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See detailEffect of Transient Creep Strain Model on the Behavior of Concrete Columns Subjected to Heating and Cooling
Gernay, Thomas ULg

in Fire Technology (2012), 48(2), 313-329

In the numerical analysis of structures in fire, the material models that are used have important implications on the global behavior of the structure. In concrete, a particular phenomenon appears when ... [more ▼]

In the numerical analysis of structures in fire, the material models that are used have important implications on the global behavior of the structure. In concrete, a particular phenomenon appears when subjected to high temperatures: the transient creep strain. Models integrating explicitly a term for transient creep strain have been proposed in the literature but, in the current Eurocode 2 model, there is no explicit term for transient creep strain. This phenomenon is included in the Eurocode 2 model, but it is implicitly considered in the mechanical strain term. A series of experimental fire tests on axially restrained concrete columns subjected to heating and cooling has been recently performed at South China University of Technology and described by Wu et al. (Fire Technol 46:231–249). In the original paper, it was shown that using the implicit model of Eurocode 2, the behavior of the columns cannot be simulated properly, especially during the cooling phase. The objective of the present paper is to perform again the fire tests simulations using a new formulation of the Eurocode 2 model that contains an explicit term for transient creep. In the first part of the paper, the explicit formulation of the Eurocode 2 model is presented. In the second part, the fire tests are modeled with the software SAFIR using, on the one hand, the implicit Eurocode model and, on the other hand, the new explicit model. It is shown that the transient creep model has significant implications on the global behavior of structural concrete members, as the residual axial load sustained by the columns at the end of the fire can differ by up to 25% of the initial applied load depending on the transient creep strain model that is used for the calculation. The experimental behavior is better matched with the new explicit model than with the current Eurocode model. Particularly, the results given by the Eurocode model during the cooling phase are unconservative as the residual axial load is overestimated. Finally, it is explained why, on the basis of an example, in a performance-based approach, these results can have important implications on the global fire resistance of a structure. [less ▲]

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See detailNumerical Modelling of Thin-Walled Stainless Steel Structural Elements in Case of Fire
Lopes, Nuno; Vila Real, Paulo; Simoes da Silva, Luís et al

in Fire Technology (2010), 46(1), 91-108

In this paper, the structural response of stainless steel thin-walled elements submitted to fire is analysed numerically by means of the geometrically and materially non-linear Finite Element program ... [more ▼]

In this paper, the structural response of stainless steel thin-walled elements submitted to fire is analysed numerically by means of the geometrically and materially non-linear Finite Element program SAFIR, including imperfections. In order to make these simulations, two main changes in the program were made: (i) the code was changed in order to deal with the stainless steel 2D material constitutive law to be used with shell elements and (ii) the possibility of the program to take into account residual stresses with shell finite elements was introduced. The stainless steel stress–strain relationship at high temperatures was based on the one presented in part 1.2 of Eurocode 3. To model the strain hardening exhibited by the stainless steels, using the shell element formulation, an approximation to the Eurocode 3 constitutive law was needed. Local and global geometrical imperfections were considered in the simulations. The paper shows the influence of the residual stresses on the ultimate load-carrying capacity of thin-walled stainless steel structural elements in case of fire. [less ▲]

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See detailAssessment of Eurocode 5 charring rate calculation methods
Cachim, Paulo; Franssen, Jean-Marc ULg

in Fire Technology (2010), 46

The base hypothesis for the assessment of fire resistance of timber structures by simple calculation models is that for temperatures above 300 ºC, timber is no longer able to sustain any load ... [more ▼]

The base hypothesis for the assessment of fire resistance of timber structures by simple calculation models is that for temperatures above 300 ºC, timber is no longer able to sustain any load. Consequently, the determination of the location of the 300 ºC isotherm, the charring depth, is decisive for the result of fire resistance calculation methods. Charring rate of timber is dependent of numerous factors, such as wood species (density, permeability or composition), moisture or direction of burning (along or across the grain). Eurocode 5, Part 1-2, presents several methods for the calculation of fire resistance of timber structures that are divided into simplified and advanced. In this paper simplified and advanced methods are compared regarding the calculation of the charring depth and residual cross section strength. Finite element simulations have been performed, employing the proposed timber properties of Eurocode 5 using finite element code SAFIR. The influence of parameters such as timber density and moisture has been investigated. The results obtained with finite element calculations were then compared with Eurocode 5 simplified models. Some inconsistencies between methods have been observed. This paper presents some proposals to overcome some of the inconsistencies as well as to extend the applicability of the models. [less ▲]

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See detailInfluence of ostacles on the development of gravity current prior to backdraft
Pérez-Jiménez, Christian; Guigay, Georges Jan; Horvat, Andrej et al

in Fire Technology (2009)

The phenomenon of backdraft is closely linked to the formation of a flammable region due to the mixing process between the unburned gases accumulated in the compartment and the fresh air entering the ... [more ▼]

The phenomenon of backdraft is closely linked to the formation of a flammable region due to the mixing process between the unburned gases accumulated in the compartment and the fresh air entering the compartment through a recently created opening. The flow of incoming fresh air is called the gravity current. Gravity current prior to backdraft has already been studied, Fleischmann (1993, Backdraft phenomena, NIST-GCR-94-646. University of California, Berkeley) and Fleischmann (1999, Numerical and experimental gravity currents related to backdrafts, Fire Safety Journal); Weng et al. (2002, Exp Fluids 33:398–404), but all simulations and experiments found in the current literature are systematically based on a perfectly regular volume, usually parallelipedic in shape, without any piece of furniture or equipment in the compartment. Yet, various obstacles are normally found in real compartments and the question is whether they affect the gravity current velocity and the level of mixing between fresh and vitiated gases. In the work reported here, gravity current prior to backdraft in compartment with obstacles is investigated by means of three-dimensional CFD numerical simulations. These simulations use as a reference case the backdraft experiment test carried out by Gojkovic (2000, Initial Backdraft. Department of Fire Safety Engineering, Lunds Tekniska Ho¨ gskola Universitet, Report 3121). The Froude number, the transit time and the ignition time are obtained from the computations and compared to the tests in order to validate the model. [less ▲]

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See detailFire tests and calculation methods for circular concrete columns
Franssen, Jean-Marc ULg; Dotreppe, Jean-Claude ULg

in Fire Technology (2003), 39(1), 89-97

The introduction sets the scene of the present paper i.e. the extensive research works performed at the University of Liege in order to derive acceptable calculation methods for the re design of concrete ... [more ▼]

The introduction sets the scene of the present paper i.e. the extensive research works performed at the University of Liege in order to derive acceptable calculation methods for the re design of concrete columns. It is explained that all previous works have been based on square or rectangular cross sections, for which corner spalling was observed very often, whereas circular section are nowadays becoming more and more popular. In order to examine the influence of the circular shape on the behavior under re conditions, an experimental research study has been performed recently at the University of Liege. This paper describes the test procedure, the observations made, and the values obtained for the re resistance. Theoretical methods have been developed for a quick, safe, and efficient design of concrete columns under re conditions. These methods have been applied successfully to the recently tested circular columns. [less ▲]

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See detailSpecial issue on properties of concrete and behavior of concrete columns subjected to fire
Franssen, Jean-Marc ULg

in Fire Technology (2003), 39(1), 97

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