Pseudo-Static Seismic Performance of Square Steel Tube Confined Concrete Bridge Piers

Literature Overview and Research Context

The study by Ou Zhijing and colleagues from Fujian University of Technology, published in the Journal of Chongqing University (2023, Vol. 46, Issue 10, pp. 40-50), investigates the seismic performance of square steel tube confined concrete bridge piers under pseudo-static loading conditions. The research is 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 Program (Grant No. 202003). This work addresses a critical need in bridge engineering, where the seismic resilience of pier structures directly determines the survivability of transportation networks during earthquakes.

Experimental Configuration and Test Methodology

Three full-scale bridge pier specimens were designed and tested to provide a comprehensive comparison of confinement effectiveness and connection detailing. The test matrix is presented in the table below.

The pseudo-static tests were conducted using displacement-controlled loading at the pier cap, simulating earthquake-induced lateral displacements. The loading protocol followed standard cyclic displacement amplitude increments to capture the progressive degradation of stiffness and strength. Instrumentation included load cells, displacement transducers, and strain gauges positioned at critical sections to monitor the structural response throughout the test.

Key Experimental Results and Structural Behavior Analysis

The destruction patterns of all three specimens exhibited a consistent overall compression-bending failure mode, which is the desired failure mechanism for ductile pier design. However, the quantitative differences in seismic performance indicators are significant and highly informative for design practice.

The 46.5% improvement in horizontal peak load for the SYZ specimen compared to the SFZ specimen demonstrates the substantial effectiveness of square steel tube confinement in enhancing the seismic capacity of concrete bridge piers. This improvement is attributed to the lateral confinement pressure exerted by the steel tube on the concrete core, which enhances the compressive strength and strain capacity of the confined concrete. The enhanced hysteresis energy dissipation further confirms that the steel tube confinement promotes more stable and energy-absorbing behavior during cyclic loading.

The comparison between SYP-GT4 and SYZ reveals that the hybrid joint connection used in the prefabricated pier does not compromise the seismic performance relative to the monolithic construction. The 24.1% improvement in displacement ductility factor and the reduced residual displacement indicate that the hybrid joint provides additional flexibility and deformation recovery capability. The fuller spindle-shaped hysteresis loop without pinching is particularly significant, as it suggests that the connection maintains integrity throughout the loading cycles without developing significant damage that would reduce lateral force capacity.

Welding and Fabrication Considerations

From a fabrication and welding quality control perspective, the construction of square steel tube confined concrete piers involves several critical welds that must be inspected and qualified. The following table outlines the key welding joints and their quality requirements.

The square steel tube confinement system requires precise dimensional control during fabrication to ensure proper fit-up with the concrete formwork. Welding-induced residual stresses and distortions must be controlled to prevent premature buckling of the steel tube under combined axial and lateral loading. Post-weld stress relief or controlled cooling may be necessary for thick-walled steel tubes used in large-diameter piers.

Engineering Practice Integration

The findings of this study have direct implications for bridge design codes and construction specifications. The demonstrated equivalence of seismic performance between prefabricated and monolithic steel tube confined piers opens new possibilities for modular bridge construction, which offers advantages in terms of construction speed, quality control, and site logistics. The hybrid joint connection detail should be further investigated under multi-hazard loading conditions, including combined seismic and hydraulic loads that are common in bridge applications.

For quality assurance in construction, the following inspection protocols are recommended: ultrasonic testing of all full-penetration welds, magnetic particle testing of surface-breaking welds, dimensional surveying of the steel tube assembly prior to concrete placement, and post-pour inspection of the concrete-steel tube bond interface using rebound hammer and ultrasonic pulse velocity methods.

Study Insights and Critical Reflection

The experimental results provide strong evidence for the adoption of square steel tube confined concrete piers in seismically active regions. However, several limitations should be acknowledged. The pseudo-static testing method, while widely used, does not fully replicate the dynamic characteristics of real earthquake loading, including inertial effects and strain rate sensitivity. The specimens were tested under displacement-controlled loading, which may not represent the actual loading conditions in a bridge system where the pier is part of a global structural system. Future research should extend to shake table testing and full-scale bridge model testing to validate the findings under more realistic conditions. The long-term fatigue performance of the hybrid joint under combined traffic and seismic loading also warrants investigation.