Pure Bending Mechanical Properties of Circular Hollow Sandwich Steel Tube Concrete Members

Literature Overview

The research by Huang Hong, Tao Zhong, and Han Linhai, published in Industrial Construction in 2006 (Vol. 36, No. 11, pp. 15–18), investigates the pure bending behavior of circular hollow sandwich steel tube concrete (CHS-SC) members. Five specimens with varying void ratios were tested under pure bending conditions, and numerical analysis was performed using established stress-strain models for steel and concrete. The study proposes simplified formulas for flexural capacity based on parametric analysis.

Core Technical Content and Methodology

Specimen Configuration

The hollow sandwich design involves an outer steel tube, an inner steel tube, and a concrete layer between them, with a hollow core. This configuration offers:

Reduced weight compared to solid steel tube concrete
Improved flexural efficiency through optimal material distribution
Potential for improved energy absorption characteristics
Design flexibility through adjustable void ratio

Specimen Parameter	Typical Range	Purpose
Outer tube diameter	200–300 mm	Primary load-bearing element
Outer tube wall thickness	8–12 mm	Controls flexural capacity
Inner tube diameter	80–120 mm	Secondary confinement
Inner tube wall thickness	4–6 mm	Additional confinement
Concrete layer thickness	20–40 mm	Core material
Void ratio	0.3–0.6	Design variable
Concrete strength	C40–C60	Core material property
Steel grade	Q235–Q345	Tube material

Pure Bending Test Setup

The pure bending test configuration ensures that the specimens are subjected to constant bending moment over the middle span, eliminating shear effects. This allows for direct measurement of flexural capacity and bending stiffness without the confounding influence of shear deformation.

Numerical Analysis Approach

The numerical model accounts for:

Bilinear steel stress-strain relationship with strain hardening
Concrete stress-strain model with confinement effect
Interface behavior between steel tubes and concrete
Geometric nonlinearity at large deformations

Key Technical Points and Interpretation

Effect of Void Ratio on Flexural Performance

The void ratio is the primary design parameter investigated in this study. The relationship between void ratio and flexural properties is non-linear:

Low void ratio (0.3–0.4): High flexural capacity but also high weight; bending stiffness is maximized
Medium void ratio (0.4–0.5): Optimal balance between capacity and weight; improved strength-to-weight ratio
High void ratio (0.5–0.6): Reduced capacity but significantly lighter; potential for reduced bending stiffness

The study demonstrates that the hollow sandwich configuration can achieve comparable flexural capacity to solid sections while reducing weight by 15–30%, depending on the void ratio selected.

Bending Stiffness and Capacity

The flexural stiffness (EI) and plastic moment capacity (Mp) are the two primary performance indicators:

Elastic stiffness: Dominated by the outer tube geometry; relatively insensitive to void ratio for moderate values
Plastic capacity: Increases with concrete strength and confinement; the inner tube provides additional confinement to the concrete layer
Post-peak behavior: The hollow sandwich configuration exhibits more gradual post-peak degradation compared to solid sections due to the distributed failure mode

Welding Considerations for Sandwich Construction

The hollow sandwich steel tube concrete construction involves multiple weld joints:

Weld Type	Location	Critical Parameters
Outer tube longitudinal weld	Outer tube circumference	Full penetration, HAZ toughness
Inner tube longitudinal weld	Inner tube circumference	Full penetration, dimensional accuracy
End plate welds	Both ends of assembly	Full penetration, distortion control
Internal support welds	Between inner and outer tubes	Spot welds or continuous fillet welds
Concrete interface	Between tubes and concrete	Bond strength, not a weld per se

The quality of each weld type directly affects the structural performance:

Outer tube weld quality: Determines the ultimate flexural capacity; defects can initiate fracture at the tension zone
Inner tube weld quality: Affects the confinement effectiveness; poor welds can lead to premature separation
End plate welds: Critical for load transfer and boundary condition accuracy; distortion can affect test results
Internal support welds: Maintain the concentricity of inner and outer tubes; inadequate welds can cause eccentricity and reduced capacity

Connection with Engineering Practice

Design Optimization

For practical design of hollow sandwich steel tube concrete members, the following optimization criteria are recommended:

Weight efficiency: Maximize flexural capacity per unit weight by selecting optimal void ratio (typically 0.4–0.5)
Manufacturability: Consider welding accessibility and inspection requirements when selecting tube dimensions and weld configurations
Cost-effectiveness: Balance material cost, fabrication cost, and structural performance
Durability: Ensure adequate concrete cover and corrosion protection for both steel tubes
Seismic compatibility: Design for ductile behavior with adequate deformation capacity

Fabrication Quality Control

The fabrication of hollow sandwich members requires careful attention to:

Dimensional accuracy: Concentricity of inner and outer tubes within ±2 mm; straightness within 1:1000
Weld quality: 100% UT of all full-penetration welds; RT of critical joints; MT of all weld surfaces
Concrete placement: Proper vibration to ensure complete filling of the annular space; avoid voids and honeycombing
Curing: Adequate curing period before structural testing or installation; minimum 7 days for standard curing
Final inspection: Dimensional verification, surface quality check, and documentation of all weld inspection results

Application Scenarios

The hollow sandwich steel tube concrete configuration is particularly suitable for:

Long-span beam applications where weight reduction is critical
Seismic retrofitting of existing structures where added weight must be minimized
Bridge piers and columns where high flexural capacity with moderate weight is required
Special structures where energy absorption under impact loading is desired

Key Questions and Reflections

A fundamental question raised by this study is the long-term durability of the hollow sandwich configuration. The internal concrete layer, while providing confinement and additional capacity, is not directly accessible for inspection or repair. Any degradation of the concrete or corrosion of the inner tube would be difficult to detect and impossible to repair without disassembly.

Another reflection concerns the interaction between the inner tube and the concrete under cyclic loading. The relative slip between the inner tube and concrete, which may occur due to differential deformation, can lead to fatigue damage at the interface. This is particularly relevant for seismic applications where repeated loading cycles are expected.

Study Insights and Implications

The research on hollow sandwich steel tube concrete members demonstrates the potential of innovative composite configurations to achieve improved structural efficiency. The simplified flexural capacity formulas developed in this study provide practical design tools that can be incorporated into engineering software and design manuals.

For pipe fabrication and welding engineers, the hollow sandwich configuration presents unique challenges and opportunities. The multi-tube construction requires precise dimensional control and high-quality welds at multiple locations, but the resulting structural efficiency can justify the additional fabrication complexity. The key to successful implementation lies in rigorous quality control at every fabrication stage, from tube manufacturing through final assembly and inspection.

The study also highlights the importance of numerical modeling in understanding complex composite behavior. The validated analytical model can be extended to other loading conditions (axial compression, shear, torsion) and can be used for parametric optimization studies that would be impractical through physical testing alone. This integrated approach of experimentation, numerical analysis, and simplified design formulas represents the state-of-the-art in structural engineering research and provides a template for future investigations into advanced composite structural systems.