Calculation of Axial Compression Bearing Capacity of Composite Steel Tube Concrete Columns

Literature Overview

This 2010 paper by Zhang Zhiquan, Zhao Junhai, Zhang Yufen, and Li Xiaowei, published in the Journal of Chang'an University (Science & Engineering Edition), presents a method for calculating the axial compression bearing capacity of composite steel tube concrete columns. The authors from the College of Civil Engineering and Architecture at Chang'an University conducted this research under the auspices of the Shaanxi Provincial Natural Science Foundation (SJ08E214). The study addresses a fundamental question in structural engineering: how to accurately predict the axial compression capacity of composite columns that combine steel tubes and concrete cores.

Core Technical Content

The composite steel tube concrete column is a structural system that combines a steel tube (circular or polygonal) with a concrete core. The steel tube provides lateral confinement to the concrete, which enhances the concrete's compressive strength and ductility. The concrete core, in turn, provides support to the steel tube, delaying local buckling and enhancing the overall stability of the column.

The paper develops a theoretical model for the axial compression bearing capacity based on the following assumptions:

1. The concrete core is confined by the steel tube, and the confining pressure is uniform around the circumference.
2. The stress-strain relationship of the confined concrete follows a modified Mander model.
3. The steel tube behaves elastically up to the yield point, and then yields uniformly.
4. The steel tube and concrete deform compatibly, with no slip between them.

The bearing capacity equation is derived as:

Where:

• A_c = cross-sectional area of concrete core
• f_c = unconfined concrete compressive strength
• f_cc = confined concrete compressive strength
• f_y = yield strength of steel tube
• A_s = cross-sectional area of steel tube
• k = confinement factor = (A_c - A_s) / A_s

Experimental Validation

The theoretical model is validated against experimental data from the literature and from a series of tests conducted by the authors. The comparison between theoretical predictions and experimental results is shown below:

The results demonstrate that the theoretical model provides accurate predictions, with the ratio of theoretical to experimental values consistently between 0.97 and 0.98. This level of accuracy is sufficient for practical design purposes.

Stress-Strain Relationship of Confined Concrete

A critical component of the bearing capacity calculation is the stress-strain relationship of the confined concrete. The paper adopts a modified Mander model that accounts for the confinement pressure:

Where:

• ε_c = strain at peak stress for unconfined concrete
• t = wall thickness of steel tube
• D = outer diameter of steel tube

Engineering Practice and Design Guidelines

The paper provides practical design guidelines for the application of composite steel tube concrete columns:

1. Slenderness ratio limits: The slenderness ratio (L/D) should not exceed 30 for short columns and 50 for slender columns, beyond which buckling becomes the governing failure mode.
2. Concrete strength range: The concrete compressive strength should be in the range of C20 to C80. For concrete strengths above C80, the confinement effect becomes less effective due to the brittle nature of high-strength concrete.
3. Steel tube material: The steel tube should be made of structural steel with a yield strength of 235–460 MPa. Higher strength steels are not recommended because they reduce the ductility of the composite column.
4. Connection design: The connection between the steel tube and the concrete core must ensure full composite action. This can be achieved by welding internal stiffeners or by using mechanical interlocks.

Comparison with Design Codes

Study Insights and Implications

The most significant contribution of this paper is the development of a simple yet accurate bearing capacity equation that is validated against experimental data. The equation is easy to implement in design practice and provides predictions that are within 3% of experimental values, which is well within the acceptable range for structural design.

The paper also highlights the importance of the confinement factor k in determining the bearing capacity of composite columns. The confinement factor is a function of the ratio of concrete area to steel tube area, and it directly influences the confined concrete strength. This means that the geometry of the composite column is a critical design parameter, and engineers must optimize the steel tube dimensions to achieve the desired confinement effect.

From a broader perspective, the composite steel tube concrete column represents a rational structural system that combines the advantages of steel and concrete. The steel tube provides ductility and confinement, while the concrete provides compressive strength and fire resistance. The composite action between the two materials results in a column system that is superior to either material used alone.

In summary, this paper provides a practical and accurate method for calculating the axial compression bearing capacity of composite steel tube concrete columns. The theoretical model is validated against experimental data, and the design guidelines are applicable to a wide range of structural applications. The paper is a valuable resource for engineers involved in the design of composite columns, and its findings contribute to the ongoing development of composite structural systems.