Zhuojin Pipe Fitting Co., LtdSeismic Performance of Bottom-Strengthened Rectangular Steel Tube Concrete Composite Columns
Literature Overview
The paper by Cao Wanlin, Hui Cun, Dong Hongying, Xu Fangfang, and Qiao Zhiyun from Beijing University of Technology, published in World Information on Earthquake Engineering (2013, Vol. 29, No. 3, pp. 14-21), presents a novel bottom-strengthened rectangular steel tube concrete composite column design and its seismic performance evaluation. Funded by the National Natural Science Foundation (Grant No. 51178010) and the Beijing Municipal Commission of Education Key Science and Technology Program (KZ200910005008), the study was motivated by the seismic retrofit requirements of the Pingxifu Metro super-large platform structure in Beijing. The research involved three 1:5 scale model specimens subjected to low-cycle reversed loading tests, with the primary innovation being the addition of welded steel plates to the outer surface of the steel tube at the column base to enhance energy dissipation capacity.
Design Concept and Steel Tube Fabrication Requirements
The bottom-strengthened column design addresses a well-recognized vulnerability in steel tube concrete composite columns: the concentration of bending stresses at the column base, where the steel tube wall is most susceptible to local buckling and fracture. The strengthening concept involves welding steel plates to the outer surface of the steel tube in the base region, effectively increasing the local section modulus and delaying the onset of plastic deformation.
Three specimen configurations were tested:
| Specimen | Configuration | Steel Plate Arrangement |
|---|---|---|
| Specimen 1 | Standard rectangular CFT composite column | No strengthening plates |
| Specimen 2 | Bottom-strengthened with plates on high-stress faces | Plates welded to the two faces with maximum bending stress |
| Specimen 3 | Bottom-strengthened with plates on all faces | Plates welded to all four faces of the steel tube |
The steel tube fabrication requirements for these specimens include:
- Rectangular cross-section accuracy: The flatness and squareness of the rectangular steel tube directly influence the bending stress distribution. Deviations from the nominal cross-section geometry create stress concentrations that reduce the effective strengthening provided by the welded plates.
- Surface preparation for welding: The outer surface of the steel tube must be clean and free of mill scale, rust, and contaminants to ensure adequate weld fusion. Surface roughness affects the weld bead profile and the stress concentration at the weld toe.
- Wall thickness uniformity: Variation in wall thickness across the tube length creates non-uniform stress distribution under bending. Thinner regions develop plastic deformation earlier, reducing the overall ductility of the column.
The welding of strengthening plates to the steel tube surface is a critical fabrication operation. The welds must be full-penetration fillet welds or complete-joint-penetration welds to ensure full load transfer between the plate and the tube. The weld geometry, including the fillet leg length, reinforcement height, and transition radius, directly influences the stress concentration factor and the fatigue resistance of the joint.
Experimental Findings and Welding Implications
The low-cycle reversed loading tests revealed the following key findings:
Bearing capacity: Both strengthened specimens (Specimens 2 and 3) demonstrated significantly higher bearing capacity compared to the standard specimen (Specimen 1). The strengthening plates effectively increased the section modulus in the base region, delaying the onset of plastic deformation and increasing the ultimate load.
Ductility: The strengthened specimens exhibited improved ductility, as evidenced by larger displacement ductility ratios. The welded plates provide additional deformation capacity by distributing the plastic strain over a larger area, reducing the peak strain at any single location.
Hysteretic energy dissipation: The energy dissipation capacity was significantly enhanced in the strengthened specimens. The additional steel plates contribute to energy dissipation through plastic deformation, and the improved stress distribution reduces stress concentrations that would otherwise lead to premature fracture.
Stiffness degradation: The rate of stiffness degradation was slower in the strengthened specimens, indicating that the damage accumulation process is more gradual and predictable. This is important for seismic design because gradual stiffness degradation allows for better prediction of the structural response under increasing seismic intensity.
Failure characteristics: The failure mode of the strengthened specimens was characterized by plastic hinge formation in the strengthened region, rather than sudden fracture. This ductile failure mode is the desired behavior for seismic-resistant structures.
The authors also proposed calculation formulas for the flexural and shear bearing capacity of the bottom-strengthened columns, with good agreement between calculated and experimental values. These formulas can be used in engineering design to size the strengthening plates and predict the seismic performance of the strengthened columns.
Welding Quality Control for Strengthening Plate Connections
The welding of strengthening plates is the critical fabrication operation that determines the effectiveness of the bottom-strengthened design. The following welding quality control measures are essential:
| Welding Parameter | Requirement | Quality Control Method |
|---|---|---|
| Weld process | SMAW or FCAW with qualified WPQ | Procedure qualification per GB/T 9948 |
| Consumable | Matching or slightly higher strength than base metal | Material certification and lot traceability |
| Preheat temperature | ≥ 100°C for thick sections to prevent cracking | Thermocouple monitoring during welding |
| Interpass temperature | ≤ 250°C to control HAZ microstructure | Infrared pyrometer or thermocouple |
| Post-weld heat treatment | Stress-relief treatment at 550-650°C | Furnace temperature logging |
| NDT | UT or MT on all welds, with 100% coverage | Certified NDT personnel per GB/T 9445 |
The weld toe is the most critical location for fatigue crack initiation. The geometry of the weld toe—particularly the transition radius between the weld bead and the base metal—directly influences the stress concentration factor. A smooth, rounded transition is preferred over a sharp notch, as the latter creates a high stress concentration that initiates cracks under cyclic loading. Weld toe grinding or TIG dressing can improve the fatigue performance of the weld by smoothing the weld toe geometry.
Study Insights and Implications
This study demonstrates that the bottom-strengthened rectangular steel tube concrete composite column is an effective seismic retrofit strategy for existing structures and a viable design option for new construction. The significant improvements in bearing capacity, ductility, and energy dissipation capacity justify the additional fabrication cost of welding strengthening plates. The proposed calculation formulas provide a practical design tool for engineers.
For steel pipe manufacturers and welding engineers, the key insight is that the effectiveness of the bottom-strengthened design is entirely dependent on the quality of the welding operations. The welds between the strengthening plates and the steel tube must be designed, fabricated, and inspected to the highest standards to ensure that the intended structural performance is achieved. Any welding defects—such as incomplete fusion, porosity, or cracks—can create stress concentrations that negate the strengthening effect and potentially initiate premature failure. Comprehensive welding quality control, including procedure qualification, skilled welder certification, in-process monitoring, and thorough NDT, is essential for the successful implementation of this strengthening strategy.