Seismic Performance of Internally Confined Square Steel Tube Concrete Columns

Literature Overview

This study by Zhang Tao et al., published in "World Information on Earthquake Engineering" (2022, Vol. 38, No. 4), addresses a fundamental weakness of conventional square steel tube concrete (CFT) columns: the relatively poor lateral confinement provided by square tubes compared to circular tubes. The authors propose a novel internally confined square CFT column incorporating transverse tie reinforcement within the concrete core and investigate its seismic performance through ABAQUS finite element analysis. The work is funded by Henan Provincial Science and Technology Projects and graduate education reform programs.

Proposed Internally Confined System

The innovation lies in adding internal transverse reinforcement (拉筋) within the concrete core of the square steel tube to enhance confinement and delay local buckling of the tube wall under cyclic seismic loading. The authors evaluated two internal confinement configurations:

The transverse tie reinforcement configuration outperformed the circular ring type, attributed to more effective confinement stress distribution in the rectangular concrete core geometry.

Finite Element Modeling Approach

The ABAQUS model employed the following material constitutive formulations:

• Steel tube: Kinematic hardening model with cyclic stress-strain relationship calibrated from coupon tests, incorporating Bauschinger effect and strain hardening.
• Concrete core: Concrete Damaged Plasticity (CDP) model with damage parameters calibrated to uniaxial and triaxial compression tests, capturing plastic damage evolution under cyclic loading.
• Internal ties: Elastic-plastic material model with bilinear stress-strain behavior.
• Interface interaction: Penalty contact formulation with friction coefficient of 0.4 to simulate steel-concrete bond behavior.

The model was validated against existing experimental data, demonstrating good agreement in load-displacement hysteresis loops, with deviations in peak load generally within 5-8%.

Key Parametric Findings

The parametric study revealed several important design relationships:

• Steel ratio effect: Increasing the section steel ratio (含钢率) from 15% to 25% improved ultimate bearing capacity by approximately 18-22%, while also enhancing energy dissipation capacity through increased steel tube plastic deformation.
• Slenderness ratio effect: Higher slenderness ratios improved ultimate capacity but reduced ductility, indicating a trade-off between strength and deformation capacity that must be balanced in seismic design.
• Axial compression ratio: Within the range of 0.3 to 0.6, increasing axial compression ratio enhanced bearing capacity; however, beyond 0.6, the beneficial effect diminished and ductility degraded significantly.
• Optimal tie arrangement length: The authors determined optimal placement lengths for transverse ties at different axial compression ratios, with denser confinement required near column ends where plastic hinges form.
• Volume stirrup ratio: A reasonable volume stirrup ratio (体积配箍率) was identified as 1.5-2.5% for optimal seismic performance.

Welding Quality and Fabrication Challenges

From a fabrication and welding quality perspective, the internally confined CFT column introduces additional complexity:

1. Internal tie welding: The transverse ties must be welded to the inner surface of the steel tube, which requires access for welding operations. In practice, this may necessitate the use of narrow-gap welding techniques or pre-fabrication of tie assemblies before tube closure.
2. Weld defect sensitivity: The internal welds are difficult to inspect visually and require ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) per NB/T 47013. Common defects in internal welds include porosity from inadequate gas shielding and lack of fusion from poor root preparation.
3. Residual stress interaction: The combination of tube-to-tube welds, internal tie welds, and any external reinforcement welds creates a complex residual stress field that may affect fatigue performance under cyclic seismic loading. Welding sequence optimization is critical to minimize residual stress concentrations.

Energy Dissipation Mechanism Analysis

The study's discussion of energy dissipation (耗能性能) is particularly relevant to seismic design philosophy. The internally confined column dissipates energy through three primary mechanisms:

• Plastic deformation of the steel tube walls (particularly at corners where bending stress concentrates).
• Slippage and bond degradation at the steel-concrete interface.
• Plastic deformation and potential fracture of the internal transverse ties.

The confinement reinforcement enhances the first and third mechanisms while controlling the second, resulting in more stable hysteresis loops and higher cumulative energy dissipation compared to conventional CFT columns.

Engineering Implementation Considerations

For practical implementation of this internally confined system in seismic regions, the following measures are recommended:

• Use of low-alloy structural steel (Q345 or Q390) for both the tube and internal ties to ensure compatible weldability and ductility.
• Application of submerged arc welding (SAW) or flux-cored arc welding (FCAW) for internal tie welds to ensure consistent weld quality with minimal operator skill dependency.
• Implementation of magnetic particle testing (MT) or dye penetrant testing (PT) for surface inspection of internal welds where accessible, supplemented by UT for subsurface defect detection.
• Consideration of post-weld vibration stress relief (PWVR) to reduce residual stresses in the confined core region.

Study Insights and Implications

This research presents a practical and economical solution to the well-known confinement deficiency of square CFT columns in seismic applications. The internal transverse tie reinforcement system is conceptually simple and can be integrated into existing fabrication workflows with minimal process modification. The finite element validation provides confidence in the design methodology, though further experimental verification with full-scale cyclic loading tests would strengthen the basis for code adoption. The identified optimal parameters for tie arrangement and volume stirrup ratio provide actionable design guidance for practicing engineers.