•Comparison of numerical modeling strategies for localized fires on structures.•Benchmarking against experimental tests with steel temperature measurements.•Decision flowchart to select the adequate ...fire-thermal modeling approach.•Localized fires create large temperature gradients in structural members.•FDS-FEM strategies depend on whether the member influences the fire development.
This paper investigates different numerical modeling strategies for evaluating temperatures in structures subjected to localized fires. Three simple fire models and two methods to interface computational fluid dynamics models with finite element thermal analyses are reviewed. Based on the field of application of each model, a decision flowchart is proposed to select the fire model and strategy suited to the problem. Validation is conducted against three experiments on steel members subjected to localized fires. Results show that the coupling between computational fluid dynamics and finite element analysis through the adiabatic surface temperature is a versatile method able to capture the member temperature in the different experiments in this study. The use of an automatic transfer interface using a transfer file with gas temperatures and radiant intensities is an efficient alternative when the member is outside the fire area and far enough that it does not significantly influence the fire development. The member should not be present in the FDS model when using the interface method. The simple models tend to yield more conservative predictions compared with the experimental results.
The convective heat transfer coefficient hc characterizes the heat transfer capacity between the fluid-solid interface in a thermal convection phenomenon. At present, values adopted from ...specifications are mostly used to define the hc coefficients in localized fire researches. In this paper, the specific scenario of the steel column surrounded by fire plume is selected as a typical case to explore the surface hc change rules of the steel column under the action of the fire source at the bottom. Different from the application of the concept of adiabatic surface temperature (AST) to achieve fluid-solid thermal coupling, this paper takes the scenario with the fire source heat release rate (HRR) being 81 kW as an example and conducts a two-way coupling simulation analysis of the fire and the thermal models by means of CFD codes. First, the validity of this simulation methodology is verified by comparing the results of the numerical model and the experiment. Following this, the distribution laws of the spatial velocity and temperature fields are analyzed, and the hc coefficient of the steel column surface is explored in detail using theoretical formula and traditional empirical formula respectively. It is noted that the hc results obtained based on theoretical formula can comprehensively consider the velocity boundary layer and temperature boundary layer situations above the column surface, which is a direct result of the definition of the convective heat transfer coefficient. It is found that the hc‾ is lower at the bottom of the column and increases with height, with relative maximum value in the lower and middle parts of the column, reaching around 40 W/m2 K. The hc‾ results decrease along the axial direction, with the value of upper part of the column being about 20 W/m2 K. In contrast, the hc‾ distributions along the height obtained by the empirical formulas are similar to the distribution of fire plume velocity along the height. In other words, it considers only the effect of the velocity boundary layer, while the effect of significant changes in fluid temperature on convective heat transfer behavior is less considered. Compared to the results based on the theoretical definition, the hc‾ results obtained by empirical formulas are significantly smaller in the lower and middle parts of the column, and higher at the top of the column. Finally, a new hc‾ practical formula for the column surfaces surrounded by fire plume is proposed, which agrees well with the results based on the theoretical definition in several cases.
•hc based on theoretical definition comprehensively considers velocity and temperature boundary layers, and shows large differences from that based on empirical formulas.•A new practical formula for the column surface hc coefficient surrounded by fire source is proposed.
This paper presents a method to calculate the non-uniform temperature field of steel members protected by intumescent coatings under localized fires. In this proposed method, the steel member is ...divided into a number of segments along the length according to the localized fire temperature field, and each segment divided into plate elements with different surrounding fire temperatures. The temperature of each plate element is assumed uniform and is calculated by the lumped mass equation with the thermal conductivity of intumescent coatings approximated using a three-stage thermal conductivity model. Heat conduction between different steel plate elements is ignored. Comparisons of steel temperature results between measured results and the proposed simplified temperature calculation method show very good agreement in temperature distributions along the steel member length, in the cross-section and detailed temperature–time relations at critical and high temperature locations, with differences in calculated and measured average temperatures in different segments being less than 7.2 %. Comparisons of load carrying capacities of axially-loaded steel members using measured temperature distributions and calculated temperature distributions show small (<6%) differences.
•A simplified method calculating temperature distributions in steel members protected by intumescent coatings under localized fires is proposed.•Steel temperature distributions calculated by simplified method show good agreement with measured results.•Comparisons of loading capacities of steel columns using measured and calculated temperature distributions show small (<6%) differences.
•Localized fire tests on hot rolled I beams.•Fire tests on I-beams at steady and transient state.•Effect of linitial load ratio on failure mode and critical temperature of beams.•Effect of location ...of local fire on beam failure.•Numerical modelling of local fire tests of I-beams.
The behaviour of hot-rolled steel I-section beams was studied experimentally and numerically in uniform temperatures in the past, but only limited experimental data are available for localized fire tests. The present paper reports ten experimental tests for short to medium-range laterally unrestrained I-beams of Class 1 section with a non-dimensional slenderness ratio in the range of 0.74 to 1.1. The localized fire behaviour of unrestrained I-beams is experimentally studied with a high thermal zone having a temperature range of 595 to 1200 °C. Three-point loading is provided considering simply-supported boundary conditions with pre-load and peak load (ultimate load) in the range of 0.26 to 0.76 of the design load capacity of a beam member at ambient temperature. The two types of fire scenarios are considered (a) fire concentration at the middle of the span and (b) fire concentration near the support of the beam under steady and transient experimental conditions. The geometrical imperfection is measured using an industrial blue-light optical scanner. A numerical validation study was also conducted using the robust FE modelling software ABAQUS. The experimental study concluded that failure mode can depend on fire location, load ratios, and span length. In the case of localized fire at the middle of the span, lateral torsional buckling happened, and when the fire was near support, local buckling happened at higher load ratios. However, local buckling happened in the beam for lower to mid-load ratios. It is observed that the steady and transient fire tests provided a negligible effect on the response of the beam member. The experimental and numerical results are in good agreement, with a mean percentage variation of 1.5 %, with a maximum value of up to 4.67 %. The study provides quite a simple experimental setup that can easily be replicated for different types of members that enhance the fire safety study of structure. Further, a parametric study is conducted under simply-supported and varying end restraints condition at supports.
This paper describes an analytical model developed for design purposes with the aim to assess the radiative heat flux to a vertical element subjected to a localised fire. It focuses on the ...calculation of the radiative heat flux, which represents the main source of heating of vertical members that are not engulfed into the fire. The model exploits the concept of virtual solid flame whose height and temperature are determined according to the correlations provided in Annex C of EN1991-1-2 and relies on an analytical formulation easy to implement in any spreadsheet. A major enhancement of the proposed model is the capability of providing a way to predict the heat flux on all faces of a vertical member. Moreover, it is deemed valid for any range of heat fluxes that can be expected in a localised fire and it is inherently conservative to be used for design calculations. The proposed model was validated against several experimental tests performed within the European LOCAFI project, as well as against tests found in the literature. Moreover, a parametric numerical analysis was carried out by means of Computational Fluid Dynamics (CFD) numerical simulations that were calibrated on the basis of the experimental outcomes. In this way, the validation of the analytical model was also performed numerically and useful information about the main parameters that significantly influenced the numerical simulation of localised fires fire was drawn. Conservative agreement in terms of heat flux predictions between the proposed model and experimental as well as numerical outcomes was found.
•Analytical model to predict radiative heat flux.•Heat flux prediction for faces composing a vertical element.•Flame shape approximated according to a cone.•Analytical model provided conservative results in terms of predicted heat flux.•Analytical model can be applied for design purposes.
Most of localised fire models focused on the heating impact on horizontal members at the ceiling level or the temperature inside the fire plume. Limited models describe the heat received by vertical ...surfaces due to a localised fire. As localised burning is prevailing in modern large compartments with sufficient ventilation, suitable fire models to quantify the heat fluxes on vertical components are under essential need. Advancing from the point source model that assumed radiation emitting from a virtual concentrated point, this paper proposes a line source model accounting for the radiant energy discharged along a vertical line to the target surface to improve the heating impact estimation. Non-uniform radiative power in slices and flame radius distributions along the flame height are derived with normalisation and necessary assumption. Multiple datasets obtained from experimental tests and numerical simulation are used to validate the proposed model, demonstrating good performance with mostly 10% or lower deviation. In built environment, the convective heat nearby the localised fire or in the smoke layer are found to be the main source of errors in prediction. Further modification is necessary to carefully consider the convective components if estimating the fire impact of a localised fire in large compartments.
•A 3D steel frame was analysed to study its resistance to fire-induced progressive collapse.•It is feasible to perform an explicit dynamic analysis for structures in fire by scaling a real fire time ...to seconds.•The uneven load transferring in the two planar direction plays a key role in the collapse modes.•The frame withstands the fire of single columns heated (load ratio of 0.25) but collapses for a higher load ratio of 0.5.•The frame collapses for a corner fire and long edge fire due to the uneven load redistribution.
This paper numerically investigates progressive collapse resistance of three-dimensional steel frames with reinforced concrete slabs exposed to localized fire using LS-DYNA. An explicit dynamic analysis is carried out and the real fire time is scaled down to save the computational cost. The prototype of the model is based on the eight-storey building in Cardington tests. The scenario of heating individual columns on the ground floor is first studied followed by a sever case of simultaneously heating four columns in one compartment. The collapse modes and load redistribution scheme of the frame subjected to different load ratios and fire locations are investigated. The modelling parameters such as mesh size, initial imperfection, timescale are first studied by validating against the fire test data. It is found that the mesh size has little influence on the responses of members at elevated temperature. The initial imperfection has significant effect on the load-bearing capacity of columns. The quasi-static behaviour of structures under a fire duration of hours can be simulated in a dynamic analysis by being scaled to seconds without causing oscillation of responses. The numerical results of the 3D frame show that the frame does not collapse in the case of single column heated for a fire design load (load ratio of 0.25 for columns). By increasing the load ratio to 0.5 as for the ambient design, progressive collapse occurs. For a fire of four columns heated, the frame collapses for the corner bay fire and long edge bay fire but withstands for the internal and short edge bay fires. The collapse modes are dominated by the uneven load redistribution in the two horizontal directions and the fire location, which cannot be simulated by a 2D model. The loads previously sustained by the buckled heated column are transferred more along the short span than the long span. The critical temperature of a column in a frame is significantly lower than that given in EC3, due to the fact that the translational and rotational restraint increase its load ratio and reduce its effective length, respectively. The critical temperature for the global collapse of the frame is about 50–100°C higher than that of individual heated columns.
This paper aims at deriving fire fragility curves for a prototype steel pipe-rack in an industrial plant subjected to localised fires. In particular, starting from a reference case study, ...uncertainties related to the structural capacity and the size of the localised fires caused by a hole in a tank or a hole in a pipe are included in the analyses. Thus, the influence of uncertainties in the derivation of the fragility functions was highlighted by comparing four sets of analyses in which both demand and capacity uncertainties were progressively introduced. Moreover, alongside the cloud analysis (CA), the suitability of the multiple stripe analysis (MSA) to build relevant probabilistic fire demand models was assessed. Fire fragility curves were derived by considering the interstorey drift ratio (ISDR) as engineering demand parameter (EDP) and by assessing different relevant intensity measures (IMs) that represent the severity of localised fires. It was found that by introducing uncertainties in the steel yield strength, lower probabilities to exceed the life safety and the near collapse limit states with respect to the reference case study were observed. Moreover, the inclusion of further uncertainties, described with continuous physically-based probability functions of the size of the fire diameter, affected the probabilistic models by lowering the probability of exceedance. These functions provide a more realistic description of the fire scenario, enabling a better representation of the structural vulnerability. For this case study, the CA exhibited better suitability for the derivation of fire fragility curves than the MSA. All the analysis results are thoroughly discussed in the paper.
•Structural experiments conducted on a 6 m long steel beam with variable connections and exposed to a localized fire.•The specimen tested at transient localized fire failed at higher bottom flange ...temperature than at steady-state localized fire.•Axial restraint provided by the double-angle connection resulted in lower failure temperature.•The results highlight that the heating conditions and end restraints influence the behavior of steel beams under localized fire.
This paper presents an experimental behavior of unprotected W16 × 26 steel beams subjected to localized fire. Beam specimens were either simply supported or connected to steel columns via all bolted double angle connections. Two test fires, steady or transient heating, were produced using a natural gas fueled burner located below the center of the beams. Thermal and structural responses of the steel beams as well as the heat release rate and applied forces were measured. The tests showed that all specimens failed by lateral-torsional buckling regardless of fire loading and end support conditions. The results highlight that the heating conditions and end restraints influence the behavior of steel beams under localized fire. The experimental results presented can be used for validation of predictive models in order to improve deign methods for localized fire conditions.
A fire in a storage warehouse creates a scenario that the load bearing members, notably storage rack uprights, are exposed to localised fires, generating the temperature field whose distribution is ...not uniform along the member length. This non-uniformity in temperature distribution is different from the assumption implied in the current structural fire design guidelines. This paper investigates the resistance of the uprights subjected to localised fires, in particular, fires at the bottom of the uprights, by accounting for the temperature gradients along the member length. Finite Element (FE) models of cold-formed steel storage rack uprights are developed, and are validated against experimental results from aspects of both heat transfer and buckling behaviour. The validated FE models are employed in a parametric study to investigate the resistance and buckling behaviour of the uprights under localised fires. The parametric study results show that the failure modes of the uprights are very sensitive to the localised fires. Moreover, it is observed that the Direct Strength Method (DSM), in the current form, does not produce accurate predictions for the cold-formed steel storage rack uprights under localised fires. Hence, modified DSM global curves are proposed in the paper to allow for the effects of localised fires, and prove to be sufficiently accurate through reliability analysis.
PDF
