The Experiment Research And Nonlinear Analysis Of Fire Damage Identification Of Reinforced Concrete Beams Based On Dynamic Characteristics

In the process of construction fires,beam-and-plate components will suffer extensive damage,which is more serious than other damaged components.Especially for beam members,material properties inevitably deteriorate,so there is a certain degree of damage to the structural properties.It can be seen that it is particularly important to adopt reasonable and effective detection method to evaluate the damage of beam-type after fire,because it can provide a basis for the repair and evaluation of the components after the fire.This paper studied the reinforced concrete beams subjected to fire by the combination of experimental research and finite element analysis.According to the change of the selected power fingerprint before and after the fire,the position and extent of the damage are effectively identified.At the same time,the thermal elastoplastic theory is used to analyze the nonlinear change of the reinforced concrete beam subjected to fire,and the bending moment is solved by using FORTRAN.The fundamental reason for the change of dynamic characteristics of the structure is explained from the change of the bending stiffness of the section with the time of the fire.The main research work and achievementsare listed as follow:1.Fire tests and modal test were carried out on five reinforced concrete beams with different reinforcement ratios and fire time,The test results show that:1)Under the ISO834 standard heating mode,the temperature field obtained from the full-bottom fire test of simply supported beams is in good agreement with the one-dimensional heat conduction theory;2)The change of dynamic characteristics after fire is related to the reinforcement ratio and fire time.The increase of the reinforcement ratio weakens the damage caused by the fire to the beam to a small extent,making it less influential on dynamic characteristics,and the dynamic characteristics change greatly with the extension of fire time;3)In the process of identifying the damage by dynamic fingerprints such as frequency square ratio,vibration mode variation and vibration mode curvature difference,the sensitivity of the vibration shape difference to the damage location is higher.It is concluded that single side fire on a simply supported beam will cause large-area approximate uniformity and segmental damage.2.The temperature field of the reinforced concrete beam is simulated by selecting reasonable material performance parameters,and the obtained temperature field is introduced into the modal model for analyzing the dynamic damage characteristics after fire.The results indicate that: 1)The simulated temperature field basically coincide with the measured one,because of the thermal inertia of the concrete,the temperature cloud map gradually transits from convex to concave along the beam height relative to the fire surface.2)The damage results of the simply supported beam were identified by dynamic fingerprints such as frequency,vibration mode and vibration shape difference.The conclusions obtained are basically consistent with the test.3)Compared with the full-surface fire model,the frequency value of the reinforced concrete beam after partial fire is improved,the vibration mode changes obviously,and the position where the damage occurs can be roughly discriminated,and the recognition accuracy of the vibration shape curvature difference is more sensitive and the recognition effect is better,it is shown that it is feasible to use multiple damage indicators to identify fire damage of reinforced concrete beams.3.The nonlinear change of high temperature performance and mechanical properties of reinforced concrete beams under fire is the essential reason for the change of structural dynamic characteristics.Therefore,the nonlinear analysis of reinforced concrete beams under fire is carried out by the theory of thermal elastoplastic increment.The iterative method is used to solve the set of moment-curvature equations established by the “strip method”,and finally the output of the result is realized by FORTRAN programming.The results show that the bending moment-curvature curve increases nonlinearly before reaching the ultimate bending moment.According to this,the nonlinear variation of the bending stiffness and deflection of the mid-section cross-section with the fire time is obtained,Furthermore,the intrinsic reason why the frequency ratios of the test pieces are no longer changed according to the order of the square ratio is clarified.