模拟局部火灾下受火焰冲击的约束梁性能

钢结构可能会受附近火源的影响而被局部加热。局部火灾所造成的火焰冲击可能会使钢构件暴露于高温之下,从而导致结构失效。对受局部火灾火焰冲击的约束梁的热力学性能进行数值研究。对4个不同尺寸和约束的钢梁进行试验。研究其在火灾发展阶段和稳定阶段的性能。与ISO834火灾标准相比较。研究发现受火焰冲击的钢梁内部的温度分布很不均匀,多是穿过钢梁或在钢梁边沿。在梁的长度方向,火源附近的温度要比远离火源的温度高几百度。由于不同的温度分布,受火焰冲击的约束钢梁的变形模式与受ISO834火灾标准的梁相比明显不同。受局部火灾影响的约束钢的失效温度可能会高于或低于ISO834火灾标准下的约束梁的失效温度。如果潜在的实际火灾为局部火灾,那么根据标准火灾所做的设计可能就不够安全。     

Simulating the behavior of restrained steel beams to flame impingement from localized-fires

Steel structures may be exposed to localized heating by a fire source nearby. Flame impingement from localized fire may lead to high temperatures in the exposed steel members, which may lead to structural failure. This paper numerically investigates the thermal and mechanical behaviors of restrained steel beams exposed to flame impingement from localized fires. Four steel beams with different dimensions and restraints were considered. Both developing and steady burning fires were investigated. The standard ISO834 fire was also used for comparison. The study finds that the temperature distributions within the steel beams subjected to flame impingement are highly non-uniform both across and along the beams. Along the beam length, the temperatures near the fire source may be hundreds of degrees higher than those far from the fire source. Due to different temperature distributions, the deformation mode for restrained steel beam subjected to flame impingement may be significantly different from that of a beam subjected to the standard ISO834 fire. The failure temperatures for restrained steel beams subjected to localized fires may be higher or lower than those for restrained beams subjected to the standard ISO834 fire. Reliance on the standard fire may lead to an unconservative design if the potential real fires are localized fires.     

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.     

Thermal response of steel framing members in open car park fires

For open car park structures, adopting a performance-based structural fire design is often justified and allowed because the fire does not reach flashover. However, this design approach requires an accurate assessment of temperatures in structural members exposed to car fires. This paper describes a numerical study on the thermal exposure on steel framing members in open car park fires. Steel temperatures are computed by the coupling of computational fluid dynamics and finite element modeling, and by analytical models from the Eurocodes. In addition, the influence of galvanization on the steel temperature evolution is assessed. Results show that temperatures in unprotected beams and columns are influenced by the section geometry, car fire scenario, modeling approach, and use of galvanization. Galvanization slightly delays and reduces peak temperature. Regarding the different models, CFD-FEM (CFD: computational fluid dynamics, FEM: finite-element method) coupled models predict lower temperatures than the Hasemi model, because the latter conservatively assumes that the fire flame continuously touches the ceiling. Further, the Hasemi model cannot account for the effect of reduced emissivity from galvanization on the absorbed heat flux. Detailed temperature distributions obtained in the steel members can be used to complete efficient structural fire designs based on the member sections, structure layout, and use of galvanization.     

Assessment of Fire Dynamics Simulator for Heat Flux and Flame Heights Predictions from Fires in SBI Tests

This paper presents an experimental and numerical study of heat flux and flame heights from fires generated in single burning item (SBI) tests. Thin steel plate probes were developed, as an inexpensive and reliable alternative to heat flux gauges, to measure the surface heat flux, whilst flame heights were determined by analyzing the instantaneous images extracted from the videos of the experiments by a CCD camera. Experimental results obtained at different heat release rates were subsequently used to assess the accuracy of the computational fluid dynamics (CFD) code, Fire dynamics simulator (FDS, V4.07). Simulation results indicated that though predicting reasonably flame heights FDS underpredicts significantly the surface heat flux at higher heat release rates. Consequently, a sensitivity study of the parameters used in the radiation and soot models in FDS was conducted.     

火灾下蠕变对轴向约束钢柱屈曲的影响

目前,大多采用考虑蠕变的Harmathy蠕变模型进行耐火性分析。Harmathy蠕变模型仅能预测恒定应力下一定精度的蠕变,不适用于应力变化的情况。轴向约束钢柱遇到火灾时,火灾过程中应力将随着时间和温度的变化而迅速变化。该文研究了火灾下蠕变对轴向受压钢柱屈曲性能的影响。使用ANSYS软件中能够预测任何时间、应力或温度下钢材蠕变应力的蠕变模型来预测蠕变应力。分别对考虑和不考虑蠕变的情况进行数值模拟,并对两种情况下的屈曲温度和轴向变形结果进行对比分析。快火和慢火工况都考虑在内。研究结果显示,慢火工况下考虑蠕变的轴向约束钢柱屈曲温度高于不考虑蠕变的屈曲温度,快火工况下考虑蠕变的轴向约束钢柱屈曲温度可能高于也可能低于不考虑蠕变的屈曲温度。     

Creep effect on buckling of axially restrained steel columns in real fires

At present, Harmathy creep model is used in most fire resistance analysis, which explicitly consider creep. Harmathy creep model only predicts creep strains with acceptable accuracy for the case of constant stresses, but becomes invalid for the case of variable stresses. For the case of axially restrained steel columns subjected to fire, the fire induced stresses vary considerably and rapidly with time and temperature. In this paper, the effect of creep on the buckling behavior of axially restrained steel columns in real fires has been investigated. A creep model in ANSYS, which is capable of predicting creep strain regardless of any coupling between time and either stress or temperature of steel, is used to predict creep strains. The results for buckling temperatures and axial deformations, predicted with and without considering creep in numerical simulations, are compared. Both fast and slow fires are considered. The study found that for axially restrained steel columns in slow fires, considering creep gives higher buckling temperatures than those not considering creep; and for axially restrained steel columns in fast fires, considering creep might give higher or lower buckling temperatures than those not considering creep.     

Sensitivity Study on Using Different Formulae for Calculating the Temperature of Insulated Steel Members in Natural Fires

The applicability of using different formulae for calculating the temperature of insulated steel members exposed to natural fires which include heating and cooling phases has been investigated. The widely referenced ‘Swedish’ fire curves and measured temperature time curves in real fire tests are adopted to represent different natural fire environments. Parameters including insulation thickness, section factor, and protection material are considered in sensitivity studies. The steel temperatures predicted by different formulae are compared with the numerical results predicted by FEM. Comparatively, the formula given by Chinese Code CECS200 gives best prediction of steel temperatures. Studies also show that the approach adopted by Eurocode 3 for avoiding the negative steel temperature increment at the early heating phase in standard fire should not be used for calculation in natural fires. The formula given by CECS200 is recommended as the most satisfactory formula for the temperature calculation of insulated steel members in natural fires.     

Numerical modelling of light gauge cold-formed steel compression members subjected to distortional buckling at elevated temperatures

Fire safety design of building structures has received greater attention in recent times due to continuing loss of properties and lives during fires. However, fire performance of light gauge cold-formed steel structures is not well understood despite its increased usage in buildings. Cold-formed steel compression members are susceptible to various buckling modes such as local and distortional buckling and their ultimate strength behaviour is governed by these buckling modes. Therefore a research project based on experimental and numerical studies was undertaken to investigate the distortional buckling behaviour of light gauge cold-formed steel compression members under simulated fire conditions. Lipped channel sections with and without additional lips were selected with three thicknesses of 0.6, 0.8, and 0.95 mm and both low and high strength steels (G250 and G550 steels). More than 150 compression tests were undertaken first at ambient and elevated temperatures. Finite element models of the tested compression members were then developed by including the degradation of mechanical properties with increasing temperatures. Comparison of finite element analysis and experimental results showed that the developed finite element models were capable of simulating the distortional buckling and strength behaviour at ambient and elevated temperatures up to 800 °C. The validated model was used to determine the effects of mechanical properties, geometric imperfections and residual stresses on the distortional buckling behaviour and strength of cold-formed steel columns. This paper presents the details of the numerical study and the results. It demonstrated the importance of using accurate mechanical properties at elevated temperatures in order to obtain reliable strength characteristics of cold-formed steel columns under fire conditions.     

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.     

Behavior of prestressed stayed steel columns under fire conditions

Prestressed stayed steel columns experience loss of strength and stiffness when exposed to fire conditions. This paper presents results from experimental studies on the behavior of prestressed stayed circular steel columns under fire conditions. Two full scale prestressed stayed steel columns were tested by subjecting the columns to simultaneous gravity (mechanical) loading and fire conditions. In these fire tests, the varied parameters include load level and level of prestressing. Cross sectional temperatures, axial deformations, as well as fire resistance during the fire tests were recorded and measured. The results indicate that prestressed stayed steel columns undergo various failures modes under different combinations of load and prestress ratios. Specifically, load level significantly influence the fire response of prestressed stayed steel columns with higher load level leading to higher contraction and lower fire resistance.     

Experimental behaviour of a steel structure under natural fire

Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths.This paper reports on the results of a collaborative research project (Tensile membrane action and robustness of structural steel joints under natural fire, European Community FP5 project HPRI—CV 5535) involving the following institutions: Czech Technical University (Czech Republic), University of Coimbra (Portugal), Slovak Technical University (Slovak Republic) and Building Research Establishment (United Kingdom). It consists of an experimental programme to investigate the global structural behaviour of a compartment on the 8-storey steel–concrete composite frame building at the Cardington laboratory during a BRE large-scale fire test, aimed at the examination of the temperature development within the various structural elements, the corresponding (dynamic) distribution of internal forces and the behaviour of the composite slab, beams, columns and connections.     

Evaluating Models for Predicting Full-Scale Fire Behaviour of Polyurethane Foam Using Cone Calorimeter Data

Two models that can be used to predict full-scale heat release rates of polyurethane foam slabs were evaluated in this study. Predictions were compared with results of furniture calorimeter tests of 10 cm thick polyurethane foam specimens which were ignited in the centre or on the edge. Furniture calorimeter results indicated that peak heat release rates and fire growth rates were higher during centre ignition tests than edge ignition tests. For both situations, the growth phase of the heat release rate curves measured in the full-scale tests was successfully predicted using t ² design fires; the choice of a specific t ² fire depended on the surface area of the specimen and ignition location. A model originally developed during the European Combustion Behaviour of Upholstered Furniture (CBUF) project was also evaluated using heat release rate data from cone calorimeter tests and flame area burning rates measured using infrared video records of the furniture calorimeter tests. This model was able to successfully predict the initial growth phase of the fires and predictions of peak heat release rates were within 17% of measured values. The model had less success in predicting heat release rates later in the growth phase and during the decay phase of the fires, and did not appear to capture all of the physics of the full-scale tests, in particular foam melting and subsequent liquid pool burning. As the model did show promise, future work is planned to address these shortcomings and to develop improved flame spread models for polyurethane foam.     

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。     

Bemessung von Holzdecken aus Hohlkastenelementen im Brandfall

Fire behaviour of timber slabs made of hollow core elements This paper presents a simplified design method for the calculation of the fire resistance of timber slabs made of hollow core elements. The simplified design method is based on the reduced cross‐section method according to Eurocode 5 and takes into account two different charring phases, before and after the lower fire‐exposed layer is completely charred. For simplicity linear relationships between charring depth and time are assumed for each phase. The first part of the paper describes the simplified calculation model, in the second part results of fire tests are compared to the calculation model.     

火灾下钢结构楼板的薄膜作用

通过对真实火灾中的足尺火灾试验和观察显示，合组合楼板和承载钢梁的建筑物的结构承载力比现行杭大设计方法的建议值高出许多。因此规范中规定所有承载钢梁都要添加被动防火保护是不必要的。现行设计方法和实际结构性能之间产生这种差异是由于设计方法中忽略了楼板的薄膜作用。本根据国外有关资料给出了几种简单计算方法，允许在钢结构杭大设计中考虑楼板的薄膜作用。从而可以更精确地评估火灾下建筑物的真实承载能力，在给定的耐火时间内能减少相当数量钢梁的防火保护。     

Punching shear tests on flat concrete slabs exposed to fire

Underground parking structures often consist of flat slabs connected by columns, for which punching shear is often the most critical design criterion. In fire conditions, the punching load can increase due to restraint of the thermal curvature of the slab or due to the expansion of the columns. This increase of the punching load is discussed in the paper by means of a literature review. On the other hand, during fire the punching resistance of the slab decreases due to a gradual reduction of the material properties. This reduction in bearing capacity is studied by means of real scale fire tests, consisting of 6 slabs measuring 3.2×3.5×0.25 m with a connected column stub and tested for punching shear with a specially designed loading frame. Two reference tests are executed at ambient temperature conditions and four slabs are submitted to ISO 834 curve for 120 min. Comparison of the test data with the expected increased axial load due to thermal restraint found in the literature, shows a potential danger for premature punching failure of flat slab-column connections exposed to fire.     

真实火灾下单层门式刚架抗倒塌性能试验研究

设计完成了一个单层单跨门式刚架厂房的足尺火灾试验,得到了主要构件的温度及位移发展规律,分析了真实火灾下门式刚架厂房结构的受力响应。结果显示：真实火灾下门式刚架的温升曲线与标准升温曲线有较大差别,燃烧室中的上部构件达到较高温度而提前失效,下部构件温度较低;在火灾下,未做防火保护的钢结构很短时间内就会发生垮塌,在火场及构件到达峰值温度前结构已产生较大位移。试验研究发现,受火柱的柱顶出现了热膨胀伸长、轴向压缩、轴向破坏三个阶段,且受顶部热烟气聚集的影响,各柱的柱顶轴向位移均大于柱中位移。试验成果可为门式刚架结构抗火数值模拟研究及结构防火设计提供参考。     

On the trajectories of embers initially elevated or lofted by small scale ground fire plumes in high winds

Millions of acres and hundreds of structures are destroyed annually by wildfires. With many of these fires extending long distances due to spotting, detailed knowledge of ember transport by external and flame-generated winds is critical for fundamental understanding and prediction of the inception and evolution of such fires. This work presents a model that treats the burning and wind carrying of embers, and numerically compares their trajectories for spherical, cylindrical, and disk geometries. The embers may be launched at predetermined heights or lofted by a fire buoyant plume. Various terrain conditions and variable wind properties are considered. Results show that for embers of equal initial mass, disks propagate the farthest and have the highest remaining mass fraction upon impacting the ground. Spheres are carried the shortest distance, and cylinders have the smallest mass fraction upon impact. For disks in the range of diameters examined, initial diameter has no effect on the distance carried. Charring and extinction criteria are investigated for cylinders and spheres. Higher surface burning temperatures are found to lead to shorter propagation distances.     

Warehouse commodity classification from fundamental principles. Part II: Flame heights and flame spread

In warehouse storage applications, it is important to classify the burning behavior of commodities and rank them according to their material flammability for early fire detection and suppression operations. In this study, a preliminary approach towards commodity classification is presented that models the early stage of large-scale warehouse fires by decoupling the problem into separate processes of heat and mass transfer. Two existing nondimensional parameters are used to represent the physical phenomena at the large-scale: a mass transfer number that directly incorporates the material properties of a fuel, and the soot yield of the fuel that controls the radiation observed in the large-scale. To facilitate modeling, a mass transfer number (or B-number) was experimentally obtained using mass-loss (burning rate) measurements from bench-scale tests, following from a procedure that was developed in Part I of this paper.Two fuels are considered: corrugated cardboard and polystyrene. Corrugated cardboard provides a source of flaming combustion in a warehouse and is usually the first item to ignite and sustain flame spread. Polystyrene is typically used as the most hazardous product in large-scale fire testing. The nondimensional mass transfer number was then used to model in-rack flame heights on 6.1-9.1 m (20-30 ft) stacks of ‘C’ flute corrugated cardboard boxes on rack-storage during the initial period of flame spread (involving flame spread over the corrugated cardboard face only). Good agreement was observed between the model and large-scale experiments during the initial stages of fire growth, and a comparison to previous correlations for in-rack flame heights is included.