Study On Disease Mechanism Analysis And Reinforcement Disposal Strategy Of Small Box Girder In Expressway

Prestressed concrete small box girder bridges have been widely used in expressway construction because of their reasonable structure and low cost.However,as the service life increases,a series of small box girder diseases have appeared one after another.If the treatment is improperly or not timely,the diseases will develop further and even threaten the service life of the small box girder.In this paper,combining the structure and force characteristics of small box girder of the expressway,based on the classification and analysis of its disease occurrence mechanism and reinforcement method,based on the ABAQUS finite element model,a simulation analysis of the occurrence mechanism of various diseases is carried out.Based on the research results,corresponding reinforcement methods are formulated for the loss of prestress and insufficient web thickness,and the classification and comparative analysis are carried out,and finally verified by engineering examples.This paper takes the 25 m prestressed concrete small box girder commonly used in expressways as the research object,and the main conclusions are as follows:(1)The full bridge model of 25 m prestressed concrete small box girder was established through ABAQUS finite element software,and the influence of prestress loss,web thickness,bottom plate thickness,etc.on the cracking of prestressed concrete small box girder was compared and analyzed.Research has shown that the loss of prestress has a vital influence on the cracking load of box girder.With the increase of prestress loss,the cracking load of box girder dropped sharply from600 kN to 410 kN.At the same time,the maximum increase of the main tensile stress of the key nodes of the web is 0.87MPa;the increase of the thickness of the small box girder web can improve the crack distribution density of the box girder web,but it cannot reduce the distribution range of the box girder cracks.The reduction in the thickness of the bottom plate of the small box girder will reduce the tensile stress reserve of the bottom plate of the box girder and cause more concrete cracking damage.(2)The subroutine D-Load is used to simulate the moving vehicles randomly passing the bridge,and MATLAB is used to output the stress amplitude of the vehicles.Combined with the S-N fatigue life algorithm specified by the Ministry of Railways,the influence of overloaded vehicles on the fatigue life of the small box girder is analyzed.The study shows that under the effect of the standard axle load of the vehicle,the stress amplitude of the mid-span floor reinforcement is concentrated below 15 MPa,and the stress amplitude of the mid-span floor concrete is concentrated below 3 MPa.When the axle load is 30% overweight,the fatigue life of the small box girder is shortened by more than 40 times.(3)Targeted reinforcement treatment methods have been formulated for the prestress loss,thin webs and other diseases,and the reinforcement effects of different reinforcement forms have been compared and analyzed through ABAQUS.Research shows that the longitudinally pasted steel plate on the bottom of the small box girder can effectively improve the problem of excessive mid-span and lower deflection caused by the loss of prestress.Comparing the reinforcement effects of steel plates with three lengths,comprehensively analyzing the deflection,crack distribution,steel bar stress,and economic factors,it is best to use 15 m steel plates to strengthen the 25 m small box girder;The reinforcement of small box girder webs with obliquely pasted steel plates has no obvious effect on improving the bearing capacity of the beams,but it can increase the shear reserves of the webs and effectively limit the development of web cracks.It is an effective auxiliary reinforcement method.(4)Through MIDAS CIVIL,a certain viaduct of Lianhuo Expressway was modeled and calculated,the causes of bridge cracks were analyzed,and targeted reinforcement measures were formulated.Through the analysis of the bending,shearing,and cracking resistance of the box girder of the supporting project,it is found that all the requirements are met except for individual positions.However,it is not inconsistent with the existing disease situation of the bridge.The analysis is caused by overloaded vehicles and heavy traffic.Therefore,the webs are pasted with steel plates for auxiliary reinforcement.