Damage Assessment And Experimental Study Of Reinforced Concrete Floor Slab After High Temperature

Reinforced concrete floor slab is one of the main load-bearing components.High temperature will damage the structure and affect the normal use function.At present,for the damage assessment of the structure or component of a building after a fire^[1],only the determination of the damage location can not be carried out quantitative analysis.How to quantify the damage is a difficult problem that our researchers have to overcome.It is also an important subject of this article.This paper mainly conducts dynamic test（dynamic load）and bearing capacity test after fire（static load）on 4 reinforced concrete slabs.Evaluate the damage of structures or components after a fire through damage depth,frequency reduction,stiffness reduction,load-deflection,etc.The incremental constitutive model of steel and concrete is established through temperature field parameters to reduce the bending stiffness of reinforced concrete floor after fire;through the frequency and stiffness changes of dynamic load test,and the relationship between load and deflection of static load test,The neutral axis and elastic modulus of the floor slab before and after the fire are analyzed,and the damage of the building structure or components after the fire is evaluated from the two aspects of material performance and dynamic characteristics.The main research work and results of this paper are as follows:（1）Check domestic and foreign related research,and determine the research content,research direction and research methods of this article.According to the laboratory conditions and the requirements of the"Concrete Design Code",four reinforced concrete two-way slabs were designed,and the experimental research of the reinforced concrete floor before and after the fire was carried out.（2）Four reinforced concrete two-way slabs were subjected to a dead load test under fire and a dynamic test before and after the fire.The temperature of the experimental furnace and the temperature in the floor slabs B1～B4 were collected through the Agilent 34980A software,and the temperature field was analyzed,and it was concluded that the temperature reached mild Fire time and damage depth of damage and severe damage;use DH3816N to collect the frequency of B1～B4 floor slabs at different temperatures,and use the frequency equation of the floor slab under fire to analyze the first-order and second-order frequencies to get the slight damage Depth;through the analysis of the first-order and second-order formations of the floor slab,it is found that the vibration mode of the floor before and after the high temperature is basically consistent,and the high temperature will not affect its formation.This also proves that the assumption of the temperature field along the thickness direction is certain.Sex.（3）Carry out bearing capacity test of 4 reinforced concrete floor slabs after high temperature（different support methods）,analyze the changes of the neutral axis along the long and short sides of the floor through strain analysis,as well as the law of neutral axis changes of different support forms,and analyze the temperature field The change law of the neutral axis formed a contrast,and it was found that the temperature had little effect on the neutral axis,and the bearing capacity had a great influence on the neutral axis,showing a nonlinear growth.（4）Analyze the elastic modulus and stiffness through temperature field and frequency.It is found that the elastic modulus reduction at high temperature is related to the reinforcement ratio and plate thickness.;the elastic modulus of B1～B4 floor slabs at high temperature is obtained from the temperature field.The reduction rate is between 47.88%～51.46%;the elastic modulus reduction rate at high temperature obtained by the second-order frequency is between 44.46%～53.09%,and the error of the elastic modulus obtained from the temperature field is 1.30%～5.87%,It also further verifies the reliability of the temperature field function;the stiffness reduction rate of the floor slab at high temperature is between 77.44%and 82.32%,indicating that the stiffness reduction at high temperature is related to the reinforcement ratio and plate thickness.The higher the reinforcement ratio is The higher the reduction,the thicker the board,the lower the reduction.The stiffness is analyzed through the load deflection curve,and it is found that the support method,the plate thickness and the reinforcement ratio also have an effect on the stiffness.Especially in the case of the same plate thickness,the change speed of the four-side simply supported stiffness is 7times that of the four-point support,which proves that the support method is the main factor affecting the bending stiffness.（5）Perform nonlinear analysis on reinforced concrete structures or components after high temperature,perform elastoplastic analysis on the basis of the high temperature performance of reference materials and related mechanical properties,establish an incremental constitutive relationship model of steel and concrete,and perform nonlinear equations Group solving and dynamics solving,obtained frequency and formation,compared with the actual,found that the two are basically consistent,which proves that the nonlinear analysis has certain reliability.