Axial Compressive Performance of Hybrid FRP–Concrete–Steel Double-Skin Tubular Columns with Varying Slenderness Ratios in Square-Circle Configuration
Publication: Journal of Composites for Construction
Volume 28, Issue 6
Abstract
Hybrid double-skin tubular columns (DSTCs) are a promising form of composite columns in which the concrete is sandwiched between inner steel and outer fiber-reinforced polymer (FRP) tubes. A modified form of DSTC incorporating stiffened steel tubes demonstrates more potential benefits than the standard unstiffened design. The inclusion of stiffeners in the steel tube enhances composite interaction among column components, effectively delaying local buckling deformations and potentially improving the behavior of confined concrete. The present study performs both experimental and analytical investigations to measure the impact of stiffeners on the confined concrete behavior of square-circle (SC) configured specimens, considering various column slenderness ratios. Thirty DSTC specimens were experimentally tested with variations in steel stiffener properties, including number, configuration (maintaining a similar combined area), thickness, and width; and unconfined concrete strength. The test results showed a substantial improvement in axial load capacity in the stiffened SC-shaped specimens, reaching up to approximately 37% in this study. Subsequently, the test data were utilized to propose a stress–strain confined concrete model specific to SC-shaped specimens with varying slenderness ratios. This proposed model was employed for calibration study using detailed finite-element (FE) analysis, demonstrating excellent agreement with the test data. A parametric investigation was then conducted through FE analysis to optimize the rib-stiffener properties. Finally, the findings from both the experiments and the FE analysis were utilized to develop a simplified equation that predicts the axial load capacity of stiffened SC-DSTCs.
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Data Availability Statement
The experimental and NLFFA-based data can be obtained from the corresponding author upon request.
Notation
The following symbols are used in this paper:
- Acg
- total cross-sectional area of the core concrete;
- Ag
- gross cross-sectional area;
- As
- total cross-sectional area of the steel tube;
- Ast
- total cross-sectional area of stiffeners;
- Bo
- outer-to-outer specimen width;
- Dc
- outer-to-outer depth of the concrete core;
- De
- deflection corresponding to the elastic axial load;
- Dp
- deflection corresponding to the peak axial load;
- Ds
- outer-to-outer diameter of the steel tube;
- Du
- deflection corresponding to the ultimate axial load;
- Ec
- concrete modulus of elasticity;
- Es
- steel modulus of elasticity;
- Esec
- ratio of confined concrete stress to strain;
- Ex
- hoop directional elastic modulus of FRP;
- Ey = Ez
- transversely isotropic modulus of elasticity of FRP;
- f
- steel axial stress;
- fc
- confined concrete axial stress;
- modified unconfined compressive strength of concrete;
- unconfined concrete compressive strength;
- fs
- axial stress in the steel tube;
- fs,st
- axial stress in the stiffeners;
- fy
- yield stress for the steel tube;
- fy,st
- yield stress for stiffeners;
- fuf
- ultimate hoop-directional tensile strength of FRP;
- Gx
- hoop-directional shear modulus of FRP;
- Gy = Gz
- transverse isotropic shear modulus of FRP;
- Ig
- moment of inertia of the cross section;
- K0, K1, and K3
- first, second, and third damage function constants in a microplane failure model, respectively;
- Leff
- effective length of the specimen;
- Paxial
- average axial load;
- Pe
- axial elastic load;
- Pe,NLFEA
- finite-element-based elastic axial load;
- Pp
- peak axial load;
- Pp,NLFEA
- finite-element-based peak axial load;
- Pu
- ultimate axial load;
- Pu,NLFEA
- finite-element-based ultimate axial load;
- rg
- radius of gyration of the gross cross section;
- S1, S2, and S3
- proposed modification factors for confinement effect;
- ts
- thickness of the steel tube;
- αmic
- maximum damage parameter in the microplane failure model;
- βmic
- factor for the rate of damage in the microplane failure model;
- critical equivalent strain-energy density in the microplane failure model;
- γc
- reduction factor for ;
- rm
- a ratio based on the modulus of elasticity of unconfined and confined concrete;
- μxy = μxz
- transversely isotropic Poisson’s ratio for FRP; and
- μyz
- hoop-directional Poisson’s ratio for FRP.
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© 2024 American Society of Civil Engineers.
History
Received: Nov 29, 2023
Accepted: Jul 17, 2024
Published online: Sep 24, 2024
Published in print: Dec 1, 2024
Discussion open until: Feb 24, 2025
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