Experimental Study On Axial Compression Performance Of Circularized Square Concrete-filled Steel Tube Columns Strengthened By CFRP

The appearance of concrete-filled steel tube has a history of several decades.It has been widely used in the development of urban underground space,bridges and building construction.Beacause of its good mechanical performance and convenient construction technology,concrete-filled steel tube play an increasingly important role in various civil engineering.Meanwhile,square concrete-filled steel tube(SCFST)has the advantages of easy joint structure and convenient construction,its engineering applications have gradually increased in recent years.However,because of the increase in service life,the change in service function,the increase in load and the corrosion of steel pipes,some existing SCFSTs face the reinforcement requirements of increased bearing capacity and durability.Compared with concrete-filled steel tubes,SCFST has relatively weaker restraint capacity for filling concrete.How to improve the bearing capacity and durability of existing SCFST members at the same time has become one of the hot issues in the research field of SCFST.Carbon Fiber Reinforced Polymer(CFRP)has been widely used in civil engineering reinforcement and reconstruction with its advantages of light weight,high strength and corrosion resistance.The utilization of CFRP to strengthen the existing SCFST columns can significantly improve the bearing capacity and durability of the structure.However,because square steel tubes cannot be chamfered like ordinary rectangular concrete columns,the CFRP-reinforced SCFST column fractured due to stress concentration at the corners,which cannot fully exert the hoop restraint effect.The paper conducted an experimental study on the axial compression performance of SCFST columns reinforced with CFRP and rounded treatment,considering the difference in force,it is divided into A loading method(steel pipe bears vertical load)and B loading method(steel pipe does not bear vertical load),and mainly completed the following tasks:(1)Axial compression tests of 24 CFRP-reinforced SCFST short columns were completed,and the effects of section form,loading method and number of CFRP layers on the bearing capacity and deformation of SCFST columns were studied.The test results show that compared with the direct reinforcement of SCFST columns by CFRP,the bearing capacity and deformation of SCFST columns reinforced by CFRP rounding treatment are improved to a certain extent,and the rounding treatment of cross-section improves the reinforcement efficiency.(2)According to the load-displacement curve and load-strain curve measured by the test,the failure mode of the SCFST column in the rounding treatment of CFRP reinforcement is similar to that of the CFRP constrained circular concrete column,showing bilinear feature;through the analysis of the force mechanism,it is found that it is compared with A Loading mode,in the B loading mode,the steel tube and CFRP have a stronger hoop restraint effect on the core concrete,and the improvement of the core concrete’s compressive strength is more obvious.(3)Based on the experimental data,combined with the "Technical Specification for Concrete Filled Steel Tube Structures",Teng and Mander confined concrete strength calculation models,a calculation formula for the bearing capacity of SCFST columns for CFRP reinforcement and rounding treatment is established,and the test value and the calculated value are in good agreement.The applicability of Lam and Teng,Lai and Yang stress-strain relationship curves for CFRP reinforcement rounding treatment SCFST was verified.The verification results show that the test curve is the closest to the Lam and Teng model calculation curve.