圆形钢管混凝土柱接触热阻的理论分析

精确地分析火灾下结构构件温度场对于安全可靠的防火设计尤为重要。而目前,在计算钢-混凝土组合构件火灾下的温度场时,一般都忽略了钢与混凝土界面处接触热阻的影响。对结构钢和混凝土热物理参数进行了综合考虑,通过求解热传导方程导出了圆形钢管混凝土柱中钢管内壁与核心混凝土间接触热阻的解析解,为进一步精确分析火灾下圆形钢管混凝土柱温度场提供了一种新的方法。     

圆形钢管混凝土柱中钢-混凝土界面接触热阻的理论分析

目前现有不多的计算受火条件下钢-混凝土组合结构温度场方法中,一个主要的不足是忽略了钢与混凝土界面处接触热阻的影响,这将对钢-混凝土组合结构温度场的精确分析、高温下结构(构件)的力学性能以及结构(构件)耐火极限的精确分析产生一定程度的影响。采用综合考虑钢材和混凝土热物理参数的方法进行分析推导,通过求解热传导方程导出了圆形钢管混凝土柱中钢管内壁与核心混凝土间接触热阻的数学表达式。研究为进一步精确分析高温下钢管混凝土柱的力学性能及耐火极限提供了一种新的方法。     

Numerical study on temperature distribution of high-strength concrete-filled steel tubes subjected to a fire

High-strength concrete-filled steel tube (CFST) columns offer a number of benefits and are widely used in high-rise building construction. This paper presents a brand new thermal modeling approach in comparison with tremendous test data for the heat transfer analysis. A heat transfer model has been developed to predict the thermal response of high-strength CFST columns under standard fire conditions with consideration of a number of parameters: steel and furnace emissivity, thermal interface conductance and concrete strength. The verified numerical models discussed the variation of emissivity of steel surfaces and thermal interface conductance in fire. It can be concluded that the influence of emissivity and thermal interface conductance is considerable in the numerical analysis. It is also demonstrated that thermal behavior of high-strength CFST columns subjected to fire during heating and cooling stages, and providing the guidance on predicting thermal response of high-strength concrete-filled steel tube columns.     

轴心受压双钢管混凝土柱火灾后剩余承载力的研究

研究了双钢管混凝土柱耐火承栽力以及火灾后的剩余承载力。通过选取合适的钢材与混凝土热工性能模型。在有限元程序中模拟了ISO-834标准火灾升温曲线下双钢管混凝土柱的截面升温过程。得出了截面温度场分布以及不同部位的升温过程曲线。在此基础上结合钢管、混凝土高温下以及高温后的应力-应变本构模型分别对高温下与高温后轴心受压双钢管混凝土短柱进行了受力全过程数值分析,得到了高温下（后）轴心受压双钢管混凝土短柱的受力特点。     

Fire design method for concrete filled tubular columns based on equivalent concrete core cross-section

In this work, a method for a realistic cross-sectional temperature prediction and a simplified fire design method for circular concrete filled tubular columns under axial load are presented. The generalized lack of simple proposals for computing the cross-sectional temperature field of CFT columns when their fire resistance is evaluated is evident. Even Eurocode 4 Part 1-2, which provides one of the most used fire design methods for composite columns, does not give any indications to the designers for computing the cross-sectional temperatures. Given the clear necessity of having an available method for that purpose, in this paper a set of equations for computing the temperature distribution of circular CFT columns filled with normal strength concrete is provided. First, a finite differences thermal model is presented and satisfactorily validated against experimental results for any type of concrete infill. This model consideres the gap at steel–concrete interface, the moisture content in concrete and the temperature dependent properties of both materials. Using this model, a thermal parametric analysis is executed and from the corresponding statistical analysis of the data generated, the practical expressions are derived. The second part of the paper deals with the development of a fire design method for axially loaded CFT columns based on the general rules stablished in Eurocode 4 Part 1-1 and employing the concept of room temperature equivalent concrete core cross-section. In order to propose simple equations, a multiple nonlinear regression analysis is made with the numerical results generated through a thermo-mechanical parametric analysis. Once more, predicted results are compared to experimental values giving a reasonable accuracy and slightly safe results.     

火灾下钢管混凝土柱受热分析模型

建立火灾下钢管混凝土柱的受热分析模型以便于在抗火分析和防火设计中应用。首先基于传热学的基本理论确定了火灾下钢管混凝土柱与火场的热交换过程,然后根据能量守恒原理和热力学理论确定了钢管混凝土柱截面内的导热过程,推导了有限差分形式的截面温度场计算的基本方程,最后编制了数值计算程序来模拟钢管混凝土柱截面内的温度场。分析模型中重点考虑了水分和接触热阻对截面温度场的影响。数值程序的计算结果与试验结果符合得较好。     

Behavior of Steel–Concrete Partially Composite Beams Subjected to Fire—Part 2: Analytical Study

This paper presents the development of an analytical model of steel–concrete partially composite beams subjected to fire. The model includes consideration of temperature dependent material properties, temperature dependent interface slip between concrete and steel, non-uniform temperature distributions throughout the cross-section and the effect of different rates of thermal expansion at the concrete–steel interface. Model predictions showed good agreement with the results of fire tests on two composite beams reported in an earlier companion paper as well as with limited experimental data published in literature. An extensive parametric study was undertaken by using the proposed model. Parameters considered in this study included geometric dimensions of the composite beam, material grades of steel and concrete, shear connection ratio, reinforcing steel ratio in the concrete slab, and load level on the beam. The parametric study clearly shows that shear connection ratio and load level significantly influence the fire performance of partially composite beams. The critical temperatures with shear connection ratio of 50%, 75% and 100% are 645°C, 602°C and 548°C, respectively, under load level of 0.6. The critical temperatures under load ratio of 0.5, 0.6 and 0.7 are 468°C, 553°C and 633°C respectively, with a shear connection ratio of 50%.     

An analytical method to calculate temperatures of components of reverse channel connection to concrete filled steel section under fire conditions

The paper describes the development of an analytical model for calculating the temperatures of different components of reverse channel connections to concrete filled steel tubular sections. The proposed analytical model is based on the Lumped Capacitance Method and includes the calculation of the quantity of heat transferred at the interface between the steel tube and the concrete core. For unprotected connections, the connection is divided into two regions. One part of the connection is almost unaffected by the presence of the concrete core, on the contrary, temperature of the second part of the connection is significantly affected by the heat transfer at the steel/concrete interface. For protected connections, the entire connection is assumed to have the same temperature. Equivalent two-dimensional section factors are proposed for these connection parts. The analytical model is developed based on the results of extensive parametric studies under standard and different parametric fire conditions and validated against fire test results. The proposed analytical method is applicable to both fire protected and unprotected connections, including both the heating and cooling periods of the fire attack, and incorporating the effects of in-fill concrete.     

钢混凝土界面接触热阻试验研究

火灾条件下钢管与混凝土间界面热阻对结构温度场有很大影响。利用INSTRON 8874型高温材料试验机的高温接触热阻试验装置对钢-混凝土界面接触热阻进行了试验研究,根据各测点的温度时间历程曲线,利用多项式拟和方法外推得到钢、混凝土界面处温度值,通过热传导方程和接触热阻定义得到了界面接触热阻。试验结果表明,不同界面压力下钢管混凝土界面的接触热阻数值比较稳定,与文献结果相比有一定可靠性;无界面压力下钢管混凝土界面的接触热阻数值离散性大,随温度变化明显。     

Thermal bowing on steel columns embedded on walls under fire conditions

The contact of steel columns with building walls is responsible for huge thermal gradients within its cross-section during fire. Current regulatory codes for fire design of steel members provide a formulation to assess the load-bearing capacity of these members assuming uniform temperature through the cross-section; however, this is not what happens in the major part of the cases in real structures where the columns are embedded on walls. The walls on one hand will provide a temperature reduction on the columns, which is somehow favourable in terms of its fire resistance, on the other hand the differential heating on the columns cross-section may lead to unfavourable stresses (bending moments) responsible for instability (thermal bowing). Considering that the structural behaviour of columns is strongly dependent of the second order effects this is an important phenomenon which may lead to a significant reduction on its fire resistance. This paper presents the results of a numerical study to assess the influence of the differential heating on the fire design of steel columns. New interaction axial force–bending moment diagrams for non-uniformly heated H steel columns are proposed.     

Fire behaviour of high strength self-consolidating concrete filled steel tubular stub columns

This paper reports an investigation into the behaviour of high strength SCC (self-consolidating concrete) filled steel tubular stub columns exposed to standard fire. A series of tests were carried out to obtain the temperature distribution, axial deformation, limiting temperature of steel and fire endurance of the SCC filled steel tubular stub columns. In addition, a finite element analysis (FEA) model was proposed and used to simulate the fire behaviour of the columns. In the FEA modeling, a sensitivity study was conducted to determine the concrete fracture energy and the contact property of the steel and concrete interface. The verified FEA model was used to analyse the structural behaviour of the columns under fire exposure, such as strain, stress, the load sharing between the steel tube and concrete and local buckling of the steel tube, to gain an insight into the failure mechanism of the columns.     

单层单跨钢框架抗火性能的试验研究

本文对2榀H型钢单层单跨钢框架进行抗火性能的试验研究。试验采用足尺试件,钢梁长3400mm,钢柱高3200mm,混凝土板宽1000mm。试验中考虑不同受火工况对钢框架抗火性能的影响。通过试验得出了梁、板、柱温度场变化规律和钢框架变形情况。试验结果表明:火灾下,由于钢筋混凝土板的存在,钢梁沿截面高度各点温度不一致,即钢梁沿截面高度存在温度变化的温度场;梁柱全部受火的单层单跨钢框架(节点受到保护)破坏方式为钢柱压屈破坏,破坏位置在受到保护的钢节点下部的钢柱上。本文研究结果为今后钢结构抗火性能的进一步研究提供参考数据。     

1截面形式对高温下钢管混凝土柱轴向加载性能的影响

钢管混凝土柱能够最大限度地发挥材料特性,现广泛用于建筑结构中。钢管混凝土柱承载能力高,能量吸收能力强,结构的抗火性能好。描述了含4种截面形式的一系列钢管混凝土柱在标准火下的抗火性能。按照以下方式将钢管混凝土柱分为三组:室温下截面强度相等、钢管截面面积相等、混凝土截面面积相等。采用ABAQUS软件分析组合柱的温度分布、临界温度和过火时间等情况。基于典型参数的分析和对比,讨论了不同截面形式钢管混凝土柱的温度分布和抗火性能。结果显示,圆截面钢管混凝土柱抗火性能最佳,接下来是椭圆截面、正方形截面和矩形截面。在该研究的基础上,给出了高温下钢管混凝土柱的简化设计方程。     

Numerical investigations on the thermo-mechanical behavior of steel-to-timber joints exposed to fire

A three-dimensional finite element model is developed to predict the thermo-mechanical behavior of steel-to-timber doweled joints in tension parallel to grain exposed to fire. To manage the plastic yielding of the materials, the mechanical model is based on the von Mises criterion for steel and the Hill criterion for timber. In fire, the material characteristics depend on the temperature. Two different meshes are used for the thermal and the thermo-mechanical models. The thermal model is continuous, to take account of the thermal continuity between the joint components. The thermo-mechanical model is discontinuous, to consider the contact evolution between the joint components. The thermal model is used to predict the evolution of the temperature field inside the joint which depend on the gas temperature. It is validated on the basis of measured temperatures during fire tests. The complex transformations in wood during fire are represented by apparent values of thermo-physical characteristics proposed in the bibliography and calibrated on the basis of the experimental measurements. The mechanical model is validated by comparison with the experimental results of joints in normal conditions. The thermo-mechanical model is validated by considering the experimental failure times of some joints. The numerical models showed a good capacity to simulate the behavior of the timber joints in cold and in fire situations. These developed and tested models can be used as a general tool to analyze the behavior of a large variety of joint configurations to constitute a data base that can be used in safe and economic practice of fire engineering of wood joints.     

预测火灾下钢构件温度的简单方法

为评价钢结构构件的抗火性能,需要了解构件横截面的温度。给出了一种预测火灾下有外保护层的钢构件截面温度的简单方法。利用标准火灾条件下的简化假设得到该方法,并推广到设计火灾情况。该方法适用于有保护层和没有保护层的型钢构件。将预测温度与ANSYS有限元分析结果进行比较,验证了该方法的有效性。此外,该方法的预测结果也与"最佳拟合"法的预测温度进行了比较。与试验结果、有限元计算结果和"最佳拟合"法预测结果的比较表明,所提出的简化方法能够较好地预测各种火灾下型钢构件的热梯度和温度历史。该方法简单明了,适合设计时使用。     

钢骨混凝土柱的耐火性能和抗火设计方法(Ⅰ)

确定了高温下组成钢骨混凝土的钢材和混凝土的热工参数和力-热本构关系模型,利用有限元法计算了钢骨混凝土柱截面温度场,计算结果得到实验结果的验证。利用数值方法对钢骨混凝土柱耐火极限及火灾下荷载-变形关系曲线进行了计算分析,理论计算结果和实验结果吻合良好。在此基础上,分析了截面尺寸、构件长细比、截面含钢率、截面配筋率、荷载偏心率、钢骨和钢筋屈服强度、混凝土强度、截面高宽比等参数对耐火极限以及火灾下构件承载力的影响规律。最后,提出了钢骨混凝土柱耐火极限的实用计算公式。本文是第一部分。     

钢结构梁柱节点耐火性能的数值模拟

建立数值模型,研究钢结构节点的耐火性能。采用2种方法将火灾模拟中的热分析转变为结构分析。根据ISO834标准火灾曲线,对钢结构节点进行足尺试验,验证本方法的正确性。通过火灾动态模拟器获得壁面温度,将其对应的截面作为火灾截面置于结构表面进行分析,数值结果与试验一致。此外研究了火炉尺寸的影响和对绝缘材料的敏感性分析。分析两根翼缘削弱梁,并与无翼缘削弱梁的模拟结果对比。经历相同时间之后,两种构件均能承受严重的火灾,并发生跑道现象。     

钢骨混凝土柱的耐火性能和抗火设计方法(II)

确定了高温下组成钢骨混凝土的钢材和混凝土的热工参数和力-热本构关系模型,利用有限元法计算了钢骨混凝土柱截面温度场,计算结果得到实验结果的验证。利用数值方法对钢骨混凝土柱耐火极限及火灾下荷载-变形关系曲线进行了计算分析,理论计算结果和实验结果吻合良好。在此基础上,分析了截面尺寸、构件长细比、截面含钢率、截面配筋率、荷载偏心率、钢骨和钢筋屈服强度、混凝土强度、截面高宽比等参数对耐火极限以及火灾下构件承载力的影响规律。最后,提出了钢骨混凝土柱耐火极限的实用计算公式。本文是第二部分。     

Analytical prediction of composite beams response in fire situations

In this paper an extension of the method of the Fourier series expansion to the fire analysis of composite beams is presented. In particular the extension concerns the introduction of the temperature dependent interaction of all the components: steel beams, concrete slab and steel connectors. These last are considered of finite stiffness, and a proper account is given to the combined effect of thermal degradation of the properties, and stress amplification caused by the differential thermal expansion across the interface.The proposed method compares very well with some experimental fire tests of simply supported and framed composite beams. Due to its relative simplicity and speed, it can be used for design purposes in evaluating the critical temperature in terms of critical deflection. Finally we recall that the method is capable of dealing with every type of fastening distribution, such as discontinuous or variable length.     

Fire performance of cold-formed steel wall panels and prediction of their fire resistance rating

Recent research at the Queensland University of Technology has investigated the structural and thermal behaviour of load bearing Light gauge Steel Frame (LSF) wall systems made of 1.15 mm G500 steel studs and varying plasterboard and insulation configurations (cavity and external insulation) using full scale fire tests. Suitable finite element models of LSF walls were then developed and validated by comparing with test results. In this study, the validated finite element models of LSF wall panels subject to standard fire conditions were used in a detailed parametric study to investigate the effects of important parameters such as steel grade and thickness, plasterboard screw spacing, plasterboard lateral restraint, insulation materials and load ratio on their performance under standard fire conditions. Suitable equations were proposed to predict the time–temperature profiles of LSF wall studs with eight different plasterboard-insulation configurations, and used in the finite element analyses. Finite element parametric studies produced extensive fire performance data for the LSF wall panels in the form of load ratio versus time and critical hot flange (failure) temperature curves for eight wall configurations. This data demonstrated the superior fire performance of externally insulated LSF wall panels made of different steel grades and thicknesses. It also led to the development of a set of equations to predict the important relationship between the load ratio and the critical hot flange temperature of LSF wall studs. Finally this paper proposes a simplified method to predict the fire resistance rating of LSF walls based on the two proposed set of equations for the load ratio–hot flange temperature and the time–temperature relationships.