Zhuojin Pipe Fitting Co., LtdHysteretic Performance of Circular CFRP-CFST Compression-Bending Members
Literature Overview
This 2011 paper by Che Yuan et al., published in "China Civil Engineering Journal," presents the results of 12 cyclic loading tests on circular carbon fiber reinforced polymer (CFRP)-wrapped concrete-filled steel tube (CFST) compression-bending members. The research, conducted at Dalian University of Technology and Shenyang Jianzhu University, investigates the combined effect of CFRP external wrapping on the hysteretic behavior of CFST members under combined axial compression and bending. The study addresses the growing interest in CFRP as a strengthening and confinement material for steel-concrete composite structures, particularly in seismic retrofitting applications.
Experimental Program and Test Parameters
Twelve specimens were tested under low-cycle reversed loading, with the primary variables being the axial compression ratio and the longitudinal CFRP enhancement coefficient. All specimens featured circular steel tubes filled with concrete, externally wrapped with CFRP sheets in both longitudinal and circumferential directions.
| Parameter | Range | Number of Levels |
|---|---|---|
| Axial compression ratio | 0 (pure bending) to moderate | Multiple |
| CFRP longitudinal enhancement coefficient | Variable | Multiple |
| Steel tube diameter | Standard | 1 |
| Steel tube wall thickness | Standard | 1 |
| Concrete strength | Standard | 1 |
| Loading mode | Cyclic reversed | All |
Core Technical Findings
CFRP Confinement and Enhancement Effects
The experimental results demonstrate that CFRP wrapping provides effective circumferential confinement and longitudinal enhancement to CFST members. The local buckling of the steel tube was either delayed or completely eliminated in CFRP-wrapped specimens. This is a significant finding, as local buckling is often the dominant failure mode in CFST members under compression and bending, and its prevention leads to improved overall structural performance.
The load-deflection curves and moment-curvature curves for all specimens exhibited full and stable hysteretic loops, indicating excellent energy dissipation capacity. The shape of the hysteretic loops was particularly favorable for seismic applications, with minimal pinching and good load-reversal characteristics.
Load-Carrying Behavior Under Cyclic Loading
A critical distinction was observed between specimens with and without axial compression:
- No axial compression (pure bending): The load-carrying capacity remained stable throughout the entire loading history, with no degradation in the later stages of loading. The CFRP wrapping effectively maintained the structural integrity of the member even at large displacements.
- With axial compression: The load-carrying capacity showed significant degradation in the later stages of loading. This is attributed to the combined effect of axial compression and bending, which accelerates the development of local damage and reduces the residual strength.
Strain Distribution and Deformation Characteristics
The study revealed that the steel tube and CFRP work synergistically in both longitudinal and circumferential directions. At any given point on the member, the longitudinal strain and circumferential strain have opposite signs, consistent with the Poisson effect. The deflection curve of all specimens approximated a sinusoidal half-wave shape, indicating that the members behaved as flexural members throughout the loading process.
| Observation | Description |
|---|---|
| Longitudinal vs. circumferential strain | Opposite signs at the same point |
| Deflection curve shape | Approximate sinusoidal half-wave |
| Hysteretic loop shape | Full and stable |
| Local buckling | Delayed or eliminated by CFRP |
| Strength degradation | Not significant |
Parametric Analysis and Design Implications
Effects of Axial Compression Ratio
The axial compression ratio has a dual effect on seismic performance. Within a certain range, increasing the axial compression ratio is beneficial for seismic performance, as it provides additional confinement and delays the onset of yielding. However, beyond this optimal range, the axial compression ratio becomes detrimental, reducing ductility and cumulative energy dissipation capacity.
Effects of CFRP Longitudinal Enhancement Coefficient
Increasing the longitudinal CFRP enhancement coefficient improves the flexural load-carrying capacity and stiffness of the member, while also slowing down stiffness degradation. However, this improvement comes at the cost of reduced ductility and cumulative energy dissipation. This trade-off is important for designers to consider: in regions where load capacity and stiffness are the primary concerns, higher CFRP enhancement is beneficial, while in regions where ductility and energy dissipation are prioritized, a more moderate CFRP enhancement may be appropriate.
Strength Degradation and Stiffness Degradation
The study found that strength degradation was not significant across the tested specimens. This is attributed to the continuous confinement provided by the CFRP wrapping, which prevents the progressive loss of concrete strength and maintains the integrity of the steel tube. Stiffness degradation, while present, was mitigated by the CFRP wrapping, with higher CFRP enhancement coefficients resulting in slower stiffness degradation.
Study Insights and Reflections
This research provides valuable experimental data on the cyclic behavior of CFRP-wrapped CFST members, which is essential for the development of design guidelines for seismic retrofitting of steel-concrete composite structures. The finding that CFRP can effectively prevent local buckling of the steel tube opens up new possibilities for enhancing the seismic performance of existing CFST structures without significant additions of steel material. The observed trade-off between stiffness improvement and ductility reduction with increasing CFRP enhancement is a critical design consideration that should be incorporated into future design codes.
From a practical standpoint, the synergistic behavior of CFRP and steel tube in both longitudinal and circumferential directions suggests that optimal CFRP wrapping configurations should consider both directions of reinforcement. The sinusoidal half-wave deflection curve indicates that the CFRP-wrapped CFST members behave as conventional flexural members, which simplifies the analytical modeling and design calculations. Future research should extend these studies to include the effects of CFRP wrapping configuration (full wrap vs. partial wrap), CFRP fiber orientation, and the long-term durability of CFRP in corrosive environments. The findings also have implications for the design of CFRP-wrapped CFST columns in high-rise buildings, where combined axial and flexural demands are common.