Zhuojin Pipe Fitting Co., LtdPseudo-Static Testing of Single-Sided Bolt Connected Circular Hollow Sandwich Steel Tube-Concrete Column Joints
Literature Overview
The paper by Wang Jingfeng, Guo Xiang, Zhang Na, and Guo Lei from Hefei University of Technology presents the results of pseudo-static testing on four circular hollow sandwich steel tube-concrete (CHS-CFST) column joints with single-sided bolt end-plate connections to steel beams. Published in 2019 in the Journal of Hefei University of Technology (Natural Science), this study was supported by the National Natural Science Foundation of China (Grant No. 51478158) and the Ministry of Education New Century Excellent Talent Support Program (NCET-12-0838). The research investigates the seismic performance and failure mechanisms of hollow sandwich CFST column joints under low-cycle reversed loading, with specific attention to the effects of end-plate form and column hollow rate.
Core Technical Findings
The study conducted pseudo-static tests on four joints with circular hollow sandwich steel tube-concrete columns connected to steel beams via single-sided bolt end-plate connections. The hollow sandwich configuration consists of an outer steel tube, an inner steel tube, and a concrete layer sandwiched between the two tubes. This configuration offers the advantage of reduced self-weight while maintaining structural capacity, but introduces additional complexity in terms of connection design and fabrication.
The test results demonstrated that these end-plate connected joints exhibit typical semi-rigid characteristics with reliable connection behavior and good energy dissipation performance. The hysteresis loops were full and stable, indicating good ductility and energy absorption capacity under cyclic loading.
Under the same hollow rate conditions, the projecting end-plate connection joints exhibited higher ultimate load-bearing capacity and energy dissipation capacity compared to flush end-plate connection joints. This is attributed to the larger moment arm provided by the projecting end-plate, which allows for more efficient force transfer between the beam and the column.
Under the same end-plate form conditions, the ultimate load-bearing capacity and energy dissipation capacity of the joints decreased as the column hollow rate increased. This is because a higher hollow rate reduces the effective concrete confinement area and the overall section stiffness, leading to earlier local buckling and reduced load-carrying capacity.
| Test Parameter | Configuration A | Configuration B | Effect on Joint Performance |
|---|---|---|---|
| End-plate form | Projecting type | Flush type | Projecting type provides higher capacity and energy dissipation |
| Hollow rate | Lower rate | Higher rate | Lower rate provides higher capacity and energy dissipation |
| Joint behavior | Semi-rigid | Semi-rigid | Full hysteresis loops, good ductility |
Interpretation of Technical Points
The hollow sandwich steel tube-concrete column represents an innovative structural concept that combines the advantages of composite action with the weight reduction benefits of hollow sections. From a metallurgical perspective, the sandwich configuration creates a unique stress state in both the steel tubes and the interstitial concrete. The outer tube experiences compressive axial forces and bending moments, while the inner tube provides additional confinement to the concrete layer. The interaction between the two tubes and the concrete is governed by friction and bonding at the steel-concrete interfaces.
The single-sided bolt end-plate connection is a practical and economical connection type that has been widely used in steel structures. The bolted connection transfers forces through bearing and friction between the end-plate and the column surface, with the bolts providing the clamping force. The reliability of this connection depends on the bolt pretension, the flatness of the contact surfaces, and the quality of the bolt installation.
The finding that projecting end-plates outperform flush end-plates is consistent with the fundamental mechanics of moment connections. The projecting end-plate provides a larger lever arm for the bolt forces, which results in higher moment capacity for the same bolt group. However, the projecting configuration also introduces additional fabrication complexity and may require more careful alignment during erection.
The reduction in joint capacity with increasing hollow rate is a critical finding for design purposes. The hollow rate directly affects the effective concrete area, the confinement pressure, and the overall section properties. In terms of welding and fabrication, the hollow sandwich configuration requires additional welding operations to connect the inner and outer tubes, which introduces additional potential failure points and quality control challenges.
Integration with Engineering Practice
In engineering practice, the hollow sandwich CFST column offers attractive benefits in terms of weight reduction and potentially improved seismic performance. However, the increased fabrication complexity must be carefully managed through proper welding procedures and quality control. The sandwich concrete layer must be properly placed and consolidated to ensure uniform thickness and adequate bonding with both steel tubes.
The bolted end-plate connection requires careful attention to bolt pretensioning and contact surface preparation. In accordance with standards such as ASTM F3125 or GB/T 1231, the bolt pretension should be verified through calibrated tensioning equipment, and the contact surfaces should be prepared to ensure adequate friction capacity. The flatness of the end-plate and column surface should be within tight tolerances to prevent uneven bearing stresses.
From a non-destructive testing perspective, the bolted connection requires visual inspection of the welds at the beam end-plate attachment, as well as bolt tightening verification. Ultrasonic testing may be employed to detect internal defects in the welded connections, particularly at the beam-to-end-plate welds where cyclic loading stresses are concentrated.
The study's findings on the effects of hollow rate and end-plate form provide valuable guidance for the design of hollow sandwich CFST structures. Designers should select an appropriate hollow rate that balances weight savings with structural capacity, and should prefer projecting end-plates where moment capacity is a critical design consideration.
Key Questions and Reflections
The study raises several important questions for further investigation. First, the long-term behavior of the hollow sandwich configuration under sustained loading and environmental exposure is not addressed. The interstitial concrete layer may be susceptible to corrosion or moisture ingress, which could degrade the composite action over time. In aggressive environments, the inner tube may provide additional protection, but the outer tube remains exposed to atmospheric conditions.
Second, the study does not consider the effect of welding residual stresses on the joint behavior. The welding operations required to fabricate the hollow sandwich column introduce significant residual stresses that can affect the local buckling behavior and the composite action between the tubes and the concrete. In practice, post-weld stress relief or controlled welding sequences may be necessary to minimize these effects.
Third, the study focuses on single-sided bolt connections, but double-sided bolt connections may provide improved load transfer and seismic performance. A comparative study of single-sided versus double-sided bolt connections would provide additional design guidance.
From a quality control perspective, the hollow sandwich configuration requires more extensive NDT coverage than conventional solid CFST columns. The inner tube welds, which are difficult to access, require special attention. Electromagnetic testing methods or phased array ultrasonic testing (PAUT) may be necessary to ensure weld quality in these hard-to-access areas.
Study Insights and Implications
The most significant insight from this study is that hollow sandwich CFST column joints with bolted end-plate connections can achieve satisfactory seismic performance, provided that the hollow rate is appropriately controlled and the end-plate form is properly selected. The semi-rigid behavior of these joints is consistent with modern seismic design philosophy, which emphasizes ductile deformation and energy dissipation over rigid force transfer.
For practitioners in the steel pipe and welding industry, the hollow sandwich configuration represents both an opportunity and a challenge. The opportunity lies in the potential for weight savings and improved structural efficiency, while the challenge lies in the increased fabrication complexity and quality control requirements. Proper welding procedures, rigorous NDT protocols, and careful bolt installation are essential to ensure the reliability of these connections.
The study also highlights the importance of pseudo-static testing as a tool for evaluating the seismic performance of structural connections. While pseudo-static tests do not replicate the exact dynamic characteristics of earthquake loading, they provide valuable information on the hysteretic behavior, ductility, and energy dissipation capacity of connections under cyclic loading.
Looking forward, I believe that the hollow sandwich CFST concept has significant potential for application in tall buildings, long-span structures, and seismic retrofit projects where weight reduction is a critical design objective. However, further research is needed to address long-term durability, fabrication standardization, and design code development to fully realize the potential of this innovative structural system.