Analysis And Application Of Stress-strain Relationship Of FRP Confined Concrete Columns Under Different Loading

Currently,many buildings in the world have developed local failure as a result of long-term exposure to external forces.Additionally,varying degrees of damage caused by natural disasters,such as earthquakes,typhoons and mudflows,affect the normal usage of many buildings.These problems require reinforcement or structural repair of the damaged buildings.In recent years,a reinforcement material,Fiber Reinforced Polymer(FRP),has been widely used in building structure repair or reinforcement due to its ability to effectively improve the strength and ductility of building structures.Reinforced concrete(RC)columns are major load-bearing members of building structures that bear various types of external loads.However,existing experimental studies and theoretical analyses have primarily focused on the mechanical performance of FRP-confined concrete columns under axial compression,whereas research on the mechanical properties of FRP-confined concrete columns under other loading paths are scarce.Hence,the mechanical behavior of FRP confined concrete under different load types need to be extensively studied.Based on the extensive collection,sorting out existing research,the stress-strain relationship,the ultimate stress and strain of FRP confined concrete column under axial loading are studied.The effect of strain rate and different load paths on the mechanical behavior of FRP confined concrete column are also studied.The main conclusions of this research are listed as follows:(1)An experimental database is created by using the test data obtained from published literature for FRP-confined concrete columns with circular,square,rectangular or oval sections.Based on the database,the existing stress-strain models are evaluated.A unified stress-strain model for the FRP-confined different cross-sectional concrete columns is also proposed.The proposed stress-strain model and existing models are compared by using test data.(2)Based on test data collected from the existing literatures,the existing ultimate stress models of FRP confined concrete columns with different cross-section shapes are evaluated.According to the evaluation results,artificial neural networks model is developed to predict the compression strength of FRP confined column with different cross-sections.(3)Based on the theoretical analysis and experimental verification,it is proved that the energy balance method proposed by Mander et al.can not be directly applied to calculate the ultimate strain of FRP confined concrete.(4)Based on the modified energy method and database,a ultimate strain model of FRP confined concrete column under axial loading and under eccentric loading is developed.The ultimate strain model can be used to calculate the ultimate strain of FRP confined concrete column with different cross-section.Compared with existing ultimate model,the proposed model has good performance.(5)An experimental study on axial loading of FRP-confined concrete under various strain rates and layers of wraps is conducted.The stress-strain relationship of FRP confined concrete column under different strain rates is also investigated.Test results are combined with published test data to propose a stress-strain relationship model and compressive strength model that consider strain rates.The finite element model is also used to validate the proposed compressive strength model.(6)An experimental test for FRP confined concrete column under constant axial load and increasing bending moment is presented.Compared with another two load paths of constant load eccentricity with increasing axial load and pure centric load,the responses of the column under these two different loading schemes are very different.The test results show that loading path has significant effect on the stress-strain response of confined concrete.(7)After evaluation the axial stress-strain model and constant load eccentricity with increasing axial load stress-strain model using experimental data,it is concluded that stress-strain models for FRP confined concrete columns under different loading schemes are different.A new stress-strain model and ultimate strain model for FRP confined circular concrete columns under constant axial force and increasing load eccentricity is developed in this work for engineering applications.The accurate proposed model is evaluated by using experimental data of FRP confined reinforced concrete column under this load path.(8)The effect of stress-strain relationship of FRP confined concrete on FRP strengthened RC piers is studied by using numerical analysis method.The bending moment-curvature curves of FRP reinforced RC piers are calculated by using Opensees software.With gradual increasing of strain and step-by-step numerical integration method,the sectional moment-extreme strain relationship of FRP confined RC concrete piers is obtained by using Matlab software.The stress-strain relationships of FRP-confined concrete under different loading paths are considered in the Matlab program,and the numerical results are compared with the experimental results.