Axial Compression Capacity of Circular Hollow Sandwich Steel Tube Concrete Short Columns

Literature Overview

This theoretical and numerical study by Wang Xianong, Gao Congchao, and Wang Nannan (2013), published in the Journal of Hebei Engineering University (Natural Science Edition), investigates the axial compression load-bearing capacity of circular hollow sandwich steel tube concrete (HSSC) short columns. The authors employ the double shear unified strength theory to analyze the interaction between the core concrete and the inner and outer steel tubes under axial compression.

The HSSC structural system represents an advanced composite construction technique where a concrete core is sandwiched between two concentric steel tubes, creating a double-confinement configuration. This configuration offers potential advantages in terms of load-bearing capacity, ductility, and durability compared to conventional single-tube SRC members.

Theoretical Framework and Derivation

The study employs the double shear unified strength theory, a sophisticated constitutive model that accounts for the influence of intermediate principal stress on the failure criterion of materials. This theory is particularly appropriate for the analysis of confined concrete, where the triaxial stress state significantly influences the material behavior.

The analytical framework proceeds through the following steps:

1. Stress state analysis: Under axial compression, the core concrete exerts lateral pressure on both the inner and outer steel tubes. The inner tube experiences compressive radial stress from the concrete, while the outer tube experiences tensile radial stress as it resists the outward expansion of the inner tube.
2. Horizontal bearing stress calculation: The horizontal (radial) bearing stresses in both the inner and outer steel tubes are calculated based on equilibrium conditions and the double shear unified strength theory.
3. Vertical ultimate bearing stress derivation: Using the double shear unified strength theory, the vertical ultimate bearing stresses in the inner and outer steel tubes are derived. The theory accounts for the biaxial stress state (axial compression combined with radial stress) experienced by each tube.
4. Core concrete capacity calculation: The load-bearing capacity of the core concrete is calculated considering the enhanced compressive strength due to the double confinement provided by both the inner and outer steel tubes.
5. Total capacity formula: The total axial compression load-bearing capacity of the HSSC short column is obtained by summing the contributions from the inner steel tube, outer steel tube, and core concrete.

The simplified calculation formula derived in the study was solved numerically, and the calculated results were compared with experimental data from domestic literature. The comparison showed good agreement, validating the analytical approach.

Double Shear Unified Strength Theory Application

The double shear unified strength theory is a generalized strength criterion that encompasses several classical criteria as special cases. For the analysis of HSSC short columns, the theory provides the following advantages:

• Intermediate principal stress consideration: Unlike the Mohr-Coulomb or Drucker-Prager criteria, which neglect the intermediate principal stress, the double shear unified strength theory accounts for all three principal stresses. This is particularly important for the analysis of confined concrete, where the intermediate principal stress can be significant.
• Unified treatment of different materials: The theory can be applied to both the steel tubes and the core concrete using a unified framework, simplifying the analytical process.
• Physical interpretability: The theory provides physically meaningful parameters that can be directly related to material properties, such as uniaxial compressive strength, uniaxial tensile strength, and the Lode angle parameter.

The application of this theory to HSSC short columns represents a significant advancement in the analytical treatment of double-confinement composite structures. The derived simplified formula provides a practical tool for engineers to estimate the axial compression capacity of HSSC members without resorting to complex numerical analysis.

Fabrication and Welding Considerations

The HSSC structural system introduces several unique fabrication and welding challenges:

The positioning of the inner steel tube within the outer steel tube is a critical fabrication step. The concentricity of the two tubes directly influences the uniformity of the concrete annular gap, which in turn affects the compaction quality and the distribution of the confinement pressure. Precision machining of the tube ends and the use of positioning fixtures are essential to achieve the required concentricity.

The concrete filling through the annular gap between the inner and outer tubes is a challenging operation. The narrow gap can lead to poor compaction and the formation of voids, which would significantly reduce the effective confinement and load-bearing capacity. Vibration-assisted filling techniques and the use of self-compacting concrete are recommended to ensure proper filling. Ultrasonic testing (UT) should be performed to verify the absence of voids.

The weld connections between the inner and outer tubes, typically at the stiffener rings, are critical for maintaining the structural continuity of the HSSC member. These welds must be designed for full fusion and inspected using non-destructive testing methods to ensure the absence of defects. The heat-affected zone of these welds should be evaluated for potential microstructural changes that could affect the fatigue resistance of the connection.

Engineering Practice and Design Recommendations

Based on the theoretical and numerical findings, the following recommendations are proposed for the design and fabrication of HSSC short columns:

1. Concentricity control: The concentricity of the inner and outer steel tubes should be maintained within tight tolerances to ensure uniform concrete filling and consistent confinement pressure distribution.
2. Concrete filling quality: The concrete filling operation should be carefully controlled to ensure proper compaction and the absence of voids. Self-compacting concrete and vibration-assisted techniques are recommended.
3. Weld quality assurance: All weld connections, particularly the inner tube end seals and the stiffener ring welds, should be inspected using appropriate non-destructive testing methods to ensure full fusion and the absence of defects.
4. Analytical design methodology: The simplified calculation formula derived using the double shear unified strength theory should be incorporated into the design methodology for HSSC short columns, providing a rational basis for capacity assessment.

Study Insights and Reflections

The application of the double shear unified strength theory to the analysis of HSSC short columns represents a significant advancement in the analytical treatment of double-confinement composite structures. The derived simplified formula provides a practical tool for engineers, bridging the gap between complex theoretical analysis and practical design applications. The good agreement between the calculated and experimental results validates the analytical approach and provides confidence in its application to the design of HSSC members.

The HSSC structural system offers promising advantages in terms of load-bearing capacity, ductility, and durability, but its successful implementation requires careful attention to fabrication quality, particularly the concentricity of the steel tubes and the quality of the concrete filling. The welding quality of the critical connections must be ensured through rigorous quality control and non-destructive testing procedures. As the understanding of HSSC structural behavior continues to evolve, further research is needed to address the long-term performance, fatigue behavior, and seismic response of these advanced composite members.