Thomas Gernay

Collapse of concrete columns during and after the cooling phase of a fire

A study has been performed on the collapse of reinforced concrete columns subjected to natural fire conditions during and after the cooling phase of the fire. The aim is, first, to highlight the phenomenon of collapse of concrete columns during and after the cooling phase of a fire and then, to analyze the influence of some determinant parameters. The main mechanisms that lead to this type of failure are found to be the delayed increase of the temperature in the central zones of the element and the additional loss of concrete strength during the cooling phase of the fire. A parametric analysis considering different fires and geometric properties of the column shows that critical conditions with respect to delayed failure arise for short-duration fires and for columns with low slenderness or massive sections. Language: en

Modeling structures in fire with SAFIR®: Theoretical background and capabilities

Purpose This paper aims to describe the theoretical background and main hypotheses at the basis of SAFIR®, a nonlinear finite element software for modeling structures in fire. The paper also explains how to use the software at its full extent. The discussed numerical modeling principles can be applied with other similar software. Design/methodology/approach Following a general overview of the organization of the software, the thermal analysis part is explained, with the basic equations and the different possibilities to apply thermal boundary conditions (compartment fire, localized fire, etc.). Next, the mechanical analysis part is detailed, including the time integration procedures and the different types of finite elements: beam, truss, shell, spring and solid. Finally, the material laws are described. The software capabilities and limitations are discussed throughout the paper. Findings By accommodating multiple types of finite elements and materials, by allowing the user to consider virtually any section type and to input the fire attack in multiple forms, the software SAFIR® is a comprehensive tool for investigating the behavior of structures in the fire situation. Meanwhile, being developed exclusively for its well-defined field of application, it remains relatively easy to use. Originality value The paper will improve the knowledge of readers (researchers, designers and authorities) about numerical modeling used in structural fire engineering in general and the capabilities of a particular software largely used in the fire engineering community.

Structural behaviour of concrete columns under natural fires

Purpose – The paper aims to give an insight into the behaviour 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 behaviour in cooling.Findings – Failure of concrete columns dur...

Tools for Measuring a City’s Resilience in a Fire Following Earthquake Scenario

The paper provides a framework to evaluate the response of buildings in a community subject to fire following earthquake. First, a model is developed to determine the probability of ignition in buildings of a community due to an earthquake. Second, fragility functions are developed for buildings subject to fire, to quantify the structural damage and the expected losses. The ignition model, combined with the fragility functions, can be implemented in a GIS based risk management platform to evaluate economical losses in a region from fire following an earthquake.

Hybrid Fire Testing: Discussion on stability and implementation of a new method in a virtual environment

Purpose The purpose of this paper is to propose a method for hybrid fire testing (HFT) which is unconditionally stable, ensures equilibrium and compatibility at the interface and captures the global behavior of the analyzed structure. HFT is a technique that allows assessing experimentally the fire performance of a structural element under real boundary conditions that capture the effect of the surrounding structure. Design/methodology/approach The paper starts with the analysis of the method used in the few previous HFT. Based on the analytical study of a simple one degree-of-freedom elastic system, it is shown that this previous method is fundamentally unstable in certain configurations that cannot be easily predicted in advance. Therefore, a new method is introduced to overcome the stability problem. The method is applied in a virtual hybrid test on a 2D reinforced concrete beam part of a moment-resisting frame. Findings It is shown through analytical developments and applicative examples that the stability of the method used in previous HFT depends on the stiffness ratio between the two substructures. The method is unstable when implemented in force control on a physical substructure that is less stiff than the surrounding structure. Conversely, the method is unstable when implemented in displacement control on a physical substructure stiffer than the remainder. In multi-degrees-of-freedom tests where the temperature will affect the stiffness of the elements, it is generally not possible to ensure continuous stability throughout the test using this former method. Therefore, a new method is proposed where the stability is not dependent on the stiffness ratio between the two substructures. Application of the new method in a virtual HFT proved to be stable, to ensure compatibility and equilibrium at the interface and to reproduce accurately the global structural behavior. Originality/value The paper provides a method to perform hybrid fire tests which overcomes the stability problem lying in the former method. The efficiency of the new method is demonstrated in a virtual HFT with three degrees-of-freedom at the interface, the next step being its implementation in a real (laboratory) hybrid test.

Sensitivity of elevated temperature load carrying capacity of thin-walled steel members to local imperfections

The local buckling capacity of fire exposed thin-walled steel cross sections is affected by the reduction in strength and stiffness due to elevated temperatures and by the amplitude of the initial local imperfections. Several researchers have proposed design methods to calculate the capacity of these steel members at elevated temperatures, but they used different methodologies and different amplitude of local imperfections in the extensive numerical analyses that are typically at the base of these methods. This variability in hypotheses happens because there is no clear provision defining the local imperfection amplitude for fire design in the codes (European or US). EN 1993-1-5 proposes amplitude values of local imperfections for ambient temperature design, while EN 1090-2 defines a differentmaximum allowed size of fabrication tolerance during production. Meanwhile, other sizes of local imperfections have also been proposed in the literature, with values different than those from EN 1993-1-5 and EN 1090-2. This paper reviews the existing code provisions and compares the existing design models and their assumptions for thin-walled steel cross sections. Finite element analyses are then conducted on isolated steel plates at elevated temperatures to investigate the effect of local imperfections. Finally, specific amplitude of local imperfections is proposed for fire design of thinwalled steel members. the order of 2 between the extremes. Table 1 shows the governing parameters (i.e. assumptions) used in the models based on numerical analyses. The parameters a, b and t are the length, width and thickness of the plate, respectively. It can be seen that different authors assumed different values for these parameters, which naturally lead to different results. Furthermore, other researchers are using amplitude of imperfections even different from those of Table 1 (e.g. Saif et al, 2013). The results are affected by the amplitude of the initial local imperfections, by the geometry of the local imperfections (number of half-waves) and by the dimensions of the plate (ratio a/b) (Gerald et al, 1957). The effect of the number of half-waves and a/b ratio is discussed in another study (Maraveas et al, 2017). This paper presents an investigation on the influence of the amplitude of the initial imperfections. Considering code and standard provisions, the literature and new numerical results of simulations on isolated plates, recommendations are made for the amplitude of imperfection to be used for the design of slender plates at elevated temperature. Figure 1. Comparison of proposed design and code methods for capacity of slender plates at 500 o C, (a) for stiffened plates (web) and (b) for unstiffened plates (flange). Table 1. Governing analysis parameters used in the numerical simulations by different authors. Reference a/b Number ofhalf-waves Amplitude of imperfections Franssen et al, 2014 flange: 2 web: 1 1 flange: b/50 = 0.020 b web: b/200 = 0.005 b Couto et al, 2014 4 flange: 1, web: 4 flange: 80% b/50 = 0.016 b web: 80% b/100 = 0.008 b Quiel et al, 2010 5 flange: 3, web: 5 flange: 0.156 t web: 0.100 t 2 CODE AND STANDARD PROVISIONS

Fire Performance of Columns Made of Normal and High Strength Concrete: A Comparative Analysis

The use of high strength concrete (HSC) in multi-story buildings has become increasingly popular. Selection of HSC over normal strength concrete (NSC) allows for reducing the dimensions of the columns sections. However, this reduction has consequences on the structural performance in case of fire, as smaller cross sections lead to faster temperature increase in the section core. Besides, HSC experiences higher rates of strength loss with temperature and a higher susceptibility to spalling than NSC. The fire performance of a column can thus be affected by selecting HSC over NSC. This research performs a comparison of the fire performance of HSC and NSC columns, based on numerical simulations by finite element method. The thermal and structural analyses of the columns are conducted with the software SAFIR®. The variation of concrete strength with temperature for the different concrete classes is adopted from Eurocode. Different configurations are compared, including columns with the same load bearing capacity and columns with the same cross section. The relative loss of load bearing capacity during the fire is found to be more pronounced for HSC columns than for NSC columns. The impact on fire resistance rating is discussed. These results suggest that consideration of fire loading limits the opportunities for use of HSC, especially when the objective is to reduce the dimensions of the columns sections.

Experimental tests and numerical modelling on slender steel columns at high temperatures

Purpose The purpose of this paper is to gain from experimental tests an insight into the failure mode of slender steel columns subjected to fire. The tests will also be used to validate a numerical model. Design/methodology/approach A series of experimental fire tests were conducted on eight full-scale steel columns made of slender I-shaped Class 4 sections. Six columns were made of welded sections (some prismatic and some tapered members), and two columns were made of hot rolled sections. The nominal length of the columns was 2.7 meters with the whole length being heated. The load was applied at ambient temperature after which the temperature was increased under constant load. The load was applied concentrically on some tests and with an eccentricity in other tests. Heating was applied by electrical resistances enclosed in ceramic pads. Numerical simulations were performed with the software SAFIR® using shell elements. Findings The tests have allowed determining the appropriate method of application of the electrical heating system for obtaining a uniform temperature distribution in the members. Failure of the columns during the tests occurred by combination of local and global buckling. The numerical model reproduced correctly the failure modes as well as the critical temperatures. Originality/value The numerical model that has been validated has been used in subsequent parametric analyses performed to derive design equations to be used in practice. This series of test results can be used by the scientific community to validate their own numerical or analytical models for the fire resistance of slender steel columns.

Comparative fire analysis of steel-concrete composite buildings designed following performance-based and U.S. prescriptive approaches

Performance-based structural fire design provides a rational methodology for designing modern buildings with cost-effective solutions. However, in the United States, fire design still largely relies on design at the component level using prescriptive approaches. With performance-based approaches, there is an opportunity to benefit from increased flexibility and reduced cost in the design, but these advantages need to be explicitly described and disseminated to promote this shift in paradigm. In this paper, a comparative analysis is conducted on multi-story steel-concrete buildings designed following performance-based and U.S. prescriptive approaches. The steel-concrete composite structure allows taking advantage of tensile membrane action in the slab during fire, and therefore removing the fire protection on secondary beam elements. The nonlinear finite element software SAFIR is used to model the behavior of the buildings under the standard ASTM fire and a natural fire determined using the two-zone fire model CFAST. The numerical simulations show that performance-based design can be used to achieve the required level of safety currently enforced in the U.S. prescriptive guidelines, while providing an opportunity for cost reduction in fire protection material. the thickness of spray fire protection for the prescribed fire rating to be applied on the elements. Meanwhile, previous research shows that, when system-level performance is considered, fire protection on secondary beam elements is not necessary due to the development of a membrane action in the concrete slab during fire (Bailey and Moore, 2000; Gillie et al., 2002; Vassart et al., 2012). In the second part of this research, performance-based approach based on numerical simulations is adopted to design the fire protection in the building while taking into account the interaction of structural members at the system level. Different alternatives are studied considering different peripheral beam sections and amount of steel mesh in the slab. Two types of fire exposure are considered, namely the standard ASTM E119 fire and a natural fire. The prescriptive design philosophy is based on the use of standard fire curves such as the ASTM E119 fire. Consideration of the same fire exposure for the structure designed with the performance-based approach allows comparing the response of the two designs for a same thermal load, hence focusing on the structural behavior. The safety level can be discussed in terms of amount of time that the structure withstands the applied loads under this standard fire. Yet, performance-based approach entails the possibility to consider a natural fire exposure evaluated by considering the real characteristics of the compartment. Natural fires are by nature very different from standard fire and therefore can lead to distinct structural response. So, this study also investigates the response of the different alternative designs under a natural fire which is determined using a two-zone fire model implemented in the computer program CFAST. 2 PROTOTYPE BUILDING 2.1 Multi-story building The prototype building studied in this paper is a nine-story steel frame office building. The building is 45.72 m by 45.72 m in plan, consisting of five bays of 9.14 m in the two directions. The structure is composed of four moment resisting frames on the perimeter, as the lateral load resisting system, and four interior gravity frames, see Figure 1. The columns of the interior frames are continuous on the nine-story but the beams have pinned connections (statically determinate beams). The total height of the building is 37.18 m, divided between a 5.49 m high first floor and eight other floors each with a height of 3.96 m.

A method for hybrid fire testing: Development, implementation and numerical application

Hybrid Fire Testing (HFT) is a technique that allows assessing experimentally the fire performance of a structural element under real boundary conditions that capture the effect of the surrounding structure. To enable HFT, there is a need for a method that is unconditionally stable, ensures equilibrium and compatibility at the interface and captures the global behaviour of the analysed structure. A few attempts at conducting HFT have been described in the literature, but it can be shown, based on the analytical study of a simple one degree-of-freedom elastic system, that the considered method was fundamentally unstable in certain configurations which depend on the relative stiffness between the two substructures, but which cannot be easily predicted in advance. In this paper, a new method is introduced to overcome the stability problem and it is shown through analytical developments and applicative examples that the stability of the new method does not depend on the stiffness ratio between the two substructures. The new method is applied in a virtual hybrid test on a 2D reinforced concrete beam part of a moment resisting frame, showing that stability, equilibrium and compatibility are ensured on the considered multiple degree-of-freedom system. Besides, the virtual HFT succeeds in reproducing the global behaviour of the analysed structure. The method development and implementation in a virtual (numerical) setting is described, the next step being its implementation in a real (laboratory) hybrid test. (NS) the response of which is analyzed aside during the test in a finite element model or by a predetermined matrix. The NS refers to the remainder of the structure and the response of this structure will have an influence on the boundary conditions of the tested element. Few attempts at HFT have been done in the past. The first documented attempt to perform HFT was made by Korzen et al. (2002) at BAM (Germany) on a column specimen extracted from a building. Only the axial degree-of-freedom is controlled during the test and the NS is defined by a constant matrix. Robert et al. (2010) at CERIB (France) presents a HFT performed on a concrete slab where the behavior of the NS is modeled by a predetermined matrix. Three degrees-of-freedom are controlled, namely the axial elongation and the rotations on the two supports. Mostafaei (2013a, 2013b) at NRC (Canada) performed a HFT on a concrete column while the remainder structure, i.e. a moment resisting frame with some parts frame exposed to fire, was modeled in the nonlinear finite element software SAFIR (Franssen, 2005). More recently, researchers worked on the development of the HFT methodology and validation has been done in the numerical environment (Tondini et al., 2016) or by experiments performed on small-scale specimens, i.e. Whyte et al. (2016) and Schulthess et al. (2016). In most cases, the methodology applied in the former HFT has been tailored for the analyzed case studies and for the capability of the fire facility where the test has been performed. The research objective is here to develop a methodology which is applicable independently on the type of the case study and the capability of the furnace. In this paper, the methodology considered in the former HFT performed on real structural elements, i.e. Korzen, Robert and Mostafaei, will be analyzed in details. Moreover, for a better understanding, the methodology considered in these three hybrid fire tests will be referred as the “first generation method”. It will be shown that the considered methodology is applicable only for specific cases and a new solution will be proposed in order to perform HFT in a general context, independently on the analyzed case study. The capability of the proposed method will be analyzed on a case study consisting of a concrete beam extracted from a moment resisting frame. 2 INTEREST OF HYBRID FIRE TESTING In order to highlight the potential of HFT, the behavior of a concrete beam extracted from a moment resisting frame will be analyzed numerically in different configurations (details about the analyzed moment resisting frame and the fire load can be found in Section 5). The configurations considered are: (1) a test of the whole structure, only possible in a virtual environment (2) a test on the beam simply supported with no bending moment introduced at the supports (3) a test on the beam simply supported with constant bending moments introduced at the supports (through cantilever extensions, for example), (4) test on the beam with rotations fixed on the supports and free thermal expansion, (5) test on the beam under fixed rotations and fixed thermal expansions and (6) a hybrid fire test. For all these cases, the evolution of the mid-span displacements as a function of time is presented in Figure 1. Figure 1. Mid-span displacement of the beam in different testing configurations -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0 60 120 180 240 M id -s p a n d is p la c e m e n t (m ) Time (min)