Zhuojin Pipe Fitting Co., LtdExperimental Study on Connection Device Between Rectangular CFST Column and Beam-Slab Floor
Literature Overview
This 2021 paper by Kong Yatao and colleagues, published in Building Structures (Vol. 51, No. 12), presents experimental research on a connection device for rectangular steel tube-concrete filled columns (CFST) in super-high-rise buildings. The study was conducted for the China Life Insurance super-high-rise building at the Beijing CBD Z13 plot, where the external frame columns utilize rectangular CFST construction. Funded by the National Natural Science Foundation of China (Grant 51778036) and the Changjiang Scholars and Innovative Research Team Program (IRT_7R06), this work addresses a critical structural engineering challenge: the connection between rectangular CFST columns and concrete beam-slab floor systems.
The authors from China Construction First Engineering Bureau, Zhongshe Antai Engineering Consulting, and Beijing University of Civil Engineering and Architecture developed a novel connection device and subjected two test specimens to low-cycle reversed loading to evaluate seismic performance. The key finding is that considering the floor slab contribution significantly improves load-bearing capacity and overall stiffness, while slightly reducing ductility. The paper also reports that when the slab contribution is considered, the steel angle brackets bear less shear, allowing for a 30% reduction in bracket cross-section design.
Rectangular CFST Column Characteristics
Rectangular steel tube-concrete filled columns combine the structural efficiency of concrete-filled steel tubes with the practical advantages of rectangular geometry for architectural and functional design. The rectangular cross-section provides better utilization of floor space and facilitates easier connection to beam and slab systems compared to circular sections. The concrete core provides compressive strength and fire resistance, while the steel tube provides lateral confinement and tensile strength.
For super-high-rise buildings, rectangular CFST columns are particularly advantageous because they can be oriented to match the primary lateral load directions, with the longer dimension perpendicular to the primary wind or seismic direction for maximum lateral stiffness. The confinement effect of the rectangular steel tube on the concrete core is different from that of circular tubes, as the corners of the rectangular tube provide less effective confinement than the uniform hoop stress in a circular tube. This results in a slightly lower confinement efficiency, which must be accounted for in design.
| Parameter | Typical Value for Super-High-Rise | Design Consideration |
|---|---|---|
| Column cross-section | 600×800 to 1000×1200 mm | Larger sections for higher axial loads |
| Steel tube wall thickness | 12–25 mm | Must satisfy local buckling criteria |
| Concrete strength | C40–C60 | High strength for axial load capacity |
| Steel tube grade | Q345–Q460 | Higher grades for increased confinement |
| Axial load ratio | 0.4–0.7 | Higher ratios in super-high-rises |
| Slenderness ratio | 40–60 | Must satisfy stability requirements |
Connection Device Design
The connection device developed in this study addresses the challenge of transferring beam end moments and shears from the concrete beam-slab system to the rectangular CFST column. The device incorporates steel angle brackets (dog legs) that are welded to the column steel tube, providing anchor points for the beam reinforcement and shear transfer.
The connection device design must account for several load paths. The beam end moment is transferred through the beam reinforcement into the steel angle brackets, which then transfer the load to the column steel tube and concrete core. The beam end shear is transferred through shear connectors or dowel bars embedded in the concrete floor slab. The axial load from the beam is transferred through the column concrete core and steel tube.
The key design innovation is the integration of the floor slab into the load transfer mechanism. By designing the connection device to engage the floor slab, the effective width of the beam is increased, which reduces the shear demand on the steel angle brackets. This allows for a more efficient design with smaller bracket sections while maintaining adequate load transfer capacity.
Experimental Program and Test Results
Two test specimens were fabricated and tested under low-cycle reversed loading. One specimen included the floor slab contribution, while the other did not. The specimens were loaded in a quasi-static manner to simulate cyclic seismic loading, with displacement-controlled loading protocols that progressively increased the lateral displacement amplitude.
| Test Specimen | Configuration | Peak Load | Displacement at Peak | Ductility Ratio | Energy Dissipation |
|---|---|---|---|---|---|
| Specimen 1 | Without slab | Lower | Moderate | Higher | Lower |
| Specimen 2 | With slab | Higher | Higher | Slightly lower | Higher |
The hysteresis curves of the specimens with slab contribution showed fuller loops, indicating better energy dissipation capacity. The skeleton curves demonstrated that the slab contribution increased the peak load and overall stiffness. The degradation of stiffness and strength under cyclic loading was more gradual in the specimen with slab contribution, suggesting improved seismic resilience.
The failure mode observed in both specimens involved plastic hinge formation in the beam end region, with significant local buckling of the beam compression flange. The steel angle brackets and column steel tube remained within the elastic range throughout the testing, confirming that the connection device design achieved the desired "strong column, weak beam" and "strong joint, weak member" seismic design objectives.
Floor Slab Contribution and Design Optimization
The most significant finding of this study is the quantification of the floor slab contribution to the connection device performance. When the slab is included in the design, the steel angle brackets bear less shear because the slab acts as a lever arm that reduces the moment demand on the brackets. The authors recommend that the steel angle brackets be designed for 30% of the beam end load when the slab contribution is considered, achieving significant steel savings while maintaining adequate structural performance.
This finding has important economic implications for super-high-rise building construction, where the cumulative steel savings from optimized connection designs across hundreds of column-beam connections can be substantial. The 30% reduction in bracket cross-section also simplifies fabrication and welding, reducing construction costs and improving constructability.
Key Reflections and Study Insights
This paper provides valuable experimental data on the seismic performance of rectangular CFST column connections in super-high-rise buildings. The quantification of the floor slab contribution is particularly significant, as it enables more efficient connection designs that reduce material usage while maintaining or improving structural performance.
From a welding and fabrication perspective, the connection device described in this paper involves welding of steel angle brackets to the rectangular CFST column steel tube. This type of connection requires careful attention to weld quality, as the brackets must transfer significant seismic forces. The welds must be designed and qualified according to applicable standards such as GB 50661 or AWS D1.1, with appropriate weld procedures and non-destructive testing to ensure weld integrity.
The experimental results demonstrating that the steel angle brackets remain elastic while plastic hinges form in the beam end region confirm the effectiveness of the "strong joint, weak member" design philosophy. This outcome is achieved through careful design of the connection device to ensure that the bracket capacity exceeds the beam plastic moment capacity. The 30% design reduction recommendation provides a practical design guideline that balances efficiency with safety.
The study also highlights the importance of considering composite action in connection design. The floor slab contribution, which is often neglected in conventional design practice, can significantly improve connection performance. This finding encourages engineers to adopt more sophisticated design approaches that fully exploit composite action, leading to more efficient and economical structural designs.
In conclusion, this paper presents important experimental findings that have direct implications for the design and construction of rectangular CFST column connections in super-high-rise buildings. The quantification of floor slab contribution and the recommended 30% design reduction for steel angle brackets provide practical guidelines for engineers. The welding and fabrication requirements for the connection device must be carefully managed to ensure that the structural performance demonstrated in testing is achieved in actual construction.