Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Multi-Polygon Multi-Cavity CFST Self-Consolidating High-Strength Concrete Short Columns
Engineering Background and Project Motivation
This research by Xu Lihua and colleagues from Wuhan University, published in China Civil Engineering Journal in 2017, was motivated by the need to understand the structural behavior of multi-polygon multi-cavity steel tube columns used in the Tianjin Gaoyin 117 Tower, one of the tallest buildings in the world. The giant columns of this iconic structure employ a complex multi-polygon multi-cavity cross-sectional geometry filled with self-consolidating high-strength concrete (SC-HSC), representing a significant departure from conventional circular or square CFST column designs. Understanding the axial compression behavior of such novel cross-sections is essential for the safe and efficient design of super-tall buildings.
Specimen Design and Experimental Setup
Seven scaled specimens at a 1/20 scale were fabricated and tested under axial compression. The specimens represent a subset of the full column cross-sections used in the actual building, with parameters varied to isolate the effects of concrete strength, steel tube wall thickness, and the presence or absence of a steel reinforcement cage.
| Parameter | Variations | Engineering Rationale |
|---|---|---|
| Concrete strength | Multiple grades of SC-HSC | Evaluate contribution of concrete to axial capacity |
| Steel tube wall thickness | Multiple thicknesses | Assess confinement effect and ductility contribution |
| Steel reinforcement cage | Present vs. absent | Determine necessity of internal reinforcement |
| Specimen scale | 1/20 of actual column | Practical laboratory testing constraint |
| Loading condition | Axial compression | Primary loading mode for building columns |
The use of self-consolidating high-strength concrete is particularly noteworthy because SC-HSC eliminates the need for mechanical vibration during placement, which is advantageous for complex multi-cavity cross-sections where traditional vibration may be difficult to apply uniformly. The high strength of the concrete contributes significantly to the axial load capacity while the multi-cavity geometry provides additional confinement and redundancy.
Loading Stages and Failure Behavior
The axial compression test results reveal that the loading process of multi-polygon multi-cavity SC-HSC short columns can be divided into four distinct stages:
- Elastic stage: The load-deformation response is linear, and the steel tube and concrete work together in a proportional manner. No visible cracking or deformation occurs.
- Cracking stage: Micro-cracks begin to form in the concrete, particularly at the cavity corners where stress concentration is highest. The load-deformation curve begins to deviate from linearity.
- Yielding stage: The steel tube begins to yield locally, and the concrete enters a nonlinear stress-strain regime. The lateral expansion of the steel tube provides confinement to the core concrete, enhancing its compressive strength beyond the unconfined concrete strength.
- Post-peak stage: After reaching the ultimate load capacity, the column exhibits a gradual or sudden post-peak strength decline depending on the level of confinement provided by the steel tube wall thickness.
A particularly important observation is that up to 90% of the ultimate load capacity, the specimen geometry shows no visible deformation, indicating a high degree of reserve capacity and a ductile failure mode. This behavior is favorable for seismic design because it provides adequate warning before structural failure.
Parametric Influence on Axial Capacity
The parametric study reveals clear trends regarding the influence of each variable on the axial compression capacity:
| Parameter | Effect on Capacity | Effect on Ductility | Relative Importance |
|---|---|---|---|
| Concrete strength | Most significant increase in capacity | Limited effect on ductility | Highest for capacity |
| Steel tube wall thickness | Moderate increase in capacity | Significant improvement in ductility | High for ductility |
| Steel reinforcement cage | No significant effect on capacity | No significant effect on ductility | Low |
The finding that the steel reinforcement cage has no significant effect on axial capacity is noteworthy and has practical implications for construction cost optimization. In conventional reinforced concrete columns, the steel reinforcement cage is the primary load-bearing element, but in CFST columns, the steel tube itself provides the necessary tensile and confinement capacity, making the internal reinforcement cage potentially redundant for axial loading conditions.
Practical Capacity Calculation Formula
Based on the experimental results and referencing relevant domestic and international codes, the researchers developed a practical calculation formula for the axial compression capacity of multi-polygon multi-cavity SC-HSC short columns. The formula accounts for the combined contribution of the steel tube and the confined concrete core, with appropriate modification factors for the multi-cavity geometry and the self-consolidating concrete properties.
The development of a practical formula is of significant value to practicing engineers because it provides a direct design tool that can be applied in the preliminary design stage without requiring complex finite element analysis. The formula's validity is supported by the experimental data, and its application should be verified through detailed numerical analysis for critical structural elements.
Welding and Fabrication Considerations
From a fabrication and welding perspective, the multi-polygon multi-cavity geometry presents several challenges that warrant careful attention during construction. The multiple internal partitions and cavity walls require numerous T-joint and lap joints between steel plates, each of which must achieve full penetration and be inspected for defects. The complexity of the geometry increases the risk of weld defects such as lack of fusion, incomplete penetration, and angular distortion. Furthermore, the self-consolidating concrete must be placed through limited access openings into the complex internal cavity system, which requires careful planning of the pouring sequence and the placement of internal vibration-free concrete layers.
Study Insights and Engineering Value
This research bridges the gap between the innovative structural design of the Tianjin 117 Tower and the practical engineering requirements for designing similar multi-polygon CFST columns in other super-tall buildings. The finding that the steel reinforcement cage is not essential for axial compression capacity could lead to significant material savings in future projects, although its necessity for shear and seismic performance should be evaluated separately. The practical capacity formula developed in this study provides a valuable design tool, but engineers should recognize that the formula is calibrated against 1/20 scale specimens, and size effects on the behavior of full-scale columns should be considered in detailed design. The study contributes meaningfully to the structural engineering community's understanding of complex CFST cross-sections and demonstrates that multi-polygon multi-cavity configurations offer a viable and efficient structural solution for super-tall building applications.