Seismic Performance of Prefabricated Square Steel Tube Confined Concrete Bridge Piers with Hybrid Joints

Literature Overview

The paper by Ou Zhijing, Cao Lei, and Xue Wenhao from the Fujian Provincial Key Laboratory of New Civil Engineering Technology and Informatization (Fujian University of Technology), published in Bridge Construction (2023, Vol. 53, No. 1, pp. 40-47), investigates the seismic performance of prefabricated square cross-section concrete bridge piers under three configurations: hybrid joint with square steel tube confinement, grouted sleeve joint with square steel tube confinement, and grouted sleeve joint without confinement. The research was supported by the National Natural Science Foundation of China (Grant 51878172), the Fujian Provincial Natural Science Foundation (2022J01929), and the Fujian Provincial Transportation Science and Technology Project (202003). The study combines pseudo-static testing with finite element analysis to evaluate failure modes, ductility, energy dissipation, strength degradation, stiffness degradation, and residual displacement.

Core Technical Findings

All three pier specimens exhibited compression-bending failure patterns, which is the expected mode for reinforced concrete piers under combined axial and lateral loading. The key comparative findings are summarized in the table below.

Performance Indicator	Hybrid Joint + Steel Tube Confinement	Grouted Sleeve + Steel Tube Confinement	Grouted Sleeve + No Confinement
Peak Lateral Load	Highest	Intermediate	Lowest
Displacement Ductility Factor	Highest	Intermediate	Lowest
Hysteresis Loop Fullness	Full, no pinching	Noticeable pinching	Moderate pinching
Energy Dissipation Capacity	Superior	Moderate	Lower
Residual Displacement	Relatively controlled	Larger	Largest

The hybrid joint, which combines a grouted sleeve connection with steel tube shear keys, produced hysteresis loops that were notably fuller and free of significant pinching compared to the pure grouted sleeve configuration. This is a critical observation for engineers concerned with the cumulative damage and fatigue behavior of prefabricated piers under cyclic seismic loading. The absence of pinching indicates that the connection maintains stable energy dissipation throughout the loading-unloading cycles without progressive loss of connection integrity.

Parametric Study and Design Recommendations

The finite element analysis revealed several important parametric relationships for the hybrid joint configuration. Increasing the axial compression ratio improves the load-bearing capacity but reduces ductility. Decreasing the slenderness ratio similarly enhances capacity at the expense of ductility. Increasing the confinement coefficient improves both capacity and ductility simultaneously, making it the most favorable design parameter to optimize.

Design Parameter	Recommended Range	Effect on Capacity	Effect on Ductility
Axial compression ratio	≤ 0.3	Increasing raises capacity	Increasing reduces ductility
Slenderness ratio	≤ 12	Decreasing raises capacity	Decreasing reduces ductility
Confinement coefficient	0.58 ~ 0.90	Increasing raises capacity	Increasing raises ductility

These recommendations are directly applicable to the preliminary design of prefabricated bridge piers in seismic regions. The confinement coefficient range of 0.58 to 0.90 provides a practical window for selecting the square steel tube dimensions relative to the concrete core cross-section.

Engineering Practice Insights

From a welding and pipe fabrication perspective, the square steel tube confinement element is essentially a cold-formed or hot-fabricated steel box section that must be manufactured to tight dimensional tolerances. The corner welds of the square tube are critical stress concentration zones, and any porosity, incomplete fusion, or undercut in these welds would compromise the confinement effectiveness under cyclic loading. The recommended welding process for such applications should be GTAW or FCAW with full-penetration groove welds at the corners, followed by MT or PT inspection of all weld seams.

The steel tube shear keys integrated into the hybrid joint introduce additional welding complexity. These shear keys must be welded to both the square tube and the connected pier segments, creating multi-directional weld joints. The welding sequence must be carefully planned to minimize residual stress accumulation and distortion, particularly in the square tube walls. Preheating to 100-150°C is advisable for thick-walled square tubes to reduce hydrogen-induced cracking susceptibility in the heat-affected zone.

A key engineering reflection from this study is that the hybrid joint concept represents a significant advancement in prefabricated seismic-resistant construction. The combination of grouted sleeves for alignment and load transfer with steel tube shear keys for shear resistance creates a redundant connection system. This redundancy is particularly valuable in seismic zones where connection failure can lead to progressive collapse. Engineers designing similar systems should pay special attention to the interaction between the grout material properties and the steel shear key geometry, as the bond strength between grout and steel tube inner surface directly affects the load transfer mechanism.

Study Insights and Implications

This research provides a solid foundation for the seismic design of prefabricated bridge piers in regions where rapid construction and seismic resilience are both required. The recommended design parameters offer clear guidance for engineers, and the hybrid joint concept should be further explored for other prefabricated structural elements such as columns, walls, and shear links. The study also highlights the importance of combining experimental testing with numerical simulation to validate and refine design approaches for complex structural systems. Future work should investigate the long-term durability of the hybrid joint under freeze-thaw cycles and chloride exposure, as well as the effect of different steel grades on the seismic performance of the confined piers.