Axial Compression Stress-Strain Analytical Modeling Of FRP-Confined Noncircular RC Columns

In the past two decades,Fiber Reinforced Polymer(FRP)has been widely used in the field of seismic retrofitting of concrete structures due to its high tensile strength,good corrosion resistance and ease of construction.Reinforced concrete(RC)columns are important load-bearing components in structures.FRP composite material is used to wrap the RC column,which can effectively improve the ductility and seismic performance of the RC columns.The uniaxial compression constitutive model of FRP-confined concrete is the basis for studying the seismic performance of FRP-reinforced concrete columns.At present,there are many studies on the uniaxial compression performance of FRP-confined concrete around the world,but the following problems still exist:(1)The research was mostly on FRP-confined small-size cylinders.However,research on noncircular sections such as square,rectangular and elliptical was limited.Besides,the influence of size effect was not considered and the stress-strain model was based on test results on small-size specimens,therefore the applicability and accuracy of large-scale components in the actual structure need to be further verified.(2)The stress-strain model was based on the axial compression test results of FRP-confined plain concrete cylinders.The effect of double confinement due to stirrups and external FRP in the actual column members were not well considered for noncircular RC columns.(3)The cyclic compression constitutive model of confined concrete was the basis of the seismic performance analysis of FRP-confined RC columns.The longitudinal reinforcement in the column had a significant influence on the FRP-confined concrete loading and unloading curves.However,there is little research on the cyclic compression performance of the RFP confined columns.In order to accurately predict the axial compressive strength and ductility of FRP-confined large-size noncircular section RC columns,it is necessary to continue the research work and propose a cyclic stress-strain constitutive model for uniaxial compression,which can be technical support to the revision of concrete structures retrofitting code.Aiming at the deficiencies of current compression constitutive model for FRP-confined columns with noncircular sections,this dissertation collect ed a large number of axial compression test data of FRP-confined RC columns with square,rectangular and elliptical cross-sections,and systematically studied the confinement mechanism of FRP-confined noncircular section RC columns.A cyclic constitutive analysis model of confined concrete considering the size effect and the influence of steel bars was proposed.The main research contents are as follows:(1)For the RFP confined RC columns with the rectangular section with an aspect ratio less than 2,a stress-strain model was proposed based on 456 sets of experimental data.The modified model can characterize the softening behavior of the stress-strain relationship after the peak,and considered the effect of size effect,section aspect ratio,chamfer radius and transverse reinforcement ratio on the compressive strength and strain of FRP confined columns.The predictive method to identify whether the confinement is strong confinement or weak confinement was proposed.Compared with the existing model,this model achieved significant improvement in the prediction accuracy in terms of compressive strength and strain.(2)For rectangular sections with an aspect ratio greater than 2,the FRP confinement efficiency is greatly reduced.Two methods for improving confinement efficiency were proposed.One is to modify the section from rectangular to elliptical by adding concrete and then confined by external FRP,another method was to drill holes on the short side face of the rectangular section,and then use FRP anchor to increase the pulling action in the short side direction,thereby improving the confinement efficiency.Aiming at the above-mentioned methods,the confinement mechanism is deeply analyzed.The monotonically compressive model of FRP confined RC columns with rectangular cross-sections,and the model of RC columns with elliptical sections was proposed,which considered the combined action of the outer FRP sheet as well as FRP anchor.(3)Although there is limited research work on FRP-confined RC rectangular columns subjected to cyclic compression tests,there is still no cyclic compression stress-strain model which has been proposed.In order to fill this gap,based on the previous experimental test results of the research group and a small number of other test results,this paper analyzed the influence of longitudinal reinforcement and stirrup on the unloading curve,residual strain and reloading curve of RC columns,and proposed cyclic compressive stress-strain model for FRP-confined RC columns with rectangular sections,which lays the foundation for the seismic performance analysis of the FRP-confined rectangular RC column.(4)Concrete columns with FRP longitudinal and lateral hoops are increasingly used in marine and bridge structures.Unlike the ideal elastic plasticity of traditional steel bars,FRP stirrups are linear elastic materials,so the confinement mechanism is different from that of steel reinforcement.For the FRP bars reinforced concrete columns with rectangular cross-sections,the stress-strain model of monotonic compression performance has yet to be described.Based on the test results of others,a compressive model that considers the cross-sectional geometry and the properties of FRP materials is proposed.