以往人们评估结构的抗火性能都是针对单独构件进行的,忽略了结构连续性和约束的影响,然而结构中构件的火灾行为与单独构件相比存在较明显的差别,且考虑到实际火灾只在有限时间内升温,随后就会由于可燃物消耗殆尽而降温。为此,本文从试验、数值模拟、参数分析和实用计算方法等方面,较系统地研究了混凝土约束构件升降温全过程的火灾行为,以及火灾蔓延下(涉及升降温)混凝土框架的耐火性能。本文的主要工作和结论如下:1.开展了8根足尺混凝土约束梁的明火试验,考察了升降温作用下梁端轴向和转动约束、荷载比、升温时间等参数对约束梁高温变形及内力的影响趋势。试验结果表明:升降温过程中试件轴力呈现出先较快增大而后逐渐减小并渐趋平缓的趋势,降温结束后仍存在明显的残余轴压力;升温时间对试件左、右两端弯矩最大值的平均值影响有限;转动约束刚度比改变对试件左、右两端附加弯矩最大值的平均值影响不大。2.利用SAFIR程序,开展了混凝土约束梁升降温全过程的轴力和梁端弯矩分析,考察了转动约束刚度比、轴向约束刚度比、截面宽度、荷载比、全截面配筋率和升温时间等参数对ISO834标准升降温作用下混凝土约束梁轴力和梁端弯矩的影响规律,分别通过288种工况和3456种工况的计算分析,建议给出了该类构件升降温全过程的轴力和梁端弯矩的实用计算方法。研究结果表明:①对于轴向/转动约束梁,无论是单调升温还是先升温后降温,轴力比总体都呈现出先逐渐增大而后平缓变化,然后以较大速率降低的趋势,主要区别在于先升温后降温时平缓段的长度比单调升温时更长;②先升温后降温时梁端弯矩的变化总体呈现出与单调升温时类似的特性,即先逐渐增大而后不断减小并渐趋平缓或有所回升的变化趋势,主要区别在于后期因降温而呈现出持续降低的现象。3.开展了混凝土约束柱的升降温全过程轴力和半高处弯矩分析,考察了轴向/转动约束刚度比、截面边长、荷载比、荷载偏心率、配筋率、升温时间等参数对ISO834标准升降温作用下混凝土约束柱的轴力和半高处弯矩的影响规律,分别通过2880种工况和192种工况的计算分析,建议给出了该类构件升降温全过程的轴力和半高处弯矩的实用计算方法。研究结果表明:①对于轴向/转动约束混凝土柱,无论单调升温还是先升温后降温,轴力变化系数总体都呈现出先逐渐增大而后有所减小或基本保持稳定,最后以较大速率持续降低的趋势,主要区别在于后期单调升温对应的降低速率一般比先升温后降温时更大;②升降温过程中,转动约束柱的半高处弯矩变化系数首先迅速减小继而缓慢降低并渐趋平缓,单调升温时的变化趋势与此基本类似,但后期可能出现变化系数增大的现象。4.研究了混凝土约束构件升降温全过程的截面抗弯承载力变化情况,以及升温时间、截面宽度/高度、全截面配筋率、截面轴力比、混凝土轴心抗压强度等参数对升降温全过程的截面抗弯承载力的影响规律,通过2160种工况的计算分析,建议给出了相应的实用计算公式。结果表明:①轴向约束产生的轴压力使截面抗弯承载力有一定程度的提高;②先升温后降温时,降温阶段N-M曲线内缩速率逐渐减缓,降温一段时间后N-M曲线趋于稳定和对称。③作用在截面上轴力一定时,先升温后降温的截面抗弯承载力总体呈现出先逐渐降低而后有所恢复的趋势,而单调升温的截面抗弯承载力则持续降低。随着轴力比的增大,高温下截面抗弯承载力的降低速率持续增大。5.研究了火灾水平蔓延和竖向蔓延对混凝土框架高温行为的影响规律,对比了火灾水平/竖向蔓延、多跨/多层同时受火和单层单跨受火时,混凝土构件变形和内力的变化情况。结果表明:①火灾水平蔓延与否对受火梁跨中挠度的峰值影响较小,火灾水平蔓延的起始跨的梁轴力峰值比单跨受火时受火跨的梁轴力峰值大很多,火灾水平蔓延的起始跨的梁端弯矩峰值与单跨受火时受火跨的梁端弯矩峰值基本相同,多跨同时受火和火灾水平蔓延情况下受火边跨的梁端弯矩峰值均比中跨大;②多层同时受火和火灾竖向蔓延情况下,相应受火柱的轴力最大增幅大致相当,且比单层单跨受火时大很多;③火灾竖向蔓延情况下,受火柱的侧向位移曲线大致平行,侧向位移峰值与多层同时受火情况大致相同,且均比单层单跨受火时大。
电加热器
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