| Infrastructures,e.g.,bridges,culverts,etc.,should meet the requirements of high-loading vehicles in the projects for national defense and disaster relief.Thus,their structural members,e.g.,beams,plates,columns,etc.,should have high load-carrying capacity.Traditional methods of bonding steel plate can improve the load-carrying capacity and stiffness of structures within certain limits resulted from the critical reinforcement ratio,while increased cross-section reinforcement method can substantially improve the load-carrying capacity,but will affect the weight of structures obviously,besides the time-consuming construction and maintenance cycle of wet operation.Neither of the two methods above is suitable.Therefore,this thesis proposes the strengthening method of bolt-bonding U-shaped steel box,which can improve the load-carrying capacity greatly and has the advantages of easy operation and easy application.This strengthening technique combines the web of U-shaped steel with the side of concrete beam by bolts and structural adhesives,and keeps the box-shape space without concrete between the bottom of U-shaped steel and that of reinforced concrete beam,which can reduce the weight of structures.This type of steel-concrete composite structure gives full play to the tensile properties of steel and the compressive properties of concrete,resulting in a structure with better strength,stiffness and ductility.In view of the fact that there are no relevant research results and design methods to guide the application of this technique,the research team conducted a study on the basic mechanical performance of the reinforced concrete beams using the strengthening technique.This article focuses on the flexural fatigue behavior of the strengthened beams.The major accomplishments and results achieved include:1.The static loading test of two reinforced concrete beams and the fatigue loading test of ten ones were completed,including each unstrengthened beam for static loading test and fatigue loading test,one strengthened beam bolt-bonded with U-shaped steel box for static loading test and nine ones for fatigue test.The mechanical mechanism and failure law of strengthened beams bolt-bonded with U-shaped steel box under the action of fatigue loading were studied.The test results show that:1)The strengthened beams bolt-bonded with U-shaped steel box can show good cooperative work between U-shaped steel box and beam under both static loading and fatigue loading;2)Compared with the unstrengthened beam,the strengthened beams show a significant improvement in the load-carrying capacity,stiffness,and fatigue life;3)The fatigue damage of the strengthened beams bolt-bonded with U-shaped steel box originally occurs as a crack at the lateral and bottom weld toe of the U-shaped steel,the crack propagates until fracturing of both the bottom of the U-shaped steel and the tensile zone of the web after the fatigue loadling continues to cycle 20,000 to 60,000 times,and subsequently the partial web in compressive zone and the concrete beam rapidly separate with rupture of the concrete in compression zone.2?The mechanical properties of the strengthened beams strengthed with bolt-bonding U-shaped steel box under static loading and fatigue loading are analyzed.The results show that:1)the cross-sectional strain of the strengthened composite beam is in good agreement with plane section assumption under both static loading and fatigue loading;2)the material strains of concrete,rebar and steel plates and the deflection development of structures follow the three-phase evolution rule under fatigue loading;3)Based on the experimental results and theoretical derivation,the formulas for calculating cracking load and ultimate load under static loading are proposed;4)According to the strain evolution law of the concrete at the edge of the compression zone of the strengthened beam under fatigue loading,the factors influencing the cumulative residual strain of the concrete are discussed,and the corresponding calculation formulas are proposed.3?The development law of the mid-span deflection and the factors affecting the stiffness of the strengthened beams bolt-bonded with U-shaped steel box under fatigue loading are discussed.Then the short-term stiffness and the fatigue stiffness of the strengthened beams are analyzed based on the stiffness analysis method for reinforced concrete beam and the reduced stiffness method for composite beam respectively,corresponding calculation models are established,and the deformation coefficient of residual deflection is proposed to calculate the deflection of the strengthened beams under fatigue loading.Results show that the calculated results are in good agreement with the experimental results.4?Based on the fatigue tests of the strengthened beams bolt-bonded with U-shaped steel box,the fatigue life distributions of the strengthened beams with different design parameters are summarized,and the cumulative damage development of such strengthened beams is analyzed using the cumulative damage values.Then,combining with the stiffness degradation law and fatigue damage of the strengthened beams,a fatigue life prediction method with stiffness degradation is proposed for the strengthened beams based on the theory of damage mechanics,and the S-N curve of steel plate is established by the method of nominal stress to predict the fatigue life of such strengthened beams.5.The constitutive models of concrete and steel under the influence of fatigue loading are established,and then the finite element numerical simulation is carried out for the fatigue behavior of the strengthened beams bolt-bonded with U-shaped steel box,analyzing the deformation performance of the strengthed beams and the evolution process of fatigue damage with fatigue cycles.Software Fe-safe is also used to predict the fatigue life of the strengthened beam.The analysis results above are compared with test results.Subsequently,the finite element model with verified accuracy is used to expand the parameter analysis of the strengthened beams.The study finds that:1)The fatigue behavior of such strengthened beam is greatly affected by the web thickness of the U-shaped steel box,the bottom plate thickness is the second,and the height of the bottom box is the least;2)The fatigue life of such strengthened beam monotonically increases with the increase of the web thickness,but the relationships with the height of the bottom box and the thickness of the bottom plate are not monotonous,with the fact that the 6 mm thickness of the bottom plate and the 120 mm height of the bottom box can be used as the critical point for increasing the fatigue life.Therefore,when applying the strengthened method of bolt-bonding U-shaped steel box to significantly improve the bending fatigue life of reinforced concrete beams,it should be preferable way to increase the web thickness with the upper limit of 10mm. |
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