Fire Resistance Of Reinforced Concrete Restrained Members During Heating And Cooling Phases

Most of previous studies have mainly concentrated on the fire performance of isolated RC members. However, it has long been recognized that the fire resistance of isolated structural members is obviously different from that in whole structures. Moreover, the temperature of most realistic fires increases only for a finite period and then decreases after the fuel supply is consumed. Therefore, through test, numerical simulation, parameter analysis and practical calculation method, this paper investigates the fire resistance of RC restrained members exposed to fire with cooling phase and frames exposed to fire spread. The main work and conclusions include as follows:1. Eight RC restrained beams were tested in fire with cooling phase. The influences of some parameters (such as axial/rotational restraint stiffness, load ratio, and heating time) on deformations and internal forces of the restrained beams were discussed. Test results show that: (a) the axial force in restrained beam subjected to heating and cooling processes increases first and then drops gradually; the residual axial force is significant after the cooling phase, and the maximum axial force ratios of the specimens are range from 0.05 to 0.1; (b) the influence of heating time on the mean value of maximum bending moments at the left and right beam ends is limited, implying that the mean value of maximum bending moments reaches within 60 min of the heating; and (c) the change of rotational restraint stiffness has little effect on the mean value of maximum additional bending moments at the left and right beam ends.2. Using the computer program of SAFIR, the influences of some parameters (i.e., rotational restraint ratio, axial restraint ratio, section width, load ratio, reinforcement ratio and heating time, etc.) on axial force and bending moment of concrete restrained beam exposed to ISO834 standard fire with cooling phase are analyzed. Based on the simulation results of 288 and 3456 cases respectively, practical calculation methods for axial force and bending moment at the end of beam subjected to fire with cooling phase is proposed. Simulation results show that: (a) for axially-and-rotationally restrained beams in fire with or without cooling phase, the axial force ratio increases gradually first, then varies gently, and finally decreases quickly, but the gentle variation stage corresponding to fire with cooling phase is longer than that without cooling phase; (b) the bending moment at the end of beam increases to the peak first, then drops gradually and then becomes gentle or recovers slightly in fire without cooling phase; and (c) the bending moment at the end of beam in fire with cooling phase is similar to that without cooling phase, the main difference lies in the phenomenon of declining slightly in the later stage due to the cooling.3. The influences of some parameters (i.e., axial/rotational restraint ratio, sectional dimension, load ratio, load eccentricity ratio, reinforcement ratio, and heating time, etc.) on axial forces and bending moment at the mid-height of concrete restrained columns exposed to ISO834 standard fire with cooling phase are analyzed. Based on the simulation results of 2880 and 192 cases respectively, practical calculation methods for axial force and bending moment at the mid-height of column subjected to fire with cooling phase is proposed. Simulation results show that: (a) for axially-and-rotationally restrained columns in fire with or without cooling phase, the enhancement coefficient of axial force increases gradually first, then reduces gently or keeps constant, and finally decreases quickly, but in the late stage the enhancement coefficient corresponding to fire without cooling phase drops more significant than that with cooling phase; and (b) the enhancement coefficient of bending moment at the mid-height of axially-and-rotationally restrained column decreases rapidly first and then drops gradually in fire with or without cooling phase.4. Sectional bending capacity of concrete restrained elements is studied using the self-developed program, explaining the phenomenon that RC restrained beams subjected to large bending moment do not fail significantly during heating and cooling phases. The results indicate that: (a) bending capacity of RC elements is improved under the axial force generated by axial restraint; (b) with time increasing, N-M envelop diagrams of elements exposed to fire without cooling phase, reduce inward gradually, and especially rapidly in early period, and become more asymmetric; and (c) After a period of cooling, N-M envelop diagrams of elements exposed to fire with cooling phase tend to be stable and symmetric.5. The effects of horizontal and vertical fire spread on concrete frames are studied, comparing deformations and internal forces of concrete elements exposed to horizontal /vertical fire spread and uniform fire cross multi-spans/multi-storeys, etc. Simulation results show that: (a) for horizontal fire spread, the peak value of axial force of the beam that fire breaks out first is much larger than that of single span exposed to fire; the peak value of bending moment of the end of the beam that fire breaks out first is almost same to that of single span exposed to fire. (b) the increase amplitude of axial force of columns exposed to uniform fire across multi-storeys is almost same to the corresponding ones with vertical fire spread, and is much larger than that of single storey exposed to fire; the curves of lateral displacement of the columns exposed to vertical fire spread is almost mutual parallel, and the peak values of lateral displacement is almost same to that exposed to uniform fire across multi-storeys, and is much larger than that of single storey exposed to fire.