Bending Performance of Square Hollow Sandwich Concrete-Filled Steel Tube Composite Members

Literature Overview

This paper by Li Minglun and colleagues from Shenyang Jianzhu University investigates the flexural behavior of square hollow sandwich concrete-filled steel tube (HSCFST) composite members through finite element analysis. The research is supported by multiple national and provincial funding bodies, including the National Natural Science Foundation of China (51808351), reflecting the significance of this hybrid structural system in modern construction engineering. The study addresses a practical need in structural design: how to optimize bending capacity while controlling material usage and self-weight through the innovative hollow sandwich configuration.

Core Technical Concept

The hollow sandwich concrete-filled steel tube concept involves an outer square steel tube with an inner concrete core, separated by an air gap or lightweight infill layer, creating a composite cross-section that leverages the confinement effect of the outer steel tube while reducing overall weight. Unlike conventional solid CFST columns, this configuration introduces additional complexity in stress distribution and load transfer mechanisms, particularly under flexural loading where bending moments create non-uniform compressive and tensile stresses across the section.

The authors establish five characteristic points on the load-deflection curve to systematically analyze the mechanical behavior at different stages of loading:

Characteristic Point	Loading Stage	Key Mechanical Feature
Point 1	Elastic stage	Linear stress distribution, no yielding
Point 2	Initial yielding	Steel tube begins yielding at extreme fibers
Point 3	Partial yielding	Concrete core begins to crack
Point 4	Yield plateau	Significant plastic deformation in steel and concrete
Point 5	Ultimate failure	Section reaches maximum bending capacity

Key Technical Parameters and Influencing Factors

The parametric study reveals several critical factors governing the bending performance of these members:

Hollow ratio: The ratio of the hollow section area to the total cross-sectional area directly influences both the bending stiffness and the ultimate capacity. Excessive hollow ratios reduce the effective concrete confinement zone and diminish the composite action between steel and concrete.
Reinforcement ratio: Longitudinal steel reinforcement significantly enhances the bending capacity, particularly in the tensile zone where it resists cracking and contributes directly to flexural resistance.
Material strength: The compressive strength of the concrete core and the yield strength of the outer steel tube both play decisive roles in determining the ultimate moment capacity.
Section width-to-height ratio: This geometric parameter affects the distribution of bending stresses and the likelihood of local buckling in the steel tube walls.

Bending Capacity Calculation Method

The authors propose a superposition-based calculation formula for the bending capacity of the HSCFST composite member. This approach decomposes the total bending resistance into contributions from individual components:

The bending resistance provided by the outer steel tube acting as a flexural member.
The bending resistance provided by the concrete core, accounting for the confinement effect from the steel tube.
The contribution of longitudinal reinforcement bars in both tension and compression zones.

The validation results demonstrate that the ratio of calculated to finite element results has a mean value of 1.030 and a standard deviation of 0.080, indicating good accuracy and consistency of the proposed formula. This level of accuracy (approximately 3% average deviation with 8% variability) is acceptable for practical engineering design purposes, particularly during preliminary design stages.

Engineering Practice Insights

From a steel pipe manufacturing perspective, the hollow sandwich configuration presents specific fabrication challenges. The outer square steel tube must be manufactured with precise dimensional tolerances to ensure proper fit with internal components. The welding joints connecting the outer tube to any internal support structures must maintain structural integrity under cyclic bending loads. Key quality control points include:

Dimensional accuracy of the square tube (side length tolerance, wall thickness uniformity)
Weld quality at any junction between the outer tube and internal reinforcement
Surface finish of the inner tube surface to ensure proper bond with concrete
Residual stress management during the welding process to avoid premature local buckling

The study's finding that reinforcement ratio and concrete strength have the most significant influence on bending capacity suggests that material selection and reinforcement detailing are more critical than geometric modifications for optimizing performance. This insight is valuable for engineers designing economical solutions that balance cost, constructability, and structural performance.

Study Reflections

This research contributes meaningfully to the understanding of hybrid sandwich structural systems, but several questions remain for future investigation. The finite element model validation appears adequate based on the reported accuracy metrics, yet experimental verification of the proposed calculation formula would strengthen confidence in its practical application. Additionally, the long-term durability of the hollow sandwich configuration under environmental exposure, particularly in corrosive atmospheres where the air gap could trap moisture, warrants further study. The proposed superposition method, while accurate for the studied parameters, may require modification for extreme geometric proportions or unusual loading conditions beyond the scope of this parametric study.