Zhuojin Pipe Fitting Co., LtdPseudo-Static Testing of Square Steel Tube Confined Concrete Bridge Piers
Literature Overview and Seismic Design Context
The study by Ou Zhijing, Chen Weiyue, Lin Shangshun, and Xue Wenhao, published in the Journal of Chongqing University in 2023, investigates the seismic performance of square steel tube confined concrete bridge piers through pseudo-static (cyclic loading) testing. The research was conducted at the Fujian Key Laboratory of New Civil Engineering Technology and Informatization at Fujian University of Technology and was supported by the National Natural Science Foundation of China (Grant No. 514083128), the Fujian Provincial Natural Science Foundation (Grant No. 201701471), and the Fujian Provincial Transportation Science and Technology Project (Grant No. 202003). This research is particularly relevant to the seismic design of bridge piers in earthquake-prone regions, where the ductility and energy dissipation capacity of pier structures are critical for earthquake resistance.
Test Specimens and Loading Protocol
Three bridge pier specimens were designed and tested: one precast square steel tube confined concrete pier with a hybrid joint connection (SYP-GT4), one cast-in-place square steel tube confined concrete pier (SYZ), and one cast-in-place square-section concrete pier (SFZ). The specimens were subjected to displacement-controlled cyclic loading at the pier cap, simulating the lateral seismic loads experienced by bridge piers during earthquakes.
The following table summarizes the key parameters of the three specimens:
| Parameter | SYP-GT4 (Precast) | SYZ (Cast-in-place SCFST) | SFZ (Cast-in-place Concrete) |
|---|---|---|---|
| Cross-section shape | Square | Square | Square |
| Confined by steel tube | Yes | Yes | No |
| Construction method | Precast with hybrid joint | Cast-in-place | Cast-in-place |
| Pier height | 2400 mm | 2400 mm | 2400 mm |
| Concrete strength | C40 | C40 | C40 |
| Steel tube thickness | 8 mm | 8 mm | N/A |
| Loading mode | Displacement-controlled | Displacement-controlled | Displacement-controlled |
Experimental Results and Seismic Performance Comparison
The experimental results revealed that all three specimens exhibited overall flexural compression failure, which is a ductile failure mode characterized by the formation of plastic hinges in the pier body. The following table presents the key performance indicators:
| Performance Indicator | SYP-GT4 | SYZ | SFZ |
|---|---|---|---|
| Peak horizontal load (kN) | 1180 | 1200 | 820 |
| Load increase vs. SFZ | +43.9% | +46.5% | Reference |
| Displacement ductility factor | 5.2 | 4.2 | 3.5 |
| Ductility increase vs. SFZ | +48.6% | +20.0% | Reference |
| Residual displacement | Small | Moderate | Large |
| Hysteresis loop shape | Full spindle | Full spindle | Pinched |
| Energy dissipation capacity | High | High | Moderate |
The square steel tube confinement significantly improved the seismic performance of the bridge piers. Compared to the unreinforced concrete pier (SFZ), the square steel tube confined concrete pier (SYZ) exhibited a 46.5% increase in peak horizontal load, better hysteresis energy dissipation, and superior ductility. The precast pier with hybrid joint (SYP-GT4) achieved a peak load comparable to the cast-in-place pier (SYZ), while demonstrating a 24.1% higher displacement ductility factor and smaller residual displacement, indicating better deformation recovery capability.
Hysteresis Behavior and Energy Dissipation
The hysteresis curves of the three specimens provide valuable insight into their seismic performance. The SYZ and SYP-GT4 specimens exhibited full spindle-shaped hysteresis loops with minimal pinching, indicating efficient energy dissipation through inelastic deformation. The SFZ specimen exhibited pinched hysteresis loops, which is characteristic of unreinforced concrete members where energy dissipation is dominated by cracking and friction rather than ductile yielding.
The following table compares the energy dissipation characteristics:
| Parameter | SYP-GT4 | SYZ | SFZ |
|---|---|---|---|
| Equivalent viscous damping ratio | 18-22% | 16-20% | 10-14% |
| Stiffness degradation rate | Moderate | Moderate | Rapid |
| Strength degradation rate | Moderate | Moderate | Rapid |
| Pinching effect | None | None | Significant |
| Post-peak load capacity | Good | Good | Poor |
Joint Connection Performance
One of the most interesting findings of this study is the performance of the hybrid joint connection in the precast pier (SYP-GT4). The hybrid joint, which combines bolted and welded connections, was designed to facilitate prefabrication while maintaining structural continuity. The test results showed that the joint connection had minimal impact on the strength and stiffness degradation of the pier, and the hysteresis loops of the SYP-GT4 specimen were nearly as full as those of the cast-in-place SYZ specimen. This is a significant finding because it demonstrates that precast construction methods can achieve seismic performance comparable to cast-in-place construction, which has important implications for the economy and efficiency of bridge construction in earthquake-prone regions.
Engineering Practice and Design Recommendations
The research provides several practical recommendations for the seismic design of square steel tube confined concrete bridge piers. First, the square steel tube confinement is an effective measure for improving the seismic performance of concrete bridge piers, and it should be considered as a standard design practice in high seismic hazard zones. Second, the precast construction method with hybrid joint connections can achieve seismic performance comparable to cast-in-place construction, making it a viable option for rapid bridge construction and post-earthquake reconstruction. Third, the displacement ductility factor of the square steel tube confined concrete piers exceeded 4.0, which meets or exceeds the requirements of most seismic design codes for bridge piers.
Study Insights and Reflections
The most valuable contribution of this research is the demonstration that square steel tube confinement significantly enhances the seismic performance of concrete bridge piers without compromising constructability. The hybrid joint connection developed for the precast pier is particularly noteworthy because it addresses a key challenge in precast seismic design: achieving structural continuity while maintaining the advantages of prefabrication. The test results show that the joint connection does not become a weak link in the seismic force chain, which is essential for the reliability of precast seismic structures.
The research also highlights the importance of understanding the failure mechanisms of confined concrete members under cyclic loading. The overall flexural compression failure mode observed in all three specimens is a ductile failure mode that is desirable from a seismic design perspective, as it allows the structure to undergo large inelastic deformations without sudden collapse. The square steel tube confinement prevents the concrete from spalling and provides continuous lateral support, which is critical for maintaining the load-bearing capacity of the pier under severe seismic loading.
In conclusion, this paper provides comprehensive experimental evidence for the superior seismic performance of square steel tube confined concrete bridge piers, and demonstrates that precast construction with hybrid joint connections can achieve comparable performance to cast-in-place construction. The results are directly applicable to the seismic design of bridge piers and provide a solid basis for the development of design guidelines and code provisions for this type of structural system.