Zhuojin Pipe Fitting Co., LtdMechanical Performance of Hollow Sandwich Concrete-Filled Steel Tube Columns
Literature Overview
The paper by Lu Fangwei, Liu Xiao, Li Siping, and Sun Guojun (2007), published in Highway Traffic Science and Technology, presents a theoretical analysis of the mechanical behavior of hollow sandwich concrete-filled steel tube (CFST) short columns. The hollow sandwich CFST column consists of an outer steel tube, an inner steel tube, and a concrete core, with a hollow cavity between the inner and outer tubes. The authors develop theoretical formulas for the composite elastic modulus and the full stress-strain curve of the hollow sandwich CFST column, based on the triaxial constitutive models for concrete and steel. The theoretical model is validated against experimental data from the literature.
Structural Configuration and Design Rationale
The hollow sandwich CFST column is a hybrid structural member that combines the advantages of multiple materials in a single cross-section:
- Outer steel tube: Provides external confinement and lateral stability.
- Inner steel tube: Provides additional axial load capacity and internal confinement.
- Concrete core: Provides compressive strength and energy absorption capacity.
- Hollow cavity: Reduces self-weight while maintaining structural performance.
The hollow cavity between the inner and outer tubes can be filled with lightweight materials (such as aerogel or polyurethane foam) for thermal insulation or left empty for weight reduction. The design concept is particularly attractive for bridge piers, where the combination of high load capacity and low self-weight is essential.
Theoretical Model Development
Constitutive Models
The theoretical analysis is based on the following constitutive assumptions:
- Concrete: A triaxial constitutive model is adopted, accounting for the confinement effect from the steel tubes. The confinement pressure is calculated based on the equilibrium of the concrete core and the steel tubes.
- Steel: A bilinear or multilinear stress-strain model is used for both the inner and outer steel tubes, with the same material grade assumed for both tubes.
- Deformation compatibility: The axial strains of the concrete core and both steel tubes are assumed to be equal at any given cross-section, ensuring compatibility of deformation.
Composite Elastic Modulus
The composite elastic modulus (E_comp) of the hollow sandwich CFST column is derived using the parallel spring model (rule of mixtures), which assumes that all components share the same axial strain:
| Component | Contribution to E_comp | Notes |
|---|---|---|
| Outer steel tube | E_s × A_s_outer / A_total | E_s is steel elastic modulus; A_s_outer is outer tube area |
| Inner steel tube | E_s × A_s_inner / A_total | Same steel grade assumed |
| Concrete core | E_c × A_c / A_total | E_c is concrete elastic modulus; A_c is concrete area |
| Hollow cavity | 0 | No contribution to stiffness |
The composite elastic modulus is simply the weighted average of the component elastic moduli, weighted by their respective cross-sectional areas. This formula is straightforward and easily implemented in structural analysis programs.
Full Stress-Strain Curve
The theoretical model for the full stress-strain curve is developed by considering the progressive yielding and hardening of the steel tubes and the confinement-enhanced behavior of the concrete core. The curve is divided into several stages:
- Elastic stage: All components behave elastically; the composite modulus applies.
- Steel yielding stage: The steel tubes yield while the concrete remains elastic.
- Concrete crushing stage: The concrete reaches its peak stress under confinement; the steel tubes continue to harden.
- Post-peak stage: The concrete degrades while the steel tubes provide continued load capacity.
The theoretical model is characterized by a simple functional form with few parameters, making it suitable for programmatic implementation in structural analysis software.
Validation Against Experimental Data
The authors validate the theoretical model against experimental results from the literature. The comparison shows good agreement between the theoretical predictions and the experimental observations, particularly for the elastic modulus, peak load, and post-peak behavior. The model captures the essential features of the hollow sandwich CFST column's mechanical response, including the enhanced ductility and energy absorption capacity compared to conventional CFST columns.
Engineering Practice Implications
From a steel pipe and welding engineering perspective, the hollow sandwich CFST column concept has several important implications:
- Steel tube fabrication: Both the inner and outer steel tubes must be manufactured to tight dimensional tolerances to ensure proper assembly and uniform concrete infill. The tubes are typically seamless or longitudinally welded (ERW or LSAW) tubes, with the outer tube diameter ranging from 300 mm to over 1000 mm depending on the structural application.
- Welding of inner and outer tubes: The inner and outer tubes must be connected to form a rigid assembly before concrete infill. This connection is typically achieved using transverse welds or bolted connections at regular intervals. The weld quality is critical because it governs the composite action between the two steel tubes.
- Concrete infill process: The concrete must be placed in the annular space between the inner and outer tubes. This is challenging due to the confined geometry and the need to avoid voids. Pumping methods with appropriate vibrator placement are essential. The concrete mix design should include superplasticizers to ensure workability in the confined space.
- Quality control: Non-destructive testing (NDT) is essential to verify the quality of the concrete infill. Ultrasonic testing (UT) can detect voids and honeycombing in the annular space. Radiographic testing (RT) can be used for critical columns to verify the complete concrete fill.
Key Questions and Reflections
The theoretical model assumes perfect deformation compatibility between all components, which is an idealization. In practice, slippage between the steel tubes and the concrete core can occur, particularly under high axial loads. This slippage reduces the composite action and can lead to premature failure. Engineers should account for this slippage in the detailed design, either through the use of shear connectors or by applying a safety factor to the theoretical capacity.
Additionally, the model assumes that both steel tubes have the same material grade. In practice, different grades may be used for the inner and outer tubes to optimize cost and performance. The theoretical framework can be extended to accommodate different steel grades, but the deformation compatibility assumption must be revisited.
Study Insights and Outlook
The theoretical analysis by Lu et al. provides a simple and effective model for predicting the mechanical behavior of hollow sandwich CFST columns. The model's simplicity and low parameter count make it suitable for practical engineering applications, including parametric studies and optimization. The validation against experimental data confirms the model's accuracy for the essential features of the mechanical response. For steel pipe manufacturers and welding engineers, the study highlights the importance of dimensional accuracy in steel tube fabrication, weld quality in the connection between inner and outer tubes, and concrete infill quality in the annular space. Future research should address the effects of slippage, different steel grades for inner and outer tubes, and the long-term behavior under sustained loading and environmental degradation. The hollow sandwich CFST column concept offers a promising solution for bridge piers and other structural applications where high load capacity and low self-weight are required.