Zhuojin Pipe Fitting Co., LtdMechanical Behavior Analysis of Concrete-Filled Steel Tube Push-Out Tests
Literature Overview
Liu Jieming, Li Ning, and Sun Yazhen (2011), from Shenyang Jianzhu University and the Shenyang Housing Development and Construction Management Office, present an analytical study of the push-out test for concrete-filled steel tubes (CFST). The research, supported by multiple funding sources including the National Natural Science Foundation (Grant No. 51078065), develops a micro-element analytical method to derive the bond shear stress distribution in the adhesive layer between the steel tube and concrete core.
Core Technical Approach
The micro-element analytical method treats the adhesive layer as a thin interface and derives a differential equation for the bond shear stress in the linear elastic range. Given boundary conditions, the solution yields a hyperbolic sine function for the shear stress distribution. The analysis also derives equations for the core concrete axial force versus axial displacement, the steel tube axial force versus axial displacement, and the relative axial displacement between the concrete and steel tube.
| Analytical Variable | Mathematical Form | Physical Interpretation |
|---|---|---|
| Bond shear stress | Hyperbolic sine function | Stress concentration at loading end |
| Concrete axial force | Exponential decay | Load transfer from steel to concrete |
| Steel tube axial force | Exponential growth | Progressive load sharing |
| Relative axial displacement | Hyperbolic cosine | Slippage behavior |
Key Findings and Engineering Implications
The analytical results show that bond shear stress exhibits stress concentration at the loading end, with the concentration severity decreasing as the shear modulus increases. The relative axial displacement is positive at the loading end but may become negative further away due to the progressive increase in steel tube axial force. This non-uniform stress distribution has direct implications for the design of CFST connections and the assessment of bond failure modes.
For steel pipe manufacturing and welding, the push-out test results inform the specification of the steel tube inner surface preparation and the bond quality requirements. The stress concentration at the loading end suggests that weld defects at connection interfaces—such as incomplete fusion, undercut, or porosity—can initiate bond failure under cyclic or seismic loading. Engineers should pay particular attention to the quality of the steel tube inner surface, ensuring it is free from mill scale, oxidation, and welding spatter before concrete placement. The analytical framework also provides a basis for simulating slippage phenomena in finite element models, which is essential for predicting the seismic performance of CFST structures.
Study Insights and Reflections
This paper is significant because it provides a theoretical foundation for understanding the bond behavior in CFST systems, which is often treated as an empirical parameter in design codes. The hyperbolic sine solution for shear stress distribution is elegant and provides clear physical insight into the load transfer mechanism. For welding engineers, the findings underscore the importance of maintaining dimensional accuracy and surface quality in steel tubes, as even minor deviations can exacerbate stress concentrations at bond interfaces. The analytical approach also opens avenues for optimizing the adhesive layer thickness and material properties to improve the overall structural performance of CFST members.