蔓延火灾下钢框架结构抗火性能试验研究

为研究蔓延火灾下钢框架结构建筑室内火灾温度场、钢构件温度分布和位移的发展规律,对1个足尺的两层钢框架结构进行蔓延火灾试验,测量试验区域内关键位置的空气温度、钢构件温度和位移。试验结果表明：试验中火源房间内的火灾过程呈现4个明显的发展阶段,受火70min时室内火灾达到全盛,测得火灾烟气层最高温度730℃;受火82min时火灾从火源房间蔓延至邻近房间,导致了邻近房间内各受火钢构件温度峰值出现在不同时刻,同时,各构件历经扩散热烟气加热、直接受火加热和冷却降温3个阶段,呈现反复升降温的受火过程;受火过程中,钢构件温度变化显著滞后于火场温度,受火钢柱先后升温产生向上的轴向变形。与传统室内火灾相比,蔓延火灾扩大了火场范围和钢结构受火范围,对结构安全和人员安全造成更大威胁,因此在进行结构抗火设计时应得到充分考虑。     

Research on fire resistance test of a steel frame building under spreading fire

为研究蔓延火灾下钢框架结构建筑室内火灾温度场、钢构件温度分布和位移的发展规律，对1个足尺的两层钢框架结构进行蔓延火灾试验，测量试验区域内关键位置的空气温度、钢构件温度和位移。试验结果表明：试验中火源房间内的火灾过程呈现4个明显的发展阶段，受火70min时室内火灾达到全盛，测得火灾烟气层最高温度730℃；受火82min时火灾从火源房间蔓延至邻近房间，导致了邻近房间内各受火钢构件温度峰值出现在不同时刻，同时，各构件历经扩散热烟气加热、直接受火加热和冷却降温3个阶段，呈现反复升降温的受火过程；受火过程中，钢构件温度变化显著滞后于火场温度，受火钢柱先后升温产生向上的轴向变形。与传统室内火灾相比，蔓延火灾扩大了火场范围和钢结构受火范围，对结构安全和人员安全造成更大威胁，因此在进行结构抗火设计时应得到充分考虑。     

Stability analysis of large‐span steel truss structure under local cooling from fire extinguishing system

In case of a fire in large‐span steel structure buildings, the firefighters usually use water to cool the steelwork. However, during the cooling by the spraying water, the collapse of these structures is unclear. In this paper, a large‐span center is taken as an example to study the overall mechanical response process under local cooling conditions in fire. The Fire Dynamics Simulator has been used to investigate the temperature field distribution rule of large‐span buildings under fire. Using the coupled thermal‐mechanical method to study the overall mechanical response process of steel structure under local abrupt drop in temperature, displacement and stress variations of the large‐span steel structure under local cooling have been analyzed. Results of this study show that the temperature distribution in large‐span structures is nonuniform in fire. Using spraying water to cool the structural members in fire can cause the structural stress field redistributed that may lead to further damage to the structure.     

Experimental behaviour of composite slabs during the heating and cooling fire stages

Most theoretical and experimental research investigating the effect of fire on structures has previously concentrated only on the structural behaviour during the heating stages of the fire, partly due to the fact that internationally accepted standard fire tests only consider this stage of the fire. Evidence from real fires in real buildings has highlighted that the cooling phase of a fire is equally important and it is possible for structures to fail during this stage of the fire even though they have survived the heating stage up to a maximum fire temperature. This paper provides an insight into the behaviour of composite slabs under different fire scenarios considering both the heating and cooling phase of the fire. Extensive test data is presented which shows the redistribution of moments and strains in the deck and steel mesh, together with displacements during the full duration of the fire. The results show that the behaviour of composite slabs is dependent on the heating rate, the maximum temperature reached and the cooling rate. In terms of overall performance, displacements and the temperature on the non-fire side of the slab are important. For the tests presented in this paper it was shown that one fire scenario resulted in the maximum displacement but another fire scenario resulted in the maximum temperature on the unexposed face. In addition the maximum temperature of the unexposed side of the slab and the mesh reinforcement within the slab occurring during the cooling stages of the fire. This highlights the fact that the performance of structures must be checked in design under a range of possible fire scenarios, which must include both the heating and cooling stages of a fire.     

Outline of a stochastic decision-making model for building fire safety and protection

An outline is given of stochastic state-transition models that have been developed to predict life loss and monetary loss for multi-storey, multi-compartment buildings subject to fire. The stochastic models consider the extent and the effects of smoke and flame spread. Results obtained from the stochastic models can be used to select an appropriate design strategy for the provision of fire safety and protection in buildings, based on the dual attainment of a satisfactory level of predicted life loss and an optimum level of fire-cost expectation. The fire-cost expectation considers costs due to fire losses and the provision of fire-safety and protection measures.     

Robustness assessment of planar steel frames caused by failure of a side column under localized fire

Progressive collapse of a building structure under fire is a disaster that may cause heavy casualties and serious economic loss. However, there is a lack of codified method to assess fire‐induced progressive collapse of building structures. A global–local analysis method (GLAM) has recently been proposed by the authors and their colleagues to assess progressive collapse of steel buildings under localized fire, and its application on fire scenarios that causes one inner column to fail has been verified. This paper extends the application of GLAM to fire scenarios that causes a side column to fail in a planar steel frame. The predictions of the GLAM were validated against the results obtained from nonlinear dynamic analysis of the whole frame model. Besides, effects of location of the heated column at different storeys and load level of the frame were also studied. The results show that GLAM gives the same collapse predictions to the case studies with detailed nonlinear dynamic analysis. The differences between the critical load obtained from GLAM and that provided by the nonlinear dynamic analysis is within 7%. Therefore, GLAM has good applicability on robustness assessment of planar steel frames caused by failure of a side column under localized fire.     

Dynamic modeling of fire spread in building

Modeling fire spread in a building is a key factor of a fire risk analysis used for fire safety designs of large buildings. In this paper, a dynamic model of fire spread considering fire spread in both horizontal and vertical directions is described. The algorithms for simulating the fire spread process in buildings and calculating dynamic probability of fire spread for each compartment at each time step of simulation are proposed. The formulae used in calculating the input data for the dynamic fire spread model are derived. The dynamic fire spread model can easily be applied for any building including high-rise buildings. A detailed example of calculation of fire spread in a two-storey office building is described.     

Light steel framing for residential buildings

Building systems with light steel members, gypsum plasterboards and mineral wool have a wide spread use in the US, Australia and Japan and are gaining market in some European countries. The systems have often load-bearing walls and the floors may be of lightweight steel profiles or concrete. Such systems are suited for industrial production and can contribute to a more efficient building process. Examples of components and systems are given in the paper. For low and medium rise buildings it is natural to use the walls as stabilising for horizontal loads from wind and imperfections. In Sweden and Finland this has been common practice for single-family timber houses since the 1970s. The paper describes the traditional design model for walls with single-layer gypsum plasterboards and an extension to double layers, which are needed for fire protection in multi-storey buildings.     

Experimental investigation of post-fire mechanical properties of cold-formed steels

Cold-formed steel members are widely used in residential, industrial and commercial buildings as primary load-bearing elements. During fire events, they will be exposed to elevated temperatures. If the general appearance of the structure is satisfactory after a fire event then the question that has to be answered is how the load bearing capacity of cold-formed steel members in these buildings has been affected. Hence after such fire events there is a need to evaluate the residual strength of these members. However, the post-fire behaviour of cold-formed steel members has not been investigated in the past. This means conservative decisions are likely to be made in relation to fire exposed cold-formed steel buildings. Therefore an experimental study was undertaken to investigate the post-fire mechanical properties of cold-formed steels. Tensile coupons taken from cold-formed steel sheets of three different steel grades and thicknesses were exposed to different elevated temperatures up to 800 °C, and were then allowed to cool down to ambient temperature before they were tested to failure. Tensile coupon tests were conducted to obtain their post-fire stress–strain curves and associated mechanical properties (yield stress, Young׳s modulus, ultimate strength and ductility). It was found that the post-fire mechanical properties of cold-formed steels are reduced below the original ambient temperature mechanical properties if they had been exposed to temperatures exceeding 300 °C. Hence a new set of equations is proposed to predict the post-fire mechanical properties of cold-formed steels. Such post-fire mechanical property assessments allow structural and fire engineers to make an accurate prediction of the safety of fire exposed cold-formed steel buildings. This paper presents the details of this experimental study and the results of post-fire mechanical properties of cold-formed steels. It also includes the results of a post-fire evaluation of cold-formed steel walls.     

An effective thermal property framework for softwood in parametric design fires: Comparison of the Eurocode 5 parametric charring approach and advanced calculation models

Timber, like other structural materials such as concrete and steel, has its own Eurocode (Eurocode 5 part 1.2) for the structural fire design of buildings. However unlike other fire parts of the Eurocodes it is not widely adopted due to its inherent limitations. With the exception of a single Annex, the timber Eurocode (EN 1995-1-2) is only applicable to standard fire exposure. Annex A gives guidance on the charring rates of initially un-protected timber members in parametric fires, however in the UK the use of the Annex is prohibited by the national Annex to the code.The concrete and steel industries have undoubtedly benefited from performance based design whereby the structural fire design strategy is centred on a design fire (typically a parametric fire), which is more credible than the standard fire curve. Such an approach has resulted in more flexible, innovative buildings which have been designed based upon fundamental structural mechanics at elevated temperature, using advanced numerical models. At present however the same principals cannot be applied to the advanced fire design of timber buildings due to current limitations in the timber Eurocode. Where advanced calculation procedures are considered by the code (Annex B), much like many of the methods contained therein, the procedures are only applicable to standard fire exposure.The scope of applicability of the code stems from a fundamental problem regarding a lack of understanding of the heat transfer characteristics of timber in natural fires. The thermo-physical properties contained in the code are ‘effective’ properties. This essentially means that they are calibrated against test results to account for a lack of understanding regarding mass transfer, cracking and ablation both within the timber and char layer. Such calibrations have only been performed on timber members exposed to standard furnace conditions.To attempt to overcome this barrier and extend the scope of thermo-physical properties in the code a study has been undertaken to establish how the conductivity properties of the char layer influence the depth of char in parametric fires. Through calibration of an effective conductivity of the char layer against the parametric charring method contained in Annex A of EN 1995-1-2, it has been possible to establish a relationship between ‘heating rate’ and the effective conductivity of the char layer, in the heating phase of parametric fires. The modified conductivity model is shown to be applicable to a range of densities and moisture contents of timber and also variations in heating rate and fire load density. The latter is a direct result of the method used in the adaptation of the properties. The modified model is objectively critiqued and proposed further work is discussed in detail. The applicability of the modified model in the cooling phase of fires is also discussed.     

火灾下钢结构和组合结构的非线性模拟

近年来Sheffield大学开发了一种计算机程序Vulcan，用来对火灾下组合结构的性能进行三维模拟。本简要叙述了程序的理论基础。1995—1996年在Cardington进行了三个组合框架的抗火试验，各试验中相邻结构所提供的平面内约束程度不同，通过模拟这三个试验来显示受热区域内约束对结构性能的影响。为了阐明薄膜作用的影响及其和边界约束的关系，所有的试验都分别用几何线性以及非线性的板单元进行了分析。为此，首先对引起这种作用的钢筋的性能做了一系列参数研究。很明显，板内的薄膜作用对隔间最终的完整性很重要，并且在模拟火灾下这种类型的结构时应加以考虑。     

钢管混凝土柱耐火极限研究

钢管混凝土具有承载力高、耐火性能好等一系列优点,正被越来越广泛应用于高层建筑及其它工业建筑中。本文对火灾下钢管混凝土柱的力学性能进行了分析,采用纤维模型法建立了理论分析模型,计算了钢管混凝土柱在标准升温曲线下的耐火极限,并和以往的实验结果比较,二者符合较好。     

Determination of fire resistance ratings for glulam connectors within US high rise timber buildings

Multi-storey mass timber buildings constructed with cross laminated timber and glulam are being developed globally. Where engineered timber such as glulam is utilized, the column to beam connections need to be constructed with a fire resistance rating equal to that of the connecting members. The preferred glulam connectors are either a concealed steel plate with bolts and dowels; or a concealed proprietary screw-in sleeve type connector. The fire resistance of connectors for glulam members is an unresolved design issue, as there is no clear methodology to assess their capacity under fire, when the timber is exposed and not clad behind fire protective plasterboard. There is limited fire test data on concealed connectors under shear forces, which is the normal loading condition within a constructed building. Fire test data is also limited on full-size specimens. Correlations developed to date to calculate concealed connector fire resistance have only limited application.A methodology for the design of glulam beam to column connections has been developed based on an extensive literature review, examining the key issues for connection failure. It has been determined that char rate for the timber at the connection needs to be increased above the normally accepted design values, due to the influence of the steel connectors. Secondly, the reduction in timber strength behind the char layer needs to be accounted for, by including a greater depth of reduced strength and stiffness timber, such that the connection can effectively transfer the applied forces through the timber to the steel connector. The methodology detailed within this paper provides a simple approach to evaluate the timber cover to the concealed steel connector, where the timber strength and stiffness are effective.     

Development of realistic design fire time-temperature curves for the testing of cold-formed steel wall systems

Fire resistance rating of light gauge steel frame (LSF) wall systems is obtained from fire tests based on the standard fire time-temperature curve. However, fire severity has increased in modern buildings due to higher fuel loads as a result of modern furniture and light weight constructions that make use of thermoplastics materials, synthetic foams and fabrics. Some of these materials are high in calorific values and increase both the spread of fire growth and heat release rate, thus increasing the fire severity beyond that of the standard fire curve. Further, the standard fire curve does not include a decay phase that is present in natural fires. Despite the increasing usage of LSF walls, their behavior in real building fires is not fully understood. This paper presents the details of a research study aimed at developing realistic design fire curves for use in the fire tests of LSF walls. It includes a review of the characteristics of building fires, previously developed fire time-temperature curves, computer models and available parametric equations. The paper highlights that real building fire time-temperature curves depend on the fuel load representing the combustible building contents, ventilation openings and thermal properties of wall lining materials, and provides suitable values of many required parameters including fuel loads in residential buildings. Finally, realistic design fire time-temperature curves simulating the fire conditions in modern residential buildings are proposed for the testing of LSF walls.     

Experimental study on high strength structural steel S460 in fire and after cooling down

In order to reveal more information on material behaviour of high strength structural steel S460 both in fire and after fire, an experimental research has been carried out. The elastic modulus, yield strength and ultimate strength of S460 in fire and after cooling down are presented herein. A comparison study with current European, American, British and Australian design standards of steel structures shows that the material behaviour of high strength steel S460 in fire is different from mild steels and the current leading design standards are not applicable to high strength steel S460. This paper highlights the necessity of employing a unique elevated-temperature and post-fire material behaviour of high strength steel S460 to conduct safe fire-resistance design and evaluation after fire on steel structures with S460. Moreover, the residual material behaviour of high strength steel S460 after cooling down is found promising for its reuse.     

砖木结构古建筑不同风速下火灾蔓延规律研究

为探明砖木结构古建筑火场下火灾蔓延的变化规律,从火灾蔓延特性、热释放速率、温度的变化等方面探究不同风速影响下的火灾蔓延变化情况。选取中国典型砖木结构三原城隍庙为实例,通过古建筑BIM 参数化建模,合理设置火灾场景进行火场模拟分析。结果表明：风速作用下,火焰蔓延过程变化波动较大,呈现一定的周期性,在一定范围内,风速越大,古建筑热释放速率达到峰值的时间越短、一次峰值与二次峰值的时间间隔越短、达到轰燃的时间越早。热浮力的作用将导致火焰向上蔓延至屋面,屋面位置温度最高,上部结构危险更高。     

预应力钢棒张拉锚固体系的研究及工程应用

预应力钢棒张拉锚固体系强度高,具有良好的耐腐蚀和防火性能,被成功地运用于大型公共钢结构建筑中。研究表明,预应力钢棒体系能有效控制结构变形、增加结构刚度。开发国产预应力钢棒体系和张拉设备用于实际工程是完全可能和必要的。     

Study on nominal values of mechanical properties of high strength steel at elevated temperature and after fire exposure

高强钢高温下和高温后的力学性能是进行高强钢结构抗火设计和火灾后评估的重要基础。我国GB 51249—2017《建筑钢结构防火技术规范》和欧洲规范EC3中针对普通低碳钢提出了高温下屈服强度和弹性模量计算公式，但其不适用于高强钢。国内外学者对高温下和高温后高强钢力学性能已开展了一系列试验研究，但由于钢材强度等级、试验设备、加热速率和加载制度等影响，导致试验结果离散性较大，不能应用于实际工程中。同时不同学者提出的力学性能指标计算式各不相同，均不具有普遍适用性。采用数理统计中t分布与置信区间的方法对高强钢高温下和高温后力学性能试验数据进行统计分析，得到不同温度下力学性能指标具有95%保证率的标准值，拟合出高强钢高温下和高温后力学性能指标的计算式，并与GB 51249—2017和欧洲规范EC3预测结果进行对比。结果表明：自然冷却和浸水冷却条件下，高强钢高温后屈服强度发生明显下降的转折点分别是600℃和 500℃；高温下高强钢的屈服强度折减系数低于普通结构钢；高强钢弹性模量折减系数在作用温度小于600℃时低于普通结构钢的，而在温度大于600℃时高于普通结构钢的。     

Q345冷成型钢高温力学性能试验制度参数的影响研究

钢材的火灾全过程高温本构是开展冷成型钢结构抗火研究的重要输入数据。为此,开展国内常用Q345冷成型钢高温力学性能试验研究,定量考察峰值温度保温时间、降温速率、温度历程以及稳态与瞬态试验方法等试验制度参数对其力学性能的影响,结果表明： 高温稳态试验中,峰值温度保温时间和降温速率对钢材高温力学性能影响不明显,相对百分偏差均介于-10%~10%之间;若温度历程中各次升温过程峰值温度中的最高温度和拉伸温度均相同,则钢材在多次升降温过程下的高温力学性能与其在一次升降温过程降温段的高温材性相互接近; 高温瞬态试验中,温度历程对钢材高温试验应变影响显著;相同拉伸温度下,升温与降温段的应变相对偏差最高可达12904%; 稳态与瞬态试验方法对考虑温度历程的冷成型钢高温应变影响亦非常明显,试验参数范围内,相同拉伸温度下应变相对百分偏差最大可达14851%。总之,Q345冷成型钢的火灾全过程高温本构需考虑温度历程中各次升温过程峰值温度中的最高温度和拉伸温度的影响,且稳态与瞬态试验方法所构建的高温本构模型并不等效。