Local Buckling Behavior Of FRP Confined Rectangular Concrete-filled Steel Tubular Columns

Fiber reinforced polymer（FRP）has been widely used in civil building structures in recent years.Due to its superior mechanical properties,FRP confined rectangular concrete-filled steel tube（CFST）columns are becoming one of the research hotspots in structural engineering.The local buckling behavior of FRP confined rectangular concrete-filled steel tubular columns is one of the key problems in the research and design of FRP confined square and rectangular concrete-filled steel tubular columns.In this thesis,the local buckling model and local buckling performance of FRP confined rectangular concrete-filled steel tubular columns are studied in depth.The main research work and results are as follows:（1）Based on the energy variation method,this thesis,adopts two completely different research ideas,assuming that the loaded side is fixed constraint and the unloaded side is elastic and rotatable constraint,respectively,proposes the local buckling model of FRP confined rectangular concrete-filled steel tubular column.In other words,the effect of external FRP on rectangular concrete-filled steel tube column is regarded as the constraint stress on the surface of steel tube.The local buckling model of FRP confined rectangular concrete-filled steel tubular（CFST）column is proposed,that is,the modelⅠin this thesis.The FRP and steel pipe are regarded as a whole,namely,anisotropic composite plate.The local buckling model of FRP confined rectangular concrete-filled steel tubular column（CFST）is proposed,which is the modelⅡof this thesis.The rationality of the model is verified by comparing the calculation results of the model with the existing experimental data.（2）The local buckling behavior of FRP confined rectangular concrete-filled steel tubular columns is studied by using the proposed local buckling model.The effects of the width-thickness ratio of steel plate（b/t_s）,the length-width ratio of concrete filled steel tube（D/B）,the tensile stiffness ratio of FRP to steel plate（η）and the elastic constraint factor of non-loading end（χ）on the local buckling stress of FRP confined rectangular concrete filled steel tube（CFST）columns were investigated.The results show that the local buckling stress of FRP confined rectangular concrete-filled steel tubular column increases with the decrease of the width-thickness ratio（b/t_s）of steel plate,the increase of the length-width ratio（D/B）of concrete filled steel tubular section,or the increase of the tensile stiffness ratio（η）of FRP to steel plate.On the contrary,the local buckling stress of FRP confined rectangular concrete-filled steel tubular column decreases.In addition,the constraint size of non-loading end also affects the local buckling stress of FRP confined rectangular concrete-filled steel tubular columns.The local buckling stress of FRP confined rectangular concrete-filled steel tubular column increases with the increase of the elastic constraint factor（χ）at the unloaded end.（3）The results show that the ratio of width to thickness of steel plate（b/t_s）has the most significant effect on the local buckling stress of FRP confined CFST columns,followed by length-width ratio（D/B）,the tensile stiffness ratio of FRP to steel plate（η）,and the elastic constraint factor（χ）has the least effect.Finally,based on the proposed local buckling model,the reasonable width-thickness ratio limits of different types of FRP confined rectangular concrete-filled steel tubular columns are given for reference in engineering design.At the same time,the thickness of FRP sheet is equivalent to the increased thickness of steel plate,and a simplified formula for calculating the critical local buckling stress of FRP confined rectangular concrete-filled steel tubular column is given.