Apparent Elasto-Plastic Constitutive Equation and Ultimate Bearing Capacity of Axially Compressed Steel Tube Confined Concrete Short Columns

Literature Overview

This paper by Tang Guodong and Tang Yu, published in "China Journal of Highway and Transport" (1991, Vol. 4, No. 4), presents a theoretical analysis of steel tube confined concrete (STC) short columns under axial compression. The authors derived analytical expressions for the proportional limit, ultimate bearing capacity, and an apparent elasto-plastic constitutive equation using continuum mechanics methods. The results were validated against experimental data, demonstrating good accuracy in describing the initial elasto-plastic behavior of STC columns.

Theoretical Framework

The analysis is based on continuum mechanics, treating the steel tube confined concrete column as a composite material. The key insight is that the steel tube provides lateral confinement to the concrete core, which in turn increases the triaxial compressive strength of the concrete. This confinement effect is the fundamental mechanism that gives STC columns their superior load-bearing capacity compared to plain concrete columns.

Assumptions and Model Development

The model assumes that the steel tube and concrete core deform together, with no slip between the two components. The steel tube is assumed to behave elastically up to a certain strain level, after which plastic deformation occurs. The concrete is modeled as a material whose compressive strength increases with the lateral confining pressure provided by the steel tube.

The proportional limit is defined as the stress level at which the load-strain curve deviates from linearity. Below this stress, the column behaves elastically, and the apparent elastic modulus can be calculated from the combined stiffness of the steel tube and concrete core.

Apparent Elasto-Plastic Constitutive Equation

The constitutive equation relates the axial stress to the axial strain for the composite column. The equation captures the transition from elastic to plastic behavior and accounts for the confinement effect of the steel tube on the concrete. The apparent elastic modulus is higher than that of plain concrete because the steel tube contributes additional stiffness. The peak strength is also significantly higher due to the triaxial confinement effect.

Key Results and Verification

The paper demonstrates that the analytical model accurately predicts the initial elasto-plastic behavior of STC columns. The model is particularly useful for the elastic and early plastic stages, which are the most critical for serviceability design.

Reflections and Engineering Implications

This paper represents an early and important contribution to the theoretical understanding of steel tube confined concrete columns. For structural engineers designing STC members, the constitutive equation provides a tool for predicting column behavior under various loading conditions. The model is particularly valuable for columns where the steel tube provides significant confinement, as is typical in high-strength concrete applications.

However, it is important to recognize the limitations of this model. The analysis is based on short columns, which means buckling effects are not considered. For slender STC columns, the stability behavior must be analyzed separately. Additionally, the model assumes perfect bond between the steel tube and concrete, which may not hold in all practical situations, particularly under cyclic or reversed loading conditions. The model is also applicable to circular cross-sections, and its extension to other cross-sectional shapes requires additional considerations. Despite these limitations, the paper provides a solid foundation for further research and practical applications of STC columns in highway and bridge engineering.