Study On Fire Response Of RC Frame Beam Based On Time-Varying Restrained Model

Under elevated fire temperatures,reinforced concrete beams are subjected to axial and rotational restraints from adjacent beams and columns in the overall structure.In actual fire scenarios,fire spread and deterioration of adjacent components make the restraint strength of reinforced concrete beams changed.The change of the end restraints with time will directly affect the displacement and internal force redistribution of the flexural members such as reinforced concrete beams during heating and cooling phases.In order to study the influence of time-varying restraints on the fire response of reinforced concrete beams,the time-varying characteristics of the end restraints of reinforced concrete beams during heating and cooling phases were studied.Main tasks are as follows:(1)The mechanical properties of reinforced concrete beams at normal temperature were tested considering the end restraint conditions of two different axial and rotational restrained stiffness ratios.The vertical and axial displacement,crack development and ultimate bearing capacity of the beam were analyzed.The results show that the smaller the restraint stiffness ratio,the greater the vertical and axial displacement of reinforced concrete beams;and the increase of end restraint stiffness ratio improves the ultimate bearing capacity of beams.(2)A furnace was developed,which can set a program to make the components warm up according to ISO834 standard heating curve.Based on the end restraint conditions of two different initial axial and rotational restrained stiffness ratios,the time-varying characteristics of restraint and the fire resistance test of structural response during the whole process of beam heating and cooling under constant load were carried out.The temperature field distribution of the beam section,the vertical,axial and angular displacement,the bending moment at the end of the beam and the axial force changing over time fire were analyzed.The research shows that the vertical and angular displacement,beam end bending moment and axial force of the beam show peaks after heating for about 60 minutes,and have time-varying characteristics throughout the whole process of heating and cooling,which can provide a scientific basis for the performance-based fire resistance design of the structure.(3)The residual bearing capacity tests of restrained reinforced concrete beams after cooling was carried out.The vertical and axial displacement,crack development,ultimate bearing capacity and failure characteristics of concrete beams under two different initial axial and rotational restraint stiffness ratios were analyzed.The results show that the higher the initial restraint stiffness ratio is,the greater the ultimate bearing capacity of the beam after elevated temperatures.Compared with normal temperature components,the ultimate bearing capacity of beams subjected to elevated temperatures decreases significantly.And the smaller the initial restraint stiffness ratio,the more obvious the loss of bearing capacity after elevated temperatures.(4)Based on the test and VULCAN program analysis software,the thermal-structural coupling numerical analysis of reinforced concrete beams of various end restraint conditions during heating and cooling phases under local elevated temperatures was carried out.The results show that the end restraints of beam has time-varying characteristics during the heating and cooling phases.The vertical,axial deformation and angular deformation,beam end bending moment and axial force also have time-varying characteristics,which can provide a reference for the dynamic response evaluation of the beam under fire and elevated temperatures.(5)Based on the VULCAN program analysis software,the thermal-structural coupling numerical analysis of reinforced concrete beams and columns of various end restraint conditions during heating and cooling phases under elevated temperatures of frame structures was carried out.The results show that the vertical,axial and angular displacements of the beams have more obvious time-varying response under elevated temperatures of frame structures,while the beam end bending moment and axial force are less than those under local elevated temperatures,which can provide a reference for the safety evaluation of frame structure under elevated temperatures.