Theoretical Analysis And Experimental Study On Fire Damage Identification Of Reinforced Concrete Columns Based On Dynamic Characteristics

In recent years,structural damage diagnosis based on dynamic characteristics has become a very active field of research in civil engineering.At present,a lot of research has been done on the identification of reinforced concrete beams at home and abroad and achieved some results.However,there are few studies on the high temperature damage of reinforced concrete columns.In this paper,the fire damage identification of reinforced concrete columns based on dynamic theory is studied,and selects the reasonable modal parameters to quantitatively evaluate the damage.So the damage to the structure can be effectively judged,also to ensure the normal use of the structure.In this paper,the modal data of reinforced concrete columns before and after high temperature are analyzed;also the damage caused by high temperature was evaluated.The main contents and achievements of this paper are as follows:1.At present,there are two kinds of beam theory models for damage identification of beam structure(Euler-Bernoulli theory of beam model,Timoshenko beam theory model).Based on the Timoshenko beam model,the vibration equation of the experimental column considering the influence of axial force and eccentricity is analyzed by kinetic theory.Then the solution of the vibration differential equation is obtained by solving the vibration differential equation with eccentricity and axial force.The theoretical results show that: the natural frequency of the column decreases as the axial force increases,and the Timoshenko beam model considers the influence of the moment of inertia and the shear deformation on the natural frequency of the column,so the ratio of the first three frequencies in the solution is compared with that of the Euler-Bernoulli beam model before the third order frequency ratio 1: 4: 9 becomes smaller.2.The damage caused by the fire to the test column is mainly caused by the change of the elastic modulus of the concrete,which causes the loss of bending rigidity of the test column.The loss of the stiffness of the concrete column is mainly related to the temperature field.In order to simplify the calculation,the internal temperature change of the single-sided fire column is regarded as one-dimensional heat conduction temperature field.The temperature field is a function of time,that is,stiffness and fire time there is a certain connection.And then the stiffness along the reinforced concrete column cross-section height of the direction of integration,the final fire under the concrete column stiffness and time.The theoretical analysis shows that the greater the loss of rigidity as the fire time increases,the greater the change of the modal parameters before and after the fire.3.Under the action of the fire by the fire part of the concrete due to high temperature damage and quit work.This makes the cross section inertia moment of the concrete column change.The bending stiffness of the column section also changes due to the change of the moment of inertia,and then the dynamic characteristics of the structure also will change.The damage depth d is introduced by the change of the moment of inertia of the section.Which can be brought into the law of stiffness change with time,we can get the relationship between fire damage depth d and fire time.Theoretical results show that: At that time t = 0,we can see that the damage depth d = 0,and with the increase in fire time damage depth gradually increased.Part of the concrete by the fire surface due to high temperature damage and quit work.The concrete column is pressed by the shaft to become a small eccentric compression state.The change in eccentricity also affects its own dynamic characteristics.4.In this paper,five reinforced concrete columns were designed and manufactured,and the fire test and modal test were carried out respectively,and one of them is a room temperature comparison test specimen,four for the fire test pieces.The results showed that:(1)the axial pressure reduces the natural frequency of the column.The eccentricity increases the intrinsic frequency of the column.For the general component,the influence of axial pressure on the natural frequency is greater than that of eccentricity,so the natural frequency of the column is always reduced.(2)From the test data we can see that the first order frequency is most sensitive to the damage of the structure,also it contains the most damage information.Therefore,the damage detection in the actual project uses the first-order frequency change to identify the best results.(3)The deterioration of the column by the fire time can be reflected by the decrease in the natural frequency.After one and half hours of fire,the base frequency decreased by about 30%;after two hours of fire,the base frequency is reduced by about 40%.(4)The COMAC value obtained from the vibration mode is used as the post-fire damage index,which can well reflect the damage position of the structure.COMAC values are more sensitive to local damage to the structure.