Axial Compression Mechanism of Hollow Sandwich Steel Tube Concrete Columns

Literature Overview

Published in 2006 in the Industrial Construction journal, this study by Huang Hong, Tao Zhong, and Han Linhai from Fuzhou University and Tsinghua University investigates the load-bearing mechanism of circular hollow sandwich steel tube concrete (HSCFT) columns under axial compression. The research was supported by the National Science Fund for Distinguished Young Scholars (grant 50425823) and Fujian Province high-level talent research startup funding. Using ABAQUS finite element software, the authors modeled the complete load-deformation relationship and analyzed how internal and external steel tubes share loads with the concrete core, examining the influence of hollow ratio, nominal steel ratio, and material strength on the interaction forces.

Core Technical Findings

The hollow sandwich concept represents an innovative structural system that combines the advantages of steel tube concrete columns with the weight reduction benefits of hollow sections. The key mechanical behavior analyzed includes:

The finite element results demonstrated good agreement with experimental data, validating the modeling approach for predicting the complete nonlinear behavior of HSCFT columns. The interaction force analysis reveals that the outer tube primarily resists the initial elastic load while the inner tube and concrete share increasing proportions of load as the column approaches ultimate capacity.

Interpretation of Technical Points

The hollow sandwich configuration creates a unique stress state compared to conventional solid CFT columns. The hollow core introduces an additional free surface that affects the confining pressure distribution on the concrete. In a solid CFT column, the steel tube provides uniform radial confinement to the concrete core. In the HSCFT configuration, the inner tube provides confinement to the annular concrete section between the inner and outer tubes, while the outer tube confines the concrete from the exterior. This dual-confinement mechanism creates a more complex stress field but potentially allows for more efficient material utilization.

The load sharing analysis reveals that during the elastic stage, the outer tube carries a disproportionately large share of the axial load due to its larger cross-sectional area. As concrete begins to crush and expand radially, the inner tube experiences increasing compressive force from the confined concrete, effectively transferring load from the outer tube to the inner tube through the concrete medium. This load redistribution mechanism is the fundamental basis for the composite action in HSCFT columns.

Process and Standards Analysis

The hollow ratio is a critical design parameter that balances weight reduction against structural capacity. From a manufacturing perspective, the inner tube of an HSCFT column must be precisely positioned and secured during construction to ensure uniform concrete fill in the annular space. This poses practical challenges in construction:

• Fabrication: The inner tube must be fabricated with dimensional tolerances consistent with ASME B16.9 or equivalent standards for steel tube components.
• Positioning: Temporary bracing must maintain concentricity between inner and outer tubes throughout the concrete pouring process.
• Concrete placement: The annular space between tubes requires specialized concrete placement methods to avoid voids and ensure uniform compaction.
• Quality inspection: Ultrasonic testing of the annular concrete section is more challenging than for solid CFT columns due to the presence of two steel interfaces.

The study's analysis of material strength effects highlights the importance of material compatibility. When the inner tube has significantly higher yield strength than the outer tube, the load redistribution may lead to premature yielding of the outer tube before the inner tube reaches its capacity, creating an inefficient design. Optimal material matching should consider the load redistribution ratio predicted by the finite element analysis.

Engineering Practice Integration

For engineers designing HSCFT columns in high-rise buildings or industrial structures, this study provides the following design guidance:

1. Hollow ratio selection: A hollow ratio between 0.3 and 0.5 provides the best balance of weight savings and structural efficiency. Ratios above 0.6 may compromise the composite action due to insufficient concrete confinement area.
2. Steel ratio optimization: The nominal steel ratio should be calculated considering both tubes combined. Ratios below 20% may not provide adequate composite action, while ratios above 50% may not justify the additional steel cost relative to capacity gain.
3. Finite element verification: All HSCFT column designs should be verified using nonlinear finite element analysis that captures the complete load-deformation behavior, including post-yield hardening and concrete crushing.
4. Construction monitoring: During concrete pouring, strain monitoring of both tubes should be conducted to verify that the actual load distribution matches the predicted behavior.

Key Questions and Reflections

The study raises important questions about the long-term durability of HSCFT columns. The hollow core, while beneficial for weight reduction, creates a potential pathway for moisture ingress that could accelerate corrosion of the inner tube. The annular concrete section between tubes may be more susceptible to carbonation and chloride penetration than a solid concrete core due to its reduced cross-section and increased surface-to-volume ratio. These durability concerns are not addressed in the study but are critical for practical application in aggressive environments.

Furthermore, the study focuses on axial compression behavior but does not extend to combined loading conditions (axial plus bending, axial plus shear) that are more representative of actual structural demands. The interaction between hollow ratio and lateral stability under eccentric loading could be significantly different from the axial compression case, warranting further investigation.

Study Insights and Implications

The hollow sandwich CFT column concept offers a promising structural system for applications where weight reduction is critical, such as tall buildings in seismic zones or long-span bridge piers. The finite element methodology validated in this study provides a reliable tool for predicting the complex composite behavior of HSCFT columns. However, practical adoption requires addressing the construction challenges of inner tube positioning, the durability concerns of the hollow core, and the extension of design methods to combined loading conditions. The study's parameter analysis framework is directly applicable to design optimization, allowing engineers to systematically evaluate the trade-offs between hollow ratio, steel ratio, and material strength for specific structural applications.