Zhuojin Pipe Fitting Co., LtdInfluence of Weld Seam Configuration and Stiffener Plates on Seismic Performance of Rectangular CFST Beam-Column Joints
Literature Overview
Wang Zhaocheng, Zhao Nan, Zhang Chuntao, and Wu Changgen from Southwest University of Science and Technology and China Construction Engineering Group conducted a study on the seismic performance of rectangular steel tube concrete (CFST) column to H-beam joints. The research investigated the effects of weld seam configuration, specifically full-penetration welds versus fillet welds, and the presence or absence of stiffener plates in the joint core zone. Three full-scale joint specimens were tested under quasi-static loading, and finite element analysis was performed to complement the experimental results. The study was supported by the National Natural Science Foundation of China (51508482), the Tibet Science and Technology Key Project (CGZH2018000014), and China Construction Engineering Group R&D projects (ZJKG-2020-KT-03, ZJKG-2021-KT-03).
Joint Configuration and Design Variables
The beam-column joint under investigation connects a rectangular hollow section (RHS) CFST column to an H-section steel beam. This connection type is increasingly used in modern structural engineering due to the favorable structural efficiency of rectangular CFST columns, which offer high load-bearing capacity, good ductility, and compact cross-sectional dimensions. However, the connection of an H-beam to a rectangular CFST column presents unique challenges due to the geometric discontinuity and the need to transfer both bending moments and shear forces through the joint interface.
| Design Variable | Levels Investigated |
|---|---|
| Weld seam type | Full-penetration weld, fillet weld |
| Stiffener plates | Present, absent |
| Number of specimens | 3 full-scale joints |
| Loading mode | Quasi-static cyclic loading |
| Analysis method | Experimental testing + finite element analysis |
The two variables studied, weld seam type and stiffener plate configuration, are directly related to practical construction considerations. Full-penetration welds provide complete fusion through the entire weld thickness but require more skilled welders, longer welding times, and more rigorous non-destructive testing. Fillet welds are faster and more economical but provide less resistance to through-thickness stresses. Stiffener plates in the joint core zone increase the local stiffness and bearing capacity of the column web but add material weight and fabrication complexity.
Experimental Results and Failure Modes
The quasi-static testing revealed that the joints exhibited elastic-plastic failure behavior, with the primary failure mode being tearing of the H-beam flange, particularly the lower flange. This failure mode is consistent with the well-documented behavior of moment-resisting beam-column connections, where the beam flange is subjected to severe tension-compression cyclic loading that leads to fracture at the weld interface.
The lower flange tearing was the most pronounced failure characteristic. This is attributed to the combined effect of direct tensile stress in the lower flange and the prying action that develops as the joint rotates. The prying action amplifies the tensile demand on the lower flange weld, making it the critical failure location.
The comparative analysis of weld seam types showed that the weld configuration had a relatively minor influence on the joint's load-bearing capacity. Both full-penetration and fillet welds provided comparable strength, though the full-penetration weld likely offered better fatigue resistance and more uniform stress distribution at the weld toe.
Stiffener Plate Effect
The presence of stiffener plates in the joint core zone had a more pronounced effect on the joint's load-bearing capacity compared to the weld seam type. Stiffener plates increase the local web bearing capacity of the rectangular CFST column, thereby reducing the tendency for web crippling and local buckling in the column wall. This allows the joint to develop higher moment resistance before the beam flange tears.
However, the stiffener plates did not significantly influence the initial stiffness or the stiffness degradation process of the joints. This finding suggests that the global joint stiffness is governed primarily by the beam flexural stiffness and the column axial stiffness, rather than by the local web stiffness of the column. The stiffener plates primarily affect the ultimate strength rather than the elastic or near-elastic behavior.
Finite Element Analysis Validation
The finite element models developed for this study incorporated shell elements for the steel components and solid elements for the concrete infill, with appropriate interface elements to model the steel-concrete interaction. The models were calibrated against the experimental results and showed good agreement in terms of load-displacement curves, failure modes, and stiffness degradation. This validation provides confidence in using the finite element models for further parametric studies and design optimization.
The finite element analysis also provides insight into the internal stress distributions and deformation mechanisms that are difficult to observe directly in physical testing. Stress contour plots at critical locations, such as the beam flange weld and the column web, can identify stress concentrations and potential failure initiators.
Welding Process Considerations
From a welding engineering perspective, the study highlights several important considerations for the fabrication of these joints. Full-penetration welds in the beam flange to column wall connection require careful preheating, interpass temperature control, and post-weld heat treatment to manage residual stresses and prevent hydrogen-induced cracking, particularly in thicker sections. The weld metal selection must be compatible with the base metal and provide adequate toughness at the service temperature. Non-destructive testing, typically including ultrasonic testing (UT) for volumetric defects and magnetic particle testing (MT) for surface defects, is essential to verify weld quality.
Fillet welds, while simpler to execute, are susceptible to toe cracking under cyclic loading. The weld geometry, including the weld leg size, throat thickness, and weld toe radius, significantly influences the fatigue performance. Smooth weld toes and full weld penetration at the critical locations are recommended to maximize fatigue life.
Seismic Design Implications
The study demonstrates that rectangular CFST column to H-beam joints can achieve satisfactory ductility and load-bearing capacity when properly detailed. The recommended joint configuration, incorporating appropriate weld design and stiffener plates, meets the seismic design requirements specified in current codes. The elastic-plastic failure mode, with beam flange tearing as the primary damage mechanism, is acceptable from a seismic design perspective as it provides a clear damage indicator and allows for energy dissipation through plastic deformation.
Summary
This research provides valuable experimental and analytical evidence for the seismic design of rectangular CFST column to H-beam joints. The finding that stiffener plates have a greater influence on joint capacity than weld seam type offers practical guidance for designers who need to balance structural performance with fabrication cost and complexity. The validated finite element models serve as a reliable tool for further investigation of alternative joint configurations and detailing options. The study contributes to the growing body of knowledge on CFST structures and supports the continued adoption of these efficient structural systems in seismic regions.