Competitive Stress-Strain Models For Structural Analysis Of FRP-RC Columns Under Axial And Lateral Loads

Using fiber reinforced polymers(FRPs)composite materials as external reinforcement jackets becoming a widely accepted method for strengthening reinforced concrete structures due to its ability to increase the shear and flexural strengths and ductility of deficient structural elements such as columns,beams/girders,slabs/decks,and walls.Moreover,FRP composites have mechanical and chemical advantages such as high strength to weight ratio and superior fatigue behavior.In addition,they are non-corrosive,non-magnetic,nonconductive,and generally resistant to chemicals.The issue of accurate predicting the lateral response of the reinforced concrete(RC)columns confined with FRP as externally confinement jackets has received a considerable critical attention by researchers in this filed.Thus,this study aims to investigate the accurate simulation of FRP-RC columns that subjected to cyclic lateral loads in the existence of axial compression loads;moreover establishing precise design guidelines for the FRP details of deficient columns based on the given ductility demands through the available standard seismic-design codes.To achieve the aims of this dissertation,the first step was to collect all existing available stressstrain models of FRP-confined concrete,which reached to around more than 110 models for circular columns and 40 models for noncircular columns from about 130 studies to make a comprehensive classification for these models based on;the type of models(design/analysisoriented models),the shape of the stress-strain curve,and the type of the mathematical equation of each model;this classification has been presented in Chapter 2.After that,14 designoriented stress-strain models of FRP-confined concrete were nominated from the available literature to examine the accuracy of these models to predict the cyclic responses of nine circular(RC)columns.The examined columns were externally wrapped with FRP composites and subjected to both constant axial loading and cyclic lateral loading.All of the studied stressstrain models were implemented in Open Sees software as a new uniaxial material.The numerical test results showed that the general response of an FRP-confined RC column to cyclic loading could be predicted using design-oriented stress-strain models;however,the local stressstrain law obtained from concentric compression tests did not very well reflect the local behavior of the compression zone of members in flexure with axial force.The higher the column axial load ratio is,the lower the ratio between the numerical and experimental ultimate column lateral displacements is,and thus,the predicted failure mode does not match well with the experimental results,these results have been presented in Chapter 3.The previous study was very helpful to make a clear decision about the suitable mathematical forms that should be used to predict the stress-strain behavior of FRP-confined concrete.So,a stress-strain model of FRP-confined concrete based on the lateral confinement stiffness was adopted to simulate the lateral response of RC columns retrofitted with external FRP jackets and tested under axial and lateral loads.The adopted model and other five-stress-strain models(established in former studies)were comparatively studied to simulate the seismic response of eight RC-circular columns retrofitted with FRP jackets and experimentally tested under both axial and lateral loads.Compared to the experimental results,the simulation results indicated that all stress-strain models could not identify properly the ultimate lateral displacements of the simulated columns.The adopted stress-strain model was revised to consider the effect of a new key influential parameter(eccentricity ratio),which showed a critical impact on the simulation of the seismic response of RC-columns under combined bending and axial loadings.Finally,the proposed model was evaluated in predicting the lateral response of additional three columns and the simulation results exhibited a good agreement with the experimental results.The proposed model and its results have been illustrated in Chapter 4.The next chapter(Chapter 5)describes synthesis and evaluation of the same effort has been done for the noncircular columns.Whereas,9 models that represent the classification groups were selected to assess the accuracy of these models to capture the cyclic response of 7 square/rectangular reinforced concrete(RC)columns that were tested under axial and lateral loads.The results indicated that the selected models failed to predict the failure mode and in turn the ultimate deformation of the examined columns.This inadequate prediction is caused by the effect of a new key parameter(eccentricity ratio),which displays a significant effect on the local behavior of compressed concrete under axial and bending loadings.Subsequently,one of the examined models has been revised to consider the eccentricity effect.Although several eccentricity-based stress-strain curves for the compressed concrete of FRP-confined RC columns should be developed to accurately simulate their lateral response,one eccentricity-based design-oriented model could be safely applied to evaluate/predict the response of FRP-RC columns under axial and lateral loadings.Subsequently,a software program has been constructed for the design of the FRP details of RC-columns confined externally with FRP composite jackets.The proposed software has many advantages to simplify the definition of the required column details and the output results(moment-curvature,load-deformation,or both)using Graphical User Interface(GUI).Additionally,the significant results of the design of FRP-RC columns have been exhibited in Chapter 6.Finally,the summary of this investigation and the overall conclusions based on the results have been presented by the end of thesis.Moreover,some recommendations for further research work are also included.