Push-Out Test Study on Loujiaxia Steel Tube Concrete Bridge Tower

Literature Overview

This paper by Liu Xueming and Liu Shizhong (Highway Engineering, 2017, Vol. 42, No. 4), supported by the National Natural Science Foundation of China (Grant No. 51368032), investigates the bond-slip behavior of steel tube concrete (STC) composite structures through push-out testing on a scaled model of the Loujiaxia Suspension Bridge tower. The Loujiaxia bridge represents the largest steel-concrete composite structure bridge tower in China at the time of publication. The study combines experimental push-out testing with ANSYS nonlinear finite element simulation, and further investigates the influence of shear studs on the bond-slip performance.

Experimental Program and Test Setup

The push-out test is the standard method for evaluating the bond-slip behavior at the steel-concrete interface in composite structures. For the Loujiaxia bridge tower scaled model:

The push-out test configuration typically involves a steel tube embedded in a concrete cylinder, with a steel plate welded to the top of the tube. A hydraulic jack applies axial load to the steel plate, pushing the tube out of the concrete. The relative displacement between steel and concrete is measured using LVDTs positioned at multiple heights along the interface.

Bond-Slip Behavior and Results Analysis

The key findings from the push-out testing reveal important characteristics of the steel-concrete interface behavior:

The study reports the ultimate bond strength and the slip displacement at bond failure, which are critical parameters for the design of STC structures. The comparison between experimental results and ANSYS nonlinear finite element simulation demonstrates that the finite element model can effectively capture the complex nonlinear behavior of the steel-concrete interface.

The influence of shear studs on bond-slip performance is particularly noteworthy. The study finds that:

• Before initial bond failure: Shear studs have minimal effect on the initial bond stress-slip relationship. The interface bonding is dominated by chemical adhesion, mechanical interlock, and friction.
• After initial bond failure: Shear studs significantly reduce the relative slip between steel tube and concrete. The studs act as mechanical connectors that maintain composite action even after the adhesive bond has been compromised.

This finding has profound implications for the design philosophy of STC structures. It suggests that the steel-concrete interface provides initial bond resistance that is largely independent of shear stud provision, but that shear studs are essential for maintaining long-term composite action under repeated loading or after partial bond degradation.

Finite Element Modeling Approach

The ANSYS nonlinear finite element simulation employs several advanced modeling techniques to capture the complex steel-concrete interaction:

• Contact elements: Nonlinear contact elements with friction and adhesion capabilities model the steel-concrete interface.
• Constitutive models: Concrete damaged plasticity model for concrete behavior; bilinear isotropic hardening model for steel.
• Mesh refinement: Fine mesh at the steel-concrete interface to accurately capture bond stress distribution.
• Stud modeling: Discrete elements or embedded region technique for shear stud representation.

The validation of the finite element model against experimental results provides confidence in using the model for parametric studies that would be impractical or impossible to conduct experimentally. This approach allows engineers to investigate the effects of varying parameters such as concrete strength, steel tube dimensions, stud spacing, and stud diameter on bond-slip performance.

Engineering Practice Implications

For the design of STC bridge towers and similar composite structures, this research provides several actionable insights:

1. Stud spacing optimization: The finding that studs primarily affect post-bond-failure behavior suggests that stud spacing can be optimized for the specific loading scenario. In structures subjected to predominantly elastic service loads, wider stud spacing may be acceptable, while structures with significant cyclic or impact loading require closer stud spacing.
2. Interface treatment importance: Since the initial bond strength is independent of studs, attention should be paid to interface preparation methods that enhance initial bonding, such as surface roughening of the steel tube interior, use of bonding agents, or vibratory concrete placement to ensure full compaction against the tube wall.
3. Long-term performance: The progressive nature of bond degradation highlighted in this study has implications for long-term serviceability. Engineers should consider the cumulative effects of thermal cycling, shrinkage, and creep on the steel-concrete bond over the design life of the structure.
4. Construction quality control: The effectiveness of the composite action depends heavily on construction quality, particularly the completeness of concrete filling inside the steel tube. Ultrasonic testing or radiographic inspection should be employed to verify full concrete fill before proceeding with subsequent construction stages.

Study Insights and Conclusions

The push-out test research on the Loujiaxia bridge tower represents a significant contribution to the understanding of steel-concrete composite behavior in large-scale bridge structures. The combination of experimental testing with validated nonlinear finite element simulation provides a powerful methodology for investigating complex interface behavior that cannot be fully captured by analytical formulas alone.

The key insight that shear studs primarily influence post-bond-failure behavior rather than initial bond strength challenges the conventional design assumption that studs proportionally increase bond capacity. This suggests that design codes may benefit from more nuanced treatment of the steel-concrete interface, distinguishing between initial adhesion and long-term mechanical interlock contributions.

For practicing engineers, the practical recommendations are clear: ensure excellent construction quality for the steel-concrete interface through proper concrete placement and compaction; design shear stud arrangements based on the expected post-bond-failure loading scenarios rather than assuming uniform contribution throughout the loading history; and employ validated finite element models for detailed analysis of critical STC structural components. The methodology established in this study provides a robust framework for future research into composite structure behavior under complex loading conditions, including seismic and fatigue scenarios that are increasingly relevant in modern bridge engineering practice.