Research On The Bearing Capacity Of Inclined Section Of Ultra-high Performance Concrete Beam Under Fire

Oblique section failure is the most prominent failure mode when the concrete beam breaks and collapses under fire.The failure of the inclined section is brittle failure.During the failure,the fracture collapses suddenly,and the prevention and control is poor.The fire safety risk of the new ultra-high performance concrete(UHPC)structure is large,especially the load-bearing performance of the UHPC beam oblique section under fire needs to be studied urgently.The study of the load-bearing performance and failure mechanism of the UHPC beam oblique section under fire is required.Safety control theory and fire resistance performance-based design methods have important academic value,significant social and economic benefits,and broad application prospects.Therefore,this paper focuses on the above-mentioned key scientific and technical issues to carry out the research on the load-bearing performance of UHPC beams under fire.The main work is as follows:(1)Based on the shear failure theory of concrete beam oblique section,analyze the mechanism of serious damage and collapse(collapse)of concrete beam under fire,compare the material and thermal performance of UHPC beam and concrete beam under fire,and the shear transfer of oblique section The difference between the mechanism and the fire resistance design method is to conduct theoretical analysis,identification and quantification of the high-temperature burst of the shear section in the load-bearing performance of the UHPC beam under fire and the change in the shear resistance transmission mechanism caused by the nonlinear temperature field Fire safety of UHPC beam inclined cross section.The results show that the failure of the diagonal section is the main reason for the collapse(collapse)of the concrete beam under fire.The risk of UHPC beam diagonal section bearing failure under fire is significantly higher than that of ordinary concrete beams.(2)The UHPC beam temperature field calculation model was established according to the heat conduction theory,the thermal-mechanical coupling effect was taken into consideration,and the high-temperature creep and transient thermal strain of the material were introduced to construct a refined finite element analysis model of the UHPC beam diagonal section load under fire.And carried out experimental verification.The results show that under the same heating conditions,the temperature of the UHPC beam at the same location is significantly higher than that of the ordinary concrete beam.The coupled thermo-mechanical finite element model can reveal the transmission mechanism and damage evolution process of the UHPC beam oblique section under fire,and the failure mode of oblique section under fire.Similar to the test results,the deformation and fire resistance limits are in good agreement with the test results,with scientific rationality and good accuracy.(3)Using the refined finite element model of the UHPC beam under oblique section load under fire,the UHPC beam is developed with the parameters of shear span ratio,load level,stirrup ratio and stirrup configuration,longitudinal reinforcement ratio,cross-sectional size,etc.Under the constant load according to the ISO834 standard heating conditions,the expansion parameter analysis of the oblique section bearing performance,find out the key parameters that affect the UHPC beam oblique section bearing performance under fire and the influence law of each key parameter,and reveal the UHPC beam oblique under fire under thermal-mechanical coupling Cross-section bearing failure mode.The results show that the bearing failure of diagonal section of UHPC beam under fire is brittle failure,the ductility of failure displacement is significantly higher than that of ordinary concrete beam,and it increases with the increase of the shear span ratio.The shear span ratio increases from 1.5 to 3.5,and the fire resistance limit increases by about 1.2 ~ 1.5h;the fire resistance of UHPC beams with web reinforcement is less than that of beams without web reinforcement under the same conditions;the coupling rate of UHPC beams is increased from 0.16% to 0.27%,and the fire resistance of specimens is increased by about 0.5 ~ 1.0h.(4)Comprehensively considering the influence parameters of the shear bearing capacity of the UHPC beam under fire and the influence law of each parameter,the safety control and fire resistance design methods of the shear bearing performance of the UHPC beam under fire are proposed,including: controlling the minimum cross-sectional size of the UHPC beam and The minimum protective layer thickness,controlling the shear-span ratio of the UHPC beam,avoiding the diagonal pull failure mode under fire,and appropriately increasing the longitudinal reinforcement ratio and the coupling ratio to improve the safety performance of the UHPC beam diagonal section load under fire.