Axial Local Compression Mechanical Properties of Conical Hollow Sandwich Concrete-Filled Steel Tube Short Columns with End Plates

Literature Overview

The paper by Ren Qingxin, Lv Yanbo, Jia Lianguang, and Mo Yaqing from the School of Civil Engineering at Shenyang Jianzhu University investigates the axial local compression behavior of conical hollow sandwich concrete-filled steel tube (CHSCFST) short columns with end plates. Published in the journal "Industrial Construction" (工业建筑) in 2013 (Vol. 43, No. 4, pp. 149–155), this work was supported by the Liaoning Provincial Doctoral Start-up Fund (20091064), the Ministry of Housing and Urban-Rural Development Science and Technology Program (2011-K2-16), and the National Natural Science Foundation of China (51208135A).

Structural Configuration and Research Motivation

The conical hollow sandwich CFST (CHSCFST) is a composite structural member that combines three components:

Inner steel tube — Provides inner confinement and acts as a formwork during construction
Outer steel tube — Provides outer confinement and load-bearing capacity
Sandwich concrete layer — Filled between the inner and outer tubes, providing additional confinement and load transfer

The conical geometry introduces a taper ratio that varies along the column length, creating a non-uniform cross-section. The hollow core (formed by the inner tube) reduces self-weight while maintaining structural integrity. The end plates are critical components that ensure load transfer and confinement at the column ends.

This configuration is particularly relevant for:

Industrial structures requiring non-uniform load paths (e.g., silos, towers)
Seismic-resistant structures where the conical geometry provides progressive energy dissipation
Applications where weight reduction is critical but structural capacity must be maintained

Finite Element Modeling

The authors used ABAQUS finite element analysis software to model the CHSCFST short columns under axial local compression. The numerical model was validated against experimental results, and the following comparison was reported:

Parameter	Experimental	Finite Element	Deviation
Load-displacement curve shape	Full process	Full process	Good agreement
Failure mode	Observed	Predicted	Consistent
Peak load	Measured	Calculated	Within acceptable range

The good agreement between FE results and experimental data validates the numerical model's reliability for parametric studies.

Stress Distribution Analysis

The FE analysis revealed the following stress distribution characteristics during the full loading process:

Upper End Plate Stress Distribution

The upper end plate experiences non-uniform stress distribution due to the local compression loading. The stress concentration occurs near the loaded area, with gradual stress diffusion toward the column body. The end plate stiffness significantly influences the load transfer efficiency.

Inner and Outer Steel Tube Stress Distribution

Outer tube: Carries the majority of the axial load, with higher compressive stress in the region directly beneath the loaded area. The stress distribution becomes more uniform as the load propagates downward.
Inner tube: Experiences lower stress levels due to the load transfer path through the sandwich concrete. The inner tube acts primarily as a confinement element rather than a primary load-bearing component.

Sandwich Concrete Stress State

The sandwich concrete layer is subjected to a complex multiaxial stress state:

Axial compression from the end plate loading
Lateral confinement from both the inner and outer steel tubes
Shear stress at the steel-concrete interfaces due to differential deformation

Steel-Concrete Interaction

The interaction between the inner/outer steel tubes and the sandwich concrete was analyzed in detail. The key findings include:

Friction and dilation at the interfaces contribute to load transfer
The concrete dilation is restrained by the steel tubes, creating a confinement effect
The confinement effect is more pronounced in the sandwich layer compared to conventional CFST because of the double-tube configuration

Parametric Study Results

The authors conducted a systematic parametric study investigating the influence of five key parameters on the ultimate load capacity and load-displacement behavior:

Parameter	Effect on Ultimate Load	Effect on Load-Displacement Curve
Taper ratio (cone angle)	Higher taper ratio increases load capacity	Steeper ascending branch
Material strength (concrete and steel)	Higher strength increases load capacity	Higher peak load, potentially reduced ductility
Hollow ratio (inner tube diameter/outer tube diameter)	Moderate hollow ratio optimizes capacity-to-weight	Excessive hollow ratio reduces capacity
Local compression area ratio	Higher area ratio increases capacity	More uniform stress distribution
End plate stiffness	Higher stiffness increases capacity	More efficient load transfer, sharper peak

Key Parametric Insights

Taper ratio: The conical geometry creates a natural load-spreading mechanism. As the taper ratio increases, the effective loaded area at any cross-section increases, distributing the local compression more evenly and reducing stress concentrations.
Hollow ratio: There exists an optimal hollow ratio that balances weight reduction with structural capacity. Excessive hollow ratios compromise the sandwich concrete layer thickness, reducing the confinement effect and the effective cross-sectional area for load transfer.
End plate stiffness: The end plate acts as a load-distribution element. Insufficient stiffness leads to local buckling of the steel tubes at the loaded area, while excessive stiffness may cause stress concentration at the plate-column interface. The optimal stiffness ensures efficient load transfer without premature failure.

Engineering Practice Implications

Design optimization: The parametric study provides quantitative guidance for optimizing the CHSCFST column design. Engineers can use the identified parameter influences to select appropriate geometric and material parameters for specific applications.
Local compression vulnerability: The study highlights that local compression is a critical failure mode for CHSCFST columns, particularly when the loaded area is small relative to the cross-section. Design codes should incorporate local compression checks for such members.
End plate design: The end plate stiffness is a critical design parameter that is often overlooked. Engineers should ensure adequate end plate thickness and connection details to prevent premature failure at the column ends.

Critical Reflection

The study relies heavily on finite element analysis for the parametric investigation, which is efficient but requires careful validation. The FE model's accuracy depends on the constitutive models used for concrete and steel, as well as the contact modeling between the steel tubes and concrete. The authors should have provided more details on the material models and contact algorithms used. Additionally, the study would benefit from experimental validation of the parametric trends, particularly for extreme parameter combinations.

Study Insights

This paper contributes to the understanding of a relatively novel composite structural configuration — the conical hollow sandwich CFST. The combination of conical geometry, hollow core, and double-tube sandwich construction creates a structurally efficient member with unique mechanical behavior. The parametric study provides practical design guidance, while the stress distribution analysis offers insights into the load transfer mechanisms. Engineers working on innovative composite structures should consider the CHSCFST concept for applications where non-uniform loading, weight reduction, and enhanced confinement are required. The study also emphasizes the importance of end plate design in ensuring the structural integrity of composite columns.