Mechanical Behavior of Steel Tube Concrete Composite Columns under Axial Loading

Literature Overview

This study by Huang Deng, Huang Yuan, Chen Guirong, Yang Yang, and Peng Bo from the College of Civil Engineering at Hunan University was published in Earthquake Engineering and Engineering Dynamics (2017, Vol. 37, No. 4, pp. 129–137). Funded by the National Natural Science Foundation of China (grants 51478174 and 51338004), the Hunan Provincial Natural Science Foundation (14JJ3054), and the Central Universities Research Fund (2016HNDX), the paper investigates the load-bearing performance of steel tube-concrete composite columns through both analytical modelling and parametric analysis. The research addresses a structurally important but under-explored configuration where concrete is placed both inside and outside a steel tube, creating a hybrid composite section that leverages the confinement effect of the steel tube on inner concrete while utilizing outer concrete for additional load capacity.

Core Technical Content

The authors developed a MATLAB-based calculation program using the strip method to simulate the nonlinear behavior of steel tube-concrete composite columns. The strip method discretizes the cross-section into thin strips, computing stress-strain relationships for each material layer at every loading increment. This approach captures the progressive redistribution of axial force between inner and outer concrete as plastic deformation develops across the section. The computed moment-curvature and load-displacement curves were validated against experimental data, confirming the accuracy of the analytical model.

The parametric analysis examined five key parameters: nominal axial compression ratio (n), composite ratio (m), outer concrete longitudinal reinforcement ratio (ρ), hoop confinement coefficient (θ), and outer concrete stirrup characteristic value (λ). These parameters collectively govern the ductility, load redistribution mechanism, and failure mode of the composite column.

Key Technical Parameters and Their Influence

The study derived recommended limits for the nominal axial compression ratio and corresponding minimum stirrup characteristic values to ensure a ductility demand of u△ ≥ 3, meaning the displacement ductility ratio should not fall below three times the yield displacement.

Engineering Practice Integration

From a metallurgical and structural engineering perspective, the composite column concept has significant implications for the selection and specification of steel tubes used in construction. The steel tube must provide effective confinement to the inner concrete, which demands adequate wall thickness, yield strength, and ductility of the steel material. Common grades such as Q235B, Q345B, and Q390B (per GB/T 1591) are typically specified, but the confinement effectiveness depends not only on yield strength but also on the geometric proportions (diameter-to-wall-thickness ratio) of the tube.

The composite ratio parameter directly relates to the fabrication process. When outer concrete is cast around a steel tube, the interface bonding between the steel tube surface and the outer concrete becomes critical. Surface treatment of the steel tube—such as roughening, drilling, or welding shear studs—can significantly improve the composite action. In practice, this means that the steel tube used for composite columns should be procured with surface quality specifications that support reliable bond development.

The axial force redistribution mechanism highlighted in this study has direct consequences for welding design at column connections. As the column approaches plastic hinge formation, the redistribution of forces between inner and outer concrete creates complex stress states at welded joints. Engineers should ensure that welded connections at composite column ends are designed with adequate toughness and fatigue resistance, particularly where the steel tube meets the column base plate or top plate.

Study Insights and Reflections

The strip method implementation in MATLAB represents a practical approach that bridges analytical simplicity and numerical accuracy. For practicing engineers, this provides a tool that can be adapted to project-specific conditions without requiring expensive finite element software. The validation against experimental data lends confidence to the model's predictive capability.

The recommendation of u△ ≥ 3 as a minimum ductility criterion aligns with seismic design philosophy, ensuring that columns can undergo significant inelastic deformation without catastrophic failure. However, in regions of high seismicity, higher ductility demands (u△ ≥ 5 or even 6) may be warranted, and the parametric relationships derived here could be extrapolated to those more stringent requirements.

One area deserving further investigation is the long-term behavior of the composite column under sustained loading and the potential for creep and shrinkage effects in the outer concrete to alter the axial force distribution over time. This is particularly relevant for steel tubes made from low-alloy steels where the thermal expansion coefficient mismatch with concrete may induce interface stresses.

Reference Value and Outlook

This work provides a solid analytical foundation for the design of steel tube-concrete composite columns, offering practical design recommendations that can be directly incorporated into structural design codes. The parametric study methodology is replicable and can be extended to incorporate additional factors such as cyclic loading, shear demand, and fire resistance. For the steel pipe industry, the study underscores the importance of producing steel tubes with consistent mechanical properties and surface quality to ensure reliable composite action in structural applications.