Analysis On Structural Stressing State Characteristics Of Reinforced Concrete Box Girder Strengthened By CFRP

Reinforced concrete(RC)box girder is widely used in the construction of urban overpass and viaduct due to its characteristics of carried out cross-section,low dead weight,excellent bending and torsion resistance.With the increase of service life of the bridge,as well as the effects of disasters such as load,environmental erosion or earthquake,the working performance of RC box girder bridge will degrade and even no longer meet the requirements of normal use.In addition,with the development of economy,the requirement of bridge service capacity is constantly improved.It is an inevitable choice for the safety and benefit consideration to strengthen the bridge.At present,carbon fiber materials(CFRP)with light weight,high strength and easy operation are generally selected to strengthen the RC box girder bridge.In the research of CFRP strengthening,the improvement of the ultimate load capacity of the specimen is usually applied to estimate the overall load capacity of the bridge.Some theoretical formulas have been proposed to evaluate the performance of bridge by strengthened by CFRP.However,due to the influence of parameter and load conditions,these formulas are usually semi-empirical and semi-theoretical.There is no systematic and unified understanding of the change process of the stressing state of the reinforced structure or component.Many structural working behavior characteristics hidden in the test ed data have not been explored.Therefore,this paper applies the theory of structural stressing state to analyze the tested data of RC box girder strengthened by CFRP,and reveals the general working rules of strengthened box girder under the action of bending,shear and torsion,as follows:(1)The connotation and analysis method of structural stressing state theory is introduced.This paper considers the characteristics of the tested data,proposes the method of transforming the tested strain data to stressing state parameter the stress generalized strain energy density(GSED),describes the stressing state mode and the modeling method of its characteristic parameter,introduces the method of detecting leap characteristic structural stressing state(M-K criterion).Then,the structural working rules to be revealed in the stressing state analysis of the strengthened box girder are clarified.(2)The GSED is obtained by modeling the experimental strain data of box girder.The characteristic parameter(GSED sum)is constructed to describe box girder's stressing state,and corresponding curve is plotted to reflect the change characteristics of the box girder's stress state.The characteristic loads of box girder are obtained by applying M-K criterion to the curve.Then,the stressing state model of the box girder is constructed,such as the strain distribution model of the measuring points and the stressing state model of the plane section,etc.,and the variation features of the stressing state model corresponding to the above characteristic loads are investigated to verify the change features of the box girder's stressing state.(3)Through comparing the GSED sum of RC box girder under different strengthening schemes,(stressing state characteristic parameter curve),failure load,and strain distribution model of stirrup(stressing state model),the effect of CFRP on the stressing state characteristics of box girder is analyzed.The results show that the application of CFRP sheet at the bottom of RC box-girder can be regarded as longitudinal strengthened constrains for tested box-girder,and using U configuration with separate CFRP strips to strengthen RC box-girder can be seen as lateral restraint for tested box-girder.(4)The spatial interpolation method combined with finite element simulation,numerical shape function method,is used to expand the test data to further analyze the characteristics of the box girder's stressing state.Firstly,verify the precision of the numerical shape function method extension data,then interpolate the tested data to obtain the strain field of the section.Based on the material constitutive,the stress field of cross section also can be obtained.the strain energy density of the section can be obtained by integration.The evolution characteristics of the response field mode and the variation characteristics of strain energy density are consistent with the variation characteristics of the stressing state of the structure revealed above,and the objectivity and regularity of the characteristic loads are verified.(5)ABAQUS is used to simulate and analyze the box girder,and the simulated data are compared with the measured data to verify the accuracy of the simulation results.Similar to the tested data,the simulated data are modeled and analyzed,and the characteristic loads in the working process of the box girder were identified by M-K criterion,and then the working performance of the reinforced material and its influence on the working features of concrete are investigated after the box girder is strengthened by CFRP.