Bending Performance of Conical Hollow Sandwich Steel Tube Concrete Members

Literature Overview

The paper by Liu Han, Shi Yanli, Fan Jiahao, and Wang Wenda from Lanzhou University of Technology investigates the pure bending behavior of conical hollow sandwich steel tube concrete members. Published in Progress in Steel Building Structures in 2021, the research was supported by the National Natural Science Foundation (51768038) and the Gansu Provincial Construction Science and Technology Plan Project (JK2021-42). This work addresses a specialized structural component that combines the advantages of sandwich construction with conical geometry, offering potential for efficient structural systems in applications requiring tapered members.

Structural Configuration and Geometry

The conical hollow sandwich steel tube concrete member consists of an outer steel tube and an inner steel tube with a concrete layer sandwiched between them, all arranged in a conical (tapered) configuration. This geometry differs from conventional cylindrical steel tube concrete members in that the cross-section varies along the member length, creating a tapered profile that can be advantageous in certain structural applications such as columns in tapering frames or members in spatial structures.

Geometric Parameters

Parameter	Symbol	Typical Range	Description
Outer tube diameter (large end)	D_o1	200–400 mm	Larger end diameter
Outer tube diameter (small end)	D_o2	100–200 mm	Smaller end diameter
Inner tube diameter (large end)	D_i1	100–200 mm	Inner tube at large end
Inner tube diameter (small end)	D_i2	50–100 mm	Inner tube at small end
Cone angle	α	5°–15°	Half-angle of taper
Hollow ratio	η	0.3–0.6	Ratio of inner to outer diameter
Length-to-diameter ratio	L/D	3–6	Slenderness parameter
Outer tube thickness ratio	t_o/D_o	0.015–0.03	Wall thickness parameter

Numerical Modeling and Validation

The authors conducted numerical simulations based on previously validated modeling methods, applying them to the specific geometry of the conical hollow sandwich member. The finite element model captures the complex stress state arising from the combination of conical geometry, sandwich construction, and pure bending loading.

Material Models and Contact Interface

The numerical model employs appropriate material models for each constituent:

Component	Material Model	Key Properties
Outer steel tube	Elastic-plastic with isotropic hardening	Yield strength, hardening modulus
Inner steel tube	Elastic-plastic with isotropic hardening	Yield strength, hardening modulus
Concrete core	Concrete damaged plasticity	Compressive/tensile strength, confinement effect
Contact surfaces	Penalty contact with friction	Friction coefficient, tangential stiffness

The contact interaction between the steel tubes and the concrete core is critical for capturing the composite action that provides the member with its enhanced performance. The frictional contact model allows for relative slip between the steel and concrete surfaces, which is particularly relevant in the conical geometry where differential deformation between the tubes and core can be significant.

Bending Performance and Load-Deflection Behavior

The analysis reveals that the conical hollow sandwich steel tube concrete member exhibits three distinct stages in its moment-deflection relationship:

Stage 1: Elastic Behavior

In the initial loading stage, all components behave elastically, and the moment-deflection relationship is linear. The composite action between the steel tubes and concrete core is fully effective, with no slip or separation at the interfaces. The effective stiffness of the member is determined by the combined flexural rigidity of all components.

Stage 2: Elastic-Plastic Transition

As loading increases, yielding begins in the outer steel tube at the location of maximum bending stress. The stress distribution redistributes as plasticity spreads, and the concrete core begins to carry a larger share of the load through composite action. The moment-deflection curve becomes nonlinear but continues to increase with displacement.

Stage 3: Plastic Behavior

In the final stage, extensive plastic deformation occurs, and the member reaches its ultimate bending capacity. The failure location shifts progressively toward the smaller cross-section as the cone angle increases, reflecting the reduced capacity at the tapered end. The member maintains load-carrying capacity through plastic hinge formation, demonstrating good ductility.

Parametric Analysis and Design Recommendations

The study systematically investigates the influence of several geometric and material parameters on the bending performance of the conical hollow sandwich member.

Effect of Cone Angle

Increasing the cone angle causes the failure location to shift progressively toward the minimum cross-section. This is expected, as the smaller end has less material and therefore lower capacity. However, the rate of capacity reduction with increasing cone angle is not linear, suggesting that there exists an optimal cone angle range that balances geometric efficiency with structural performance.

Cone Angle	Failure Location	Ultimate Capacity	Ductility
5°	Near midspan	Highest	Best
10°	Slightly toward small end	Moderate reduction	Slight reduction
15°	Near small end	Significant reduction	Reduced

Effect of Hollow Ratio

The hollow ratio—the ratio of inner tube diameter to outer tube diameter—has a relatively minor influence on the deformation curve of the member. This finding is somewhat counterintuitive, as increasing the hollow ratio reduces the concrete core area and therefore the composite contribution. However, the analysis suggests that within the studied range, the steel tubes provide sufficient flexural capacity that the concrete contribution is not dominant.

Effect of Other Parameters

Parameter	Influence on Capacity	Influence on Deformation
Outer tube thickness ratio	Moderate increase with thicker walls	Slight stiffness increase
Outer tube strength	Significant increase	Minimal change
Inner tube strength	Significant increase	Minimal change
Concrete strength	Small increase	Minimal change
Cone angle	Significant decrease with larger angle	Failure location shift
Hollow ratio	Moderate effect	Minor effect on deformation curve

Proposed Bending Capacity Calculation Method

Based on the numerical results, the authors propose a calculation method for the bending capacity of conical hollow sandwich steel tube concrete members. The method accounts for the composite action between the steel tubes and concrete core, as well as the geometric effects of the conical shape. The proposed formulas show good agreement with the numerical simulation results, providing a practical tool for preliminary design.

Engineering Applications and Considerations

The conical hollow sandwich steel tube concrete member offers several potential advantages for structural applications:

The tapered geometry allows for efficient material utilization in members with varying load demands along their length.
The sandwich construction provides enhanced fire resistance and corrosion protection compared to bare steel tubes.
The composite action between steel and concrete yields superior ductility and energy dissipation capacity.
The hollow configuration reduces self-weight while maintaining structural capacity.

However, several practical challenges must be addressed for widespread adoption:

Fabrication complexity: The conical geometry and sandwich construction require specialized manufacturing techniques.
Concrete placement: Ensuring complete concrete fill within the annular space between conical tubes presents a practical challenge.
Connection design: The tapered geometry complicates the design of connections to other structural members.
Quality control: Verifying the integrity of the concrete-steel interface in the tapered geometry requires appropriate inspection methods.

Study Insights and Reflections

This research addresses a specialized but potentially valuable structural component that combines multiple advanced concepts—conical geometry, sandwich construction, and steel-concrete compositing—into a single member type. The numerical approach, validated against existing test data, provides a reliable basis for understanding the behavior of these members under pure bending.

The finding that the hollow ratio has limited influence on deformation behavior is practically significant, as it suggests that designers have flexibility in selecting the hollow ratio without severely compromising performance. This flexibility can be leveraged to optimize other design objectives such as weight reduction or material cost.

The proposed bending capacity calculation method, while showing good agreement with numerical results, should be validated against experimental data before being adopted for design purposes. The authors acknowledge this limitation, and I would recommend that future work include full-scale or component-scale bending tests on conical hollow sandwich members to confirm the numerical predictions and the proposed design equations.

One area of potential concern is the long-term behavior of the concrete-steel interface under sustained loading and environmental exposure. The conical geometry may introduce differential thermal expansion effects that could lead to interface degradation over time. This aspect warrants further investigation before the component is adopted for applications requiring long service life.

In summary, this research provides valuable insight into the bending performance of conical hollow sandwich steel tube concrete members, offering both quantitative data and a proposed design method. The members demonstrate good ductility and capacity, with the cone angle and steel tube properties being the most influential parameters. While practical challenges remain in fabrication and quality control, the structural performance is promising, and further development of this component type could expand the range of available structural solutions for specialized applications.