Bearing Capacity of Variable Central Angle Round-Ended Steel Tube Concrete Axially Compressed Short Columns

Literature Overview

This research by Ren Zhigang, Wang Gaoyu, and Li Peipeng from Wuhan University of Technology investigates the bearing capacity of round-ended steel tube concrete (SRC) short columns with variable central angles under axial compression. Published in the Journal of Wuhan University of Technology in 2020, the study presents experimental results from four test specimens and proposes a bearing capacity formula based on the double-shear unified strength theory. The research addresses a unique structural configuration where the steel tube cross-section combines flat segments with rounded ends of varying central angles, creating a hybrid cross-section with distinct confinement characteristics.

Core Technical Analysis

Cross-Section Geometry and Confinement Mechanism

The round-ended SRC cross-section consists of:

• Flat segments: Rectangular portions of the steel tube that provide direct lateral confinement to the concrete core
• Round-end segments: Curved portions at the ends of the flat segments, characterized by a central angle that varies from the studied specimens

The confinement mechanism differs between these two regions:

The variable central angle of the round-end segments is the key geometric parameter studied. As the central angle increases from 60° to 180°, the proportion of round-end segments increases relative to flat segments, which affects the overall confinement effectiveness and bearing capacity.

Experimental Results and Failure Modes

The experimental results reveal that for a rectangular aspect ratio of 1.0 (square cross-section), the bearing capacity increases with increasing central angle. The 180° central angle specimen exhibits the highest bearing capacity, followed by the 120° specimen. However, the 180° specimen demonstrates lower ductility, indicating a trade-off between strength and deformation capacity.

Bearing Capacity Formula Development

The authors develop a bearing capacity formula based on the double-shear unified strength theory and concrete strength zoning. The formula accounts for:

• Steel tube contribution: The axial load carried by the steel tube itself
• Concrete core contribution: The enhanced concrete strength due to lateral confinement
• Confinement effectiveness factor: A parameter that varies with the central angle and cross-section geometry
• Steel-concrete interaction: The frictional bond and interface shear transfer between steel tube and concrete

The proposed formula shows good agreement with experimental results for central angles greater than 60°, demonstrating the validity of the theoretical approach for the studied range.

Steel Tube Manufacturing and Material Considerations

Steel Tube Fabrication Requirements

The round-ended SRC cross-section presents unique manufacturing challenges:

• Cold forming process: The combination of flat and curved segments requires precise cold forming operations to achieve the desired geometry
• Welding requirements: If the steel tube is fabricated by welding, the welds at the transition between flat and curved segments must be carefully controlled to avoid stress concentrations
• Dimensional tolerances: The central angle and segment dimensions must be controlled to tight tolerances to ensure consistent confinement characteristics
• Material selection: The steel grade should be selected to provide adequate confinement without excessive cost. Q345 or Q390 grade structural steel is commonly used for SRC applications.

Material Property Effects

The study notes that increasing steel strength has a more pronounced effect on the ultimate bearing capacity than increasing concrete strength. This finding has important implications for material selection:

Standards and Design Code Context

The design of SRC members is governed by several standards:

The round-ended SRC cross-section is not explicitly covered in existing design codes, which primarily address circular and rectangular SRC members. The proposed bearing capacity formula fills this gap for the specific geometry studied, but further research is needed to extend the applicability to other cross-section configurations and loading conditions.

Engineering Practice Integration

Practical Applications

Round-ended SRC columns offer potential advantages in specific applications:

• Aesthetic considerations: The rounded ends provide a more visually appealing appearance compared to sharp-cornered rectangular columns
• Stress distribution: The curved transitions may reduce stress concentrations at cross-section corners
• Formwork efficiency: The round-ended shape may simplify formwork design for concrete placement
• Connection detailing: The rounded ends may facilitate connection design with adjacent structural elements

Design Recommendations

Based on the research findings, the following design recommendations are proposed:

1. For maximum bearing capacity, a central angle of 180° is preferred, but ductility requirements may limit the practical application
2. A central angle of 120° provides a good balance between bearing capacity and ductility
3. Central angles less than 60° should be avoided, as the bearing capacity formula shows poor agreement with experimental results in this range
4. Steel strength should be prioritized over concrete strength for maximizing bearing capacity
5. The cross-section aspect ratio should be maintained near 1.0 for optimal performance

Key Questions and Reflections

• How does the bearing capacity formula perform for long columns with slenderness effects? The study focuses on short columns, and the effects of buckling and P-Δ second-order effects are not considered.
• What is the behavior of round-ended SRC columns under eccentric loading? The study only considers pure axial compression.
• How does the confinement effectiveness vary with different concrete grades and steel grades?
• Can the round-ended SRC cross-section be efficiently manufactured using hot forming or extrusion processes?
• What is the long-term creep and shrinkage behavior of the concrete core in round-ended SRC columns?

The transition from short column to long column behavior requires additional research. The slenderness effects may interact with the variable confinement characteristics of the round-ended cross-section in complex ways that are not captured by the current bearing capacity formula.

Study Insights and Implications

This research makes a valuable contribution to the understanding of SRC member behavior with non-conventional cross-section geometries. The development of a bearing capacity formula based on the double-shear unified strength theory provides a theoretical foundation for the design of round-ended SRC columns.

For steel pipe and SRC engineers, the key insight is that cross-section geometry significantly influences the confinement effectiveness and bearing capacity of SRC members. The variable central angle parameter offers a design degree of freedom that can be exploited to optimize the strength-ductility balance. The finding that steel strength has a more pronounced effect on bearing capacity than concrete strength reinforces the importance of steel material selection in SRC design.

Future research should extend to eccentric loading, long column behavior, cyclic loading for seismic applications, and the development of simplified design equations suitable for practical engineering use. The integration of round-ended SRC technology with advanced steel pipe manufacturing processes, such as cold forming and hot rolling, could enable widespread adoption of this innovative cross-section geometry in structural applications. The proposed bearing capacity formula provides a solid starting point for design code development, and further experimental and analytical research will refine and extend its applicability.