Ellipsoidal Solid Flame Model for Structures Under Localized Fire

This paper presents a model to evaluate the thermal energy transfer between a localized fire and the surfaces exposed to it, without the flame impinging the ceiling of the semi-open compartment. Although this type of fire may not have significant consequences for the structure as a whole, it is capable of triggering other disasters such as explosions and larger fires, which is why its study becomes increasingly important. Currently, this accident is analyzed using either sophisticated or semi-empirical numerical models available in the literature. The former uses computational fluid dynamics (CFD), which acceptably reproduces the fire, although with high computational cost. In turn, the semi-empirical models generally present conservative results. The proposed model presents variants in classic simple models available in the literature with the aim of being a tool that allows designers to estimate the thermal fields resulting from this type of fires at the preliminary structure design stage. In this model, the thermal analysis is performed using a finite element program, considering relevant parameters that characterize the fire such as: heat release rate, location and equivalent diameter of the fire source, among others. Through subroutines, the finite element model considers (a) a modification of hot gases temperature field based in a classic simple model and (b) proposition of a new geometry of the flame. The estimated radiative heat flux employs a solid ellipsoidal flame whose height changes according to the heat release rate. The convective heat flux is evaluated using a model for localized fire. Efficiency and accuracy of the methodology are checked by comparing the simulation results with those obtained by sophisticated models developed in fire dynamic simulator (FDS). The cases studied consider: (a) the replication of the experimental test conducted at Luleå University and (b) an offshore platform deck under localized fire action. The results of the first case confirm that the FDS replicates the experimental measurements with high accuracy. Finally, the results show that the proposed model allows to realistically represent the temperature fields generated by the fire, with relatively low computational cost compared to the CFD models for cases (a) and (b), therefore it is possible to use it to develop preliminary analyses in other fire scenarios.     

Mechanical response of a partially restrained column exposed to localised fires

Recent trends in structural fire engineering research have focussed on the response of buildings with large open plan spaces to so-called travelling fires. These fires travel horizontally across the floor plate of a building and result in time and spatially varying thermal exposure and response of the structure to the fire. What has received little attention, however, is the effect that non-uniform thermal exposure has on columns. Recent tests conducted at SP demonstrated the effect of a small non-uniformity of thermal exposure, resulting in a thermal gradient of around 1 °C/mm, on a column exposed to a pool fire. The curvature resulting from a non-uniform thermal exposure where the column is pinned, or in cases where the column is partially restrained, will result in an eccentricity in the column’s loading and large second order effects.This paper describes the effect of thermal exposure varying in both the horizontal and vertical axes to columns by means of including this thermal boundary in a solution of classical Euler beam theory. The resulting solution allows for a variation in the stiffness of the rotational restraint at both ends of the column and a non-uniform temperature exposure through the column’s section and along its height. The resulting equations help to understand better the impact of the assumptions of ‘lumped capacitance’ on steel columns, suggesting a challenge to this assumption in some instances, as well as to enhance understanding of the impact of non-uniform fires on steel columns.     

A simple method for combining fire and structural models and its application to fire safety evaluation

To consider the actual fire characteristics in the fire response analysis of building structures and to simplify the complex relationship between the fire analysis model and the structural finite element analysis model, a spatio-temporal model of the fire temperature and heat flux boundary for heat conduction analysis is developed. The proposed model adopts a two-way orthogonal polynomial approach for fitting the discrete data from the fire simulation and obtains continuous spatial polynomial equations. It is shown to be accurate for capturing the distributions of temperature and heat flux that are required for a heat conduction analysis and a thermal mechanical coupling analysis. Finally, the model is implemented through user-subroutines UTEMP and DFLUX in ABAQUS, and it is applied to a new archive in Beijing. The results show that this method may be used to combine fire simulation and structural analysis for the safety evaluation of structures under fire.     

Using adiabatic surface temperature for thermal calculation of steel members exposed to localized fires

This paper provides a new way to calculate the temperature of steel components in localized fires. A newly developed concept named adiabatic surface temperature (AST) on fire-structural interface research has been used in the calculation to represent the fire exposed boundary conditions. Based on AST, a simple heat transfer model with the capability of considering heat conduction among different parts of the steel sections has been proposed. The popular CFD code FDS (version 5.3.0), which can output ASTs, has been adopted to simulate the heating of a steel beam and a steel column in a localized fire. Steel temperatures calculated by the proposed model agree very well with the results from CFD simulation. Studies also obviously show that using the surrounding gas temperatures for thermal calculation of steel members exposed to fire might yield unreasonable and unsafe results.     

Large eddy simulation of compartment fire with solid combustibles

For properly describing practical building fire processes with solid combustibles, the pyrolysis kinetics model of solid combustibles and the large eddy simulation (LES) approach are applied to the simulation of the thermal decomposition of the polyurethane foam (PUF) slab and the space fire spread in a compartment. The instantaneous variations of the heat release rate of the PUF slab, the smoke temperature, and the smoke interface height with time are obtained under different ventilation conditions. They are in agreement with the measured data. The ventilation conditions have distinct effects on the interactions between the pyrolysis of the PUF slab and the space fire spread. Influenced by the space fire spread, the heat flux on the top plane of the PUF slab exhibits a non-uniform distribution. The PUF slab is consumed in an asymmetric manner.     

Fire thermal boundary condition measurement using a hybrid heat flux gage

New experimental methods have been developed using a hybrid heat flux gage to quantify the thermal boundary condition to a surface exposed to fire. The hybrid heat flux gage is capable of measuring the net heat flux and exposure heat flux at gage temperatures up to 1000 °C without the need for water cooling. Using these heat fluxes at elevated surface temperatures, new methods were developed to quantify the convective heat transfer coefficient and adiabatic surface temperature. In addition, a procedure is presented for determining the convective and radiative heat flux components when the gas temperature is measured close to the gage surface. Techniques were validated in a series of experiments performed in a cone calorimeter at different heat fluxes. Cold surface heat fluxes from the hybrid heat flux gage were within 5% of heat fluxes measured using a water-cooled Schmidt-Boelter gage. Temporal adiabatic surface temperature measurements from the hybrid gage compared well with steady-state plate thermometer measurements.     

Predicting the Fire Resistance of Wood–Steel–Wood Timber Connections

This paper presents an investigation on the fire performance of wood–steel–wood timber connections with slotted-in steel plates. In the first part, a three-dimensional thermal model was employed that uses the finite element method to analyze heat transfer within timber connections exposed to the standard fire. The temperature-related properties were obtained from the literature and imported into the thermal model. A validation of the proposed thermal model was achieved by comparing predicted temperatures with experimental results. In the next phase, a reduction in the embedding strength method was adopted to estimate the load-carrying capacity of connections in fire. Based on the temperature profiles within the connection calculated by the thermal model, the reduction of the embedding strength was determined and used to calculate the load capacity at elevated temperatures. Furthermore, a formula was proposed to evaluate the fire resistance rating of timber connections and compared with the results of fire resistance tests. The parameters considered included the load level, fastener diameter and wood member thickness.     

A study of the effect of partial loss of protection on the fire resistance of steel columns

Missing fire protection material on steel structural members is generally recognized as a problem, though little information is available to quantify the effect. A finite element heat transfer analysis is used to compare the thermal response of steel columns with lost protection material when exposed to the ASTM E-119 furnace environment. Based on the predicted thermal response and using thermal endpoint criteria specified in ASTM E-119, estimates are made of the fire resistance of each column with varying amounts of missing protection. Relatively small proportional losses of fire protection material are required before significant reductions in fire resistance are realized. The degradation in fire resistance is most significant for light-weight columns and for cases where the fire protection material is missing from the flange rather than web.     

火灾下腹板开孔轻钢龙骨墙体温度场数值模拟分析

采用有限元分析软件ABAQUS建立了腹板开孔轻钢龙骨围护墙体温度场分析模型,阐述了模型中的若干关键问题,如受火面与背火面边界条件,石膏板、岩棉和钢材的热工性能参数及材料间的接触条件等,并将模拟结果与相关试验结果进行对比。在此基础上,分析了相关参数对腹板开孔轻钢龙骨墙体背火面的最高温度和平均温度的影响规律。结果表明:该模型可有效模拟该类墙体在火灾下的温度分布;龙骨截面高度与石膏板层数及布置方式均可在较大程度上影响墙体背火面的最高温度与平均温度;腹板开孔排数只对背火面最高温度影响较大,对平均温度则影响不大。     

膨胀型防火涂层膨胀尺寸非线性传热模型

膨胀型防火涂层在膨胀过程中孔隙存在的对流传热和辐射传热会降低涂层的防火性能。考虑到孔隙之间存在涂层分解产生的气体,因此通过理想气体状态方程,描述了涂层在不同升温条件下的膨胀行为。建立了以膨胀尺寸为输出项,并考虑孔隙对有效导热系数增强作用的有限元传热模型,探究不同厚度的涂层在不同升温环境下的防火性能。结果表明：膨胀尺寸模型能够较为准确地预测涂层在不同温度场下的膨胀行为,涂层最大膨胀率随干膜厚度增大而减小;基于膨胀尺寸和多孔有效导热系数的有限元传热模型模拟的钢管温度变化与试验结果吻合较好;膨胀型防火涂层在高升温速率环境中能够明显延缓钢管升温,并降低其最大受火温度;不同升温环境中,涂层的防火能力和厚度呈非线性变化。     

Measuring incident radiant heat flux using the plate thermometer

This paper shows that the plate thermometer as described in the fire resistance test standards ISO 834-1 and EN 1363-1 can be used for measuring incident radiant flux under ambient conditions as an alternative to water cooled total flux heat metres (HFMs). Measurements with a plate thermometer mounted in the cone calorimeter and exposed to different heat flux levels were analysed as well as simultaneous measurements with total HFMs and plate thermometers in large scale tests. It is shown how the incident radiant flux to a target can be derived from measurements with total HFMs and plate thermometers, respectively, and how well these two methods match. The plate thermometer is therefore deemed to be a practical alternative for measuring thermal conditions including incident radiant heat flux particularly under field conditions. It is, however, recommended that the plate thermometer should be modified when used under ambient conditions to reduce errors.     

The plate thermometer - a simple instrument for reaching harmonized fire resistance tests

The measured fire resistance of a structure tested in different furnaces in accordance with ISO 834 may differ considerably. Similarly, the fire resistance of that same structure may be 25% longer when tested in accordance with ISO than it is when tested in accordance with ASTM. These anomalies complicate the evaluation of test results and must be eliminated to reach harmonized international testing.The heat transfer to a test specimen in a test furnace at high temperature depends primarily on radiant flux rather than convection. Temperature measurement devices used to control furnaces should therefore respond to this type of heating in a way similar to that in which test specimens respond. They should have a large area so that the radiant heat transfer dominates, and they must, at the same time, have a quick thermal response.The plate thermometer is designed to have these properties. It consists of a thin steel plate, 100 mm by 100 mm and 0.7 mm thick, with an insulating fiber board on one side. A thermocouple is welded to the center of the plate. It should be placed in front of the specimen, with the insulated side facing the specimen. The exposed side will then receive the same radiant heat flux as the specimen.This paper describes the plate thermometer and gives a basic theoretical analysis of the heat transfer conditions in furnaces. Measurements with the plate thermometer in several furnaces are also reported.     

Full-field surface heat flux measurement using non-intrusive infrared thermography

A method was developed to measure full-field, transient heat flux from a fire onto a surface using infrared (IR) thermography. This research investigated metal plates that were directly exposed to fire while the unexposed side temperature of the plate was measured using IR thermography. These temperatures were then used in a two-dimensional finite difference inverse heat transfer analysis to quantify the heat flux. The method was demonstrated through a series of experiments with direct fire exposures onto vertical and horizontal plates. Fires were produced using a propane sand burner and ranged from 20 to 100 kW. Point heat flux measurements were also measured using a Schmidt–Boelter heat flux gauge. It was found that heat fluxes obtained via IR thermography were within one standard deviation of those from the Schmidt–Boelter gauge. The effect of plate material was studied both numerically and experimentally for stainless steel and aluminum plates. It was found that although precision is affected by material, appropriate resolutions can be selected to obtain similar precision for both materials. Spatial and temporal resolution effects were also investigated and it was found that the precision of the heat flux measurement is inversely proportional to both spatial and temporal resolutions.     

火灾下钢结构的性能与高等分析

本提出了一种改进的模型用于真实火灾下结构的性能分析。其主要特点是用非线性有限元模拟构件，反映结构整体非线性的影响。对真实火灾采用多区域及热辐射模型进行了模拟，对瞬态热传导采用有限元法进行了计算。采用边界面理论来考虑塑性沿构件截面的连续性。对承受局部火灾的半连续框架、多高层及大跨拱形析架采用了基于性能的分析方法。其中对梁跨度，火源位置对火势的蔓延对结构的影响进行了分析。最后对大跨屋面结构在非对称热力作用下的权限状态进行了分析。     

玻璃幕墙传热系数计算方法及工程应用

在研究玻璃幕墙热传递特点的基础上,基于一维稳态热传导理论,以中空玻璃为例建立了玻璃系统传热系数计算模型;基于二维稳态热传导理论和有限单元法,采用三节点三角形单元对二维温度场进行了离散,推导了单元热传导矩阵和温度载荷列阵,并推导了热对流、热流密度、辐射以及各种边界条件耦合作用下对单元热传导矩阵和温度载荷列阵的修正公式,建立了玻璃幕墙框及附加线传热系数计算模型。利用Visual C++和ObjectARX对AutoCAD进行了二次开发,研发了玻璃幕墙传热系数计算软件TJCW,并通过算例与LBNL系列软件计算结果进行比较,验证了所编软件的正确性和有效性。最后对某工程实例中玻璃幕墙传热系数进行了节能验算。研究结果表明：建立的传热系数计算模型能够正确的计算玻璃幕墙传热系数,基于该计算模型开发出的软件能够应用于实际工程的节能分析和计算中。     

Critical evaluation of an integral model for the pyrolysis of charring materials

When field models are used to predict fires and flame spread, a solid model is required to calculate the amount of released pyrolysis gases. Here, the integral model for the pyrolysis of charring materials of Moghtaderi et al., extended with a cooling stage is considered. When an incident heat flux measured during a flame spread experiment, is imposed to the solid, the cooling stage is shown to be indispensable to solve the pyrolysis process. Next, the integral model is compared with a moving grid model. The latter uses the same physical model but does not make any assumption on the temperature profile and thus gives the true solution of the physical model. Comparison of both models reveals that the integral model has problems with suddenly varying boundary conditions and produces erroneous results for parameter studies on thickness and rear boundary condition variations. However, when compared to inert pyrolysis experiments, the integral and the moving grid model are comparable in quality. An automatic optimisation technique is described to obtain material fire properties.     

Simulation of steel beam under ceiling jet based on a wind–fire–structure coupling model

For localized fires, it is necessary to consider the thermal and mechanical responses of building elements subject to uneven heating under the influence of wind. In this paper, the thermomechanical phenomena experienced by a ceiling jet and I-beam in a structural fire were simulated. Instead of applying the concept of adiabatic surface temperature (AST) to achieve fluid–structure coupling, this paper proposes a new computational fluid dynamics–finite element method numerical simulation that combines wind, fire, thermal, and structural analyses. First, to analyze the velocity and temperature distributions, the results of the numerical model and experiment were compared in windless conditions, showing good agreement. Vortices were found in the local area formed by the upper and lower flanges of the I-beam and the web, generating a local high-temperature zone and enhancing the heat transfer of convection. In an incoming-flow scenario, the flame was blown askew significantly; the wall temperature was bimodally distributed in the axial direction. The first temperature peak was mainly caused by radiative heat transfer, while the second resulted from convective heat transfer. In terms of mechanical response, the yield strength degradation in the highest-temperature region in windless conditions was found to be significant, thus explaining the stress distribution of steel beams in the fire field. The mechanical response of the overall elements considering the incoming flows was essentially elastic.     

Extinction of surface stabilized gaseous diffusion flames: Part I Simplified numerical model and implications for solid fuels in fires

In this work a previously proposed empirical and analytical criterion for extinction is numerically extended and validated for varying fuel dilution, oxidant dilution, strain rate, and surface temperature. The output of this work is presented in two parts: the current Part I uses simple kinetics and constant thermal transfer properties and Part II uses detailed kinetics, varying thermal transfer properties, flame radiation feedback and flame suppression agents in order to demonstrate that conclusions from the simplified model are still valid. In addition this work goes beyond the concept of critical flame temperature or mass flux for extinction by including the influence of slow chemical kinetics through the Damkohler number which becomes even more important for commonly used fire retarded materials.Extinction of flames on solid fuels is modeled by decoupling the pyrolysis chemistry from the gas-phase combustion chemistry using the flame energy feedback versus pyrolysis rate curves and an energy balance at the surface. This approach has the advantage of identifying and deducing key materials properties for solid and gaseous phase from experiments. Simulations are performed in a planar stagnation-point flow diffusion flame configuration using one-step Arrhenius chemical kinetics and a simplified transport model with Lewis number equal to unity. Only quasi-steady conditions are considered for the gaseous phase even if the pyrolysis rate of the solid is transient because the response time for the solid phenomena is, in general, much larger than the response (diffusion) time for the gaseous phenomena.It is found that at high pyrolysis rates and low straining rates (infinitely fast kinetics regime) there is no leakage of oxygen to the surface of solid fuel. However, as the solid fuel extinction is approached, oxygen leakage occurs because the effective air to fuel mass stoichiometric ratio becomes less than one owing to fuel dilution near the surface. At high straining rates, solid combustion cannot be sustained at any pyrolysis rate. In the infinitely fast kinetics regime, an appropriate scaling has been developed which collapses the convective heat flux curves onto a single one. In general, the critical pyrolysis fuel mass flux exhibits a universal behavior for variation of various model parameters when plotted versus a modified Damköhler number, and becomes constant when the latter is sufficiently high. Comparison with experiments is discussed, and the implications of the criterion for characterizing ignition flammability properties of solid fuels are also discussed.     

Numerical analysis on structural behaviors of concrete filled steel tube reinforced concrete (CFSTRC) columns subjected to 3-side fire

The structural behaviors of concrete filled steel tube reinforced concrete (CFSTRC) columns, which were exposed to a 3-side fire were discussed by using the non-linear finite element analysis (FEA) software ABAQUS. Details of the temperature distribution, fire resistance, failure modes, redistribution of internal force, contact stress between the steel tube and the concrete (both inside and outside of the steel tube), and the development of stress and strain within the CFSTRC columns subjected to a 3-side fire were revealed. The factors that may have affected the fire resistance of the CFSTRC columns exposed to three-side fire were analyzed. Based on the above research, the present study observed uniaxial symmetry on the cross-sectional thermal distribution of the CFSTRC, wherein a significantly lower temperature on the unexposed side was observed as compared to the exposed side. The two side verges of the surface, which were not exposed to fire, exhibited the lowest temperature. Following the end of the heating, the maximum temperature difference reached about 1065oC. The large temperature difference would bring non-uniform thermal stress and strain, and accidental eccentricity. In addition, the existence of concrete inside and outside of the steel tube prevented the steel tube from occurring local buckling, and the failure modes of CFSTRC columns acted as overall bucking. Parameters such as the fire load ratio, sectional dimension, slenderness ratio, sectional core area ratio, and external concrete compression strength significantly influenced the fire resistance of the CFSTRC columns. Finally, a simplified calculating formula was proposed to calculate the fire resistance influence factors of the CFSTRC columns subjected to three-sid fire. The formula-calculated results were well in agreement with the finite element analysis results, thereby providing a simple and feasible method for evaluating the fire-resistance design of these types of components in practical engineering.     

Compressive Response of Composites Under Combined Fire and Compression Loading

The thermal buckling of an axially restrained composite column that is exposed to a heat flux due to fire is studied by both analytical and experimental means. The column is exposed to fire from one-side and the resulting heat damage, the charred layer formation and non-uniform transient temperature distribution are calculated by the thermal model developed by Gibson et al. (Revue de l’Institute Francais du Petrole 50:69–74, 1995). For the thermal buckling analysis, the mechanical properties of the fire-damaged (charred) region are considered negligible; the degradation of the elastic properties with temperature (especially near the glass transition temperature of the matrix) in the undamaged layer, is accounted for by using experimental data for the elastic moduli. Due to the non-uniform stiffness and the effect of the ensuing thermal moment, the structure behaves like an imperfect column, and responds by bending rather than buckling in the classical Euler (bifurcation) sense. Another important effect of the non-uniform temperature is that the neutral axis moves away from the centroid of the cross section, resulting in another moment due to eccentric loading, which would tend to bend the structure away from the fire. In order to verify the mechanical response, the compressive buckling behavior of the same material subjected to simultaneous high intensity surface heating and axial compressive loading were investigated experimentally. Fire exposure was simulated by subjecting the surface of rectangular specimens to radiant heating in a cone calorimeter. Heat flux levels of 25 kW/m², 50 kW/m² and 75 kW/m² were studied. All specimens exhibited buckling and subsequent catastrophic failure, even at compressive stresses as low as 3.5 MPa under a surface heat flux of 25 kW/m². Details of the experimental procedure, including modifications made to a cone calorimeter to allow simultaneous mechanical loading are presented.