Zhuojin Pipe Fitting Co., LtdStress Mechanism Analysis of Conical Hollow Sandwich Steel Pipe Concrete Axially Compressed Short Columns
Literature Overview
This paper by Wang Wenda, Zhang Chaofeng, Wang Jingxuan, and Zheng Long, published in Journal of Architecture and Civil Engineering (2019, Vol. 36, No. 3, pp. 37-45), presents a finite element analysis of conical hollow sandwich steel pipe concrete (CHSSC) axially compressed short columns. The research was supported by the National Natural Science Foundation of China (Grant 51768038) and the Gansu Provincial University Collaborative Innovation Team Program (Grant 2018C-08), originating from the School of Civil Engineering at Lanzhou University of Technology.
Finite Element Model Validation
The authors developed a detailed finite element model using ABAQUS software and validated it against existing experimental data. The validation process is critical, as the credibility of any numerical study depends on the accuracy of the underlying model. The model captures the geometric nonlinearity of the conical configuration, the material nonlinearity of both steel and concrete, and the interaction between the inner steel pipe, concrete annulus, and outer steel pipe.
Load-Strain Curve Characteristics
The load-strain relationship of the typical CHSSC specimen exhibits four distinct stages:
| Stage | Description | Dominant Mechanism |
|---|---|---|
| Elastic stage | Linear load-strain relationship | Elastic deformation of all components |
| Elastoplastic stage | Nonlinear but increasing load | Progressive yielding of steel and concrete |
| Descending stage | Load decreases with increasing strain | Local buckling of outer steel pipe |
| Stable stage | Load stabilizes at a lower level | Post-buckling equilibrium |
This four-stage behavior is characteristic of composite steel-concrete members but is modified by the conical geometry and the hollow sandwich configuration. The presence of both an inner and outer steel pipe creates a unique load-sharing mechanism that differs from conventional SRC or HSSC columns.
Failure Mode and Load Distribution
The failure mechanism is dominated by outward buckling of the outer steel pipe, with the critical section located at approximately 0.8H from the base (where H is the column height). This asymmetric failure location is directly related to the conical geometry, which creates a non-uniform stress distribution along the column height.
| Parameter | Effect on Failure Section Location |
|---|---|
| Increasing cone angle | Failure section moves toward column top |
| Increasing hollow ratio | Failure section moves toward column top |
| Increasing outer pipe D/t ratio | Failure section moves toward column top |
| Increasing concrete strength | Minor effect on failure section location |
Interaction Force Analysis
The paper provides detailed analysis of the interaction forces between the steel pipes and the concrete core:
Outer steel pipe to concrete interaction force: Influenced by hollow ratio, cone angle, outer pipe diameter-to-thickness ratio, outer pipe yield strength, and concrete strength. The outer pipe provides the primary confinement to the concrete annulus, and the interaction force is a direct measure of this confinement effectiveness.
Inner steel pipe to concrete interaction force: Influenced by hollow ratio, cone angle, and outer pipe yield strength. The inner pipe's interaction with concrete is primarily through friction and mechanical interlock, as the inner pipe does not directly confine the concrete annulus.
Inner steel pipe D/t ratio and yield strength: These parameters have minimal influence on the interaction forces between concrete and both the inner and outer steel pipes. This finding is practically significant, as it suggests that the inner pipe dimensions can be optimized for other criteria (such as cost or weight) without significantly affecting the structural performance.
Engineering Implications for Fabrication and Welding
The conical hollow sandwich configuration presents unique fabrication challenges:
- Conical pipe fabrication: The conical geometry requires specialized forming processes such as hydroforming, spinning, or multi-pass rolling. Each method introduces different residual stress patterns and microstructural gradients in the pipe wall, which must be considered in the welding design.
- Multi-layer welding: The sandwich configuration with inner and outer steel pipes requires careful welding sequence planning. The welds between the inner pipe and concrete, and between the outer pipe and concrete, must be designed to accommodate differential thermal expansion and contraction.
- Dimensional tolerances: The interaction forces between components are sensitive to geometric parameters. Tight dimensional tolerances on the pipe diameters, wall thicknesses, and concentricity are essential to achieve the predicted structural performance.
- Post-weld heat treatment: The residual stresses from welding may interact with the complex stress state in the conical configuration. Post-weld stress relief treatment may be necessary to prevent premature yielding or buckling under service loads.
Key Reflections
This paper provides valuable insights into the mechanical behavior of a relatively novel composite structural configuration. The conical hollow sandwich design offers potential advantages in terms of weight optimization and load distribution, particularly for applications requiring variable cross-sections such as bridge piers or chimney stacks.
The finding that the inner pipe parameters have minimal influence on interaction forces is particularly useful for design optimization. It suggests that the inner pipe can be designed primarily for its role in carrying axial load, while the outer pipe governs the confinement behavior. This separation of functions simplifies the design process and may lead to more economical solutions.
However, I note that the study is limited to finite element analysis, even though the model was validated against experimental data. The actual fabrication of conical hollow sandwich columns introduces additional complexities—such as the welding of conical-to-cylindrical transitions, the quality of the concrete-steel bond in the conical geometry, and the residual stress distribution from the forming and welding processes—that are not fully captured by numerical models.
For engineering practice, this paper should be used as a guide for understanding the fundamental behavior of CHSSC columns, but detailed design should incorporate additional considerations from fabrication practice, including weld design, heat treatment, and quality control procedures.