Zhuojin Pipe Fitting Co., LtdMechanical Performance of Conical Hollow Sandwich Steel Tube Confined Concrete Short Columns Under Axial Local Compression
Literature Overview
This paper by Ren Qingxin and colleagues from Shenyang Jianzhu University, published in Industrial Construction (2013, Vol. 43, Issue 4, pp. 144–148), investigates the mechanical behavior of conical hollow sandwich steel tube confined concrete (CHS-STCC) short columns subjected to axial local compression. The research was supported by the Liaoning Provincial Doctoral Startup Fund (20091064), the Ministry of Construction Science and Technology Program (2011-K2-16), and the National Natural Science Foundation of China (51208135). The authors established a finite element (FE) model that was validated against experimental data, and then systematically analyzed the composite action between the steel tube and the interstitial concrete, the failure modes of the short columns, and the influence of key geometric and material parameters on the load-displacement response.
Core Technical Content
The CHS-STCC cross-section is a hybrid structural configuration that combines a conical outer steel shell with an inner hollow core filled with concrete. This geometry is particularly relevant for applications such as wind turbine towers, transmission line poles, and variable-diameter support columns where tapering is required for aerodynamic or load-path reasons. The "hollow sandwich" designation implies that the concrete is confined within a conical steel tube, and the hollow core reduces self-weight while maintaining structural integrity.
The finite element model was developed to capture three critical aspects:
- The nonlinear material behavior of both steel and concrete under multiaxial stress states.
- The contact interaction and slip between the steel tube inner surface and the concrete core.
- The progressive failure mechanism including local buckling of the conical shell, concrete crushing, and interface debonding.
Key Parameters Analyzed
The authors examined several influential parameters on the load-displacement curve:
| Parameter | Description | Effect on Behavior |
|---|---|---|
| Conical angle | Half-angle of the cone taper | Larger angles reduce axial stiffness and increase lateral deformation |
| Steel tube thickness | Wall thickness of the conical shell | Thicker walls improve confinement and delay local buckling |
| Concrete strength | Compressive strength grade of core concrete | Higher grades increase peak load but may reduce ductility |
| Hollow core diameter | Inner diameter of the hollow section | Larger cores reduce concrete volume and confinement effectiveness |
| Local compression ratio | Ratio of loaded area to total cross-section | Higher ratios concentrate stress and accelerate failure |
Failure Mechanism and Composite Action
The failure process of CHS-STCC short columns under axial local compression typically follows a multi-stage progression. Initially, the loaded region experiences elastic compression, and the steel tube and concrete share the load proportionally. As the load increases beyond the yield point, the concrete core begins to develop micro-cracking, and the steel tube transitions into the plastic range. At this stage, the conical geometry introduces a radial component of stress, which can either enhance or diminish confinement depending on the conical angle and loading eccentricity.
The composite action between the steel tube and the interstitial concrete is governed by the bond-slip mechanism at their interface. In conical geometries, the taper angle creates a geometric interlock that can improve load transfer but also introduces stress concentrations at the transition zones. The FE analysis revealed that the confinement effect of the conical steel tube on the core concrete is not uniform along the column height; it is most effective near the loaded end where axial compressive stresses are highest, and progressively diminishes toward the unloaded end.
The failure mode observed in both experiments and simulations involved local buckling of the conical shell followed by concrete crushing in the confined region. The hollow core, while reducing weight, also creates a stress concentration zone at the inner edge of the conical shell, which can initiate premature buckling under high compressive loads. This is a critical design consideration that must be addressed through adequate wall thickness or local stiffening.
Engineering Practice Implications
From a steel pipe manufacturing and welding perspective, the conical geometry of the CHS-STCC column presents several fabrication challenges. The tapering of the steel tube requires either cold-forming or hot-forming processes, both of which can introduce residual stresses and microstructural variations in the steel material. For welded conical tubes, the weld seams are typically located along the generatrix of the cone, and the angular deviation from a straight pipe introduces asymmetric welding conditions that can lead to uneven weld penetration and residual stress distributions.
The study's findings have direct relevance to the design of tapered support structures in civil engineering. Engineers must carefully balance the benefits of the conical geometry (weight reduction, aesthetic considerations, aerodynamic performance) against the risks of reduced confinement effectiveness and potential premature failure. The FE model developed in this paper provides a valuable tool for parametric design optimization, allowing engineers to determine optimal combinations of conical angle, wall thickness, and concrete grade before physical fabrication.
Study Insights and Reflections
This paper contributes to the understanding of a relatively niche but practically important structural configuration. The combination of conical geometry, hollow core, and composite action creates a complex structural behavior that is not well captured by existing design codes. The FE-based approach adopted by the authors is well-suited to this problem, as analytical solutions for conical composite columns are extremely difficult to derive.
One area that deserves further investigation is the effect of welding residual stresses on the performance of conical steel tubes. In practice, the steel tubes used in CHS-STCC columns are rarely seamless; they are typically longitudinally welded (HFW or LSAW). The residual stress pattern from welding can significantly affect the local buckling resistance of the conical shell, particularly in the early loading stages where the structure is still in the elastic range. Future research should integrate welding residual stress fields into the FE models to provide more realistic predictions of structural performance.
Additionally, the paper does not address the long-term behavior of CHS-STCC columns under sustained loads or cyclic loading. In practical applications such as wind turbine towers, these columns are subjected to fatigue loading and environmental degradation, which can affect the composite action between steel and concrete over time. A comprehensive design methodology should account for these factors.
The study provides a solid foundation for the rational design of conical hollow sandwich steel tube confined concrete columns, and the FE methodology can be readily extended to more complex loading conditions and geometries.