Multi-Chamber Square Steel Tube Concrete Short Columns Under Axial Compression

Literature Overview

This paper by Li Bin, Xing Xin, and Luo Kaini from Inner Mongolia University of Science and Technology and MCC Huatian Baotou Design and Research Institute presents experimental and numerical investigations on nine multi-chamber square steel tube concrete (CFST) short columns subjected to axial compression. The study was funded by the National Natural Science Foundation of China (51768056) and the Inner Mongolia Natural Science Foundation (2018MS05041), published in the journal "Advances in Steel Structure" in 2021 (Volume 23, Issue 1, pages 13-17). The research addresses a critical engineering challenge: how to enhance the ductility and deformation capacity of square CFST columns through internal chamber partitioning, while systematically evaluating the influence of chamber quantity and the wall thickness ratio between the outer steel tube and internal partition plates.

Core Technical Content and Key Findings

The fundamental problem addressed here is the premature local buckling failure observed in conventional single-chamber square CFST columns. In typical single-chamber square CFST specimens, outward bulging (drumming) initiates before the ultimate load capacity is reached. Once the ultimate capacity is achieved, this bulging accelerates rapidly, leading to weld cracking and premature failure. The confining effect of the steel tube on the core concrete is not fully mobilized before specimen failure occurs. This is a well-known limitation of square-section CFST members compared to circular sections, where the uniform confinement geometry provides more effective restraint against lateral expansion.

The proposed solution involves installing internal partition plates (diaphragms) within the square steel tube to create multiple chambers. This partitioning strategy fundamentally alters the failure mechanism by providing intermediate lateral support to the steel tube walls, effectively reducing the unsupported length of the tube walls and delaying local buckling. The experimental results demonstrate that specimens with internal partitions exhibit significantly improved ductility and deformation capacity in the later stages of loading, with material resources being more fully utilized.

A particularly insightful finding concerns the optimal wall thickness ratio, defined as t1 = (thickness of outer steel tube) / (thickness of internal partition plates). The study tested ratios of 1, 1.5, and 2.5, and found that a ratio of 1.5 yields the best deformation performance. This result has direct implications for material efficiency optimization in engineering design.

Parametric Study and Technical Parameters

The wall thickness ratio finding deserves deeper engineering interpretation. When t1 = 1.0, the partition plates are as thick as the outer tube, which provides adequate partition strength but may introduce stress concentrations at the weld junctions between partitions and the outer tube. When t1 = 2.5, the partitions are relatively thin, which may reduce their effectiveness in providing lateral support to the tube walls. The optimal value of t1 = 1.5 represents a balance between partition rigidity and weld joint quality, suggesting that partition plates should be designed at approximately two-thirds the thickness of the outer tube wall.

Engineering Practice Implications

From a steel pipe manufacturing and welding perspective, this research carries several important practical implications. First, the internal partition plates require precise welding to the inner surface of the square steel tube. The welding process must ensure full penetration and sound weld quality at these internal junctions, as weld defects at partition-tube interfaces would become critical stress concentrators under compressive loading. Common welding methods suitable for such internal configurations include SMAW (shielded metal arc welding) with appropriate electrode selection, or potentially robotic GTAW (gas tungsten arc welding) for automated production environments.

Second, the fabrication of multi-chamber square CFST columns introduces additional geometric complexity. The internal partitions must be cut, formed, and positioned with high dimensional accuracy to ensure uniform chamber dimensions. Deviations in partition positioning can lead to asymmetric loading conditions and premature local buckling in specific chambers.

Third, the finding that more chambers generally improve deformation capacity suggests a design philosophy shift. Rather than relying solely on increasing wall thickness to improve performance, engineers can achieve comparable or superior results through geometric optimization via internal partitioning, potentially reducing overall steel consumption. This has direct cost implications for large-scale construction projects.

Quality Control Considerations

The internal partition welding represents a significant quality control challenge. Non-destructive testing (NDT) methods must be carefully selected for internal weld inspection. Ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) are particularly suitable for detecting lack of fusion, porosity, and cracks in internal partition welds. Radiographic testing (RT) may be challenging due to geometric access limitations, making UT-based methods the preferred approach. Visual inspection (VT) combined with magnetic particle testing (MT) can supplement the NDT program for surface-breaking defect detection.

Study Insights and Reflections

The most valuable contribution of this research is the systematic identification of the wall thickness ratio as a critical design parameter that has been largely overlooked in previous CFST studies. The finding that t1 = 1.5 is optimal, rather than the intuitive assumption that thicker partitions are always better, demonstrates the importance of parametric optimization in structural design. For practicing engineers, this suggests that internal partition design should not be treated as a secondary consideration but should be integrated into the primary design process with careful attention to thickness proportionality.

The research also highlights a fundamental tension in CFST design: the desire to maximize ductility through geometric optimization versus the practical constraints of fabrication complexity and cost. Multi-chamber designs introduce additional welding operations, increased inspection requirements, and more complex fabrication sequences. Engineers must carefully weigh the structural performance gains against the incremental fabrication costs, particularly for large-scale projects where thousands of columns may be required.