Seismic Performance of Prestressed Assembled Bridge Piers with Embedded Steel Tubes

Literature Overview

This paper by Du Qing, Gao Song-song, and Qing Long-bang from Hebei University of Technology, published in the Journal of Chongqing Jiaotong University in 2017, introduces a novel Prestressed Assembled Pier (PEAP) model featuring embedded steel tubes as the key structural element. The research was supported by the National Natural Science Foundation of China (Grant No. 51309073). The authors conducted low-cycle reversed loading tests on three PEAP specimens and developed a numerical simulation approach using the open-source finite element program OpenSEES with a fiber model. The study addresses the growing need for rapid, damage-tolerant bridge construction in seismically active regions, where traditional monolithic piers may suffer catastrophic failure during major earthquakes.

Core Technical Points

The PEAP model replaces the conventional monolithic reinforced concrete pier with segmental construction, where individual concrete segments are pre-assembled and connected through prestressing tendons. The embedded steel tubes serve multiple critical functions: they act as internal reinforcement providing ductility and energy dissipation capacity, they facilitate the passage of prestressing tendons through the pier segments, and they enhance the overall flexural capacity of the assembly. The fiber model approach in OpenSEES allows for capturing the nonlinear material behavior of both the concrete segments and the steel tube reinforcement under cyclic loading.

Key Performance Indicators

From a metallurgical and welding perspective, the embedded steel tubes in these piers are typically manufactured from seamless steel pipes or high-frequency welded (HFW) pipes conforming to standards such as GB/T 8163 or ASTM A53. The steel tube material grade commonly used is Q235 or Q345, with wall thicknesses ranging from 6 mm to 12 mm depending on the pier diameter and seismic design category. The welding connections between steel tube segments, if any, must comply with seismic-grade welding requirements as specified in GB 50011-2010, which mandates full-penetration groove welds with 100% ultrasonic testing coverage.

Numerical Simulation and Experimental Validation

The authors employed a fiber-based finite element model in OpenSEES, which discretizes the cross-section into multiple concrete and steel fibers, each governed by uniaxial constitutive laws. This approach is particularly effective for capturing the progressive crushing of concrete segments and the yielding of embedded steel tubes under cyclic loading. The numerical results showed good agreement with experimental observations in terms of hysteresis loop shape, peak loads, and residual displacements.

Comparison of Numerical and Experimental Results

The consistent underestimation of approximately 3-4% in numerical peak loads can be attributed to the simplification of concrete confinement effects by the steel tube in the fiber model. In practice, the triaxial stress state in the concrete core, enhanced by the hoop confinement of the embedded steel tube, contributes additional strength that is difficult to capture fully with uniaxial fiber constitutive laws. This observation aligns with the findings reported in Topic 2 of this study batch, where bond stress effects on steel tube confined concrete were investigated.

Engineering Practice Implications

For engineers involved in the fabrication and welding of steel tubes for assembled bridge piers, several quality control considerations emerge from this research:

1. Pipe straightness and dimensional tolerance: The embedded steel tubes must meet tight straightness tolerances (typically L/1000 or better) to ensure uniform prestress distribution across pier segments. Non-conformances in pipe geometry can lead to localized stress concentrations during seismic events.
2. Weld quality at segment joints: Where steel tube segments are welded together, the weld metal must exhibit ductility characteristics suitable for cyclic loading. Post-weld heat treatment (PWHT) should be considered for thick-walled tubes (t > 25 mm) to relieve residual stresses that could reduce fatigue life under seismic cycling.
3. Material certification: The steel tube material must be certified for Charpy V-notch impact energy at the design temperature, typically requiring ≥ 27 J at 20°C for seismic applications. This is consistent with the requirements in SY/T 0687 for casing and tubing used in harsh environments.

Key Questions and Reflections

One important question that arises from this study is the long-term durability of the embedded steel tubes within the concrete segments. Given my background in corrosion engineering, I note that the chloride-induced corrosion of embedded steel tubes in marine or deicing-salt environments could significantly compromise the seismic performance of PEAP piers over their service life. The galvanic corrosion potential between the carbon steel tube and the surrounding concrete, especially if the concrete cover is insufficient or carbonated, warrants further investigation. Future research should incorporate accelerated corrosion testing combined with cyclic loading to assess the residual seismic capacity of corroded PEAP systems.

Another reflection concerns the standardization aspect. The PEAP concept represents a departure from conventional monolithic pier design, and current seismic design codes (GB 50011, AASHTO LRFD) do not explicitly address the design of segmental piers with embedded steel tubes. Engineers must exercise engineering judgment in applying existing code provisions, and the development of dedicated design guidelines for this system is urgently needed.

This study provides a solid foundation for understanding the seismic behavior of PEAP systems, but the practical implementation requires careful attention to steel tube material selection, welding quality, and long-term corrosion protection to ensure that the designed seismic performance is maintained throughout the service life of the bridge.