Compressive Stiffness of Double-Skin Hollow Steel Tube Concrete Columns

Literature Overview

The paper by Xia Guiyun and colleagues from Central South University and Changsha Traffic College, published in Journal of Chang'an University (2003, Vol. 23, No. 4), presents an elastic analysis of the compressive stiffness of double-skin hollow steel tube concrete (DSTC) columns. This innovative composite member consists of an outer steel tube, an inner steel tube, and a concrete core confined between the two steel tubes. The authors decompose the composite column into three components—concrete, inner steel tube, and outer steel tube—and analyze them as a quasi-planar axisymmetric problem. The study establishes a compressive stiffness calculation formula, derives the stress distribution ratios under axial loading, and investigates the influence of various parameters on the structural behavior. This research contributes to the understanding of a relatively novel composite structural system with potential applications in high-rise buildings, bridge piers, and offshore platforms.

Theoretical Analysis Framework

Problem Formulation

The double-skin hollow steel tube concrete column is analyzed as a quasi-planar axisymmetric problem. This simplification is justified by the axisymmetric geometry and loading conditions, reducing the three-dimensional problem to a two-dimensional analysis while preserving the essential mechanical behavior. The key assumptions include:

Radial displacement compatibility between concrete and both steel tubes
Longitudinal strain compatibility among all three components
Elastic behavior of all materials (linear analysis)
Uniform axial stress distribution within each component
No interfacial slip between concrete and steel tubes

Stress-Strain Relationships

For each component under axial loading:

Component	Axial Stress	Axial Strain	Radial Strain	Radial Stress
Concrete	σ_c	ε_c = σ_c/E_c	ε_r,c = -ν_c × ε_c	σ_r,c = E_c/(1-ν_c²) × (ε_r,c + ν_c × ε_c)
Inner steel tube	σ_si	ε_si = σ_si/E_s	ε_r,si = -ν_s × ε_si	σ_r,si = E_s/(1-ν_s²) × (ε_r,si + ν_s × ε_si)
Outer steel tube	σ_so	ε_so = σ_so/E_s	ε_r,so = -ν_s × ε_so	σ_r,so = E_s/(1-ν_s²) × (ε_r,so + ν_s × ε_so)

Compatibility Conditions

The critical compatibility conditions that link the three components are:

Longitudinal strain compatibility: ε_c = ε_si = ε_so = ε (all components experience the same axial strain)
Radial displacement compatibility at inner interface: The radial displacement of the inner steel tube's inner surface equals the radial displacement of the concrete's outer surface at the inner tube location
Radial displacement compatibility at outer interface: The radial displacement of the concrete's outer surface at the outer tube location equals the radial displacement of the outer steel tube's inner surface
Radial stress equilibrium: The radial stresses at interfaces must be in equilibrium between adjacent components

Key Technical Findings

Compressive Stiffness Formula

The authors derive a compressive stiffness formula for the double-skin hollow steel tube concrete column that accounts for the interaction between all three components through the compatibility conditions. The total axial stiffness (N/ε) is a function of:

Elastic moduli of concrete and steel
Poisson's ratios of concrete and steel
Geometric dimensions: inner tube inner radius, inner tube outer radius, outer tube inner radius, outer tube outer radius
Wall thicknesses of both steel tubes

The stiffness is not simply the sum of individual component stiffnesses but includes interaction terms that reflect the mutual confinement and restraint between components.

Stress Distribution Ratios

A remarkable finding is that the axial stress in the double-skin hollow steel tube concrete column is NOT distributed according to the elastic modulus ratio of the materials. This deviates from the common assumption in composite material design where stress is proportional to stiffness (E × A). The deviation arises because:

The radial displacement compatibility conditions create interfacial radial stresses that modify the axial stress distribution
The Poisson effect couples radial and axial behavior, creating interaction that is not captured by simple parallel spring models
The confinement effect of both steel tubes on the concrete core creates a triaxial stress state in the concrete that influences axial stress distribution

Parameter Influence Analysis

The study investigates the sensitivity of the stress distribution ratios to various parameters:

Parameter	Influence on Stress Distribution	Degree of Influence
Concrete Poisson's ratio (ν_c)	Moderate effect on stress ratios	Significant
Inner steel tube Poisson's ratio (ν_si)	Minor effect	Small
Outer steel tube Poisson's ratio (ν_so)	Minor effect	Small
Inner steel tube wall thickness	No significant effect on outer tube stress ratio	Negligible cross-effect
Outer steel tube wall thickness	No significant effect on inner tube stress ratio	Negligible cross-effect

The finding that inner and outer steel tube wall thicknesses have no significant influence on each other's stress distribution ratios is particularly important for design purposes. It means that the two steel tubes can be designed somewhat independently, with the concrete core serving as the primary load-sharing medium between them.

Comparison with Single-Skin CFST Columns

The double-skin configuration offers several potential advantages over conventional single-skin CFST columns:

Aspect	Single-Skin CFST	Double-Skin Hollow CFST
Concrete confinement	Single steel tube confinement	Dual confinement from inner and outer tubes
Steel utilization	Single steel tube carries axial load	Two steel tubes share axial load
Concrete volume	Full solid core	Reduced concrete volume (hollow center)
Weight	Heavier due to more concrete	Lighter due to reduced concrete
Manufacturing complexity	Simple	More complex (two tubes + concrete)
Inspection access	No internal access	Potential access through inner tube

The reduced concrete volume in the double-skin configuration can lead to significant material savings while potentially maintaining or improving structural performance through the dual confinement mechanism.

Engineering Practice Considerations

Manufacturing and Construction

The fabrication of double-skin hollow steel tube concrete columns presents unique challenges:

Tube alignment: Precise concentricity between inner and outer tubes is critical to ensure uniform concrete thickness and proper load distribution
Concrete placement: The annular space between tubes requires careful concrete placement and compaction to avoid voids and honeycombing
Concrete flowability: The confined annular space requires high-flow concrete or specialized placement methods to ensure complete filling
Curing: Uniform curing conditions are important for the concrete core, particularly in the confined annular space where moisture retention may differ from conventional elements
Quality verification: Non-destructive testing methods must be adapted to verify concrete quality within the confined space

Design Implications

From a design perspective, the findings suggest:

The elastic modulus ratio alone cannot be used to predict stress distribution in double-skin CFST columns
Poisson's ratio of concrete has a more significant influence on stress distribution than that of steel
The wall thicknesses of the two steel tubes can be optimized independently without significant interaction effects
The compressive stiffness formula should be used for accurate structural analysis rather than simplified parallel spring models
The dual confinement effect should be evaluated for its potential to enhance concrete strength beyond what single-skin confinement provides

Key Questions and Reflections

Several important questions emerge from this analysis that warrant further investigation. First, the elastic analysis presented here does not account for the nonlinear behavior that dominates the performance of CFST columns under high loads. The concrete confinement effect that increases concrete strength is inherently nonlinear and depends on the radial confining pressure, which varies with axial load level. Second, the long-term behavior under sustained loading—including concrete creep and its effect on stress redistribution between components—represents an area requiring dedicated study. Third, the seismic performance of double-skin CFST columns under cyclic loading is of considerable interest, particularly regarding the interaction between the two steel tubes during inelastic deformation. Fourth, the practical feasibility of manufacturing and inspecting these members at scale needs to be demonstrated through full-scale testing and construction experience.

Summary

This paper provides a rigorous elastic analysis of the compressive stiffness and stress distribution in double-skin hollow steel tube concrete columns. The key finding that axial stress is not distributed according to elastic modulus ratios, but rather depends on complex interaction effects governed by Poisson's ratios and geometric compatibility, challenges conventional assumptions in composite column design. The derived stiffness formula and stress distribution ratios offer tools for more accurate structural analysis of this innovative member type. The independence of the two steel tube wall thickness effects on each other's stress distribution provides design flexibility that could be exploited in practical applications. As the research community and engineering practice continue to explore advanced composite structural systems, the double-skin hollow CFST column represents a promising configuration that merits further investigation through nonlinear analysis, full-scale testing, and practical application experience.