must ensure that the stay cables are protected and monitored during the extended service period to prevent premature failure. Additionally, the removal of stay cables after concrete curing introduces a transient load redistribution that must be carefully managed to avoid dynamic effects or secondary stress concentrations in the arch rib welds.

Summary

The proposed delayed stay cable removal method represents a practical and effective approach to improving the stress performance of CFST arch bridges at the completed state. For steel pipe engineers, this study reinforces the importance of considering construction sequencing in the structural design phase, as it directly impacts the stress history and fatigue life of the steel tube components. The method offers a pathway to more efficient use of composite sections and reduced material consumption, provided that the extended stay cable service period is properly managed from both structural and corrosion protection perspectives.