Mechanical Properties Analysis of Hollow Steel Tube Concrete Axially Compressed Short Columns

Literature Overview

This paper by Zhang Weizhen, Ding Faxing, and Fang Changjing, published in China Journal of Highway and Transport (2018, Vol. 38, No. 4, pp. 136–141), investigates the mechanical behavior of hollow steel tube concrete (HSTC) short columns under axial compression. Funded by the National Natural Science Foundation (Grant No. 51578548), the study uses finite element analysis (FEA) to examine how the hollow ratio affects load-deformation behavior, stress distribution, and ultimate load capacity. The authors also develop an analytical load capacity formula based on limit equilibrium theory and validate it against both FEA results and experimental data.

Conceptual Framework and Design Rationale

Hollow steel tube concrete columns represent an innovative structural system that combines the confinement benefits of steel tube concrete with the weight reduction advantages of a hollow core. The hollow ratio (defined as the ratio of hollow diameter to steel tube inner diameter) is the key design parameter that governs the trade-off between:

• Weight reduction: Higher hollow ratio reduces self-weight, beneficial for tall structures and seismic applications.
• Load capacity: Lower hollow ratio provides more concrete for load bearing and better confinement interaction.
• Stiffness: The hollow core affects the overall flexural stiffness and buckling resistance.

This paper focuses specifically on short columns (low slenderness ratio), where material strength and confinement effects dominate over geometric instability.

Finite Element Analysis Results

The FEA model captures the nonlinear behavior of both steel and concrete materials, including concrete crushing, steel yielding, and the progressive interaction between the two materials. Key findings include:

The load-deformation curves show a characteristic pattern: initial linear elastic response, followed by progressive yielding of the steel tube, and finally a gradual strength degradation as concrete crushes and the steel tube expands outward. The hollow ratio directly influences the magnitude of the post-peak softening behavior.

A particularly important observation is that after steel tube yielding, the rate of longitudinal stress reduction and the rate of hoop stress increase are both accelerated as the hollow ratio increases. This indicates that the hollow core reduces the effectiveness of the confinement mechanism, leading to more rapid strength degradation after the peak load.

Analytical Load Capacity Formula

Based on limit equilibrium theory, the authors derive an analytical expression for the ultimate axial load capacity of hollow steel tube concrete short columns. The formula incorporates:

• Steel tube contribution: based on steel yield strength and cross-sectional area
• Core concrete contribution: accounting for the confinement effect on concrete strength
• Interaction factor: representing the mutual enhancement between steel and concrete

The validation shows good agreement between the analytical formula and FEA results. When compared with experimental data, both the analytical formula and FEA results are conservative (on the safe side), which is desirable for structural design.

Engineering Practice Considerations

For engineers considering hollow steel tube concrete columns in practical applications, several considerations emerge:

1. Hollow ratio selection: The optimal hollow ratio depends on the specific application. For weight-sensitive applications (e.g., bridge piers, offshore platforms), hollow ratios of 30–50% may be appropriate. For high-load applications where maximum capacity is needed, hollow ratios below 20% are more suitable.
2. Construction methodology: The hollow core must be formed during construction. Options include using removable cores, precast inner tubes, or cast-in-place construction with internal formwork. Each method has implications for construction quality and cost.
3. Concrete placement: Ensuring uniform concrete filling around the hollow core without voids or honeycombing is critical. The confinement effect depends on complete concrete-steel contact, and any gaps would reduce effectiveness.
4. Seismic considerations: While this study focuses on short columns under axial compression, the hollow ratio also affects seismic performance. Lower hollow ratios provide better energy dissipation and ductility under cyclic loading, but this trade-off with weight reduction must be evaluated for each specific project.
5. Fire resistance: The hollow core may affect fire resistance performance by reducing the thermal mass and creating a cavity that can accelerate heating of the steel tube. Fire protection measures may need to be specifically designed for this column type.

Key Reflections

This study makes a valuable contribution to the understanding of a relatively novel structural system. The finding that both analytical and numerical methods provide conservative estimates compared to experimental results is reassuring from a safety perspective, though it may lead to somewhat uneconomic designs. Future research should focus on:

• Long columns (high slenderness ratio) where buckling and confinement interact
• Eccentrically loaded columns where bending and axial compression combine
• Cyclic loading behavior for seismic applications
• Long-term behavior under sustained loads (creep and shrinkage effects)
• Optimal hollow ratio determination methods for specific loading conditions

The limit equilibrium approach used for the analytical formula is elegant in its simplicity, but engineers should be aware that it does not capture the full nonlinear interaction between steel and concrete. For critical applications, FEA or experimental validation is recommended to complement analytical calculations.

The practical significance of this research lies in providing a design tool for a structurally efficient system that combines the benefits of steel tube concrete with reduced weight. As construction industry trends move toward lightweight, high-performance structural systems, hollow steel tube concrete columns represent a promising technology that deserves further investigation and standardization.