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See detailEffective stress method to be used in beam finite elements to take local instabilities into account
Franssen, Jean-Marc ULg; Cowez, Baptiste; Gernay, Thomas ULg

in Proceedings of the 11th IAFSS Symposium (2014, February)

In the fire situation, Bernoulli beam finite elements are the workhorse used in numerical calculation model for simulating the behaviour of the structure. Such finite elements treat all sections as class ... [more ▼]

In the fire situation, Bernoulli beam finite elements are the workhorse used in numerical calculation model for simulating the behaviour of the structure. Such finite elements treat all sections as class 1 (stocky) sections whatever the slenderness of the plates that make the section, allowing the development of a full plastic stress distribution in the section which leads to complete plastic redistribution along the members in the structure. This type of element is thus not adapted for modeling structures that contain slender sections of class 2, 3 or 4. This document presents a new approach to take into account local instabilities in slender sections using beam finite elements. The new approach is based on an effective constitutive law of steel. The effective law is not symmetrical with respect to tension and compression because, in tension, the stress-strain relationship is not modified whereas, in compression, the stress-strain relationship is modified. [less ▲]

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See detailA multiaxial constitutive model for concrete in the fire situation: Theoretical formulation.
Gernay, Thomas ULg; Millard, Alain; Franssen, Jean-Marc ULg

in International Journal of Solids and Structures (2013), 50(22-23), 3659-3673

This paper aims to develop a multiaxial concrete model for implementation in finite element softwares dedicated to the analysis of structures in fire. The need for proper concrete model remains a ... [more ▼]

This paper aims to develop a multiaxial concrete model for implementation in finite element softwares dedicated to the analysis of structures in fire. The need for proper concrete model remains a challenging task in structural fire engineering because of the complexity of the concrete mechanical behavior characterization and the severe requirements for the material models raised by the development of performance-based design. A fully threedimensional model is developed based on the combination of elastoplasticity and damage theories. The state of damage in concrete, assumed isotropic, is modeled by means of a fourth order damage tensor to capture the unilateral effect. The concrete model comprises a limited number of parameters that can be identified by three simple tests at ambient temperature. At high temperatures, a generic transient creep model is included to take into account explicitly the effect of transient creep strain. The numerical implementation of the concrete model in a finite element software is presented and a series of numerical simulations are conducted for validation. The concrete behavior is accurately captured in a large range of temperature and stress states. A limitation appears when modeling the concrete post-peak behavior in highly confined stress states, due to the coupling assumption between damage and plasticity, but the considered levels of triaxial confinement are unusual stress states in structural concrete. [less ▲]

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See detailResistencia al fuego de armaduras de acero inoxidable
Franssen, Jean-Marc ULg; Gernay, Thomas ULg

in Bastidas, David M; Medina Sanchez, Eduardo (Eds.) Armaduras de Acero Inoxidable (2013)

Un incendio constituye un riesgo por dos razones, la toxicidad del aire y las altas temperaturas. Dependiendo de si se produce al aire libre o en un recinto cerrado la magnitud del peligro varía ... [more ▼]

Un incendio constituye un riesgo por dos razones, la toxicidad del aire y las altas temperaturas. Dependiendo de si se produce al aire libre o en un recinto cerrado la magnitud del peligro varía notablemente. En el exterior, es poco probable superar los 700ºC, sin embargo, en el interior la temperatura puede llegar a ser mucho más elevada. Infraestructuras críticas son los túneles, además de por ser recintos cerrados, por la naturaleza de los materiales inflamables que albergan. El diseño y la resistencia de los materiales son aspectos claves para garantizar un tiempo suficiente hasta el colapso. Los efectos en las estructuras de hormigón armado empiezan en el propio comportamiento de los materiales. El hormigón pierde menos capacidad a altas temperaturas que el acero, por lo tanto, utilizar aceros con mejores propiedades a altas temperaturas como los inoxidables aumentan las prestaciones del conjunto de la estructura. En este capítulo se describen las características térmicas del acero inoxidable y cómo varían sus características mecánicas en función de la temperatura. Por medio de simulaciones informáticas (software SAFIR de la Universidad de Lieja) se ha realizado un análisis térmico de armaduras de acero inoxidable de diferentes tipos y al carbono, embebidas en hormigón con distintos recubrimientos, en sección de columna y de viga. Finalmente, se ha realizado un análisis estructural de dichos elementos sometidos a diferentes cargas en puntos temporales distintos en condiciones de fuego. [less ▲]

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See detailA multiaxial constitutive model for concrete in the fire situation
Gernay, Thomas ULg; Franssen, Jean-Marc ULg

in Proceedings of the 13th Fire and Materials Conference (2013, January 28)

Performance-based design in fire engineering leads to increasing demand for advanced temperature-dependent material models for the load bearing materials used in building structures. These models must be ... [more ▼]

Performance-based design in fire engineering leads to increasing demand for advanced temperature-dependent material models for the load bearing materials used in building structures. These models must be valid in natural fire situations including cooling down phase and must be sufficiently robust for complex numerical calculations such as, for example, the analysis of tensile membrane action in composite slabs. Although structural concrete is widely used in civil engineering, proper modelling of its thermo-mechanical behaviour remains a challenging issue for engineers mainly because of the complexity of the phenomena that result from the microcracking process in this material and because of the necessity to ensure the numerical robustness of the models. This paper presents a new multiaxial concrete model based on a plastic-damage formulation and developed to meet the specific requirements of structural fire engineers and researchers. The model, which incorporates an explicit term for transient creep strain and encompasses a limited number of material parameters, has been implemented in a finite element software dedicated to the nonlinear analysis of structures in fire. The paper presents a series of numerical simulations conducted to highlight the model ability to capture the main phenomena that develop in concrete under fire (permanent strains, degradation of the elastic properties, unilateral effect) as well as its ability to be used for the fire analysis of large-scale structural elements. As an example, the new concrete model is used in the numerical analysis of a full scale fire test on a composite steel-concrete slab and it is shown that the computed and measured results agree. [less ▲]

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See detailNumerical analysis of partially fire protected composite slabs
Zaharia, Raul; Vulcu, Christian; Vassart, Olivier et al

in Steel and Composite Structures (2013), 14(1), 21-39

The paper presents a numerical investigation, done with the computer program SAFIR, in order to obtain simpler finite element models for representing the behaviour of the partially protected composite ... [more ▼]

The paper presents a numerical investigation, done with the computer program SAFIR, in order to obtain simpler finite element models for representing the behaviour of the partially protected composite steel concrete slabs in fire situation, considering the membrane action. Appropriate understanding and modelling of the particular behaviour of composite slabs allows a safe approach, but also substantial savings on the thermal insulation that has to be applied on the underlying steel structure. The influence of some critical parameters on the behaviour and fire resistance of composite slabs such as the amount of reinforcing steel, the thickness of the slab and the edge conditions is also highlighted. The numerical models are calibrated using the results of three full scale fire tests on composite slabs that have been performed in recent years. [less ▲]

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See detailStructural Behaviour of Concrete Columns under Natural Fires
Gernay, Thomas ULg; Dimia, Mohamed Salah

in Engineering Computations (2013), 30(6), 854-872

Purpose - The paper aims to give an insight into the behavior of reinforced concrete columns during and after the cooling phase of a fire. The study is based on numerical simulations as these tools are ... [more ▼]

Purpose - The paper aims to give an insight into the behavior of reinforced concrete columns during and after the cooling phase of a fire. The study is based on numerical simulations as these tools are frequently used in structural engineering. As the reliability of numerical analysis largely depends on the validity of the constitutive models, the development of a concrete model suitable for natural fire analysis is addressed in the study. Design/methodology/approach - The paper proposes theoretical considerations supported by numerical examples to discuss the capabilities and limitations of different classes of concrete models and eventually to develop a new concrete model that meets the requirements in case of natural fire analysis. Then, the study performs numerical simulations of concrete columns subjected to natural fire using the new concrete model. A parametric analysis allows for determining the main factors that affect the structural behavior in cooling. Findings – Failure of concrete columns during and after the cooling phase of a fire is a possible event. The most critical situations with respect to delayed failure arise for short fires and for columns with low slenderness or massive sections. The concrete model used in the simulations is of prime importance and the use of the Eurocode model would lead to unsafe results. Practical implications – The paper includes implications for the assessment of the fire resistance of concrete elements in a performance-based environment. Originality/value – The paper provides original information about the risk of structural collapse during cooling. [less ▲]

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See detailNew developments in SAFIR 2013 - New concrete models
Gernay, Thomas ULg

Scientific conference (2012, November 29)

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See detailA multiaxial constitutive model for concrete in the fire situation including transient creep and cooling down phases
Gernay, Thomas ULg

Doctoral thesis (2012)

Purpose - The present thesis aims to develop an efficient and reliable multiaxial concrete model for implementation in finite elements softwares dedicated to the analysis of structures in fire. The need ... [more ▼]

Purpose - The present thesis aims to develop an efficient and reliable multiaxial concrete model for implementation in finite elements softwares dedicated to the analysis of structures in fire. The need for proper concrete model remains a very challenging task in structural (fire) engineering because of the complexity of the concrete mechanical behavior characterization and the severe requirements for the material models raised by the development of performance-based design. Methodology - The thesis opted for a phenomenological approach for modeling the thermo-mechanical behavior of concrete. The specifications of the model are based on the study of published experimental data of concrete samples tests and on the specific needs related to the applications in structural fire engineering. With these specifications in mind, a state of the art review of concrete models is conducted in order to choose the general theoretical framework that best fits the criteria for the development of the new model. The thesis presents the theoretical development of the model and its numerical implementation in a finite elements software. Numerical simulations of experimental tests are then performed to verify that the model satisfy the specifications. Findings - The combination of elastoplasticity theory and damage theory allows to develop a phenomenological model suitable for concrete behavior modeling within the pragmatic and robust theoretical framework of continuum constitutive models based on smeared crack approach. The state of damage in concrete, assumed isotropic, is modeled by means of a fourth order damage tensor to capture the unilateral effect. When complex performance-based situations are considered, the effect of transient creep strain at high temperature must be taken into account by an explicit term in the strain decomposition. A generic transient creep model is therefore developed based on experimental data and the model is calibrated to yield the same results as the Eurocode implicit model in simple prescriptive situations. The concrete model comprises a limited number of parameters that can be identified by three simple tests; besides, a standard set of values to be used in predictive calculations is clearly defined for these parameters. Numerical simulations can deal with all stress states as the model is developed as fully three-dimensional. A large number of examples highlight the capabilities of the model that range from the modeling of sample tests to the modeling of large scale composite structures developing membrane action. Limitations – Due to the assumption that damage and plasticity are driven by the same internal variables in the model, a limitation appears for capturing the concrete post-peak behavior in highly confined stress states. This assumption allows for reducing the number of parameter but it restrains the domain of applicability of the model; it is suggested to adopt a different approach if the behavior in multiaxial compression at high confinement level has to be accurately captured. Another limitation of the model is related to the localization issue, which is only partly addressed in this work by means of the regularization of the crack energy. Further works should bring a more elaborated response while considering the case of reinforced concrete structures, in which numerous cracks develop. Finally, several simplifying assumptions have been adopted to restrain the scope of the research; for instance, the phenomenon of spalling has not been considered. Practical implications - The thesis includes implications for the development of advanced numerical tools for the simulation of concrete structures at ambient temperature and at high temperature. The use of such advanced tools in the design may lead to significant reduction in the building costs and to improved robustness of the structures. Value – The thesis contributes to fulfil an identified need to make available proper constitutive concrete model for implementation in finite elements softwares dedicated to the analysis of structures in fire. Special care is given to the numerical robustness of the model and to the clear definition of the material parameters as the model is intended to be used by structural (fire) engineers in real applications. [less ▲]

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See detailValidation of the Advanced Calculation Model SAFIR Through DIN EN 1991-1-2 Procedure
Zaharia, Raul; Gernay, Thomas ULg

in Proceedings of the 10th International Conference ASCCS 2012 (2012, July 03)

The validation of advanced calculation models for the fire design is an important issue for computer code developers, designers and authorities. One approach consists in the comparison with experimental ... [more ▼]

The validation of advanced calculation models for the fire design is an important issue for computer code developers, designers and authorities. One approach consists in the comparison with experimental results, which are not always available or are not always useful, due to the lack of details about the input data or the results, or uncertainties about the boundary conditions. This paper presents the validation of the special purpose computer program SAFIR (2005) for structural fire analysis, through DIN EN1991-1-2 procedure (2010), which represents an alternative to the validation through fire tests. [less ▲]

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See detailLarge-scale fire test of unprotected cellular beam acting in membrane action
Vassart, Olivier; Bailey, C.G.; Hawes, M. et al

in Proceedings of the Institution of Civil Engineers: Structures and Buildings (2012), 165(7), 327334

This paper describes a full scale fire test performed recently on a composite floor for analysing the possibility of tensile membrane action to develop when the unprotected steel beams in the central part ... [more ▼]

This paper describes a full scale fire test performed recently on a composite floor for analysing the possibility of tensile membrane action to develop when the unprotected steel beams in the central part of the floor are made of cellular beams. The natural fire was created by a wood crib fire load of 700 MJ/m² and the 9 x 15 m floor survived the fire that peaked at 1000°C and lasted for about 90 minutes. Blind predictions of the air temperature development by the software OZone and of the structural behaviour by the software SAFIR which proved quite satisfactory are also described. [less ▲]

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See detailA formulation of the Eurocode 2 concrete model at elevated temperature that includes an explicit term for transient creep
Gernay, Thomas ULg; Franssen, Jean-Marc ULg

in Fire Safety Journal (2012), 51

The first objective of this paper is to highlight the capabilities and limitations of concrete uniaxial constitutive models at elevated temperatures for thermo-mechanical behavior modeling, depending on ... [more ▼]

The first objective of this paper is to highlight the capabilities and limitations of concrete uniaxial constitutive models at elevated temperatures for thermo-mechanical behavior modeling, depending on the implicit or explicit consideration of transient creep strain in the model. The characteristics inherent to the two types of models are described and compared. It appears that one of the major limitations of implicit models concerns the unloading stiffness. Based on numerical analysis performed on loaded concrete columns subjected to natural fire, it is shown that the stress–temperature paths experienced by structural concrete are varied and complicated and that concrete material models cannot handle properly these complex situations of unsteady temperatures and stresses without explicit consideration of transient creep. The second objective of the paper is to propose a new formulation of the Eurocode 2 concrete material model that contains an explicit term for transient creep. The new model is implemented in the software SAFIR and validated against experimental data of the mechanical strain developed by concrete cylinders under different unsteady temperatures and loads. It is shown that the actual material behavior is better matched with the new explicit model than with the current implicit Eurocode 2 model. Finally, a comparison is given between experimental and computed results on a centrally loaded concrete column submitted to heating–cooling sequence. [less ▲]

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See detailA multiaxial concrete model for applications in structural fire engineering
Gernay, Thomas ULg; Millard, Alain

in Fontana, M.; Frangi, A.; Knobloch, M. (Eds.) Proceedings of the 7th International Conference on Structures in Fire (2012, June 07)

Temperature-dependent material models are required in numerical softwares dedicated to the nonlinear analysis of structures in fire. Although structural concrete is widely used in civil engineering ... [more ▼]

Temperature-dependent material models are required in numerical softwares dedicated to the nonlinear analysis of structures in fire. Although structural concrete is widely used in civil engineering, proper modelling of its thermo-mechanical behaviour remains a challenging issue for engineers mainly because of the complexity of the phenomena that result from the microcracking process in this composite material and because of the lack of numerical robustness of the models. This paper presents a new multiaxial concrete model developed for the analysis of concrete structures in fire. The multiaxial model is based on a plastic-damage formulation and incorporates an explicit term for transient creep strain. After implementation in a finite element software for structural fire engineering calculations, numerical simulations have been performed to highlight the ability of the model to capture some of the main phenomena that develop in concrete (permanent strains, degradation of the elastic properties, unilateral effect) as well as the ability to be used for the fire analysis of large-scale structural elements. [less ▲]

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See detailModeling the mechanical behavior of concrete in the fire situation
Gernay, Thomas ULg

Scientific conference (2012, April 26)

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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 detailLarge-Scale Fire Test of Unprotected Cellular Beam Acting in Membrane Action
Vassart, Olivier; Bailey, C. G.; Hawes, M. et al

in Journal of Structural Fire Engineering [=JSFE] (2011), 2(4), 259-268

This paper describes a full scale fire test performed the 27th of February 2010 on a composite floor for analysing the possibility of tensile membrane action to develop when the unprotected steel beams in ... [more ▼]

This paper describes a full scale fire test performed the 27th of February 2010 on a composite floor for analysing the possibility of tensile membrane action to develop when the unprotected steel beams in the central part of the floor are made of cellular beams. The natural fire was created by a wood crib fire load of 700 MJ/m2 and the 9 × 15 m floor survived the fire that peaked at 1000°C and lasted for 90 minutes. Blind predictions of the air temperature development by the software OZone and of the structural behaviour by the software SAFIR which proved quite satisfactory are also described. [less ▲]

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See detailNew features in SAFIR: Structural Fire Analysis of a Shell Roof Structure
Gernay, Thomas ULg; Franssen, Jean-Marc ULg

Scientific conference (2011, November 15)

The structural fire analysis of a roof structure made of two concrete shells has been performed with the software SAFIR.

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See detailNew features in SAFIR: Damage-Plastic Multiaxial Model for Concrete
Gernay, Thomas ULg; Franssen, Jean-Marc ULg

Scientific conference (2011, November 15)

The need for robust, reliable multiaxial constitutive relationships for concrete is a challenging issue in structural fire engineering. A new damage-plastic model has been developed to capture the ... [more ▼]

The need for robust, reliable multiaxial constitutive relationships for concrete is a challenging issue in structural fire engineering. A new damage-plastic model has been developed to capture the development of permanent strains and the degradation of elastic properties in concrete subjected to thermo-mechanical loading. This model has been implemented in the software SAFIR for using with SOLID finite elements (multiaxial model) and with SHELL finite elements (plane stress model). [less ▲]

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See detailNew features in SAFIR: Transient Creep in Eurocode Concrete Model
Gernay, Thomas ULg; Franssen, Jean-Marc ULg

Scientific conference (2011, November 15)

A new formulation of the Eurocode 2 concrete model at elevated temperature has been implemented in which the transient creep strain is treated by an explicit term in the strain decomposition. This new ... [more ▼]

A new formulation of the Eurocode 2 concrete model at elevated temperature has been implemented in which the transient creep strain is treated by an explicit term in the strain decomposition. This new model gives the same answer as the Eurocode model when the load and the temperature increase, but the new model is better than the Eurocode model at capturing the concrete behavior when the load or the temperature decrease owing to the explicit transient creep term. This new concrete model, called Explicit Transient Creep Eurocode model (ETC), has been implemented in the software SAFIR. [less ▲]

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See detailStructural Behavior of Concrete Columns under Natural Fires including Cooling Down Phase
Gernay, Thomas ULg; Dimia, M. S.

in Barros, Helena; Faria, Rui; Pina, Carlos (Eds.) et al Proceedings of The International Conference on Recent Advances in Nonlinear Models - Structural Concrete Applications (2011, November)

This study deals with the structural behavior of concrete columns subjected to natural fire, with special focus on the cooling down phase. As the reliability of the numerical simulations largely depends ... [more ▼]

This study deals with the structural behavior of concrete columns subjected to natural fire, with special focus on the cooling down phase. As the reliability of the numerical simulations largely depends on the validity of the constitutive models that are used, this issue is addressed in details in the first part of the paper. It is found that the current concrete model of Eurocode may not be adapted for representing the behavior of concrete during a cooling phase because the effects of transient creep are incorporated implicitly in the mechanical strain term. A formulation of the Eurocode concrete model that contains an explicit term for transient creep strain has been developed and, after implementation in the software SAFIR, it was used for the numerical simulations presented in this paper. In its second part, the study gives an insight into the possibility of collapse of reinforced concrete columns during and after the cooling phase of the natural fire. It is found that failure during the cooling phase of a fire is a possible event and, more dangerous, that a failure of the concrete columns is still possible when the fire has been completely extinguished. The main mechanisms that lead to this type of failure are found to be the delayed increase of temperature in the central zones of the element and the additional loss of concrete strength during the cooling phase of the fire. A parametric study shows that the most critical situations with respect to delayed failure arise for short fires and for columns with low slenderness or massive sections. The numerical analyses have been performed also with the concrete model of Eurocode and the results also show a risk of delayed collapse. Yet, the occurrence of collapse during or after the cooling phase is predicted more often with the explicit creep model than with the implicit model of Eurocode, which suggests that the way the transient creep effects are handled in the Eurocode model may lead to unsafe results when considering the cooling phase of a fire. [less ▲]

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