Zhuojin Pipe Fitting Co., LtdSeismic Performance of Bolted-Welded Prefabricated CFST Column-Beam Joints
Literature Overview
The paper by Rong Xian, Chen Lei, and Zhang Jianxin (2020), published in the Journal of Northeastern University (Natural Science), presents a novel prefabricated steel-concrete-filled square tube column-beam joint method combining bottom bolted connections with top welded connections (termed "bolted-welded" or "bottom-bolt-top-weld" joints). The study addresses a critical challenge in prefabricated concrete-filled steel tube (CFST) structures: ensuring adequate seismic performance at connections while maintaining the advantages of prefabricated construction. Funded by the National Natural Science Foundation of China (Grant No. 51808182) and Hebei Provincial Natural Science Foundation, the research includes quasi-static cyclic testing of three bolted-welded joints and one all-bolted joint for comparison.
Joint Configuration and Test Setup
The proposed joint configuration utilizes bolted connections at the bottom of the beam-to-column interface and welded connections at the top. This hybrid approach aims to balance the advantages of bolted connections (ease of disassembly, field installation flexibility) with the superior load-carrying capacity and stiffness of welded connections. The test specimens included:
| Specimen | Joint Type | Key Variation |
|---|---|---|
| JD1 | Bolted-welded (baseline) | Standard beam section, standard plate thickness |
| JD2 | Bolted-welded | Increased beam section size |
| JD3 | Bolted-welded | Further increased beam section size |
| JD4 | All-bolted (reference) | No welded connections |
The specimens were subjected to quasi-static cyclic loading to simulate seismic conditions, with displacement-controlled loading protocols designed to achieve progressive inelastic deformation.
Key Experimental Results
Failure Modes
All three bolted-welded joints exhibited failure primarily through beam-end local buckling and ductile fracture of the steel beam flanges. This is a favorable failure mode from a seismic design perspective, as it indicates that the joint itself remains elastic or near-elastic while the ductile beam portions absorb seismic energy through plastic deformation. The all-bolted joint (JD4) also demonstrated comparable failure behavior, suggesting that the bolted-welded configuration does not introduce critical weak links at the connection.
Hysteresis and Energy Dissipation
The hysteresis loops of the bolted-welded joints were full and well-formed, indicating stable energy dissipation capacity throughout the cyclic loading. The joints exhibited:
- Good energy dissipation capacity: The area enclosed by hysteresis loops remained substantial even at large displacement amplitudes, demonstrating effective seismic energy absorption.
- Controlled stiffness degradation: Stiffness reduction with increasing displacement amplitude was gradual and predictable, without sudden drops that could compromise structural stability.
- Controlled strength degradation: Peak load capacity was maintained or degraded gradually, consistent with ductile structural behavior.
Comparison with All-Bolted Joints
The bolted-welded joints demonstrated seismic performance comparable to all-bolted joints, validating the proposed hybrid connection concept. This is a significant finding because it suggests that the welded top connection does not adversely affect the overall joint ductility or seismic response, while potentially offering advantages in terms of stiffness and load-carrying capacity.
Effect of Beam Section Size and Plate Thickness
The study revealed that increasing the beam section size (from JD1 to JD3) resulted in significant improvements in peak load, stiffness, and energy dissipation capacity. This is consistent with fundamental structural principles: larger beam sections provide greater flexural rigidity and plastic moment capacity. Conversely, increasing the thickness of the connecting plates (贴板) did not produce significant changes in seismic performance, suggesting that the plate thickness in the tested range was already adequate and that the connection is not plate-governed in terms of strength or stiffness.
Engineering Practice Integration
The findings have direct implications for the design and construction of prefabricated CFST structures, particularly in seismic zones:
| Design Parameter | Recommended Practice | Basis |
|---|---|---|
| Joint configuration | Bottom-bolt-top-weld hybrid connection | Adequate seismic performance with prefabrication advantages |
| Beam section selection | Larger sections preferred for enhanced seismic capacity | Significant improvement in peak load and energy dissipation |
| Connecting plate thickness | Minimum adequate thickness sufficient; over-thickening unnecessary | No significant performance improvement from increased thickness |
| Failure mode target | Beam-end buckling and flange ductile fracture | Ensures joint integrity while beam absorbs seismic energy |
The practical significance of this research extends beyond the specific joint configuration studied. The concept of combining bolted and welded connections to achieve both prefabrication efficiency and seismic performance is broadly applicable to other structural systems. In the context of China's rapidly growing demand for prefabricated construction, particularly in high-rise buildings and infrastructure subject to seismic hazards, such research provides essential technical foundations for code provisions and design guidelines.
Reflections and Independent Thinking
The "bottom-bolt-top-weld" concept is elegant in its simplicity. By placing the bolted connection at the bottom, where field assembly and alignment adjustments are most critical, and the welded connection at the top, where the beam-to-column moment interaction is most demanding, the design achieves a rational division of responsibilities. The bolted bottom connection facilitates prefabricated erection, while the welded top connection ensures the high stiffness and strength required for moment-resisting action.
One aspect that merits further consideration is the constructability and inspection of the top welded connection in field conditions. Welded connections in prefabricated construction present unique challenges related to weld quality assurance, accessibility for non-destructive testing (NDT), and repair procedures. The quasi-static test results demonstrate excellent seismic performance, but the practical realization of this joint in real construction projects requires careful attention to weld procedure qualification, welder certification, and post-weld inspection protocols. Standards such as GB 50661 (Steel Structure Welding Code) and relevant welding procedure specifications (WPS) must be rigorously followed.
Additionally, the study's focus on quasi-static testing, while appropriate for characterizing the fundamental seismic behavior of the joint, does not capture the effects of high loading rates and low-cycle fatigue that may be relevant in severe seismic events. Future research incorporating low-cycle fatigue testing and finite element modeling that accounts for material rate effects would provide a more comprehensive understanding of the joint's seismic performance under realistic earthquake loading conditions.
The finding that connecting plate thickness has limited influence on seismic performance is practically valuable, as it allows for cost optimization without compromising structural safety. However, this conclusion should be applied with caution, as it is based on the specific range of plate thicknesses tested. For different beam sizes, loading conditions, or connection geometries, the optimal plate thickness may differ, and parametric studies should be conducted to establish design guidelines covering a wider range of configurations.