Axial Compression Bearing Capacity of Variable Central Angle Circular-Ended CFT Short Columns

Literature Overview

Ren Zhigang, Wang Gaoyu, and Li Peipeng from Wuhan University of Technology (2020) investigate the axial compression behavior of concrete-filled steel tubes (CFT) with circular-ended cross-sections featuring variable central angles. Supported by the National Natural Science Foundation of China, this research extends the understanding of non-circular CFT cross-sections by introducing a parametric variable—the central angle of the circular end—that allows continuous variation between circular, semi-circular, and nearly rectangular cross-sections. The study combines experimental testing of four specimens with theoretical analysis based on the double-shear unified strength theory.

Core Technical Analysis

The variable central angle design allows engineers to optimize the cross-section shape for specific loading and geometric constraints. A central angle of 180 degrees corresponds to a semi-circular end (essentially a rectangular section with rounded corners), while smaller angles produce more pronounced circular features. The study reveals that the circular end portion and the flat segment of the steel tube exert different confining pressures on the core concrete, creating a non-uniform stress state that must be accounted for in bearing capacity calculations.

The proposed bearing capacity formula based on the double-shear unified strength theory and concrete strength zoning provides a theoretical framework that accounts for the non-uniform confinement. The formula shows good agreement with experimental results for central angles greater than 60 degrees, which covers the most practical range of cross-section designs.

Process and Materials Considerations

The fabrication of variable central angle CFT sections presents unique challenges compared to standard circular or rectangular CFT members:

1. Steel tube forming: The transition from circular to flat segments requires precise roll forming or bending operations. The forming process must control strain hardening and residual stress to avoid localized weakening. For seamless tubes, hot-rolling or cold-drawing with intermediate annealing is preferred. For welded tubes, the longitudinal weld should be positioned to minimize its effect on the cross-section geometry.
2. Welding of end caps: If the CFT member requires end caps or connection plates, the welding of these components to the variable cross-section requires careful joint design. The transition from circular to flat geometry creates varying gap conditions that must be accommodated in the welding procedure.
3. Concrete filling: The non-circular cross-section may create regions where concrete flow is restricted, particularly at the corners of the flat segments. The use of self-compacting concrete or vibratory compaction through access holes is recommended to ensure complete filling.
4. Steel grade effects: The study notes that increasing steel strength has a more pronounced effect on bearing capacity than increasing concrete strength. This is consistent with the general behavior of CFT members, where the steel tube provides the primary confinement, and higher-strength steel translates directly to higher confining pressure.

Study Insights and Engineering Implications

This research contributes to the growing body of knowledge on non-circular CFT cross-sections, which are increasingly used in architectural applications where aesthetic considerations require non-standard shapes. The finding that bearing capacity increases with central angle while ductility decreases highlights the fundamental trade-off between strength and ductility in structural design. Engineers must balance these competing objectives based on the specific structural requirements: for seismic zones, a lower central angle may be preferred to ensure adequate ductility, while for non-seismic applications, a higher central angle may be acceptable to maximize bearing capacity. The theoretical framework based on the double-shear unified strength theory is a significant advancement, as it provides a physically meaningful explanation for the observed behavior rather than relying on empirical correlations alone. The research also underscores the importance of material characterization: understanding how different steel grades interact with the concrete core is essential for accurate bearing capacity prediction, and this knowledge should inform material selection decisions during the design phase.