Finite Element Analysis of Axial Compression Performance of FRP-Constrained Rectangular High-Strength CFST Long Columns

Literature Overview

The research by Du Yansheng, Gao Dinghui, Chen Zhihua, and Zheng Zihan from Tianjin University, published in the Journal of Tianjin University (Science and Technology) in 2022, investigates the axial compression performance of fiber-reinforced polymer (FRP) constrained rectangular high-strength steel tube concrete (CFST) long columns through nonlinear finite element analysis. The study was supported by the National Natural Science Foundation of China (Project No. 51808182) and the National Postdoctoral Science Foundation (Project No. 2020M670680). The finite element model was validated against experimental results, with peak load deviation averaging only 0.5% and a variance of 0.080.

Finite Element Model Validation

The nonlinear finite element analysis using ABAQUS demonstrated excellent agreement with experimental results in terms of load-displacement curves and failure modes. The peak load deviation of 0.5% with a variance of 0.080 is exceptionally low, validating the accuracy of the material constitutive relationships, element types, contact definitions, and boundary conditions used in the model. This level of accuracy is critical for parametric studies and design optimization, as it ensures that numerical predictions can be relied upon for engineering decisions.

Load-Bearing Mechanism Analysis

The finite element analysis revealed a detailed load-bearing mechanism for FRP-constrained rectangular high-strength CFST long columns. At low compressive stress levels, the thin-walled steel tube undergoes local buckling. As loading progresses, plastic strain and lateral deformation develop in the concrete at the locations of steel tube bulging. Due to the small fillet radius at the corners of the rectangular section, stress concentration occurs in the FRP at these corners, leading to FRP fracture at the most severely bulged locations.

The hoop FRP constraint plays a dual role: it delays the initiation and progression of local steel tube buckling, and it enhances the confinement strength of the concrete core. This dual mechanism results in significant improvements in load-bearing capacity.

Quantitative Performance Enhancement

The FRP confinement effect is more pronounced for normal strength concrete (C40) than for high strength concrete (C80). This is because the confinement effect is most beneficial when the concrete has greater potential for lateral expansion, which is more pronounced in lower strength grades. For high strength concrete, the inherent compressive strength is already high, and the incremental benefit from additional confinement is proportionally smaller.

Slenderness Ratio Effect

The parametric study on slenderness ratio revealed that the FRP confinement effect on load-bearing capacity decreases as the slenderness ratio increases. This is expected because slender columns are more susceptible to overall buckling, which is not effectively restrained by local FRP confinement. However, for models with a fillet radius of 20 mm, the hoop FRP still provided effective confinement and improved peak load capacity by 11% even when the slenderness ratio reached 31.3.

This finding is particularly significant because it extends the applicability of FRP confinement to longer, more slender columns, which are common in practical structural applications. The fillet radius of 20 mm appears to be an optimal geometric parameter that balances the stress concentration at corners with the effectiveness of FRP confinement.

Engineering Practice Integration

The research provides practical guidance for the design of FRP-constrained CFST columns. The validated finite element model can be used for detailed structural analysis and design optimization. The quantitative data on FRP confinement effectiveness for different concrete grades and slenderness ratios enables engineers to make informed decisions about the use of FRP in CFST column design.

For normal strength concrete applications, the 6%-7% improvement in peak load capacity represents a significant and economically attractive benefit. For high strength concrete applications, the 4%-5% improvement, while smaller, may still be beneficial in situations where incremental capacity gains are critical. The finding that FRP confinement remains effective at slenderness ratios up to 31.3 expands the range of practical applications for this technology.

Study Insights

This research by Du, Gao, Chen, and Zheng makes a significant contribution to the understanding of FRP-constrained CFST column behavior through rigorous finite element analysis validated against experimental data. The exceptionally low peak load deviation of 0.5% establishes the numerical model as a reliable tool for parametric studies and design optimization. The detailed load-bearing mechanism analysis, including the identification of local buckling, concrete plastic deformation, and FRP stress concentration at corners, provides valuable insight into the structural behavior of these composite columns. The quantitative data on FRP effectiveness for different concrete grades and slenderness ratios offers practical guidance for engineers designing FRP-reinforced CFST structures. The finding that FRP confinement remains beneficial even at high slenderness ratios is particularly valuable, as it extends the applicability of this strengthening technology to a wider range of structural applications.