Adaptation factor for moment capacity calculation of steel beams subject to temperature gradient

Adaptation factors provide a simple means to estimate the moment capacity of a steel beam subject to temperature gradient. The use of adaptation factors is introduced in Eurocode for structural steel design under fire conditions. Temperature gradients arise in a steel/concrete composite beam usually as a result of the partial protection from fire by another structural or non-structural member. For a bare steel section subject to any temperature gradient, a value of 0.7 is stipulated for the adaptation factor in the Eurocode. A value of 0.85 is stipulated for protected beam under the same fire situation. These values are used to increase the steel beam’s calculated moment capacity which is based on an assumed uniform temperature of the cross-section. Literature on the use of the adaptation factors is scarce. In this paper, the value of the adaptation factor being adopted by the Eurocode for design of steel beam in fire with non-uniform temperature distribution is examined. The use of this value in the general cases of steel/concrete composite beam construction is studied by carrying out numerical computations for various temperature gradient cases. The suitability of the adaptation factor value adopted by the code is discussed and recommendations given.     

On the fire resistance of structural steel elements derived from standard fire tests or by calculation

Due to high costs, a fire resistance test of a load-bearing structural element is usually limited to one test specimen — in a few countries, to two test specimens. Accordingly, there are no possibilities of evaluating the test results statistically.For a single test specimen, the actual quality of the structural material represents a random sample from a wide variety. This applies also to the initial imperfections of the structural elements. In consequence of this, a standard fire resistance test is generally carried out on a test specimen with a load-bearing capacity which is greater — most often significantly greater — than the load-bearing capacity related to the characteristic values of the mechanical material strength and of the imperfections of the structural member. In current practice, no corrections of the test results with respect to this are made.In a conventional analytical design, a determination of the load-bearing capacity of a structure at room temperature conditions is based on the characteristics values of the strength and imperfections. Extended to a structural fire engineering design, this procedure will give an analytically determined fire resistance of a load-bearing structural element which is lower — normally essentially lower — than the corresponding value derived from a standard fire resistance test.Available methods for a simplified calculation of the temperature of fire exposed steel structures are, as a rule, based on the assumption of a uniformly distributed temperature structure at each time of fire exposure. The ECCS Recommendations for an analytical design of steel structures exposed to a standard fire follow this kind of approach. For certain types of steel structures, for example, beams with a slab on the upper flange, a considerable temperature variation arises over the cross section as well as in the longitudinal direction during a fire resistance test. A simplified, analytical method, which neglects this influence, gives a further underestimation of the fire resistance in relation to the corresponding result obtained in a standard fire resistance test.The described discrepancies between an analytical and an experimental determination of the fire resistance are further discussed and analysed in Sections 2 and 3, with particular reference to different types of steel structures. The discussion is focussed on the loading and restraint conditions, the scatter of material properties and geometrical imperfections, and the temperature variation over the structure or structural element. The discussion is summarized in Section 4 and alternative methods of correction are outlined briefly for obtaining an improved consistency between the analytical and the experimental approaches.In Section 5, one of these methods is further developed to a design basis which can be applied easily in practice. Principally, the method is characterized by a correction of the analytically determined load-bearing capacity, based on the characteristic value of the structural material properties, the characteristic value of the imperfections of the structure, and a uniformly distributed steel temperature across and along the structure. Two different sequences of the design procedure are dealt with, defined according to Figs. 10 and 11. The resultant correction factors, ? and κ, belonging to the respective sequences, are given by Figs. 8 and 12 for columns, isostatic beams, and hyperstatic beams. The straight line curves in Figs. 9 and 13 show corresponding, simplified relationships for the ? and κ factors.The derived method of correction must be characterized as an approximate approach. This is in consequence of the present state of knowledge, which does not allow a solution of high accuracy. The task to develop a correction procedure which leads to improved consistency between an analytically and an experimentally determined fire resistance, should also be seen in the context of the inadequate reproducibility of the standard fire resistance test.     

火灾均匀温度场中正放四角锥网架结构临界温度研究

随着温度的升高,钢材的力学性能将会发生较大的变化。钢结构在均匀温度场达到承载能力极限状态时的临界温度是钢结构抗火性能的重要特征,而网架结构的临界温度主要受网架的几何特征、荷载比、构件稳定应力的影响。本文通过对不同参数条件下正放四角锥网架结构在均匀温度场中结构反应全过程分析,得到不同参数条件下正放四角锥网架结构的临界温度,为大空间建筑网架结构抗火设计实用方法提供理论依据。     

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。     

Nonlinear behaviour of unprotected composite slim floor steel beams exposed to different fire conditions

An efficient nonlinear 3D finite element model has been developed to investigate the structural performance of composite slim floor steel beams with deep profiled steel decking under fire conditions. The composite steel beams were unprotected simply supported with different cross-sectional dimensions, structural steel sections, load ratios during fire and were subjected to different fire scenarios. The nonlinear material properties of steel, composite slim concrete floor and reinforcement bars were incorporated in the model at ambient and elevated temperatures. The interface between the structural steel section and composite slim concrete floor was also considered, allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the composite beam. Furthermore the thermal properties of the interface were included in the finite element analysis. The finite element model has been validated against published fire tests on unprotected composite slim floor steel beams. The time–temperature relationships, deformed shapes at failure, time–vertical displacement relationships, failure modes and fire resistances of the composite steel beams were evaluated by the finite element model. Comparisons between predicted behaviour and that recorded in fire tests have shown that the finite element model can accurately predict the behaviour of the composite steel beams under fire conditions. Furthermore, the variables that influence the fire resistance and behaviour of the unprotected composite slim floor steel beams, comprising different load ratios during fire, cross-section geometries, beam length and fire scenarios, were investigated in parametric studies. It is shown that the failure of the composite beams under fire conditions occurred for the standard fire curve, but did not occur for the natural fires. The use of high strength structural steel considerably limited the vertical displacements after fire exposure. It is also shown that presence of additional top reinforcement mesh is necessary for composite beams exposed to short hot natural fires. The fire resistances of the composite beams obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite beams at elevated temperatures. It is shown that the EC4 predictions are generally conservative for the design of composite slim floor steel beams heated using different fire scenarios.     

Thermoelastic flexural–torsional buckling of steel arches

This paper presents the lateral buckling behaviour of steel arch members with a doubly symmetric I-shape cross-section subjected to a linear gradient temperature field over the cross-section. The steel arch is subjected to an in-plane linear temperature gradient field whilst it experiences expansion along its length due to the in-plane temperature gradient producing an in-plane curvature. As the steel arch continues to be subjected to increasing temperature differential and increasing average temperature, the bending moments and axial compressive forces in the steel arch increase and upon reaching a critical value, the steel arch bifurcates from its primary equilibrium position and fails in lateral–torsional buckling mode. A novel non-discretisation mechanical-based methodology developed recently is used to model the behaviour of the steel arch prior to buckling, whilst the classical buckling theory is used to determine the critical temperature which causes flexural–torsional buckling. The proposed methodology allows for the critical temperature gradient and critical average temperature to be ascertained using an iterative method. Using a comprehensive parametric study, the variations of the thermal gradient and the critical average temperature to various parameters are then investigated. The model proposed here provides a closed-form solution for which it forms a platform which can be used for structural steel arch design and evaluation in the development of codified approaches to fire design on a performance based design.     

考虑蠕变和残余应力释放的Q460钢柱抗火设计方法

承受荷载的钢结构在火灾下可发生明显的蠕变变形,钢结构中的焊接残余应力在火灾下也会一定程度地释放,因而高温蠕变变形和残余应力会对钢柱的耐火性能产生影响.为了准确地对高强度Q460钢柱进行抗火设计,有必要定量分析高温蠕变和残余应力释放对钢柱承载力的影响.采用电炉对2根焊接H形Q460钢柱进行耐火试验,得到无保护Q460钢柱...     

钢拱结构的弯曲-扭转热屈曲

对横截面上分布线性梯度温度场的双轴对称工字截面钢拱结构的侧向屈曲性能进行研究。钢拱结构存在面内线性梯度温度场,产生面内曲率,从而使结构伸长。继续提高温度差和平均温度,结构所受弯矩和轴向压力将增至临界值。此时,结构背离初始平衡位置,发生侧向扭转屈曲破坏。采用一种新型的非离散机械方法模拟屈曲前的钢拱性能,而采用经典屈曲理论确定弯曲-扭转屈曲的临界温度。本方法要求采用迭代法确定临界温度梯度和临界平均温度。通过全面参数分析,研究热梯度变化和临界温度。本模型能给出封闭解,可用于基于性能的钢拱结构耐火设计和分析。     

温度梯度下计算钢梁弯曲承载力计算的适应因子

适应因子提供了估计温度梯度下钢梁弯曲承载力的简单方法。欧洲的钢结构设计规范中介绍了火灾情况适应因子的应用。由于其他结构或非结构构件进行局部防火保护时,钢或混凝土组合梁内通常会出现温度梯度。当无保护的钢截面存在任何温度梯度时,欧洲规范规定适应因子取值0.7,在同样火灾情况下对有保护的梁适应因子取值0.85,这些值被用于提高钢梁的计算弯曲承载力,此时假定横截面温度均匀。有关适应因子的文献很少。本文对在火灾中,按照欧规对钢梁在不均匀温度场中适应因子的设计取值进行验算。通过各种温度梯度情况的数值计算,对其在钢或混凝土组合梁结构一般情况下的应用进行了研究。并对规范所采纳的适应因子值的稳定性进行讨论并给出了推荐值。     

Non-linear inelastic analysis of steel arches at elevated temperatures

An innovative non-discretisation mechanical-based method is developed in this paper to analyse a steel arch at elevated temperatures so that its behaviour can be quantified. The steel arch has a generic but singly-symmetric cross-section with elastic and plastic parts, and it is subjected to an arbitrary thermal profile which varies along the length of the arch as well as through the depth of the cross-section. The effects of geometric and material non-linearity as well as potential catenary action which can occur at high temperatures are taken into account in the formulation. The efficiency and accuracy of the generic model developed is demonstrated by a comparison with a finite element model undertaken using ABAQUS. The proposed method is then utilised to elucidate some significant factors, such as the magnitude of the temperature at bottom fibre of the cross-section as well as the ratio of the temperature at the top fibre to that at the bottom fibre, on the response of a steel arch member during fire loading. The proposed model provides a computationally superior formulation to that of commercial finite element packages, and forms a platform which can be used for structural steel arch design and evaluation in the development of codified approaches to fire design on a performance basis.     

大空间钢结构性能化防火设计探讨

针对大空间建筑的火灾特点,提出了先进的设计方法——性能化防火设计,介绍了大空间钢结构性能化防火设计的设计思路与分析步骤,提示了在推进和发展性能化防火设计过程中应该注意把握的有关问题。     

Non-linear thermoelastic analysis of steel arch members subjected to fire

This paper addresses the non-linear thermoelastic behaviour of steel arches acted on by a sustained uniformly distributed load, when subjected to elevated temperatures as caused by fire. The steel arch is restrained at its two ends by elastic translational springs in both the horizontal and vertical directions, as well as by counterpart elastic rotational springs, which simulate a generic semi-rigid connection, or restraint by other members in a frame, or when the arch acts as a large-span roofing element supported and restrained by columns. The study is restricted to the thermoelastic structural response of the steel material and therefore the high-temperature effects of catenary action and yielding are not considered; however the important effect of the second order term in the strain–displacement relationship is included. In order to model structural response of an elastically supported steel arch under thermal loading, an alternative geometric formulation is needed since the tangential and radial deflections and rotations as well as the axial compressive force in the member are substantial at the early stage of the fire. The formulation presented in this paper takes into account the degradation of the stiffness of the steel arch prior to yielding at elevated temperatures and it is argued that there are many situations for which analyses of a real fire situation in the thermoelastic range are valid. It is shown that the proposed model agrees well with independent solutions obtained using finite element analyses. The proposed model has significant potential for use in the analysis of restrained steel arches subjected to uniformly distributed load at elevated temperatures, such as large-span roofs and can provide a foundation for codified procedures in design.     

Analysis of experimental hydrocarbon localised fires with and without engulfed steel members

Localised fires can represent an important hazard to structural safety of buildings where a fully generalised fire cannot develop or when it is at its early stage. Plume correlations given in the codes are valid for undisturbed plume and it is not known whether the presence of a structural element engulfed into the localised fire can affect the validity of such correlations. In structural design, this may lead to highly conservative assumptions or, even, to possible misuses of the correlations. In order to provide insight into this issue, a comprehensive experimental programme aimed at providing data on hydrocarbon localised fires with and without engulfed vertical steel members was performed. In detail, a series of 22 tests of circular hydrocarbon pool fires in well-ventilated conditions of diameters ranging from 0.6 m to 2.2 m were performed with diesel and heptane. The particular aspect of these tests is that they were performed by means of a system that controlled the fuel flow and thus the rate of heat release (RHR) of the fire. The flame length and the temperatures of the fire plume measured experimentally were compared with existing plume correlations, data in the literature and the Eurocode correlations. The results show that: the presence of the column contributed to “straighten” the flame; although pool fires with same diameters were characterised by the same RHR, the flame length was different depending on the fuel type; experimental gas temperatures were lower than the temperature correlation given in the Eurocodes. In sum, the correlations included in the Eurocodes provided reasonable predictions in terms of flame length and of fire plume temperature rise around a steel vertical element located along the centreline of the localised fire.     

高温下超静定钢结构受力情况分析

采用FDS火灾动态模拟软件建立钢结构厂房火灾场景,并运用ANSYS有限元分析软件进行结构受力分析。研究表明:在高温作用情况下,超静定钢结构工程由于内力重分布会导致构件内力发生较大的变化,可能与常温下的受力设计不一致,进而造成结构失稳。由此提出,对钢结构在高温作用下的安全分析,应当进行基本钢结构单元的抗火承载力验算,而不能仅因为温度远未达到失去静态平衡稳定性的临界温度(540℃左右)就认为结构是安全的。     

Proposed design methods for lateral torsional buckling of unrestrained steel beams in fire

The behaviour of unrestrained steel I-beams has been studied by means of numerical analysis and published experimental results. The numerical model was developed using a commercial finite element program, MSC.MARC Mentat. A series of different UB and UC sections and different spans, subjected to both uniform moment and midspan point loads, are considered. The numerical predictions of the buckling moments are then compared with published experimental results. Consequently, a new approach is proposed to provide more accurate and safe predictions of the fire resistance of unrestrained beams, as well as to overcome certain weaknesses in the EC3:1.2 [European committee for standardization (CEN). Eurocode 3: Design of steel structures, Part 1.2: General rules — structural fire design, EN 1993-1-2. Brussels (Belgium); 2005] design formula. In addition, to provide a quick and simple design approach for engineers, a straightforward and rational method known as the Rankine method is introduced to predict the LTB failure load of steel beams in fire. It is shown that the Rankine approach generally provides a good lower bound value for numerical predictions.     

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.     

受长度约束钢柱的耐火性研究

火灾下钢柱的性能取决于其与包覆结构的相互作用。为提高对该现象的认识,对受长度约束的钢柱进行了一系列耐火性试验。设计了新的试验方法,并采用该方法进行试验。试验柱的长细比分别为50.6和63.3,初始荷载分别为室温下钢柱屈曲荷载设计值的30%和70%,包覆结构轴向刚度分别为13,45,63.3kN/mm。初始荷载偏心度与截面的弱惯性轴或两个主惯性轴的长度相等。结果显示,包覆结构刚度增加并不能够降低柱的临界温度,这是由于轴向刚度增加导致扭转刚度增加,而扭转刚度增加将使得柱的临界温度升高。结果还指出,初始应力水平越高,柱的临界温度越低。     

钢结构建筑防火保护关键问题及对策

通过对钢结构建筑防火保护现状进行分析,基于钢结构建筑使用年限,提出钢结构设计阶段应考虑保护材料的耐久性,合理分析防火保护材料生产商、钢结构工程施工方的违规操作,有针对性地细化工程建设阶段现场监管以及检测技术的完善,对建筑验收、工程使用管理提出切实可行的控制措施,结合工程实例揭示了钢结构防火保护管控的要点,给出设计、施工...     

S30408不锈钢高温材料性能及本构模型综述

精确掌握材料高温下的性能参数,对结构抗火设计的准确性具有十分重要的意义.以S30408奥氏体不锈钢为研究对象,在现有S30408不锈钢高温材料性能试验的基础上,研究发现S30408不锈钢在同种条件下,相比碳钢具有较低的升温速率,且S30408不锈钢在600℃时仍能保持50％以上的强度与刚度.但不锈钢具有更大的热膨胀率,...     

火灾高温下耐火保护钢梁设计荷载的分析

研究了火灾高温环境下耐火保护钢梁温度和结构的变化,运用有限元软件ANSYS模拟不同涂料厚度的耐火保护钢梁在标准温升模型、慢速和快速温升BFD模型3种温升曲线下,荷载作用不同时的最大挠度值,并与Robertson-Ryan准则中的最大挠度值比较,确定耐火保护钢梁的设计荷载值。结果表明,采用标准火灾模型代替自然火灾模型确定的设计荷载是不安全的;钢梁最高温度越高,设计荷载越低;随着涂料增厚,温升速率的改变对设计荷载值的影响逐渐明显。