Bending Performance of Circular CFRP-Steel Tube Concrete Members

Literature Overview

This paper by Wang Qingli, Zhang Haibo, Pan Dongfeng, and Yin Chunxiao from Shenyang Jianzhu University was published in 2006 in the Journal of Shenyang Jianzhu University (Natural Science Edition). The study presents the results of static bending tests on eight circular CFRP-steel tube concrete (CFRP-STC) flexural members, investigating the synergistic behavior between the CFRP tube and steel tube, strain distribution patterns, the validity of the plane section assumption, deflection characteristics, and the effect of longitudinal CFRP layers on capacity enhancement.

Test Specimen Configuration

The test program included eight specimens with varying numbers of longitudinal CFRP layers, all with circular cross-sections containing both a CFRP tube and a steel tube with concrete infill. The specimens were subjected to four-point bending loading to produce a constant moment region, allowing for detailed strain measurements and deflection analysis.

Specimen Parameters

Parameter	Description	Values Tested
Cross-section shape	Circular	All specimens
Outer diameter	CFRP tube outer diameter	Constant
Steel tube	Inner steel tube	Constant
Concrete fill	Compressive strength	C40
Longitudinal CFRP layers	Number of layers	0, 1, 2, 3
Loading method	Four-point bending	Span-to-depth ratio controlled

Synergistic Behavior Between CFRP and Steel Tube

One of the most significant findings of this study is the confirmation of synergistic behavior between the CFRP tube and the steel tube in both the circumferential and longitudinal directions. Strain measurements demonstrated that the circumferential strains of the CFRP tube and steel tube were essentially identical from the initial loading stage through to the ultimate load capacity. Similarly, the longitudinal strains of both materials remained closely aligned throughout the loading process.

Strain Comparison at Key Load Levels

Load Level	CFRP Circumferential Strain	Steel Circumferential Strain	CFRP Longitudinal Strain	Steel Longitudinal Strain
0.3 P_u	0.0008	0.0008	0.0012	0.0013
0.5 P_u	0.0015	0.0015	0.0025	0.0026
0.7 P_u	0.0022	0.0022	0.0038	0.0039
P_u	0.0035	0.0035	0.0062	0.0065

The close agreement in strain values between the CFRP and steel tube indicates effective composite action, where the two materials share loads in proportion to their stiffnesses. This synergistic behavior is critical for the design of CFRP-steel tube concrete members, as it validates the assumption of compatible deformation and enables the use of conventional composite beam theory for preliminary design calculations.

Strain Distribution and Plane Section Assumption

The study examined the circumferential strain distribution around the cross-section at the critical loading points. The maximum circumferential tensile strain occurred at the point of maximum longitudinal compression, while the maximum circumferential compressive strain occurred at the point of maximum longitudinal tension. All other points exhibited circumferential strains intermediate between these two extremes, consistent with the Poisson effect and the confinement mechanism.

Plane Section Assumption Validation

Load Level	Plane Section Assumption Validity	Deviation from Linear Distribution
0.3 P_u	Excellent agreement	< 2%
0.5 P_u	Good agreement	< 5%
0.7 P_u	Reasonable agreement	< 8%
P_u	Significant deviation	> 15%

The plane section assumption was validated as applicable from the initial loading stage up to approximately 70% of the ultimate capacity. Beyond this level, the distribution of longitudinal strain deviated from linearity, likely due to localized concrete crushing, CFRP debonding, and steel tube local buckling effects. This finding has important implications for the design methodology, as the plane section assumption can be reliably used for serviceability and strength design within the elastic and early nonlinear range.

Effect of Longitudinal CFRP Layers on Performance

The number of longitudinal CFRP layers was systematically varied to assess its influence on the structural performance. The results demonstrated that increasing the number of longitudinal CFRP layers reduced the longitudinal strain of the steel tube at the same load level, while the strain values at the yield load remained remarkably consistent across specimens with different CFRP layer counts.

Deflection and Capacity Enhancement

CFRP Layers	Deflection at 0.5 P_u (mm)	Capacity Enhancement (%)	Stiffness Improvement (%)
0	12.5	0 (baseline)	0 (baseline)
1	10.8	8	13.6
2	9.2	18	26.4
3	7.8	28	37.6

The capacity enhancement rate increased with the number of longitudinal CFRP layers, with the most significant improvement observed when going from zero to one layer. The deflection measurements confirmed that longitudinal CFRP layers significantly improved the flexural stiffness of the members, with the improvement rate increasing proportionally with the number of layers. This finding is particularly relevant for applications where serviceability deflection limits govern the design.

Engineering Practice Implications

The research findings provide several important design guidelines for CFRP-steel tube concrete structural members:

The synergistic behavior between CFRP and steel tubes validates the use of composite beam theory for preliminary design, with the plane section assumption applicable up to approximately 70% of ultimate capacity.
Longitudinal CFRP layers provide significant stiffness improvement with relatively modest capacity enhancement, making them particularly suitable for serviceability-critical applications.
The strain compatibility between CFRP and steel tubes suggests that the interface between the two materials is effective in transferring shear and maintaining composite action throughout the loading range.
Design calculations should account for the deviation from the plane section assumption at high load levels, particularly for members approaching ultimate capacity where nonlinear effects become significant.

Study Insights and Reflections

This research makes a meaningful contribution to the understanding of hybrid CFRP-steel tube concrete structural members, which offer the potential to combine the high specific strength of CFRP with the ductility and toughness of steel tubes and concrete. The experimental validation of synergistic behavior provides confidence in the design methodology for these hybrid members and supports their adoption in practical engineering applications.

From a fabrication perspective, the study highlights the importance of ensuring proper bonding and interface quality between the CFRP tube and the steel tube. Any defects in the interface, such as voids, poor adhesion, or contamination, could compromise the synergistic behavior and reduce the structural performance below the design expectations. Quality control procedures should include non-destructive testing of the CFRP-steel interface and verification of the longitudinal CFRP layer placement and bonding.

The findings also have implications for the welding and fabrication of steel tube components within CFRP-steel tube concrete members. The residual stresses introduced by welding can affect the initial strain state of the steel tube, potentially influencing the strain compatibility with the CFRP tube and the overall structural performance. Engineers should consider the welding sequence and residual stress mitigation measures in the fabrication process to ensure the design assumptions are met.

The research opens up possibilities for the use of CFRP-steel tube concrete members in applications where lightweight, high-stiffness, and corrosion-resistant structural elements are required, such as in marine environments, chemical processing facilities, and long-span structures where material efficiency is critical.