Creep Behaviour and Stress Redistribution in Steel Tube Concrete Arch Ribs

Literature Overview

The paper published in Engineering Mechanics (Vol. 24, Issue 6, 2007, pp. 79–85) by Peng Jianxin, Shao Xudong, Cheng Xiangyun, and Huang Zhengyu from the Bridge Engineering Research Institute of Hunan University addresses a critical yet often underappreciated aspect of steel tube concrete (SRC) arch bridge engineering: the long-term creep behaviour of the composite arch rib. Funded by the Ministry of Transport Western Transportation Construction Science and Technology Project, this work fills a significant gap between short-term structural analysis and the reality of decades-long service life under sustained loading.

Core Technical Framework

The authors derive a stress redistribution formula based on deformation compatibility conditions between the steel tube and the core concrete. The key innovation is the introduction of an equivalent elastic modulus of concrete, which accounts for the time-dependent creep deformation. This approach transforms the complex viscoelastic problem into a more tractable analytical framework that can be directly compared with code-based calculations.

The fundamental compatibility equation can be expressed as follows:

• The total axial deformation of the composite section must be compatible between the steel tube and the concrete core at any time t.
• The stress in the steel tube at time t is related to the initial stress and the creep-induced stress transfer.
• The equivalent elastic modulus of concrete is defined as: E_c,eq(t) = E_c / [1 + φ(t, t₀)], where φ(t, t₀) is the creep coefficient dependent on loading age t₀ and time t.

Experimental Methodology and Results

The research team constructed scaled model segments of SRC arch ribs following the similarity principle. The models were tested under serviceability limit state (SLS) conditions to validate the analytical formula. The experimental results were then compared with calculations based on the current design code.

The most striking finding is the magnitude of stress redistribution: concrete stress can decrease by as much as 52.7% over the service life, with the corresponding load being transferred to the steel tube, which experiences up to 27.3% stress increase. This has profound implications for long-term structural assessment and maintenance planning.

Engineering Practice Implications

Design Considerations

From a practical standpoint, the findings suggest that the current design codes, which typically treat the composite section as having a fixed elastic modulus ratio, may underestimate the long-term stress in the steel tube. Engineers designing SRC arch bridges should consider the following:

1. The steel tube may become the critical component in the long term, even if the concrete core governs the short-term design.
2. The stress redistribution is progressive, meaning that the structure continuously evolves from a concrete-dominant to a steel-dominant load-bearing system.
3. The steel ratio is a powerful lever for controlling creep effects; increasing the steel ratio from, say, 15% to 25% can significantly reduce the magnitude of stress transfer.

Construction and Inspection Implications

For construction monitoring and periodic inspection programmes, the following points deserve attention:

• The early service years are critical for detecting any anomalies in the composite action, as the creep rate is highest in the initial period.
• Strain gauges installed on the steel tube during construction should be monitored for the first 2–3 years to establish a baseline creep curve.
• The maximum stress concentration at the haunch section (arch foot) should be verified against the allowable stress with a creep-adjusted factor.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation. First, the paper focuses on SLS conditions, but the authors also tracked the creep development under ultimate limit state (ULS) conditions. The transition from elastic to plastic behaviour under sustained loading combined with creep is a complex phenomenon that may affect the ultimate load capacity more than the SLS stress levels alone. Second, the temperature effects on creep in outdoor-exposed arch ribs are not addressed, yet temperature cycling is a reality for bridge structures spanning hundreds of metres. Third, the concrete mix design parameters—such as cement type, aggregate gradation, and water-to-binder ratio—significantly influence creep, but the paper treats the concrete as a homogeneous material without differentiating between mix designs.

Study Insights and Implications

The most valuable contribution of this paper is the quantitative demonstration that creep is not merely a theoretical concern but a practical design issue that can shift the load-bearing role from concrete to steel by more than half the original concrete stress. For engineers involved in the design, construction, and maintenance of SRC arch bridges, this means that long-term structural assessment must account for the evolving stress state rather than relying on the as-built condition. The equivalent elastic modulus approach provides a practical tool that can be incorporated into existing finite element analysis workflows without requiring fundamentally new software capabilities. This paper should be required reading for any engineer involved in the lifecycle management of SRC arch bridge structures.