Creep Analysis of Steel Tube Concrete Large Eccentric Compression Members

Literature Overview

Guo Weiwei, Wang Yuanfeng, and Han Bing from the North China Railway Institute of Technology published this study in Engineering Mechanics (2003, Vol. 20, Issue 1, pp. 91–95), funded by the Ministry of Railways Science and Technology Development Fund (J99Z071) and a university fund (2002SM011). The paper addresses the time-dependent behavior of CFT members under large eccentric compression, a condition prevalent in bridge columns, frame columns, and transfer structures in railway and highway engineering.

Core Technical Methodology

Creep Modeling Approach

The authors employ the sequential loading theory (also known as the superposition approach) to model concrete creep under multiaxial stress states with continuously unloading conditions. This is a sophisticated choice because large eccentric compression members experience non-uniform stress distributions that evolve over time as creep develops, creating a feedback loop between stress redistribution and further creep development.

The key innovation is the integration of three technical elements:

Multiaxial stress creep model: Accounts for the interaction between axial stress and bending stress in the confined concrete core.
Sequential loading theory: Enables analysis of continuously changing stress states by decomposing the time-varying loading into incremental steps.
Iterative computation method: Simulates the time-dependent nature of creep through successive iterations until convergence is achieved.

Influencing Factors Analysis

Parameter	Effect on Creep	Engineering Significance
Steel ratio (ρ)	Higher ρ reduces concrete creep due to increased confinement	Design optimization target
Eccentricity ratio (e/h)	Higher eccentricity increases tensile creep on the convex side	Controls long-term deflection
Confining stress (σ_conf)	Higher confinement reduces creep and increases concrete strength	Key design variable for CFT

Technical Interpretation

The study establishes that the creep behavior of CFT large eccentric compression members is governed by the interaction between the steel tube confinement effect and the time-dependent stress redistribution within the composite section. As creep progresses, the concrete on the compressed side undergoes additional shortening, which increases the effective eccentricity of the applied load and shifts the neutral axis. This creates a progressive amplification effect that is particularly pronounced at high eccentricity ratios.

The iterative approach is essential because the creep strain at any time depends on the stress history, which itself is modified by the creep that has already developed. The authors demonstrate that convergence is typically achieved within 5–8 iterations for practical loading scenarios.

Comparison with Existing Methods

Method	Applicability	Accuracy	Computational Cost
Effective modulus method	Simple loading, small eccentricity	Low for large eccentricity	Low
Equivalent age method	Constant stress	Moderate	Moderate
Sequential loading theory	Varying stress, large eccentricity	High	High
Finite element with creep	Complex geometries	Highest	Very high

Engineering Practice Relevance

For steel pipe manufacturers and structural engineers, this paper has several practical implications:

Long-term deflection prediction: The creep analysis enables accurate prediction of long-term deflections in CFT columns, which is critical for serviceability design. Excessive long-term deflection can lead to cracking in adjacent concrete elements and reduced fire resistance.
Steel ratio optimization: The finding that higher steel ratios reduce creep provides a quantitative basis for optimizing the steel-to-concrete ratio in CFT design, balancing structural performance with material cost.
Weld joint considerations: In multi-story CFT structures, the steel tubes are connected by welded joints. Creep-induced stress redistribution over time can affect the fatigue performance of these welds, particularly under cyclic loading conditions.
Design code implications: The study highlights gaps in existing design codes that do not adequately account for the time-dependent behavior of CFT members under large eccentric compression, suggesting the need for code updates.

Study Insights

The paper's use of sequential loading theory represents a methodologically sound approach to a genuinely complex problem. The time-dependent behavior of CFT members is not merely an academic concern—it directly affects the long-term performance and safety of structures such as railway viaducts, highway overpasses, and industrial buildings where CFT columns are widely used.

A particularly important insight is the amplification effect of eccentricity on creep. Engineers often focus on the immediate (elastic) response of CFT members but may underestimate the long-term creep deflection, especially for members with high eccentricity ratios. This paper provides the analytical tools to quantify and control this effect.

From a fabrication standpoint, the paper reinforces the importance of accurate geometric control during steel tube manufacturing. Any initial geometric imperfections—such as ovality or straightness deviations—will interact with the creep-induced stress redistribution, potentially accelerating the development of long-term deformations.

Conclusion

This paper makes a significant contribution to the understanding of time-dependent behavior in CFT members under large eccentric compression. The sequential loading theory approach, combined with iterative computation, provides a reliable analytical framework that can be applied to practical design problems. The findings on the influence of steel ratio, eccentricity, and confining stress offer actionable guidance for engineers optimizing CFT member design. For steel pipe manufacturers, the implications extend to geometric quality requirements and weld integrity standards, as these factors directly influence the long-term creep performance of CFT structures.