Damage Identification Of Reinforced Concrete Beam Under High Temperature Basedon Dynamic Parameters

In view of the characteristics of flame plume and heat-transfer mode in the process of building fire,For beams and slabs which as the first extensive damage components,The deterioration of material performance due to high temperature will inevitably reduce the durability and bearing capacity of the component.It is necessary to make an effective evaluation of the damage degree and residual capacity of the structure by using appropriate testing methods.The theory of dynamic identification is widely used in bridge fatigue recognition at present,but it is not mature for the damage discrimination theory of the components after fire.This paper is based on the theory of dynamic damage identification and fire damage identification theory.By defining the structure fire damage according to the characteristics of fire damage and introducing transverse isotropy beam,considering the dynamic response caused by elastic modulus damage and neutral axis change under uniform fire damage conditions as well,a fire damage model considering moment of inertia change and shear deformation is established.and a new identification method for simply supported beam with fire damage due to thermal mechanical coupling is proposed,which is based on the frequency as identify the fingerprint parameter.then,The validity of the theory is verified through the fire testing,modal testing and bearing capacity experiment.The main research contents and achievements are as follows:1.The development status of fire damage identification theory domestic and foreig is analysed by investigating the actual fire process and understanding the characteristics of building fire,which is qualitatively less than quantitative.According to the characteristics of fire,the definition of fire damage is defined: Fire damage is mainly like the large area,approximately homogeneous and nonlinear gradient degradation damage along the fire surface due to material changes with the high temperature.The similarities and differences between the dynamic theory mode and the fire damage model are compared.The research status of mode identification theory is summarized,and the feasibility of fire damage using is demonstrated.2.Based on Timoshenko theory and thick plank theory,A differential equation of free vibration with shear deformation and moment of inertia is established.The natural frequencies and modes of each order are calculated according to the boundary conditions of both sides simply supported.Establish simple beam models for abaqus-finite element software simulation test.By given comparative analysis to the influence of shear deformation and moment of inertia with abaqus finite element software,Which is concluded that the two theoretical model of beam are identical in low order modes,The thick plank theory is superior In the higher order modes;The effects of shear deformation and moment of inertia on the natural vibration characteristics can not be neglected in the study of the depth-span ratio greater than 1/6 of beam or higher order mode regularities.3.A time-varying model of concrete beam under high temperature is established.According to the homogeneous graded characteristics of the fire damage and considering the uniform damage condition,the beam is assumed to be transversely isotropic beam along the height direction.The analytic equation for time and space of one-dimensional heat conduction is obtained by using the Fushi transform theory,then,Taking stiffness as the hinge,The One-dimensional analytical solution and the change equation of the elastic modulus of temperature in frequency analysis can substituted into analytic equation.Then,the regulation of the natural frequency of the simple supported beam at high temperature about time by integraling along height direction.Considering the contribution of comprehensive reinforcement ratio to beam stiffness,The comprehensive influence ratio of the reinforcement and stiffness coefficient is deduced.Finally,the dynamic damage identification method of simple supported RC-beam under fire is realized by frequency identification.After using the finite element software for simulation and comparison analysis,It is found that the theoretical value is consistent with the simulation value and the reinforcement ratio on the stiffness contribution increased after high temperature;The equivalent beam height under the frequency equivalent condition is put forward for reference to the calculation of residual load capacity after fire;The modal-curvature method and arbitrarily-two-order frequency change ratio method are used to identify the location of fire regional damage.4.By designing fire test of RC beam under the simulated standard heating mode,The modal acquisition test and Residual capacity test of the beams with different reinforcement ratio and fire time before and after fire are carried out;By comparing,verifing,analysing the test data and theoretical data,It can come to conclusions as follows:(1)In the ISO834 standard heating mode,The beam damage is basically consistent with the homogenization damage model and the internal temperature field agrees with the one-dimensional heat conduction regular pattern.(2)The natural frequency of the beam after fire decreased significantly,which has a decreasing by 40% and 52% of fundamental frequency when it is fired by 90 mins and 120 mins.More,The decrease of the higher order modes is more obvious than that of the lower modes.(3)the Testing data and the theoretical calculation data are consistent and can be used to identify references.(4)The degree of beam frequency reduction after fire is related to the macrocosm reinforcement rate and fire time;With the increase of the fire time,the frequency decreases nonlinearly,and the natural frequency of the beam increases with the increase of the reinforcement ratio.(5)The change of beam mode shapes after fire is not obvious.But the damping ratio increases significantly because of the internal defect of beam is serious and the energy dissipation capacity of it increases Substantially.