The methodologies that are used for analysing the fire behaviour of a structure that is subjected to a uniform thermal situation cannot be applied when the fire is localised. The concept of “zoning” ...can be applied: the structure is divided into several zones in which the situation is approximated as uniform. It is shown here that this division can lead to spurious forces in the structure. The structural code of the first author has been adapted in order to accommodate a continuous spatial variation of the fire environment. A series of uncoupled 2D thermal analyses is performed along the length of the beam finite elements and a series of 1D thermal analyses is performed across the thickness of the shell finite elements. After a discussion of the concept and the particularities dictated by the continuous thermal environment, the methodology utilised is explained and is shown in an example consisting of a composite steel concrete car park subjected to a localised fire of the type given in Eurocode 1.
This thesis presents the results of a research project to develop methods to carry out fire safety design of welded steel tubular trusses at elevated temperatures due to fire exposure. It deals with ...three subjects: resistance of welded tubular joints at elevated temperatures, effects of large truss deflection in fire on member design and effects of localised heating. The objectives of the project are achieved through numerical finite element modelling at elevated temperatures using the commercial Finite Element software ABAQUS v6.10-1 (2011). Validation of the simulation model for joints is based on comparison against the test results of Nguyen et al. (2010) and Kurobane et al. (1986). Validation of the simulation model for trusses is through checking against the test results of Edwards (2004) and Liu et al. (2010).For welded tubular joints, extensive numerical simulations have been conducted on T-, Y-, X-, N- and non-overlapped K-joints subjected to brace axial compression or tension, considering a wide range of geometrical parameters. Uniform temperature distribution was assumed for both the chord and brace members. Results of the numerical simulations indicate for gap K- and N-joints (two brace members, one in tension and the other in compression) and for T-, Y- and X-joints with the brace member under axial tensile load (one brace member only, in tension), it is suitable to use the same ambient temperature calculation equation as in the CIDECT (2010) or EN 1993-1-8 (CEN, 2005a) design guides and simply replace the ambient temperature strength of steel with the elevated temperature value. However, for T-, Y- and X-joints under brace compression load (one brace member only, in compression), the effect of large chord deformation should be considered. Large chord deformation changes the chord geometry and invalidates the assumed yield line mechanism at ambient temperature. For approximation, the results of this research indicate that it is acceptable to modify the ambient temperature joint strength by a reduction factor for the elastic modulus of steel at elevated temperatures. In the current fire safety design method for steel truss, a member based approach is used. In this approach, the truss member forces are calculated at ambient temperature based on linear elastic analysis. These forces are then used to calculate the truss member limiting temperatures. An extensive parametric study has been carried out to investigate whether this method is appropriate. The parametric study encompasses different design parameters over a wide range of values, including truss type, joint type, truss span-to-depth ratio, critical member slenderness, applied load ratio, number of brace members, initial imperfection and thermal elongation. The results of this research show that due to a truss undergoing large displacements at elevated temperatures, some truss members (compression brace members near the truss centre) experience large increases in member forces. Therefore, using the ambient temperature member force, as in the current truss fire safety design method, may overestimate the truss member critical temperature by 100 °C. A method has been proposed to analytically calculate the increase in brace compressive force due to large truss deformation. In this method, the maximum truss displacement is assumed to be span/30. A comparison of the results calculated using the proposed method against the truss parametric study results has shown good agreement with the two sets of results, with the calculation results generally being slightly on the safe side. When different members of a truss are heated to different temperatures due to localised fire exposure, the brace members in compression experience increased compression due to restrained thermal expansion. To calculate the critical temperature of a brace member in a localised heated truss, it is necessary to consider this effect of restrained thermal expansion. It is also necessary to consider the beneficial effects of the adjacent members being heated, which tends to reduce the increase in compressive force in the critical member under consideration. Again, an extensive set of parametric studies have been conducted, for different load ratio, slenderness and axial restraint ratio. The results of this parametric study suggest that to calculate the critical temperature of a brace member, it is not necessary to consider the effects of the third or further adjacent members being heated. For the remainder of the heated members, this thesis has proposed a linear elastic, static analysis method at ambient temperature to calculate the additional compressive force (some negative, indicating tension) in the critical member caused by the heated members (including the critical member itself and the adjacent members). The additional compressive force is then used to calculate the limiting temperature of the critical member. For this purpose, the approximate analytical equation of Wang et al. (2010) has been demonstrated to be suitable.
Complex temperature fields may develop inside structural steel elements in fire, severely affecting the local and global response of the structure. However, simplified yet well-accepted design ...procedures allow for uniform temperature distributions in the steel elements. The second generation of Eurocodes provides advanced models for localised fires where non-negligible thermal gradients both inside the sections and along the elements are expected to influence the resistance of steel elements, especially those critical to the stability of the entire structure, such as vertical members. This paper sheds some light on the consequences of employing procedures with increasing complexity in the design of unprotected axially compressed steel columns subjected to localised fires for which the effect of the smoke layer is not significant. Five procedures were compared, considering a refinement in both the thermal and the mechanical models, culminating in the application of a procedure enabling complex temperature fields both in the section and along the column, namely the LOCAFI model, and exploiting numerical simulation in SAFIR. A parametric study was conducted, encompassing different fire scenarios and structural characteristics, resulting in 27468 analyses. It was shown that in the case of localised fire scenarios that induce significant thermal gradients in the structure advanced modelling is recommended because it better captures the thermomechanical response. Detailed considerations on relevant outcomes, such as the temperature at failure, were provided. Finally, the influence of the surrounding structure on the effectiveness of the procedures was investigated extending the parametric analysis to columns with various degrees of axial restraints.
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•Design with simple thermal and mechanical models provides inaccurate predictions.•ISO834 leads to uneconomical consideration that fire protection is always required.•The LOCAFI model included in the 2nd generation of Eurocodes is highly recommended.•Thermal gradients become less relevant for higher axial restraints.•Failure cannot be related only to slenderness for complex thermal gradients.
•Analysis of load redistributions in a structure when a localized fire affects a column.•Tension builds up in the fire-exposed column due to heating-cooling sequence.•This overloads the adjacent ...columns potentially leading to progressive collapse.•The behavior is observed on a twenty-story steel frame building under localized fire.•Fire loading cannot be addressed by an event-independent column loss approach.
In progressive collapse analysis, event-independent column loss is commonly used as a design scenario. Yet this scenario does not account for the fire-induced thermal forces that develop in case of a fire. The thermal forces may cause detrimental load redistributions in the structure, notably during the cooling phase. However, as the response of entire structures during the full course of fires until burnout has received little attention, these effects are not well established. The objective of this paper is to analyze the mechanisms of load redistribution in a structural system comprising a column subjected to localized fire, with a focus on the effects of the cooling phase. Numerical simulations by nonlinear finite element method are used, after validation against experimental data. The observed mechanisms result in tension building up in the fire-exposed column and overloading the adjacent columns in compression. Consequently, the damaged vertical member redistributes a force that is larger than the force initially carried. This can lead to failure of vertical members not directly affected by the fire and trigger a progressive collapse. These mechanisms are parametrically studied on a simple system composed of a column and a linear spring. Major parameters influencing the residual tensile force in the fire-exposed column are the maximum reached temperature and the relative stiffness of the remainder of the structure. The analysis of a twenty-story steel frame building under localized fire attacking one ground level perimeter column confirms the development of these mechanisms in a real design. The results have important implications as they question the validity of an event-independent design scenario for capturing the influence of column failure due to fire loading.
This work aims to investigate the thermal behaviors of the concrete ceiling slab of a semi-open car park exposed to localized fire in hydrogen fuel cell vehicles. For this purpose, a numerical ...simulation of the hydrogen fuel cell vehicle fire was performed in the Fluid Dynamic Simulator and then coupled with a subsequent thermal analysis of concrete structure carried out in ANSYS Mechanical APDL. In particular, an automatic procedure was used to extract the output of the fire simulation and apply them as boundary conditions of the thermal model. The one-way coupling procedure involving fire simulation and transient thermal analysis has been validated by comparing it with concrete temperatures of a previous test study. Then, two parameters, the diameter of thermal pressure relief devices (1 mm, 2 mm, 3 mm, and 4 mm) and fire spread time between vehicles (0 min, 20 min, and 30 min), are taken into account to study the thermal properties of concrete. The analysis revealed that an increase in the nozzle diameter of the thermal pressure relief device leads to a rise in the maximum concrete surface temperature. The simulation results also showed that the maximum value of the heat release rate increases with a higher value of the nozzle diameter of the thermal pressure relief device and a shorter fire spread time between vehicles.
•FDS modeling of hydrogen fuel cell vehicle fires in a car park.•ANSYS modeling of a concrete slab exposed to fires.•One-way CFD-FEM coupling interface is used to connect a fire model and a thermal model.•Effects of TPRD nozzle diameters and fire spread times are investigated.
Evaluating the fire safety of hydrogen storage cylinders is crucial before hydrogen can be widely used in fuel cell vehicle applications. This research presents a comprehensive 3D hydrogen cylinder ...fire safety model that couples ANSYS FLUENT to ANSYS-mechanical and simulates a vehicle-mounted hydrogen cylinder when subjected to fire. By considering a volumetric heat source that is beneath the hydrogen cylinder and by factoring the effects of complex heat transfer (convection, radiation, etc.), fluid dynamics, and the mounting space's solid wall, the temperature distributions across the hydrogen cylinder wall are calculated with greater accuracy than existing models that over-simplify the fire source model. Moreover, the temperature distributions are input to ANSYS Mechanical to calculate the induced mechanical stress. Then, a series of parametric analyses are conducted to evaluate their influence on maximum temperature and mechanical stress. Results reveal that thermal radiation is the dominant heat transfer mechanism from the fire source to the cylinder body. Decreasing the cylinder body's thermal conductivity can reduce maximum mechanical stress by 7%.Meanwhile, increasing the cylinder body thickness introduced more thermal strain to increase the maximum mechanical stress, which is not conducive to improving fire safety.
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•An accurate 3D vehicle-mounted hydrogen cylinder fire simulation is developed.•Heat transfer and fluid dynamics of ANSYS-FLUENT are coupled to ANSYS-mechanical.•A series of parametric analyses are conducted to evaluate thermo-mechanical stress.•Small latent heat transfer meant the top wall had little effect on fire safety.•Mechanical stress can be marginally reduced by thermally insulating the cylinder.
Automated Rack Supported Warehouses (ARSW) are self‐bearing rack structures designed to support, besides self‐weight and weight of products, environmental loads (i.e., wind, snow and seismic action) ...and all the other non‐structural elements. These structures are unique in the design; indeed, they represent a topic of great interest for both the scientific community and for the manufacturers of industrial racks. However, the behavior in case of fire of these structures is still poorly known. In order to study the fire resistance of these structures to consider a natural fire curve is necessary. In particular, a vertically traveling fire model in the upright frames, starting from a localized fire has to be modelled.
This paper proposes a fire model that considered both the vertical and horizontal fire propagation, evaluated and validated against experimental test results. In addition, advanced thermo‐mechanical analyses using both natural and nominal fire curves, were performed with a finite element software (SAFIR), which allows modeling Class 4 steel elements considering the local instabilities that can occur in these slender sections. A comparison between the capacity models that the regulations propose for these structural elements, was also conducted.
•A new model for radiation calculation to any horizontal surfaces from localized fire.•The new model is based on classic fire plume theory and radiation theory of participating medium.•The mean ...relative deviations of the model against experimental and numerical data are within ± 30 %.
Fire is the most frequent hazard to buildings. The current design methods for predicting the temperatures of structures in fire are originally developed for the standard fire that assumes a uniform gas temperature field and are invalid for natural fires with high non-uniform gas temperature fields. Localized fire is a typical natural fire with high non-uniform gas temperature field, which is often considered in fire safety design. There are currently no validated simple general models for calculating the heat transfer from localized fire to exposed structures. To address the need of design method, this paper develops a refined cylinder fire model for calculating the incident radiant heat fluxes on horizontal structural members inside and outside the flame region of a localized fire. The model uses a cylinder, centered at the fire plume axis, to represent the localized fire. The height of the cylinder is not fixed, different to any other fire models. The temperatures in the cylinder are highly non-uniform in three dimensions and are determined based on the classic fire plume theory. For radiation calculation, the cylinder is evenly divided into several elements along the radius and height. Each element is assumed to be a homogenous, isotropic, and absorbing-emitting (participating) graybody. By careful consideration of the spatial gas temperature distribution, the radiation properties (emissivity and transmissivity) of the elementary graybody, and the configuration factor between the elementary graybody and the horizontal target surface, the model can get relatively accurate and conservative incident radiant heat flux. The relative deviations against both the experiment data and numerical results are within ± 30 %. The model introduces a new concept of calculating the heat fluxes from fire to structures, is a relatively simple general model (compared with the sophisticated computational fluid dynamics models), and is more precise and accurate than other simple ones. The model is recommended for use in performance-based fire safety design.
In this study, a one-way coupling methodology combining fire simulation based computational fluid dynamics (CFD) with finite element (FE) analysis is proposed to investigate the thermo-mechanical ...behaviors of composite beams with corrugated steel webs (CSWs) under localized fire exposure. Fire Dynamics Simulator (FDS) is used to simulate the local fire environment, and the adiabatic surface temperature (AST) data obtained in FDS are imported into the FE models in ABAQUS by the fds2ascii coupling tool. The validations of FDS models and the CFD-FE coupling method are verified by a localized fire test on a steel beam conducted by the National Institute of Standards and Technology (NIST). Then, the CFD-FE coupling method is applied to analyze the thermo-mechanical behaviors of composite beams with concrete flanges and CSWs exposed to localized fire. As the fire exposure time increases, the composite beam appears significant variation of the temperature gradient at elevated temperature, and induce the dynamic, uneven temperature field in the structural members. Due to the influences of the temperature gradient field, the traditional design assumptions for composite beams with CSWs are not valid when the structures suffered a localized fire. It found that the shear stress on the CSWs is no longer evenly distributed along the depth at elevated temperatures, and the concrete flange resists a considerable part of the sectional shear force, particularly of the fire-affect zone. This study hopes to provide a basis for the fire resistance design of composite girder bridges with CSWs.
•The distribution of normal stress of composite beams with CSWs changes regularly under fire condition.•The concrete flange of composite beams resists a considerable part of shear force under fire conditions.•The traditional assumption that the CSWs bear the entire shear force of a section may not apply to the beams suffered a localized fire.
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