Axial Compression Performance of GFRP Tube-Steel Tube Concrete Composite Columns

Literature Overview and Research Motivation

The paper by Li Xiaojuan and Shang Yonghui, published in Highway Engineering (2018, Vol. 43, Issue 4, pp. 165-169), investigates the axial compression behavior of composite columns that integrate an internal glass fiber reinforced polymer (GFRP) tube within a steel tube concrete (SRC) column. Funded by the Huanghuai University Young Backbone Teacher Support Program (Grant No. 16010896), this research from Huanghuai University and Central South University explores a novel composite structural system that combines the corrosion resistance of GFRP with the structural strength of steel and concrete.

The motivation for this research stems from the need to address corrosion-related durability issues in steel structures, particularly in aggressive environments such as marine, chemical, or de-icing salt environments. By incorporating an internal GFRP tube, the composite column potentially reduces the exposed steel surface area and provides an additional load-bearing component that is immune to corrosion.

Core Technical Methodology and Numerical Analysis

Constitutive Models and Numerical Framework

The research is based on numerical simulation using constitutive models developed for the individual components: GFRP, steel, and concrete. The GFRP constitutive model accounts for the anisotropic nature of the fiber-reinforced polymer, with the mechanical properties depending on the fiber winding angle. The steel constitutive model incorporates elastic-plastic behavior with strain hardening. The concrete constitutive model captures the nonlinear behavior under compression, including the transition from elastic to plastic behavior and the post-peak softening.

The numerical analysis employs a finite element approach with appropriate boundary conditions and loading protocols to simulate the axial compression behavior of the composite column. The interaction between the GFRP tube, steel tube, and concrete is modeled through contact elements that allow for relative movement and pressure transfer between the components.

Key Parameters and Their Effects

The study systematically investigates the effects of several geometric and material parameters on the axial compression performance of the composite column:

The most interesting finding is the non-monotonic effect of the fiber winding angle on the ultimate load capacity. In the elastic stage, the winding angle has no influence on the load-bearing capacity, as the GFRP tube primarily acts as a formwork and provides minimal structural contribution. However, in the elastic-plastic stage, the winding angle significantly affects the load capacity, with a parabolic relationship where the minimum occurs at an intermediate angle.

Quantitative Results

The study reports that a composite column with 0° fiber winding angle can sustain 15.7% higher load than one with 45° winding angle. This finding has direct implications for the design and manufacturing of GFRP tubes for structural applications.

The linear relationship between the steel tube wall thickness and the ultimate load capacity confirms that the steel tube remains the primary load-bearing component, even with the addition of the internal GFRP tube. The GFRP tube contributes additional capacity, but its contribution is secondary to that of the steel tube.

Connection with Steel Pipe Manufacturing and Welding Practice

The integration of GFRP tubes within steel tube concrete columns introduces several manufacturing and welding challenges. First, the steel tube must be manufactured with precise internal dimensions to accommodate the GFRP tube with the appropriate clearance. Second, the welding of the steel tube must be of high quality, as any defect in the weld can compromise the structural integrity of the composite column.

Manufacturing Considerations for Composite Columns

The welding of the steel tube in this composite column application must consider the presence of the internal GFRP tube. The welding heat input can potentially damage the GFRP tube if it is too high, causing delamination or degradation of the fiber-matrix interface. Therefore, low-heat-input welding processes such as GTAW (Tungsten Inert Gas Welding) or pulsed GMAW (Gas Metal Arc Welding) may be preferred for this application.

Welding Process Selection for Composite Columns

The presence of the GFRP tube also affects the welding procedure qualification. The thermal conductivity of GFRP is significantly lower than that of steel, which means that the heat dissipation during welding is reduced when the GFRP tube is present. This can lead to higher peak temperatures in the steel tube wall and a wider HAZ, potentially affecting the mechanical properties of the weld.

Independent Reflections and Engineering Insights

The most significant finding from this research is the confirmation that the GFRP tube provides a meaningful contribution to the axial compression capacity of the composite column, particularly in the elastic-plastic stage. However, the contribution is highly dependent on the fiber winding angle, which has important implications for the design and manufacturing of GFRP tubes.

The parabolic relationship between the fiber winding angle and the ultimate load capacity suggests that there is an optimal winding angle for maximum structural performance. The minimum at an intermediate angle (likely around 22.5° to 30°) indicates that the load-bearing efficiency of the GFRP tube is minimized when the fibers are oriented at an intermediate angle between the axial and hoop directions. This is because the axial load is most efficiently carried by fibers oriented in the axial direction (0°), while the hoop confinement effect is most effective with fibers oriented in the hoop direction (90°). An intermediate angle provides a compromise that is suboptimal for both load-carrying mechanisms.

From a steel pipe manufacturing perspective, this research suggests that the steel tube in a GFRP-steel-concrete composite column should be designed with consideration for the interaction with the GFRP tube. The steel tube wall thickness should be adequate to provide the primary structural capacity, while the GFRP tube should be designed to provide supplemental capacity and corrosion protection.

The linear relationship between the steel tube wall thickness and the ultimate load capacity is reassuring, as it indicates that the structural performance is predictable and scalable. However, this also means that the steel tube remains the dominant structural component, and the GFRP tube should not be relied upon as the primary load-bearing element.

The research also highlights the importance of concrete strength in the composite column. The linear relationship between concrete strength and ultimate load capacity confirms that the concrete core is a significant contributor to the structural performance. From a manufacturing perspective, this means that the concrete placement must be carefully controlled to ensure full filling of the space between the GFRP tube and the steel tube, without voids or segregation.

In conclusion, this paper presents a promising composite structural system that combines the corrosion resistance of GFRP with the structural strength of steel and concrete. For steel pipe manufacturers and welding engineers, the key challenges are ensuring the dimensional accuracy of the steel tube, selecting appropriate welding processes that minimize thermal damage to the GFRP tube, and controlling the concrete placement to ensure full composite action. The optimal design of the fiber winding angle is a critical parameter that should be carefully selected based on the specific application requirements. Further research on the long-term durability and cyclic loading behavior of these composite columns would be beneficial for their application in seismic regions.