Axial Compression Mechanical Properties of GFRP Tube-In-Steel Tube Concrete Composite Columns

Literature Overview

This study by Li Xiaojuan and Shang Yonghui, published in Highway Engineering (2018, Vol. 43, No. 4, pp. 165-169), investigates the axial compression mechanical properties of composite columns incorporating an internal glass fiber reinforced polymer (GFRP) tube within a steel tube concrete (SRC) column. The research employs numerical simulation based on constitutive models to analyze the effects of GFRP tube diameter, wall thickness, fiber winding angle, steel tube wall thickness, and concrete strength grade on the axial compression behavior of the composite column. Funded by the Huanghuai University Young Backbone Teacher Support Program (16010896), this work from Huanghuai University and Central South University represents an innovative approach to enhancing the structural performance of SRC columns.

Core Technical Content

The concept of incorporating a GFRP tube within an SRC column introduces a novel composite action mechanism. The GFRP tube, being lightweight and corrosion-resistant, provides additional confinement to the concrete core while not contributing significantly to the overall weight of the column. The interaction between the GFRP tube, the steel tube, and the concrete core creates a complex composite system whose mechanical behavior depends on the geometric and material properties of each component.

The numerical simulation approach employs constitutive models that capture the nonlinear behavior of each material component under axial compression. The GFRP tube behavior is modeled using laminate theory, accounting for the fiber orientation and the anisotropic mechanical properties of the composite material.

Interpretation of Key Technical Points

GFRP Tube Diameter Effect

The study reveals that the maximum axial compression load of the composite column increases with increasing GFRP tube diameter. This is attributed to the larger cross-sectional area of concrete confined by the GFRP tube, which enhances the confinement effect. However, the relationship is not strictly linear, as the confinement effectiveness depends on the interaction between the GFRP tube and the concrete core.

Wall Thickness Effect

The ultimate bearing capacity of the composite column increases with increasing GFRP tube wall thickness, and this relationship is approximately linear. This linear relationship indicates that the GFRP tube contributes directly to the load-carrying capacity of the column, with each increment of wall thickness providing a proportional increase in strength.

Fiber Winding Angle Effect

The fiber winding angle of the GFRP tube has a nuanced effect on the column behavior:

• Elastic stage: The winding angle has no effect on the elastic-stage behavior, as the axial compression load is primarily carried by the concrete and steel tube in the elastic range.
• Elastic-plastic stage: The ultimate bearing capacity first decreases and then increases with increasing winding angle, exhibiting an upward-opening parabolic relationship. The winding angle of 0° provides 15.7% higher load capacity compared to 45°.

This non-monotonic relationship suggests that there exists an optimal winding angle for maximum axial compression capacity, which may differ from the optimal angle for other loading conditions such as shear or torsion.

Steel Tube and Concrete Effects

The steel tube wall thickness has a significant effect on the axial compression mechanical properties, with the ultimate bearing capacity exhibiting a linear increase with steel tube thickness. Similarly, the concrete strength grade has a linear effect on the ultimate bearing capacity, reflecting the direct contribution of concrete strength to the overall column capacity.

Constitutive Model Development

The constitutive model for the GFRP tube-steel tube concrete composite column must account for the complex interaction between the three components. The model should capture:

• The nonlinear stress-strain behavior of concrete under triaxial confinement
• The elastic-plastic behavior of the steel tube with strain hardening
• The linear-elastic behavior of the GFRP tube up to failure
• The progressive interaction between components as deformation increases
• The failure modes of each component and their interaction

Engineering Practice Integration

Design Considerations for GFRP-Reinforced SRC Columns

The incorporation of GFRP tubes in SRC columns offers several engineering advantages:

• Corrosion resistance: GFRP tubes do not corrode, unlike steel tubes, which is advantageous in aggressive environments such as marine or chemical exposure.
• Weight reduction: GFRP tubes are significantly lighter than steel tubes, reducing the overall weight of the column and potentially reducing foundation costs.
• Non-magnetic properties: GFRP tubes are non-magnetic, which is beneficial in applications where electromagnetic interference is a concern.
• Design flexibility: The fiber winding angle can be optimized for specific loading conditions, providing design flexibility not available with conventional steel tubes.

Fabrication and Quality Control

The fabrication of GFRP tube-steel tube concrete composite columns requires attention to several quality control aspects:

• GFRP tube dimensional accuracy: The GFRP tube must be manufactured to precise dimensional tolerances to ensure proper fit within the steel tube and adequate concrete cover.
• GFRP tube surface treatment: The outer surface of the GFRP tube should be treated to promote bonding with the concrete, such as roughening or the application of bonding agents.
• Concrete placement quality: The concrete must be placed and compacted carefully around the GFRP tube to ensure full confinement and avoid voids or honeycombing.
• Steel tube welding quality: The steel tube welds must be executed to high quality standards, as weld defects can affect the confinement effectiveness and overall column performance.

Welding Challenges

The welding of steel tubes in GFRP-reinforced SRC columns presents specific challenges:

• Heat input control: Excessive heat input during steel tube welding can damage the GFRP tube, causing delamination or fiber breakage. Low-heat-input processes such as GTAW should be used.
• Weld sequencing: The welding sequence should be planned to minimize thermal distortion and avoid damaging the GFRP tube.
• Weld inspection: The welds must be inspected using non-destructive testing methods compatible with the presence of GFRP, such as visual testing and magnetic particle testing (if applicable).
• Post-weld inspection: The GFRP tube should be inspected after welding to verify that no thermal damage has occurred.

Key Questions and Reflections

This study raises important questions about the practical implementation of GFRP-reinforced SRC columns. While the numerical analysis demonstrates improved mechanical properties, the cost-effectiveness of incorporating GFRP tubes must be evaluated. The additional cost of GFRP tube manufacturing and installation must be justified by the performance improvements achieved.

The optimal fiber winding angle of 0° for axial compression is interesting, as it suggests that the GFRP tube acts primarily as a confining element rather than a load-bearing component in axial compression. This finding has implications for the design of GFRP tubes for different loading conditions, and suggests that the winding angle should be optimized based on the dominant loading mode.

Study Insights and Implications

The most significant insight from this study is the demonstration that the incorporation of a GFRP tube within an SRC column can enhance the axial compression capacity through improved concrete confinement. The linear relationship between GFRP tube wall thickness and ultimate capacity suggests that the GFRP tube contributes directly to the load-carrying capacity, making it a viable structural component rather than merely a confining element.

For pipe and welding engineers, this study highlights the potential of GFRP tubes as a complement to steel tubes in composite structural applications. The non-corrosive nature of GFRP tubes makes them particularly attractive for applications in aggressive environments, where traditional steel tubes would require additional corrosion protection. The welding challenges associated with GFRP tubes emphasize the need for careful process planning and quality control to ensure that the GFRP tube is not damaged during fabrication.

The constitutive model development presented in this study provides a framework for the analysis of multi-component composite columns, which can be extended to other composite configurations. The numerical simulation approach demonstrated here is applicable to the design and optimization of composite columns for specific structural requirements.

Reference Value and Outlook

This paper provides valuable insights into the mechanical behavior of GFRP-reinforced SRC columns under axial compression. The parametric study of GFRP tube geometry and material properties provides practical guidance for the design of such columns. Future research should focus on the experimental validation of numerical predictions, the investigation of cyclic and seismic behavior of GFRP-reinforced SRC columns, and the development of design guidelines for practical applications. The welding and fabrication aspects of GFRP-reinforced SRC columns also require further investigation, particularly regarding the development of fabrication procedures and quality standards that ensure the integrity of the GFRP tube throughout the fabrication process.