Mechanical Behaviour of Square Hollow Sandwich Steel Tube Concrete Eccentrically Compressed Columns

Literature Overview

This paper by Tao Zhong, Han Linhai, and Huang Hong from Fuzhou University, published in the China Civil Engineering Journal (Vol. 36, No. 2, 2003), presents a systematic experimental and analytical study on square hollow sandwich steel tube concrete (SHS-CFST) eccentrically compressed columns. The research was funded under the Fujian Province Key Science and Technology Program (2002H007). The authors conducted twelve full-scale tests with slenderness ratio and eccentricity ratio as the primary variables, establishing constitutive models for both the steel shell and core concrete before performing numerical analysis of the complete load-deformation response.

Core Technical Content

The sandwich construction concept involves an outer square steel tube, an inner steel tube, and concrete infilled between the two tubes. This configuration creates a composite column that leverages the confinement effect of the outer steel tube while providing additional structural efficiency through the inner tube. The study systematically varied two key geometric parameters:

The constitutive models developed for the steel and concrete components were derived from uniaxial test data and then applied within a numerical framework to simulate the full load-displacement curve. The theoretical predictions showed good agreement with experimental results, validating the modelling approach.

Interpretation of Key Technical Points

Confinement Mechanism in Sandwich Construction

The sandwich steel tube concrete column operates through a dual-confinement mechanism. The outer tube provides lateral confinement to the concrete core, enhancing its compressive strength and ductility. The inner tube contributes additional axial load capacity and helps distribute stresses more uniformly across the cross-section. This is particularly significant under eccentric loading, where bending stresses are superimposed on axial compression, creating a non-uniform stress distribution that the sandwich geometry helps mitigate.

Slenderness Ratio Effects

For slender columns, the load-bearing capacity is significantly reduced due to flexural buckling. The study demonstrates that the sandwich configuration maintains a more favourable strength-to-weight ratio compared to conventional solid CFST columns, particularly in the intermediate slenderness range. The hollow interior reduces self-weight without proportionally reducing the moment of inertia, since the outer tube retains its contribution to flexural rigidity.

Eccentricity Ratio and Failure Modes

Under eccentric compression, the failure mode transitions from crushing at low eccentricities to flexural buckling at higher eccentricities. The sandwich geometry introduces an additional complexity: the interface between the outer tube and the concrete can experience local buckling or delamination under high eccentric loads. The study's twelve specimens provide sufficient data to characterise this transition behaviour.

Engineering Practice Implications

The findings have direct relevance to the design of composite columns in high-rise buildings and long-span structures where weight efficiency is critical. The sandwich CFST column offers advantages in:

• Reduced dead load through the hollow interior, which is particularly beneficial for seismic design where inertial forces are proportional to mass.
• Improved fire resistance characteristics, as the hollow space can be filled with fire-resistant materials or used for service routing.
• Enhanced construction practicality, since the inner and outer tubes can be fabricated and transported separately before being assembled on site.

From a manufacturing perspective, the fabrication of sandwich CFST columns requires precise dimensional control of both tubes to ensure uniform concrete thickness in the annular space. The inner tube must be centred during pouring to avoid eccentric concrete distribution, which would compromise the confinement symmetry. This introduces quality control requirements related to positioning fixtures and concrete placement methods that warrant careful attention during construction.

Study Insights and Reflections

The research represents an early but important contribution to the understanding of sandwich composite columns in China. The approach of combining experimental testing with numerical analysis, validated through comparison, is a rigorous methodology that remains relevant. The use of constitutive models tailored to confined concrete conditions reflects the understanding that conventional unconfined concrete models significantly underestimate the post-peak behaviour of confined members.

A notable observation is that the sandwich configuration's performance is sensitive to the relative thicknesses of the inner and outer tubes. If the inner tube is too thin, it may not contribute meaningfully to load capacity; if too thick, it may interfere with the concrete placement and reduce the effective confinement zone. Optimising this ratio requires balancing structural efficiency with constructability.

The study's conclusions, while based on twelve specimens, provide a solid foundation for code provisions and design guidelines. However, further research on cyclic loading behaviour, connection details, and long-term durability under environmental exposure would be needed before widespread adoption in critical infrastructure applications.