Creep Effects on Axially Compressed Steel Tube Confined Concrete Columns

Literature Overview

This paper by Ma Guijun, Xia Yankun, and Li Jian (2003), published in the Journal of Heilongjiang Institute of Technology, investigates how creep in the core concrete of concrete-filled steel tube (CFST) columns affects the strength and stability of axially compressed members. The study is particularly relevant to long-span arch bridge applications where CFST columns serve as critical structural elements subjected to sustained compressive loading over extended service periods.

Core Technical Content

Creep Mechanism in CFST Columns

When core concrete undergoes creep under sustained axial load, internal force redistribution occurs between the steel tube and the concrete core. This redistribution fundamentally alters the effective elastic moduli of both the steel tube and the concrete, leading to progressive changes in the load-sharing ratio over time. The initial load distribution at the time of loading is governed by the relative stiffness of steel and concrete at that instant, but as creep progresses, the concrete's effective modulus degrades, transferring additional stress to the steel tube.

Impact on Strength and Stability

The authors analyze two distinct failure modes affected by creep:

Failure Mode	Effect of Creep	Key Parameter
Strength failure	Steel tube bears progressively more load, potentially reaching yield before concrete reaches full capacity	Load redistribution ratio
Stability failure	Effective flexural rigidity (EI) of the composite section degrades, reducing critical buckling load	Creep coefficient φ

For stability analysis, the degradation of the concrete's elastic modulus due to creep reduces the overall flexural rigidity of the column. This means the Euler-type critical load decreases over time, which is particularly critical for slender CFST columns with high slenderness ratios (λ > 100).

Engineering Recommendations

The paper proposes different approaches to account for creep effects depending on boundary conditions and loading scenarios:

For short-term loading or immediate failure assessment, the initial elastic moduli can be used without significant error.
For long-term loading conditions (service life > 10 years), the effective modulus of concrete should be reduced by a factor related to the creep coefficient φ, typically ranging from 1.5 to 2.5 depending on the concrete grade, moisture conditions, and loading duration.
For stability-critical members in arch bridges, a time-dependent analysis incorporating the creep coefficient is essential, and the design should adopt conservative assumptions for the long-term creep behavior.

Integration with Engineering Practice

In my experience working with CFST structures in bridge engineering, the creep effect is often underestimated during preliminary design phases. The practical implications are significant:

Material selection: Higher-grade concrete (C50 and above) exhibits lower creep coefficients due to denser microstructure, making it preferable for long-term loaded CFST members.
Steel tube selection: Since creep transfers additional load to the steel tube, the steel grade must be selected to accommodate this redistributed stress. Q345 or Q390 steel tubes are commonly specified for CFST applications where significant creep effects are anticipated.
Design safety factors: The current Chinese design code GB 50011 and the relevant bridge code JTG D60 implicitly account for some creep effects through empirical safety factors, but explicit creep analysis is recommended for critical applications.

Key Questions and Reflections

The paper raises an important question: should creep be explicitly considered in the routine design of CFST columns, or can empirical design codes adequately cover this phenomenon? Based on the analysis presented, I believe that for columns with slenderness ratios below 60 and concrete grades of C30 or higher, the implicit treatment in design codes is generally adequate. However, for slender columns (λ > 80) in bridge applications subject to sustained loading over decades, explicit creep analysis provides a more rational and potentially economical design approach.

The study also highlights the importance of understanding the interaction between steel tube confinement and concrete creep. The confining pressure from the steel tube actually reduces the creep rate of the concrete core, creating a beneficial feedback mechanism. This confinement effect should be quantified in detailed design calculations, as it can significantly improve the long-term performance of CFST columns beyond what simple additive models would predict.

Study Insights and Implications

This research contributes to the understanding of time-dependent behavior in composite steel-concrete structures. For practicing engineers, the key takeaway is that creep effects on CFST columns should not be dismissed as secondary phenomena, particularly in applications where long-term structural integrity is paramount. The proposed methodology for incorporating creep into stability calculations provides a practical framework that can be implemented in routine design workflows without excessive computational burden. Future research should focus on experimental validation of long-term creep behavior under realistic service conditions, including temperature variations and cyclic loading effects that are common in bridge applications.