| The fire seriously threatened the safety of people's life and property,which caused great losses to the country.The reinforced concrete(RC)structure has been widely used due to its good moldability and durability,and a large number of fire accidents will inevitably occur.Since the fire is a complex physical and chemical change,the properties of the steel and concrete materials will be deteriorated to varying degrees depending on the degree of fire.The most obvious is the reduction of the stiffness and bearing capacity of the damaged structure,which seriously causes the risk of structural failure and collapse.For this purpose,a lot of theoretical analysis and experimental research on fire resistance of RC structures has been carried out by relevant scholars at home and abroad.Due to the limitations of the test conditions and the difficulty of onsite measurement,the vibration characteristics of RC structures under fire are less studied at present.The fire damage identification research for RC structures is still rare,and the existing fire damage identification methods only consider the influence of frequency,and the vibration modes that are sensitive to local damage are not included.For this purpose,the fire damage identification of RC beams based on vibration test is studied systematically by theoretical analysis,numerical simulation and experimental verification.In addition,a fire damage identification method based on Improved support vector machine based on firefly algorithm(FA-SVR)is proposed.The feasibility of the method was verified by numerical simulation and fire test.Specific research work is as follows:(1)The research status of fire resistance of engineering structures,fire damage vibration characteristics and fire damage identification at home and abroad are discussed,and the existing problems and shortcomings are pointed out.(2)The influence of crack on the temperature field of beam is considered,and a formula for calculating the high-temperature bearing capacity of t-shaped beams is proposed.Firstly,it is assumed that cracks develop linearly.Then,the ANSYS was used to simulate the temperature field of RC beams.And the temperature field is compared with the temperature field without considering the crack beam,the influence law of the crack on the beam temperature field is obtained.Finally,the stiffness and bearing capacity are calculated according to the temperature field with considering the crack of the beam.(3)The theory of support vector machine,existing problems and improvementideas are introduced systematically.A firefly algorithm is proposed to optimize the parameters of support vector machines.The fire damage modal parameters of RC beam are analyzed by means of numerical simulation.The fire damage identification method of simply-supported RC beams based on FA-SVR is proposed.White noise is applied to test samples for verification,and the recognition results are good,which meets the requirements of evaluation index and engineering accuracy.(4)A three-span continuous beam model is established.Considering the different fire time of different beam spans,a three-step refined fire damage identification method for continuous beams based on the fire damage identification method of simplysupported beam is proposed.The method greatly reduces the workload and solves the problem of sample data explosion.White noise is applied to test samples for verification,and the recognition results are good,which meets the requirements of evaluation index and engineering accuracy.(5)Ten simply-supported t-shaped RC beams are designed,and nine of them were used for fire test under different fire time and different loads.The other one,as a comparative test piece,was not subjected to a fire test,and only a static load test was performed.The calculation formula of the high-temperature bearing capacity of the tshaped beam and the damage identification method are verified by measuring the frequency and vibration mode under fire and after fire and post-disaster bearing capacity. |
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