Bearing Capacity Calculation of Eccentrically Compressed Square Steel Tube Concrete Members

Literature Overview

This paper, published in the Journal of Daqing Petroleum Institute in 2001 by Zhang Wenfuk and colleagues from the Department of Civil Engineering at Daqing Petroleum Institute, addresses the calculation of bearing capacity for square steel tube concrete (STC) members under eccentric compression. The research was supported by the Heilongjiang Provincial Natural Science Foundation (Grant A9915). The authors selected appropriate constitutive relationships for both the steel tube and the core concrete, applied a simplified numerical method to compute the bearing capacity of composite members composed of square steel tubes and core concrete, and presented detailed calculation procedures. Validation against experimental data yielded an average error of 0.9847 and a standard deviation of 0.1100, indicating excellent agreement between calculated and measured results.

Core Technical Approach

The fundamental challenge in analyzing eccentrically compressed STC members lies in the nonlinear interaction between the steel tube and the confined concrete under combined axial and bending loads. The authors' approach rests on several key assumptions and methodological choices that merit careful examination.

Constitutive Relationships

The selection of constitutive models is critical to the accuracy of any numerical analysis of STC members. For the steel tube, the authors employed an elastic-perfectly plastic or bilinear model that captures the yielding behavior of structural steel grades commonly used in STC construction (typically Q235 or Q345). For the core concrete, the confined concrete model accounts for the lateral constraint provided by the square steel tube, which significantly enhances both the compressive strength and the ductility of the concrete compared to unconfined conditions.

The confinement effect in square tubes differs from that in circular tubes because the corners of a square section experience less lateral restraint than the mid-span regions of the flat sides. This geometric non-uniformity must be captured in the constitutive model to avoid overestimating the confined concrete strength.

Simplified Numerical Method

The simplified numerical method employed by the authors likely involves dividing the cross-section into discrete elements and integrating the stress-strain relationships over each element under a given eccentricity. The calculation procedure involves:

1. Defining the geometry and material properties of the square steel tube and core concrete.
2. Selecting appropriate stress-strain curves for both materials under confinement.
3. Applying a given axial load and eccentricity, then computing the strain distribution across the section.
4. Iteratively solving for the neutral axis position such that equilibrium of forces is satisfied.
5. Computing the resultant moment and comparing it with the applied moment.

This approach is essentially a fiber-based or layer-based integration method that is computationally efficient and well-suited for design applications.

Engineering Practice Implications

From a steel pipe manufacturing and quality control perspective, several observations emerge from this literature. The accuracy of the bearing capacity calculation depends heavily on the actual mechanical properties of the steel tube used. In practice, the yield strength of square tubes produced by cold-formed processes (such as those conforming to GB/T 3094 or ASTM A500) can vary from batch to batch, and the strain-hardening behavior near the corners of cold-formed sections may differ from the mid-span regions due to localized plastic deformation during the bending and welding operations.

For engineers involved in the design and fabrication of STC columns, the following quality control measures are recommended:

Key Questions and Reflections

The paper's reported average error of 0.9847 (which likely represents a ratio of calculated to experimental capacity) and standard deviation of 0.1100 suggest that the method is reliable for practical design purposes. However, several questions remain worth considering. First, the applicability of the method to different square tube aspect ratios (width-to-wall-thickness ratios) should be verified, as slender square tubes may exhibit local buckling of the flat sides before the composite action is fully mobilized. Second, the effect of the steel tube's initial out-of-straightness and residual stresses introduced during cold forming on the ultimate capacity should be investigated, as these factors can significantly reduce the load-bearing capacity in eccentric compression.

The paper's contribution to the field is significant, particularly for the design of STC columns in petrochemical and industrial structures where square tubes are commonly used for their ease of fabrication and connection. The simplified numerical method provides a practical tool that balances accuracy with computational efficiency, making it suitable for routine design calculations without requiring sophisticated finite element software.

Study Insights

This literature reinforces the importance of understanding the composite action between steel and concrete in STC members. For engineers involved in steel pipe procurement and fabrication, it highlights that the mechanical properties of the steel tube directly influence the structural performance of the composite member. Quality assurance during steel tube manufacturing—particularly the control of yield strength, dimensional accuracy, and surface quality—is therefore not merely a compliance exercise but a critical factor in ensuring the structural safety of STC constructions. The simplified numerical method presented here can serve as a useful cross-check against more elaborate finite element analyses in the design stage.