Axial Compression Performance of Square Hollow Sandwiched Steel Tube Concrete Composite Long Columns

Literature Overview

The paper by Li Minglun, Ren Qingxin, Wang Qinghe, and Ding Jinan, published in the Journal of Shenyang Jianzhu University (Natural Science Edition) (2023, Vol. 39, No. 1, pp. 35-43), presents a comprehensive finite element analysis of square hollow sandwiched steel tube concrete (SHS-STC) composite long columns under axial compression. The research was supported by multiple funding sources including the National Natural Science Foundation of China (51808351), the Liaoning Province "Xingliao Talent Plan" (XLYC1902027), and the Ministry of Housing and Urban-Rural Development Science and Technology Plan (2019-K-054). This work addresses an innovative structural system that combines the benefits of sandwich construction with steel tube concrete technology.

Core Technical Content

Structural Configuration

The square hollow sandwiched steel tube concrete column consists of:

• Outer square steel tube: Providing lateral confinement and forming the primary structural frame.
• Inner concrete layer: A hollow core surrounded by concrete, creating a sandwich effect.
• Composite construction: The term "composite" (叠合) refers to the combined action of different structural layers working together.

Finite Element Model Development and Validation

The authors established and verified the finite element model through comparison with experimental data. The model incorporated:

• Material nonlinearity: Both steel and concrete exhibited nonlinear stress-strain behavior.
• Geometric nonlinearity: Large deformation effects were considered, particularly important for long columns where buckling is a primary failure mode.
• Contact interaction: The interface between steel tube and concrete layers was modeled with appropriate contact definitions.

Key Findings

1. Six mechanical characteristic points: The authors defined six characteristic points on the load-deformation curve, enabling detailed analysis of stress distribution at different stages of loading.
2. Influencing factors: The axial compression performance was significantly affected by:

• Outer steel tube thickness ratio (b/t): Higher ratios reduce local buckling resistance.
• External concrete strength: Higher strength improves overall capacity but may reduce ductility.
• Slenderness ratio (λ): Higher slenderness reduces stability capacity.

1. Capacity calculation methods: Two methods were proposed:

• Superposition method: Adding the capacities of individual components.
• Reduction method: Applying reduction factors to account for composite action effects.

1. Validation results: The ratio of calculated to finite element capacity had a mean value of 1.013 and a standard deviation of 0.038, indicating excellent agreement between the proposed formulas and the numerical results.

Engineering Practice Implications

Design Methodology

The proposed superposition and reduction methods provide practical tools for designers:

• Superposition method: Suitable for preliminary design, where the total capacity is estimated as the sum of individual component capacities.
• Reduction method: More accurate for detailed design, accounting for the interaction effects between components.
• Slenderness correction: The reduction factor for slenderness should be applied based on the equivalent slenderness ratio of the composite section.

Fabrication and Quality Control

For the fabrication of square hollow sandwiched steel tube concrete columns:

• Steel tube forming: The square steel tubes must be formed with precise corner radii to avoid stress concentrations. The forming process (roll forming or press forming) must be controlled to prevent material thinning at the corners.
• Welding quality: If the square tubes are welded from plate, the weld quality is critical. Longitudinal and transverse welds should be inspected by NDT (UT or MT).
• Concrete placement: The hollow sandwiched configuration requires careful concrete placement to ensure uniform fill without voids. Vibration must be adequate but not excessive to avoid segregation.

Defect Analysis

Key Questions and Reflections

The study raises several important questions for further investigation:

1. Eccentric loading: The study focuses on pure axial compression. In practice, columns are rarely subjected to pure axial loads. The behavior under eccentric loading, where bending moments interact with axial compression, requires further study.
2. Fire resistance: The fire performance of this innovative structural system has not been addressed. The hollow core may affect the thermal behavior and fire resistance duration.
3. Seismic performance: The ductility of the composite long columns under cyclic loading is critical for seismic design. The sandwich configuration may provide inherent ductility benefits that should be quantified.

Study Insights and Implications

This research contributes to the development of innovative structural systems that combine the efficiency of hollow sections with the confinement benefits of concrete-filled steel tubes. The proposed design formulas, validated against finite element results with excellent accuracy (mean ratio 1.013, standard deviation 0.038), provide a reliable basis for engineering design. The identification of key influencing factors—particularly the steel tube thickness ratio, concrete strength, and slenderness ratio—enables designers to optimize the structural performance through appropriate parameter selection. For engineering practice, the study demonstrates that square hollow sandwiched steel tube concrete columns offer a promising alternative to conventional solid concrete-filled steel tubes, potentially reducing material usage while maintaining structural capacity.