Research On Flexural Behavior Of Reinforced Concrete Beams Strengthened With U-shaped Steel Subjected To Secondary Loading

"With the development of structural engineering,there are many new challenges in the field of structural reinforcement.The traditional strengthening methods include enlarging section method,outsourcing steel method,external bonded carbon fiber method,prestress method and so on.As one of the most important methods,steel reinforced method has been widely used in engineering for its obvious strengthening effect and simple operation.In this paper,on the basis of the existing steel reinforcement method,a new type of steel reinforcement method is put forward,which is the U-shaped steel box strengthening method.The experimental and theoretical research about it is carried out.The basic ideas,the measuring methods and so on,are introduced in the second chapter,which is about the design of reinforced concrete beams strengthened with U-shaped steel.The test studied the influence of steel plate thickness,height,initial loading degree on the beam reinforcement.Finally,the comparison test of 9 beams(strengthened and non strengthened)was carried out,and the force-deformation curves and the strain curves were measured.The third chapter is based on the experimental phenomena and data.The force-deformation curve,bearing capacity and stiffness are analyzed and find that:?the bending strength of strengthened beam is about 6-9 times before reinforcement,the tangent stiffness is about 1-2 times before reinforcement;?The section deformation can meet the plane section assumption;?The initial load have seldom influence on the performance of the strengthened beams.The increase of steel plate thickness and section height can effectively improve the bearing capacity of the beams;In the fourth chapter,based on the assumption of plane section and the constitutive model of concrete and steel,the formula of flexural capacity of beam which strengthened with U-shaped steel box is derived.The theoretical formula proposed in this paper match well with the experimental results and numerical results.This formula can reasonably reflect the strengthening mechanism and has a certain value in engineering application.In the fifth chapter,the finite element software ABAQUS is used.Based on the elastic-plastic damage constitutive model for concrete and the elastic-plastic constitutive model for steel the nonlinear force-deformation process of the beams are analyzed.The modeling method,element selection and the model parameters are introduced in detail.Finally we find that:?The U-shaped steel box changes the damage evolution of concrete which improves the energy dissipation ability of the concrete;?The U-shaped steel box reduces neutral axis of the beams then increases compressive area of the concrete.?The initial load has seldom influence on the flexural strength and stiffness of the beams.The increase of the plate thickness and height of the U-shaped steel box can effectively improve the bearing capacity of the beams,which is consistent with the experimental results.Increasing the section height of U-shaped steel box is a more economical way of strengthening.