Fire Damage Evaluation For Box Girder

Fire,as a sudden and unpredictable harmful factor,can cause great damage to a bridge.Detection,identification and reinforcement of a bridge after fire are necessary to ensure its reuse.Applying numerical simulation to an actual engineering project,this thesis systematically analyses the evaluation and repairing process of a bridge damaged by fire.Its contents and conclusions are as follows:(1)Relevant theoretical knowledge about heat conduction is introduced,and changes of thermal and mechanical properties of main materials after a fire are summarized.(2)Based on an actual project,the detection methods for material strength,carbonization depth,cracks,thickness of steel bar protective layer,bridge deflection and dynamic characteristics of the studied box girder after fire are introduced,and corresponding test results are analyzed.According to the test results,it is preliminarily predicted that under the fire,the temperature of the lower surface of the box girder is about 500 ?.The fire temperature field in the box girder is established,and the corresponding formula of temperature curve is deduced according to ISO834 standard heating curve.(3)The finite element analysis model of the box girder is established by using MIDAS FEA,and the heat transfer process and thermal stress change of the box girder during fire are simulated.In order to avoid the deviation of on-site detection and inaccurate temperature prediction,three working conditions of 500,600 and 700 Celsius are established respectively for analysis.The concept of prestress loss rate is introduced to calculate the prestress loss.The results show that the temperature gradient of fire heat transfer is non-linear.The prestress loss rates in three working conditions are 4.63%,5.85%,6.92% respectively,which are all less than 10%.(4)Based on the results of numerical simulation and theoretical calculation,the damage degree of the analyzed box girder is identified as the second level,which means it deserves a reinforcement.Five commonly used bridge reinforcement methods are enumerated,and after comparison,the bonding reinforcement method is adopted to conduct the reinforcement of the box girder.