Load-Deformation Analysis of Circular CFRP-Steel Pipe Concrete Eccentric Compression Members

Literature Overview

This paper by Jiang Guilan, Wang Qingli, and Zhou Bo from Shenyang Jianzhu University, published in the Journal of Shenyang Jianzhu University (Natural Science Edition) in 2008 (Volume 24, Issue 1, pages 81 to 85), presents a numerical investigation of the load-deformation behavior of circular CFRP-steel pipe concrete (CFRP-SPC) eccentric compression members. The research was supported by the National Natural Science Foundation of China (50408032), the Liaoning Provincial Department of Education Science and Technology Program Key Laboratory Project (20060690), the Shenyang Science and Technology Program (1063290-1-003), and the Liaoning Provincial Key Laboratory Special Program.

Research Background and Motivation

CFRP-SPC members represent a hybrid structural system that combines the advantages of:

• Carbon fiber reinforced polymer (CFRP): High tensile strength, corrosion resistance, and lightweight characteristics
• Steel pipe: High compressive strength and ductility
• Concrete core: Compressive strength and fire resistance

The eccentric compression loading condition is particularly important for structural columns subjected to combined axial loads and bending moments, which is the most common loading scenario in practical engineering. Understanding the load-deformation behavior under eccentric loading is essential for:

• Developing design codes and standards for CFRP-SPC members
• Optimizing the CFRP wrapping scheme and steel pipe dimensions
• Predicting failure modes and ultimate capacity
• Establishing acceptance criteria for structural assessment

Fiber Model Methodology

The authors employ the fiber model method to simulate the load-deformation relationship of CFRP-SPC eccentric compression members. This approach divides the cross-section into discrete fiber elements, each assigned appropriate material constitutive relationships.

Cross-Sectional Discretization

The cross-section is divided into three material regions:

1. CFRP layer: Outermost layer, modeled with a bilinear stress-strain relationship
2. Steel tube: Intermediate layer, modeled with elastic-perfectly plastic behavior
3. Concrete core: Innermost region, modeled with the confined concrete stress-strain relationship

Material Constitutive Models

Loading and Boundary Conditions

The eccentric compression is simulated by applying an axial load with an eccentricity offset from the centroid of the cross-section. The eccentricity ratio (e/h, where h is the cross-section height) is varied to study its influence on the load-deformation behavior.

Key Findings

Load-Deformation Curve Characteristics

The load-deformation curve of CFRP-SPC eccentric compression members can be divided into three distinct stages:

1. Elastic stage: The member behaves linearly with all materials in the elastic range. The stiffness is high and the deformation is small.
2. Elastic-plastic stage: The steel tube begins to yield, and the CFRP reaches its elastic limit. The stiffness decreases, and the load-deformation curve shows a gradual transition to plastic behavior.
3. Softening stage: The concrete core crushes, and the CFRP begins to fail. The load capacity decreases rapidly with increasing deformation.

Effect of Slenderness Ratio (L/D)

As the slenderness ratio increases, the peak load decreases due to the reduced stability of the member. The post-peak behavior becomes more brittle, and the failure mode transitions from concrete crushing to buckling.

Effect of Eccentricity Ratio (e/h)

The eccentricity ratio has a significant influence on the load-deformation behavior:

• Low eccentricity (e/h < 0.1): The member behaves primarily in compression, with relatively uniform stress distribution across the cross-section. The load capacity is close to the axial compression capacity.
• Medium eccentricity (e/h = 0.1-0.2): The bending moment becomes significant, and the stress distribution becomes non-uniform. The CFRP on the tension side begins to yield, while the concrete on the compression side approaches crushing.
• High eccentricity (e/h > 0.2): The member behaves primarily in bending, with the CFRP on the tension side failing before the concrete on the compression side crushes. The load capacity is significantly reduced compared to axial compression.

Comparison with Experimental Data

The numerical results show good agreement with experimental data, with the calculated values being slightly conservative (on the safe side). This conservatism is attributed to:

• Simplified material constitutive models that do not fully capture the complex behavior of CFRP under combined loading
• Assumptions about the CFRP-steel-concrete interface bond behavior
• Neglect of geometric nonlinearity effects at large deformations

Engineering Practice Considerations

CFRP Wrapping Design

Based on the analysis, the following recommendations are made for CFRP wrapping design:

• WRapping scheme: Full wrapping (360°) is preferred over partial wrapping for eccentric compression members, as it provides uniform confinement and prevents debonding under asymmetric loading.
• CFRP thickness: The CFRP thickness should be selected to balance the strengthening effect with the cost and constructability. Excessive CFRP thickness can lead to brittle failure with little warning.
• Quality control: The CFRP application must be inspected for voids, wrinkles, and inadequate impregnation, which can significantly reduce the effectiveness of the CFRP wrapping.

Steel Pipe Selection

• Steel grade: High-strength steel (such as Q345 or Q390) is recommended for the pipe to provide adequate confinement and ductility.
• Wall thickness: The wall thickness should be sufficient to provide effective confinement of the concrete core while maintaining the required slenderness ratio for buckling resistance.
• Weld quality: If the steel pipe is fabricated from welded plate, the weld quality must be ensured through non-destructive testing (NDT) to prevent weld defects from initiating failure.

Concrete Quality

• Concrete strength: The concrete compressive strength should be matched to the steel pipe strength to ensure balanced failure behavior.
• Concrete mix design: The concrete mix should be designed for adequate workability to ensure complete filling of the steel pipe without voids.
• Curing: Proper curing is essential to achieve the design concrete strength, particularly for high-strength concrete mixes.

Key Questions and Reflections

Several questions emerge from this study:

• How does the CFRP-SPC member behavior change under cyclic loading conditions relevant to seismic applications?
• What is the effect of temperature on the load-deformation behavior of CFRP-SPC members, considering the potential fire exposure in structural applications?
• Can the fiber model method be extended to predict the post-peak behavior and energy dissipation capacity of CFRP-SPC members?
• How do manufacturing defects in the CFRP wrapping (such as voids, wrinkles, or inadequate impregnation) affect the load-deformation behavior?

Study Insights and Implications

This paper provides a valuable contribution to the understanding of CFRP-SPC eccentric compression member behavior through the application of the fiber model method. The identification of three distinct stages in the load-deformation curve offers clear guidance for structural design and assessment. The systematic investigation of slenderness ratio and eccentricity ratio effects provides practical design parameters for engineers. The good agreement between numerical predictions and experimental data validates the modeling approach and gives confidence in its application to practical design problems. For steel pipe manufacturers and fabricators, the findings highlight the importance of dimensional accuracy and material quality in ensuring the predicted structural performance. The conservative nature of the numerical predictions provides an additional safety margin, which is particularly important for structural members subjected to eccentric loading where failure can be sudden and catastrophic.