Zhuojin Pipe Fitting Co., LtdEccentric Compression Performance of Steel Tube Members Reinforced with Internal Concrete Under Load
Literature Overview
The study by Hu Rui, Du Xinxin, Cai Zongyang, Cheng Xiaoyan, and Yuan Xiejin, published in Industrial Construction, Volume 47, Issue 2, 2017, presents experimental research on the eccentric compression performance of steel tube members reinforced with internal concrete filling while under existing load (load-bearing condition). Nineteen specimens with varying slenderness ratios, eccentricity ratios, and initial stress ratios were tested. The research addresses a significant practical scenario in structural retrofitting: strengthening existing steel tube members that are already carrying structural loads.
Core Technical Findings
The experimental results demonstrate that the failure characteristic of loaded internal concrete-reinforced steel tube eccentric compression members manifests as bending instability failure, with the deflection curve approximately following a sinusoidal assumption. The presence of initial stress reduces the ultimate bearing capacity of the members, with the reduction becoming more pronounced as initial stress increases.
| Test Parameter | Range/Description |
|---|---|
| Number of specimens | 19 |
| Variables studied | Slenderness ratio, eccentricity ratio, initial stress ratio |
| Failure mode | Bending instability |
| Deflection curve | Approximately sinusoidal |
| Effect of initial stress | Reduces ultimate bearing capacity |
| FEA verification | Good agreement with experimental results |
Eccentric Compression Behavior Analysis
The eccentric compression behavior of steel tube members is governed by the interaction between axial force and bending moment, modified by the P-Δ second-order effects. For the reinforced members under initial load, the behavior can be understood through the following framework:
- Initial stress effect: The pre-existing axial load reduces the effective buckling capacity of the member. The member has already undergone partial deformation under the initial load, consuming part of its deformation capacity before the additional eccentric load is applied.
- Concrete confinement effect: The internal concrete filling provides lateral support to the steel tube, delaying local buckling and enhancing overall stability. However, under eccentric loading, the concrete experiences non-uniform stress distribution with tension on the concave side.
- Slenderness interaction: Higher slenderness ratios amplify the P-Δ effects, making the member more sensitive to initial imperfections and residual stresses.
The sinusoidal deflection curve assumption is consistent with the Euler buckling theory for ideal columns and provides a practical basis for analytical design methods. The deviation from ideal sinusoidal behavior would increase with higher slenderness ratios and larger eccentricities due to material nonlinearity and geometric imperfections.
Retrofit Engineering Considerations
The study directly addresses a critical engineering scenario: the retrofit of existing steel tube structures under load. This is common in industrial applications such as:
- Strengthening of existing pipe racks or support structures
- Rehabilitation of aging steel tube columns in industrial facilities
- Capacity enhancement of steel tube members in existing structures undergoing functional changes
| Retrofit Challenge | Engineering Solution |
|---|---|
| Concrete placement under load | Low-viscosity concrete or self-compacting concrete with careful vibration |
| Air void elimination | Multiple pour layers with interlayer bonding treatment |
| Curing under load | Accelerated curing methods for early strength development |
| Load monitoring | Instrumentation during and after retrofit to verify performance |
| Quality verification | UT thickness measurement, concrete strength testing of cores |
Finite Element Analysis and Design Methodology
The good agreement between FEA results and experimental data validates the numerical modeling approach for design applications. Key modeling considerations include:
- Material models: Bilinear or multilinear isotropic hardening for steel; concrete damage plasticity model for internal concrete
- Contact definition: Frictional contact between steel tube inner wall and concrete core, accounting for potential debonding
- Initial stress imposition: Application of initial axial load before eccentric loading phase
- Geometric imperfections: Inclusion of initial out-of-straightness based on fabrication tolerances per applicable standards
- Residual stresses: Consideration of welding and forming residual stresses in the steel tube
The parametric study across different slenderness ratios and eccentricity ratios provides a comprehensive database for developing design formulas. The initial stress ratio emerges as a critical parameter that must be properly accounted for in retrofit design calculations.
Welding and Fabrication Quality for Steel Tubes
For the steel tube members being retrofitted, the original fabrication quality is a critical factor. Key quality aspects include:
- Tube straightness: Per ASTM A500 or applicable standards, out-of-straightness affects buckling capacity
- Wall thickness uniformity: Variations in wall thickness create weak points under eccentric loading
- Weld quality (for welded tubes): ERW or HFW weld seam quality affects the tube's structural integrity
- Surface condition: Corrosion or damage to the tube surface reduces effective cross-section
For the concrete filling operation, the steel tube interior surface should be cleaned and prepared to ensure adequate bond between the existing steel surface and the new concrete. Any internal coatings or mill scale should be removed. The concrete mix design should consider workability for placement in confined tube spaces, with typical slump requirements of 150-200 mm for standard vibration or higher for self-compacting mixes.
Study Insights and Reflections
This research provides valuable experimental data for the practical retrofit of steel tube structures under load. The systematic investigation of three key parameters—slenderness ratio, eccentricity ratio, and initial stress ratio—creates a comprehensive understanding of the interaction effects. The finding that initial stress significantly reduces bearing capacity has direct implications for retrofit design: engineers must account for the existing load condition when determining the required retrofit capacity.
The practical significance of this work extends to the growing need for structural retrofit and rehabilitation in industrial and infrastructure applications. As existing steel structures age and service requirements change, the ability to strengthen members under load without complete structural shutdown becomes increasingly valuable. The study's methodology and results provide a foundation for developing practical design guidelines for this important engineering application.
Future research should address the long-term performance of retrofitted members, including creep effects of concrete under sustained load, thermal compatibility between steel and concrete in varying temperature environments, and the behavior of retrofitted members under combined loading conditions that may occur in actual service.