Zhuojin Pipe Fitting Co., LtdSeismic Performance of Bottom-Strengthened I-Shaped Steel Tube Concrete Columns
Overview of the Study
This paper by Hui Cun et al. (2013), published in Earthquake Engineering and Engineering Dynamics, presents the design and experimental investigation of I-shaped steel tube concrete (STC) columns with bottom strengthening. The research is supported by the National Natural Science Foundation of China (50978005) and the Beijing High-Level Talent Program (PHR20100502). Three specimen configurations are tested under low-cycle reversed loading to evaluate seismic performance, with the primary innovation being the addition of welded steel plates to the flanges at the column base region.
Core Technical Findings
Specimen Configurations
| Model | Configuration | Description |
|---|---|---|
| Model 1 | Standard I-shaped STC column | Baseline configuration |
| Model 2 | Flange-plate strengthened | Steel plates welded to outer surfaces of both flanges at base |
| Model 3 | Perimeter-plate strengthened | Steel plates welded to all edges of both flanges at base |
Comparative Seismic Performance
| Performance Indicator | Model 1 (Standard) | Model 2 (Flange-plate) | Model 3 (Perimeter-plate) |
|---|---|---|---|
| Ultimate load capacity | Baseline | Significantly improved | Significantly improved |
| Ductility | Moderate | Significantly improved | Significantly improved |
| Hysteresis energy dissipation | Baseline | Significantly improved | Significantly improved |
| Stiffness degradation | Moderate rate | Slower degradation | Slower degradation |
The study demonstrates that the bottom-strengthened I-shaped STC columns exhibit markedly improved seismic performance compared to the standard configuration. The strengthening effectively delays local buckling of the steel tube flanges, which is typically the primary failure mechanism in I-shaped STC columns under cyclic loading.
Bearing Capacity Calculation
The authors develop calculation formulas for both normal section and inclined section bearing capacity of I-shaped STC columns. The calculation results show good agreement with experimental measurements, validating the proposed analytical approach.
Welding and Fabrication Analysis
The bottom strengthening technique relies on welding steel plates to the flanges of the I-shaped steel tube, which introduces several critical welding considerations:
- Welding sequence and distortion control: The welding of strengthening plates to the column base region can introduce significant welding distortion, particularly angular distortion and out-of-plane warping. A symmetric welding sequence is essential to minimize residual deformation. The welding procedure should follow a back-step or symmetric pattern to balance thermal inputs.
- Heat-affected zone (HAZ) properties: The HAZ in the base region of the column is a critical location for seismic performance. The welding process must be carefully controlled to avoid excessive grain growth, martensitic transformation, or hardness peaks that could lead to brittle fracture under cyclic loading. Preheating temperatures of 100-150°C and controlled interpass temperatures are recommended for Q345 or Q355 grade steel tubes.
- Weld quality inspection: The strengthening welds must be inspected using ultrasonic testing (UT) or magnetic particle testing (MT) to detect lack of fusion, porosity, or cracks. Any defects in these welds would create stress concentration points that could initiate premature failure under seismic loading.
- Residual stress management: The welding residual stresses in the base region interact with the cyclic stresses during seismic events. Post-weld stress relief (PWHT) or mechanical stress relief may be necessary for critical applications. The residual stress state can be characterized using the hole-drilling method or neutron diffraction.
Seismic Design Implications
The I-shaped STC column configuration offers several advantages for seismic-resistant structures:
- The I-shaped section provides efficient biaxial flexural capacity, which is important for moment-resisting frames.
- The concrete infill provides fire resistance and enhances the compressive capacity of the web region.
- The bottom strengthening creates a plastic hinge region away from the column base, which is beneficial for damage control and post-earthquake repairability.
However, engineers must ensure that the strengthened region is properly detailed to maintain ductility. The strengthening plates should not be so stiff that they create a rigid constraint, which would force the plastic hinge to form at an unintended location. The transition zone between strengthened and unstrengthened regions should be carefully designed.
Study Insights and Practical Recommendations
The bottom-strengthened I-shaped STC column represents a practical solution for improving seismic performance without significantly increasing material usage. The key to successful implementation lies in the welding quality and detailing of the strengthening plates. Engineers should ensure that welding procedures are qualified in accordance with GB/T 19866 or ISO 3834, and that welders are certified for the specific joint configuration and steel grade. The proposed bearing capacity formulas provide a useful design tool, but should be supplemented with finite element analysis for complex loading scenarios. Future research should investigate the effect of strengthening plate thickness, length, and material grade on seismic performance, as well as the long-term durability of the welded connections under cyclic fatigue loading.