Seismic Performance of Precast Concrete Piers with Embedded Steel Tubes

Literature Overview and Background

This paper by Du Qing, Wang Junnan, and Qing Longbang, published in Architectural Technology in 2017, investigates the seismic performance of precast concrete piers with embedded steel tubes and prestressed steel strands. The study is motivated by the need for rapid, high-quality bridge construction in earthquake-prone regions, where precast methods offer advantages in construction speed and quality control compared to cast-in-place methods. Three test specimens incorporating embedded steel tubes and prestressed strands were designed and subjected to quasi-static cyclic loading tests. Additionally, a finite element model using the ADINA software was developed to simulate the behavior of the precast composite piers under cyclic loading. The study demonstrates that high-prestress composite columns possess higher strength and better energy absorption capacity compared to conventional precast concrete piers.

Core Technical Findings

Cyclic Loading Test Results

The quasi-static cyclic tests reveal the following key behaviors:

Finite Element Model Validation

The ADINA finite element model captures the key features of the cyclic response, including the initial stiffness, yield behavior, and post-yield hardening. The model-test agreement is described as good, validating the numerical approach for parametric studies and design optimization. The finite element model accounts for:

1. Concrete cracking and crushing using a smeared crack model.
2. Steel tube yielding and local buckling using shell elements with plasticity.
3. Prestressed strand behavior using truss elements with nonlinear stress-strain relationships.
4. Steel-concrete interface interaction using contact elements.

Connection to Steel Pipe Fabrication and Welding

Steel Tube as a Confinement Element

The embedded steel tube serves as a confinement element that enhances the ductility and energy dissipation of the precast concrete pier. The steel tube must be fabricated to precise dimensional tolerances to ensure proper fit within the precast concrete form and to provide uniform confinement pressure on the core concrete. The steel tube is typically fabricated from hot-rolled steel plate using longitudinal welding, and the weld quality directly affects the confinement effectiveness.

Welding Challenges in Embedded Steel Tubes

The welding of embedded steel tubes presents specific challenges:

• The longitudinal weld seam creates a plane of reduced toughness that may act as a crack initiation site under cyclic loading.
• The circumferential splices (if any) must be located away from regions of high stress concentration, such as near the pier base or near the deck connection.
• The welding procedure must produce a HAZ with adequate Charpy V-notch toughness at the expected minimum service temperature, which for seismic applications is typically the most severe winter temperature at the bridge location.
• The post-weld residual stresses interact with the cyclic stresses during earthquake loading, potentially accelerating fatigue damage at the weld toe.

Material and Welding Specifications

For seismic applications, the following specifications are recommended for the embedded steel tubes:

• Steel grade: minimum Q345 (ASTM A572 Gr.50) with Charpy V-notch impact energy of at least 34 J at the minimum service temperature.
• Welding process: multi-pass SAW or FCAW with controlled heat input to minimize HAZ width.
• Weld inspection: 100% UT or PAUT inspection of all longitudinal and circumferential welds, with acceptance per GB/T 11345 Level 1 or EN ISO 17635 Level B.
• Post-weld treatment: mechanical peening or shot peening of the weld toe to introduce compressive residual stresses that counteract fatigue crack initiation.

Quality Control Integration

The quality control program for precast piers with embedded steel tubes must integrate fabrication quality with structural performance:

1. The steel tubes must be manufactured to dimensional tolerances of ±1.5 mm for diameter and ±0.5 mm for wall thickness.
2. The concrete mix must be designed for high ductility, with a maximum compressive strain capacity of at least 0.004 under confinement.
3. The prestressed strands must be tensioned to the design force with a tolerance of ±2% of the design tension.
4. The precast segments must be assembled with precise alignment to ensure that the embedded steel tubes form a continuous column without offset.
5. The final pier must undergo a load test or non-destructive evaluation (e.g., impact echo, ultrasonic pulse velocity) to verify structural integrity.

Study Insights and Reflections

The study by Du Qing and colleagues demonstrates that the integration of embedded steel tubes and prestressed strands into precast concrete piers significantly enhances seismic performance, but the realization of this enhanced performance depends critically on the fabrication quality of the steel tubes and the quality of their welds. The improvement in energy dissipation and ductility is not an automatic consequence of embedding a steel tube; it requires that the steel tube maintains its integrity throughout the cyclic loading, which in turn requires that the welds do not fail prematurely. The finite element validation provides confidence in the analytical model, but the model assumes ideal material behavior that may not be achieved if the steel tube welds contain defects or if the HAZ has been embrittled by excessive heat input. For the fabrication engineer, this study reinforces the principle that in seismic design, the welds are not merely joints but critical structural elements that must be designed, fabricated, and inspected with the same rigor as the steel tube itself. The precast construction method, while offering significant advantages in construction speed and quality consistency, introduces additional interfaces—between precast segments, between steel tubes and concrete, and between prestressed strands and grout—that must all be carefully designed and controlled to ensure that the composite system performs as intended during an earthquake.