Zhuojin Pipe Fitting Co., LtdCFRP and Steel Plate Composite Strengthening of Earthquake-Damaged Square CFST Frame Structures for Progressive Collapse Resistance
Literature Overview
This paper by Xu Chengxiang, Zhou Xingyu, and Ni Tiejun from Wuhan University of Science and Technology investigates the effectiveness of CFRP (Carbon Fiber Reinforced Polymer) and steel plate composite strengthening in restoring and enhancing the progressive collapse resistance of earthquake-damaged square steel tube concrete (CFST) frame structures. The study was supported by the National Natural Science Foundation of China (Grant No. 51178057) and published in Earthquake Resistance Engineering and Retrofitting in 2023.
Methodology and Modeling Approach
The researchers employed the OpenSees open-source platform to develop numerical models of three-bay, three-story square CFST frame structures. The seismic damage was simulated by reducing the material stiffness and strength parameters, creating three scenarios: unstrengthened, moderately damaged and strengthened, and severely damaged and strengthened. Low-cycle reversal analyses were conducted to validate the numerical models against experimental data, followed by incremental dynamic analysis (IDA) to evaluate progressive collapse vulnerability.
Key Modeling Parameters
| Model Configuration | Damage Level | Collapse Safety Reserve Factor |
|---|---|---|
| Unstrengthened | None | 1.8 |
| CFRP + Steel Plate Strengthened | Moderate damage | 2.18 |
| CFRP + Steel Plate Strengthened | Severe damage | 2.07 |
The collapse safety reserve factor is a quantitative metric that indicates the margin between the structure's actual resistance and the threshold for progressive collapse. A value greater than 1.0 indicates that the structure can resist progressive collapse under the specified loading conditions.
Strengthening Mechanism Analysis
The composite strengthening approach combines the high tensile strength and light weight of CFRP with the ductility and toughness of steel plates. This hybrid approach addresses the limitations of using either material alone: CFRP alone provides excellent tensile capacity but lacks ductility and can debond from the substrate under severe loading, while steel plates alone provide ductility but add significant weight and may not fully utilize the available strengthening potential.
Role of CFRP in Strengthening
CFRP wraps provide effective confinement to the concrete core within the CFST columns, enhancing the compressive strength and ductility of the concrete. The CFRP also contributes to the tensile capacity of the frame members, helping to redistribute loads away from damaged regions. However, the CFRP's performance is sensitive to surface preparation quality and adhesive bonding conditions, which must be carefully controlled during retrofitting operations.
Role of Steel Plates in Strengthening
Steel plates, typically welded or bolted to the exterior surfaces of the CFST columns and beams, provide additional bending and shear capacity. The steel plates also improve the overall ductility of the strengthened members by preventing premature local buckling of the original steel tube. The connection details between the steel plates and the existing CFST members are critical; welded connections provide superior load transfer but may introduce heat-affected zone (HAZ) concerns in the original steel tube.
Progressive Collapse Assessment
The incremental dynamic analysis results reveal that the CFRP and steel plate composite strengthening effectively restores and enhances the progressive collapse resistance of earthquake-damaged CFST frames. The moderately damaged and strengthened configuration achieves the highest collapse safety reserve factor of 2.18, representing a 21.1% improvement over the unstrengthened structure. The severely damaged and strengthened configuration achieves a factor of 2.07, still representing a 15.0% improvement over the unstrengthened baseline.
Vulnerability Curve Interpretation
The vulnerability curves derived from the IDA plots show that the strengthened structures exhibit a more gradual transition from elastic to collapse behavior compared to the unstrengthened structure. This gradual transition is indicative of better energy dissipation and load redistribution capacity, which are essential characteristics for progressive collapse resistance. The damage state distribution along the vulnerability curves indicates that the strengthened structures develop more distributed damage patterns rather than concentrated failure at critical members.
Engineering Practice Considerations
From a steel pipe and welding engineering perspective, several practical considerations emerge from this study:
- Surface preparation for CFRP application: The exterior surface of CFST members must be cleaned, ground, and primed to ensure reliable CFRP adhesion. Surface roughness should be controlled to achieve optimal bond strength, typically requiring a surface profile of 40-70 micrometers.
- Steel plate welding to CFST members: When welding steel plates to the existing steel tube, the welding process parameters must be carefully controlled to avoid excessive heat input that could degrade the mechanical properties of the original steel tube. Preheating and post-weld heat treatment may be required for thicker sections or higher strength grades.
- Residual stress management: The combined effect of CFRP and steel plate strengthening introduces additional residual stresses in the CFST members. These residual stresses can interact with the seismic damage already present in the structure, potentially reducing the effective strengthening capacity.
- Quality inspection of strengthening work: Non-destructive testing of the CFRP-to-substrate bond and the steel plate welds is essential. Ultrasonic testing can detect CFRP debonding, while magnetic particle or dye penetrant testing can verify weld integrity.
Study Insights and Reflections
The research provides compelling evidence that hybrid CFRP and steel plate strengthening is an effective approach for enhancing the progressive collapse resistance of earthquake-damaged CFST frame structures. The finding that moderate damage strengthening yields better results than severe damage strengthening is intuitive but important for practical retrofitting decisions. It suggests that early intervention after seismic damage is more cost-effective and technically feasible than attempting to restore severely damaged structures.
However, the study's reliance on numerical simulation raises questions about the long-term durability of CFRP strengthening under environmental exposure, particularly in regions with significant temperature cycling or humidity fluctuations. The bond interface between CFRP and the steel tube is vulnerable to moisture ingress, which can lead to adhesive degradation and reduced strengthening effectiveness over time.
The study also highlights an important design philosophy: progressive collapse resistance is not merely about strengthening individual members but about ensuring adequate load path redundancy throughout the structure. The CFRP and steel plate composite approach contributes to this redundancy by enhancing both the strength and ductility of critical members, enabling more effective load redistribution under extreme loading conditions.