Study On Post-fire Flexural Behavior Of Prestressed Concrete Beams And Slabs

In order to study the post-fire flexural behavior of prestressed concrete beams and slabs after fire,the static load tests at normal temperature or after fire of 7 prestressed concrete simply supported slabs,7 prestressed concrete simply supported beams and 3prestressed concrete continuous beams were carried out in this paper with theoretical analysis and finite element simulation.The main research contents and conclusions are as follows:(1)The fire tests of prestressed concrete beam and slab specimens were carried out under sustained load,and the temperature field distribution inside the specimens was observed and simulated by finite element method.The results show that the temperature change in the concrete specimen has obvious hysteresis in the process of rising and cooling,and the time of reaching the highest temperature is different in different parts of the specimen.As the moisture evaporation inside the concrete needs to absorb a large amount of heat,the temperature data measured inside the specimen will appear as platform near 100℃.The free water and the combined water content of the concrete far from the outer surface are higher,and the platform of the concrete heating curve is longer.In the finite element simulation,the thermal parameters of concrete near 100℃is adjusted,and the results obtained after the adjustment are closer to the measured values.Although there is still some error,the accuracy of finite element simulation analysis can meet the requirements of the mechanical analysis of specimens after fire.(2)The research on flexural behavior of prestressed concrete simply supported slabs show that the prestressed concrete simply supported slab has obvious bursting phenomenon in fire,the flexural rigidity of the specimen will decrease obviously in the early stage of fire,and the deformation of the specimen will increase obviously in the static load test after fire.With the extension of fire time,the residual flexural capacity decreases sharply after fire.The residual bearing capacity of the specimen damaged by the concrete explosion is very low,but the concrete in the compression zone is usually less damaged,the flexural bearing capacity and flexural rigidity of the specimens can be well restored by using the steel plate reinforcement.(3)The research on flexural behavior of prestressed concrete simply supported beams show that the flexural rigidity of decreases more than the bearing capacity after fire.Increasing the thickness of the protective layer of the prestressed tendons can improve the residual ultimate bearing capacity of the specimens after fire,on the contrary,increasing the degree of prestress can reduce the residual ultimate bearing capacity of the specimens after fire.Under the condition of equal reinforcement,the flexural behavior of the specimens of bonded and unbonded prestressed concrete simply supported beams is close to each other after fire.The ultimate bearing capacity and flexural stiffness of the prestressed simply supported beam can be calculated accurately by the cross-section conversion method.With the extension of fire time,the residual bending capacity of prestressed concrete simply supported beams decreases linearly.(4)The research on flexural behavior of prestressed concrete continuous beams show that the flexural rigidity and bearing capacity of the mid-span section of the prestressed concrete continuous beam decrease significantly with the extension of the fire time,but the flexural rigidity and bearing capacity of the support section do not decrease obviously.As a result,the redistribution capacity of internal forces of prestressed concrete continuous beams decreases after fire,the proportion of load on the support sections increases,and the failure modes of components change.The simplified analysis method proposed in this paper can be used to predict the ultimate bearing capacity and failure mode of prestressed concrete continuous beams after fire.