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钢结构因其具有强度高、重量轻、塑性好、施工速度快等优点，被广泛运用于现代建筑结构中。但是钢结构的耐火性能差，温度为 400℃时，钢材的屈服强度将降至室温下强度的一半，温度达到 600℃时，钢材基本丧失了强度和刚度，这使得火灾下钢结构局部或整体倒塌的事故频频出现。为了减小火灾带来的人员伤亡和经济损失，各国均开展了有关研究，编制了钢结构抗火规范，并提出了结构抗火计算的公式与方法。目前国内的钢结构抗火设计主要采用基于试验的构件抗火分析方法和子结构模型抗火分析方法，基于试验的分析方法费用昂贵，并且构件模型和子结构模型很难模拟构件在整体结构中的荷载分布和大小、端部约束等情况。因此，对空间框架进行整体的抗火非线性反应研究，具有重要的理论意义和工程实用价值。
本文综合归纳了国内外钢结构抗火研究动态，比较了不同文献采用的热工参数和计算公式的适用性，并选择了适合本次研究的参数和计算方法。根据传热学经典理论和有限元分析理论，利用ANSYS有限元软件，建立了钢框架在火灾条件下的传热模型与受力模型以及热－结构耦合模型，综合考虑材料非线性和几何非线性，对其进行了整体研究，包括：
（1）火灾下刚接梁和铰结梁构件的温度场和抗火性能研究，包括特征值屈曲分析和非线性屈曲分析，考察了荷载大小和约束情况对构件屈曲温度和失效形态的影响。为了使研究符合实际工程情况，在梁上施加荷载比为0.5的静力荷载，然后按照ISO标准升温曲线施加温度荷载，计算其在温度荷载作用下的反应，得出刚接梁和铰结梁在火灾下的不同失效模式，并根据钢梁的变形情况将构件在火灾下的反应划分为三个阶段：第I阶段，梁内温度低于100℃时，梁处于弹性状态，承受附加轴向压力作用；第Ⅱ阶段，梁逐渐进入塑性状态，梁内附加轴向压力逐渐减小；第Ⅲ阶段，当梁内温度超过600℃时，钢材力学性能发生较大变化，随着钢材强度的降低，梁内附加轴向压力逐渐减小，直至破坏。
（2）火灾下平面框架的抗火性能，主要研究了不同截面类型的柱（工字型截面柱与箱形截面柱）对平面钢框架抗火性能的影响。建立了两种截面类型柱的两跨平面框架，第一步施加荷载比为0.5的静力荷载，第二步按照ISO标准升温曲线施加温度荷载，通过分析两种类型平面框架的温度分布情况和力学特性，计算出平面框架梁、柱的温度－时间曲线，得出箱型截面柱比工字型截面柱的升温速度慢，耐火时间更长，抗火性能更好的结论。并通过平面框架和梁构件的极限温度的比较，说明框架抗火性能比单独构件抗火性能更好。
（3）火灾下空间钢框架的整体非线性分析。建立有防火分区的两跨空间框架模型，通过温度场计算，证实了防火墙有较好的绝热作用，能有效保护非受火区构件；此外研究了梁柱的截面温度梯度，发现受火梁截面的腹板温度最高，下翼缘其次，上翼缘最低，截面温度为非线性分布，因此规范采用的截面温度线性分布并不符合实际情况。通过对空间钢框架的热－结构耦合计算，进行火灾情况下钢结构全过程反应分析，判断出火灾下结构的薄弱环节，考察受火构件与非受火构件的相互影响，更加真实地反映出结构的抗火性能。计算结果显示节点是钢框架抗火的薄弱环节，尤其是梁、柱交接处梁翼缘根部的应力集中对梁的抗火承载力影响很大。火灾发生后，不仅仅是受火构件的内力受到高温的影响，未受火构件的内力也会发生变化。受火构件与周围结构之间的相互影响使得整体结构火灾下的内力分布是采用单根构件计算所无法模拟的，因此更加深入地对结构进行整体抗火分析是很有必要的。
本文研究成果对真实火灾作用下结构体系抗火极限状态的研究以及钢结构建筑抗火设计具有一定的指导意义和参考价值。The steel structure is widely used in modern structure for its high strength, low weigh, good plastic and quick construction. But its fire-resistance behavior is bad. When temperature is 400℃ ,the steel’s yield strength is half to the normal temperature, and when temperature is 600 ℃, it will lose it’s strength and stiffness. For present economic losses and personal casualties caused by fire, various countries research the response, establish the code for resistant fire of steel structures, and propose the analytical method for evaluating the fire resistance of steel structures .the main design method is based on experimental results at present in china ,include member analytical model and substructure analytical model this method is expersive and can hardly simulate the real action such as load distribution, load size ,and restraints in whole structure .therefore, the research on nonlinear response of space steel frame has great theoretical significance and practical value.
In this article, the research on fire resistant of steel structure is summarized, the application of the thermo-dynamical parameters and the theoretical formulas are compared. Based on the summarization of the research on fire resistant of steel structure and the comparison of the theoretical formulas, the suitable parameter and the formulas are selected for this article. According to the heat transfer study classical theory and the finite element theory, using the ANSYS software, the heat transfer model, stress model and thermal-structural coupling model is established, and the whole response is researched with the consider of the material non-linear and the geometry non-linearity.
(1) The research on temperature distribution and fire resistance of rigidly connected girder and hinged connected girder, which include eigenvalue buckling and nonlinear buckling analysis. Combined with the construction practice, the static load which the load ratio is 0.5 and the temperature load which is according to ISO temperature-time curve is applied on girder to calculate the structure’s response in fire, and the failure mode of rigidly connected girder and hinged connected girder is obtained. The response of steel girder in fire could be divided into three stages, the first stage is elastic stage, which the girder’s temperature is less then 100℃, and the girder is combined axial compression. In second stage, girder enter into plastic state and the axial compression decrease, the third stage is when the temperature is over 600℃, the steel’s mechanical performance is changed. With the decrease of steel’s strength, the axial compression in girder decrease until the girder destroied.
(2) Fire resistance of plane frame. The research on the response of plane frames in fire with different type of column, I section column and box section column. The models are established, the static load which the load ratio is 0.5 and the temperature load which is according to ISO temperature time curve is applied on frames to analysis the two type frames temperature distribution and structural behavior. With the analysis of plane frames’ temperature–time curve, it can find that the frame with box section column has better fire-resistance behavior which the heating velocity is lower and the time of fire resistance is longer, and the plane frame has better than the single member with the compare of their ultimate temperature.
(3) The research of a two span space steel frame. The model of the space steel frame with fire division is established. With the calculation of temperature, it provided that the fire wall can insulate heat and protect non-fired members. The calculation of girder temperature gradient shows that in the fired girder the web plate’s temperature is the highest, the bottom flange’s temperature is lower, and the top flange’s temperature is lowest, so that the temperature gradient in girder’s section is non-linear, which provided that the linear temperature gradient used in code is unfavorable. And through thermal-structural coupled calculation of the spatial steel frame, carry on entire process response analysis in the fire situation, have judged weak link under the fire inspected the fire component with the non-fired component mutual influence, really reflect the fire-resistance performance of the structure. The results show that beam-column joint is a weakness, specially the stress concentration in beam’s flange closed to the beam-column joint can influence bearing capacity of steel beam in fire evidently. The fire influent the internal force in not only the fired component but also the non-fired component, the inter-behavior in fired component and non-fired component make it impossible that stimulate the whole structure with single component, therefore it is necessary to analysis the whole structures’ response.
The research result in this article have the certain guiding sense and the reference value to the structure system limiting condition research under the real fire function as well as the steel structural construction fire resistance design.Abstract
The steel structure is widely used in modern construction configuration for its high strength, low weigh, good plastic character and rapid construction speed. But its fire-resistance behavior is bad: when temperature reachs to 400℃ ,the steel’s yielding strength will reduce to just one half of the normal temperature; when temperature reachs to 600 ℃, it will lose it’s strength and stiffness basically. As the result, the accidents of part or full steel’s structure collapse appeared successively. For reducing present economic losses and personal casualties caused by fire, various countries carried out relative researches, established the code for resistant fire of steel structures, and proposed the formula and method to calculate the fire resistance of steel structures .The main design method is based on experimental results at present in china, including member analytical model and substructure analytical model. This method is expensive and can hardly simulate the real action such as load distribution, load size, and restraints in whole structure. Therefore, the research on nonlinear response of space steel frame has great theoretical significance and practical value.
In this article, the research on fire resistant of steel structure is summarized and the application of the thermo-dynamical parameters and the theoretical formulas are compared. Based on the summarization of the research on fire resistant of steel structure and the comparison of the theoretical formulas, the suitable parameter and the formulas are selected for this article. According to the heat transfer study classical theory and the finite element theory, applying the ANSYS software, the heat transfer model, stress model and thermal-structural coupling model is established and the whole response is researched considering the material non-linear and the geometry non-linearity which include the following aspects:
(1) The research on temperature distribution and fire resistance of rigidly connected girder and hinged connected girder, which include eigenvalue buckling and nonlinear buckling analysis. Combined with the construction practice, the static load which the load ratio is 0.5 and the temperature load which is according to ISO temperature-time curve is applied on girder to calculate the structure’s response in fire, and the failure mode of rigidly connected girder and hinged connected girder is obtained. The response of steel girder in fire could be divided into three stages: the first stage is elastic stage, which the girder’s temperature is less then 100℃, and the girder is combined axial compression. In the second stage, girder will enter into the plastic state. At the same time, the axial compression will decrease accordingly; in the third stage, when the temperature is over 600℃, the steel’s mechanical performance will be changed. With the decrease of steel’s strength, the axial compression in girder will decrease until the girder is destroyed.
(2) Fire resistance of plane frame which focuses on the response of plane frames in fire with different type of column (I section column and box section column). The models are established. In the first step, the static load which the load ratio is 0.5 and the temperature loan is applied on frames to analysis the two type frames temperature distribution and structural behavior. In the second step, according to ISO temperature time curve, through analyzing the plane frames’ temperature–time curve, we could draw a conclusion that the frame with box section column has better fire-resistance behavior which the heating velocity is lower and the time of fire resistance is longer, and the plane frame better than the single member through comparing their ultimate temperature.
This research of this article has certain guiding sense and reference value to the structure system limiting condition research under the real fire function as well as the steel structural construction fire resistance design.
Keyword:
Nonlinear Analysis
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