Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Frustum-Shaped Steel Tube Confined Concrete Short Columns
Literature Overview
The research by Huang Xin, Jiang Jingshan, Sun Tianyang, and Jiang Wei from Nanjing Institute of Technology investigates the axial compression behavior of frustum-shaped (tapered) steel tube confined concrete (SCC) short columns. Published in Building Structures (建筑结构), Vol. 52, No. 4, 2022, pp. 100-105, this study is funded by multiple National Natural Science Foundation grants (51778282, 51678292) and Jiangsu Province programs. The frustum-shaped SCC column is a relatively novel structural element that differs from conventional cylindrical SCC columns by having a tapered steel tube shell, where the diameter ratio between the top and bottom cross-sections of the core concrete serves as the primary design parameter.
Experimental Program and Specimen Configuration
Four specimens were tested: three frustum-shaped SCC short columns and one cylindrical SCC short column for comparison. The core concrete mid-section diameter was kept constant across all specimens, while the diameter ratio α (defined as the ratio of the top surface diameter to the bottom surface diameter of the core concrete) was varied as the primary parameter. This experimental design is methodologically sound because it isolates the effect of taper geometry on structural performance while maintaining a consistent mid-section reference.
| Specimen Parameter | Description | Typical Range |
|---|---|---|
| α (diameter ratio) | Top diameter / Bottom diameter of core concrete | Varies per specimen |
| θ (inclination angle) | Angle between steel tube inner wall and ground plane | Derived from α and column height |
| Core concrete mid-section diameter | Constant across specimens | Design-dependent |
| Steel tube material | Structural steel (Q235 or Q345 typical) | Per Chinese standard GB/T 700 or GB/T 1591 |
| Concrete grade | Ordinary Portland concrete | C30-C60 typical |
Test Results and Failure Mode Analysis
The failure mode of the frustum-shaped SCC columns was characterized by bulging of the steel tube at the upper end, followed by separation between the steel tube and the core concrete. During loading, cracks developed in the core concrete and ultimately led to crushing. A notable observation was that as α increased (meaning a more pronounced taper with a larger top diameter relative to the bottom), the core concrete retained more of its original shape after loading. This suggests that the tapered geometry provides a more favorable confinement distribution as α increases.
The axial load-strain curves of the frustum-shaped SCC columns were found to be fundamentally similar to those of the cylindrical SCC column. This is an important finding because it indicates that the tapered geometry does not fundamentally alter the stress-strain response, which simplifies the analytical treatment. The frustum-shaped columns exhibited greater ductility compared to the cylindrical counterpart, which is beneficial for seismic design applications.
The ultimate bearing capacity increased with increasing α. This trend can be attributed to the enhanced confinement effect provided by the tapered steel tube geometry. As α increases, the steel tube wall at the upper portion (which is wider) provides greater lateral restraint to the concrete, thereby increasing the triaxial compressive strength of the confined concrete.
Theoretical Model Development
Based on the steel tube yielding criterion, the authors derived a simplified static equilibrium method to calculate the ultimate axial compression capacity of the frustum-shaped SCC short column. The derived formula incorporates both the diameter ratio α and the inclination angle θ between the steel tube inner wall and the ground plane. This analytical approach is practical because it avoids the complexity of full three-dimensional finite element analysis while capturing the essential mechanics of the tapered confinement.
The theoretical model rests on the following key assumptions:
- The steel tube and core concrete deform together without slip until the steel tube yields.
- After yielding, the steel tube provides uniform lateral confinement pressure on the core concrete.
- The confined concrete follows the Mander or similar confined concrete stress-strain model.
- The taper geometry modifies the effective confinement pressure through the inclination angle θ.
The verification of the formula against experimental data confirmed its rationality, though engineers should note that the accuracy of such simplified models depends on the specific material properties and geometric proportions used.
Engineering Practice Considerations
From my perspective in structural steel and composite construction, the frustum-shaped SCC column represents an interesting geometric variant that could find applications in architectural structures where tapered columns are aesthetically desirable. The key engineering considerations include:
| Design Consideration | Implication | Recommended Approach |
|---|---|---|
| Fabrication of tapered steel tube | Requires specialized rolling or forming equipment | Coordinate with steel tube manufacturer early in design |
| Concrete placement in tapered cavity | Potential for voids at the narrow end | Use vibrators with appropriate probe size; consider self-compacting concrete |
| Connection details at tapered ends | Non-standard connection geometry | Design custom connection plates or sleeves |
| Quality inspection of tapered tube | Non-uniform wall thickness measurement | UT thickness gauging at multiple angular positions |
The fabrication of tapered steel tubes presents challenges in terms of dimensional accuracy. The wall thickness uniformity must be verified at both the larger and smaller diameter ends, as well as at intermediate positions. Ultrasonic thickness testing should be performed at a minimum of three circumferential locations along the length of the tube to ensure adequate material distribution.
Study Insights and Reflections
The finding that the frustum-shaped SCC column exhibits greater ductility than the cylindrical counterpart is particularly encouraging for seismic design applications. In earthquake-resistant design, ductility is often more valuable than raw strength because it enables energy dissipation through inelastic deformation. The tapered geometry appears to promote a more distributed yielding pattern rather than a concentrated failure at a single section.
One aspect that I find particularly noteworthy is the relationship between α and the post-failure integrity of the core concrete. The observation that higher α values result in more intact core concrete after loading suggests that the tapered geometry provides a more gradual transition of confinement forces, reducing the risk of sudden concrete crushing. This progressive failure characteristic is desirable from both a safety and a repairability standpoint.
The theoretical model developed in this study, while simplified, provides a practical tool for preliminary design calculations. However, I would recommend that engineers supplement this model with finite element analysis for critical structural applications, particularly when the taper angle is steep or when the steel tube slenderness ratio is high. The interaction between the tapered steel tube and the concrete under cyclic loading conditions remains an area that warrants further investigation.
This study makes a meaningful contribution to the understanding of geometric effects on SCC column performance and opens avenues for the use of tapered steel tubes in structural applications where both functional and aesthetic requirements must be met.