Compressive Performance of Reinforced Square Steel Tube Concrete Columns

Literature Overview

This study by Zheng Liang and colleagues from North University of China, published in the Journal of Jiangsu University (Natural Science Edition) in 2015, examines the axial and eccentric compressive behaviour of square steel tube concrete (SRC) columns with and without internal reinforcement (spiral stirrups). The research combines experimental testing with nonlinear finite element analysis using ABAQUS. Funded by the National Natural Science Foundation Youth Science Fund (Project 51208473), this work addresses an important practical question: whether internal reinforcement can enhance the performance of steel tube concrete columns beyond what the steel tube confinement alone provides.

Experimental Programme and Numerical Modelling

The study involved two groups of specimens: unreinforced square SRC columns and reinforced square SRC columns with spiral stirrups. Both groups were tested under axial compression and eccentric compression conditions. The ABAQUS finite element models were calibrated against experimental results to ensure accuracy before parametric studies.

Test Condition	Unreinforced SRC	Reinforced SRC
Axial compression	Baseline behaviour	Significant increase in ultimate capacity
Eccentric compression	Capacity decreases with eccentricity	Smaller reduction rate compared to unreinforced
Failure mode	Similar between both types	Similar between both types
Deformation capacity	Baseline	Moderately improved
Post-peak stable capacity	Gradual decline with eccentricity	More stable at higher eccentricities

Analysis of Reinforcement Effects on Mechanical Behaviour

The primary finding is that adding spiral reinforcement to square steel tube concrete columns significantly improves ultimate bearing capacity while also providing moderate improvement in deformation capacity. This is attributed to the combined confinement effect: the steel tube provides external confinement, while the spiral stirrups provide internal confinement, creating a dual-confinement mechanism. The concrete core experiences multi-axial stress states from both directions, leading to enhanced compressive strength and ductility.

Under eccentric compression, both types of columns show decreasing ultimate capacity as the eccentricity ratio increases. However, the reinforced columns exhibit a smaller rate of capacity degradation, indicating that the internal reinforcement helps maintain structural integrity under combined axial and bending loads. This is particularly relevant for real-world applications where pure axial loading is rare and eccentricities inevitably develop due to construction tolerances and load asymmetries.

Welding and Fabrication Considerations

From a fabrication perspective, reinforced SRC columns introduce additional welding complexity. The spiral stirrups must be welded to the internal reinforcement cage, and the entire reinforcement assembly must be positioned within the steel tube before concrete pouring. This creates potential challenges:

Internal welding accessibility: Welding connections inside the square tube may require specialized techniques or pre-assembly outside the tube before insertion.
Residual stress interaction: The presence of internal reinforcement creates additional residual stresses that interact with those from the steel tube welding process.
Concrete placement quality: The internal reinforcement cage reduces the effective concrete placement area, requiring careful control of concrete workability and compaction to avoid voids.
Tube-to-reinforcement gap control: Maintaining adequate cover between the steel tube and internal reinforcement requires precise fabrication tolerances.

For the steel tube itself, the fabrication method (ERW, HFW, or LSAW) affects the quality of the tube walls and potential weld imperfections. Any weld defects on the tube walls could create stress concentrations that interact adversely with the internal reinforcement under combined loading.

Failure Mode Analysis and Standards Relevance

The finding that both reinforced and unreinforced columns exhibit similar failure modes is important from a design perspective. The failure morphology is governed primarily by the steel tube local buckling and concrete crushing, with the reinforcement providing additional confinement without fundamentally altering the failure mechanism. This consistency simplifies the design approach, as failure prediction models developed for unreinforced SRC columns can be adapted with appropriate modification factors for reinforced configurations.

The ABAQUS modelling approach allows for detailed investigation of the stress distribution within the composite section. The nonlinear material models for both steel and concrete, including confinement-strength relationships, are critical for accurate simulation. The agreement between numerical and experimental results validates the modelling approach and provides confidence for parametric studies that would be impractical experimentally.

Study Insights and Engineering Implications

This research demonstrates that the dual-confinement concept of combining external steel tube confinement with internal spiral reinforcement is an effective strategy for enhancing the compressive performance of SRC columns. The engineering significance is particularly relevant for high-rise buildings and heavy-load structures where columns must resist large axial forces with eccentricities. The moderate improvement in deformation capacity suggests that while the reinforcement primarily enhances strength, it also contributes to seismic resilience through improved energy dissipation. Engineers should consider the fabrication complexity and cost implications when deciding whether to include internal reinforcement, weighing these against the structural benefits in terms of capacity and stability under eccentric loading conditions.