Study On Flexural Behavior Of RC Beams Strengthened By U-shape Steel Plates As Box Structures

The reinforce method of using U shape steel as box structure is a new type methode which is based on the theory of steel-concrete structure.The method integrates U shape steel plate with concrete beam through the structural adhesive and bolt as shear connector,and the U shape steel plate is not contact with the bottom of the concrete beam,forming a box-type structure to increase the height of cross section.This reinforcemt method cobines the advantages of bonded steel plates reinforcement method and the method of enlarge cross section,has greatly improve the bearing capacity of the original structure and the utilization rate of the material is very high,at the same time the reinforcement process is very simple and the reinforcement cycle is very short.Compared with the existing reinforcement method,this new method has strong technological advantages,it has a wide application prospect in the reinforcement field of brigdes and other fields which need to improve the bearing capacity quickly and greatly.In cooperation with the project on Research on key technology of increasing the bearing capacity of engineering structure substantially and rapidly,the basic mechanical property of the RC beams strengthened by U shape steel were studied through experimental investigation and theoretical analysis,and the research result was patented for A rapid method for reinforcing concrete beam(ZL201610163624.4).The main work and conclusions are as follows:(1)Experimental studies on the flexural behavior of the reinforced RC beam were carried out,including 11 reinforced specimens under one stage loading,3 precracked reinforced specimens by U shape steel and 3 reinforced specimens under two stages longding.The height of the cross section,the thickness of the steel plate,the initial ratio of reinforcement,compressive strength of concrete,the degree of precrack and stress level index were chosen as the main variables to investigate mechanical property of the reinforced specimens.The test results show that the U shape steel can be efficiently connected with the RC beam by structural adhesive and bolt,the bearing capacity and rigidty of the specimens increased substantially,and the specimens show a good deformation ability.With the hight of box area increasing the bearing capacity and the rigidty of the reinforced RC beams increase significantly.With the increase of the thikness of the side plate,the ultimate bearing capacity and rigidty of the reinforced specimens are improved.With the thickness of the bottom plate increasing,the reinforced beams show an increasing trend in the ultimate bearing capacity and rigidty.The bearing capacity and stiffness of the reinforced specimens are less affect by the initial ratio of reinforcement of the RC beams.With the compressive strength increasing,the U shape steel is less prone to peeling,and the overall performance of the strengthened specimens are much more better.The mechanical behaviour of reinforced specimens are less affected by the precrack of the beams.The overall performance of strengthened RC beams do not show obvious change as stess level indexs changed.(2)Based on the test of flexural behavior of RC beams strengthened by U shape steel,the bearing capacity of the reinforcement specimens is analysised by the method of section internal force equilibrium,the dissertation proposes calculation method of yielding,ultimate state and shear capacity are given.Comparing with the test results,the proposed calculation methods match the experiment results well.They can be used as a reference for the engineering application.(3)The finite element software is used to analyze the reinforced concrete beams strengthened with U shape steel.The results are found to be good agreement with the experimental results.The stress state of each part of the reinforced concrete beam is analyzed,wihich further shows that the reinforcement method can give full play to the mechanical properties of different materials.(4)The characteristic strain and deformation of reinforced RC beams under serviceability load are analysised,and the calculation method of deformation is proposed,which accounting for the slip effect between U shape steel plate and RC beam,and the effect of injury that caused by the reinforcement is considered too.Comparing with the test results,the rationality of the calculation method is verified.(5)The stress on the interface which is between concrete beams and U shape steels is analyzed,the debonding types and the debonding mechanism are expounded.According to the numerical analysis,the calculation methods of interface shear stress and debond loading are proposed,Comparing with the test phenomena,the rationality of the calculation method is verified.(6)The buckling phenonenom of reinforced specimens is analysed,the analysis shows that when U shape steel has peeled from the RC beam,the local buckling would happened immediately.Based on the energe method,the critical buckling stress expressions of the side plate on different conditions,including bending,shear and bending-shear,are proposed.In order to prevent local buckling of the reinforced specimen in case of the U shape steel peelded,some suggestions are put forward.