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Contribution à l’étude du comportement mécanique de murs en béton armé dans un bâtiment soumis au feu
Nadia Otmani-Benmehidi
To cite this version:
Nadia Otmani-Benmehidi. Contribution à l’étude du comportement mécanique de murs en béton
armé dans un bâtiment soumis au feu. Rencontres Universitaires de Génie Civil, May 2015, Bayonne,
France. �hal-01167747�
reinforced concrete walls in a building subjected to fire.
Contribution à l’étude du comportement mécanique de murs en béton armé dans un bâtiment soumis au feu.
Nadia Otmani-Benmehidi
11Laboratoire LMGE, Université Badji Mokhtar- Annaba B.P.12, 23000 Algérie, benmehidi_nadia1@yahoo.fr
ABSTRACT. This study examines the mechanical behavior of reinforced concrete walls, belonging to a residential building under fire. The walls have different sections, regarding the sizing and the mechanical loading (compression forces and moments) exerted on the walls for investigation, we consider the study results carefully designed at cold. To realize this work, the software Safir is used, the latter obeys to laws “Eurocode”. It was observed that loading, heating, and sizing play a crucial role in the collapse of walls. Our results justify well the significance of firewalls, designed as reinforced concrete walls. The aim is to limit the spread of fire from a structure to another, since we get the fire resistances reaching more than 10 hours depending on the considered loading.
KEYWORDS: mechanical behavior, wall, resistance, fire, temperature.
33èmes Rencontres de l’AUGC, ISABTP/UPPA, Anglet, 27 au 29 mai 2015 2
1. Introduction
A structure must be designed and calculated so that it remains fit for use as planned. It must resist to different degrees of reliability during execution as well as that during service. Finally, the structure must have adequate durability regarding the maintenance costs. To meet the requirements outlined above, we must: choose the materials appropriately, define a design and appropriate dimensioning. For this purpose, it is imperative to provide rules specific to each country. Various researches were performed by experts in the fire field, to find out the behavior of the structures; as examples the separations and the bearer elements (concrete column, steel column…) of the building relating to a fire; which has developed fire rules. Regarding the fire behavior of bearing walls, among the authors working in this field, we mention Nadjai A [NAD 06], who performed a numerical study validated by an experimental investigation on masonry walls. Also, Cheer-Germ Go and Jun-Ren Tang [REN 12] presented an experimental investigation. In addition, reinforced concrete panels are associated to steel frames in a building due to their higher strength and stiffness, compared with unprotected steel columns in fire conditions [MOS 09].
Our work presents a contribution to the study of the behavior of reinforced concrete walls, exposed to fire, belonging to a residential building. These walls were previously studied as regards wind and earthquake [MEM 10]. The building is composed of a ground floor + 9 storey, located in Annaba (Algeria).
In a fire situation the temperature in members building, increases as a function of the combustibility relating material, also, according to the present oxygen. The fire leads the degradation of material characteristics, moreover, there are deformations and cracks which occur in structural elements, and so, the collapse of the building is unavoidable.
In order to prevent those phenomena and to minimize the spread of the disaster as quickly as possible, the firewalls in buildings are very cogent.
In this paper we will study four concrete walls; two walls with a section 20470 cm2 reinforced with bars Ø10 and two other walls having a section of 20350 cm2 (reinforced with Ø12). We consider a strip of 20 cm to reduce the work. The thermal loading is defined by the standard fire ISO 834[ENV 91]. Three walls are exposed to fire on one side; the fourth wall is exposed on two of its sides. The mechanical loadings (the compressive loads and the moments) exerted on the considered walls are from a study previously conducted at cold.
The thermal analysis gives the temperatures at every moment and at every point of the walls. These temperatures are used in the mechanical analysis. For the thermal analysis and the mechanical analysis, the tool Safir [FRA 05] is employed. This software was developed in Belgium at the University of Liege, performed for the thermal and mechanical study of structures subjected to fire, taking into account the material and geometrical nonlinearity and large displacements.
2. Modeling of walls
Table 1. Geometrical characteristics and loading of considered walls
The mechanical data file is dependent on thermal analysis for the use of the elements temperatures in function of time. The file in question contains the dimensions of the wall (height and width), the number of nodes is equal to 21.
Walls
H (m)
L (cm)
e (cm)
Ø (mm)
thermal
loading M-ult (t.m) N-ult (t)
Mu 20 2,86 470 20 10 one side 2,26 41,2
MuF 20 2,86 470 20 10 two sides 2,26 41,2
Mu (12)20 2,86 350 20 12 one side 0,14 26,32
MuCH (12)20 2,86 350 20 12 one side 2,26 41,2
The number of the beam element according to the discretization is taken equal to 10, each element contains 3 nodes. Mechanical loading is represented by a normal force and moment for each wall; the calculation is performed for a strip of 20 cm.
Concerning the boundary conditions, each wall strip is doubly supported on the lower end and simply supported on the upper end.
Figure1. Discretization and boundary conditions of walls
Figure2. Example of using reinforced concrete walls to limit the effect of fire [MOS 09].
3. Analysis tool
SAFIR can be used to perform one, two and three dimensional analyses under ambient and elevated temperature conditions. Truss, beam, shell and solid elements are available in conjunction with a range of material models incorporating stress-strain behavior at elevated temperatures including that for different materials based on Eurocodes [EC2 05] [EC3 05] Two dimensional solid elements were used for the thermal analyses of the wall cross-sections. The temperatures determined in the thermal analyses are used in the structural analyses to determine the behavior at elevated temperatures. Vassant et al. [VAS 04] carried out fire simulations on several industrial buildings using ABAQUS, ANSYS and SAFIR and concluded that the three computer programs generally gave close results.
3.1 Thermal analysis
The temperature fields within a given network are established by an integration method for time steps [FRA 05] [SCH 09]. It is assumed that conduction is the main heat transfer mechanism. Convection and radiation act essentially as heat transfer from the fire environment to the structures. Heat transfer by conduction in solid materials is described by the Fourier Eq. (1) that is solved in SAFIR according to the standard finite element procedure.
[ 1 ]
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where k thermal conductivity, W/mK;
T temperature, K;
x, y, z coordinates, m ;
Q internally generated heat, W/m³;
C specific heat, J/kgK;
ρ specific mass density, kg/m³; t time, s.
Figure3. Fire curve in accordance with ISO 834 [ENV 91], used in the analysis 3.2 Temperature in the walls
Figure 4. Temperatures in the section of wall « MuF 20 » and wall « Mu(12)20 » at failed time
In this numerical study, the thermal analysis is necessary to know the temperatures at each point of the walls, these temperatures are mandatory data for the mechanical calculation, for this purpose, we use the code "SAFIR".
We cite two cases of wall, exposed to fire (MUF 20 and Mu (12) 20). In the case of the wall "MuF 20" which is exposed to fire in two faces (Figure 4), at failed time t = 8940sec or 149min (2.43 h), we get a temperature between 900.78 and 1048.50 ° C at the surfaces in contact with the fire. Away from both faces exposed to fire, temperature decreases to 457.60 ° C. In addition, for the wall Mu (12) 20, at the failed time (25680s) the observed temperature of the bottom face (exposed to fire) varies between 970, 08 and 1222,20 ° C. Concerning, the upper face (not exposed to fire), the temperature is 213,70 °C. Of course after a long period, the temperature rises considerably.
3.3 The coating effect
The figure 4 shows that the temperature rises considerably in the wall Mu20, by comparing with Mu(12)20, after the same period and the same type of heating. Given that, the coating of the first is lower than the second one. This is confirmed for the two considered locations: 22 and 58. Furthermore, the location 22 has the biggest temperature (Tmax=1200°C) because it is near the applied standard fire. For example at time 149 min, the
location 58 has a temperature of 290°C, when we consider a coating equal to 3 cm, but, the temperature of this location become 380°C for a coating of 2.4 cm. moreover, at the same time (t=149min), the location 22 has a temperature of 610°C, for the coating 3cm and 650°C when the coating is 2.4 cm. We conclude that the coating has a significant effect on the development of the temperature. It is due to the reinforcing steel which has an elevated thermal conductivity.
Figure 4. Temperatures curves of wall « Mu 20 » and wall « Mu(12)20 » as function of time, considering the coating variation
4. Mechanical analysis
Table 2. Fire resistance of considered walls
4.1 Horizontal displacements
A noticeable advantage of reinforced concrete, it has great resistance to fire, it loses its structural integrity much more slowly than the others materials when subjected to high temperature. For these reasons, the considered reinforced concrete walls have in general, a good fire behavior. After a period of heating, the displacements occur in the reinforced concrete walls (see figure5), and keep increasing due to the reduced bearing capacity, induced by the heated internal materials until the collapse.
Walls coating (cm)
Height (m)
Ø (mm)
span (m)
Depth (m)
M-ult (t.m)
N-ult (t)
Rf (min)
Mu 20 2,4 2,86 Ø 10 4,7 0,2 2,26 41,2 179,78
MuCH (12)20 3 2,86 Ø 12 3,5 0,2 2,26 41,2 278,93
Mu (12)20 3 2,86 Ø 12 3,5 0,2 0,14 26,32 427,75
MuF 20 2,4 2,86 Ø 10 4,7 0,2 2,26 41,2 148,76
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Figure 5. Horizontal displacements produced in reinforced concrete walls after a period of heating
Among the studied walls, Mu (12) 20 has the best mechanical behavior and the best fire resistance (see table2). Indeed, his good rigidity allows him to behave in quite ductile manner. Moreover, the maximal horizontal displacement (∆h= 10 cm) is in the wall“MuCH (12)20” which is identical to the first but its mechanical loading is greater.
Concerning the two others similar walls “Mu 20” and “MuF 20”, the latter has a bad mechanical behavior because it has an important thermal loading (subjected to fire on two sides, see table 1). Furthermore, the Figure 5 shows that the displacements in “Mu 20” are the highest until 179.78 min. Moreover, they are the lowest in “MuF 20”
4.2 Comparative study
Figure6. Fire resistance of walls « Mu 20 » and « MuF 20 » depending on the load
In Figure 6 the curves show the fire resistances of two identical walls subjected to fire; the first on two sides (MuF 20)) and the second (Mu 20) on one side, considering four rates of mechanical loading (100%, 70%, 50% and 30%).We note that Mu 20 behaves better than MuF 20, as it is less affected by the fire. We also find that mechanical loading has a considerable effect on the walls in fire situation. Due to the effect of thermal expansion at the beginning of heating phase, and then, as the temperature increases, the contraction due to the material properties degrading becomes dominant, so that considered walls become incapable of supporting the loads.
The curves in Figure 7; show the fire resistances of two walls exposed to fire on one side, and submitted to different rates of mechanical loading. These two walls have not the same dimensions[5], but are subject to the same mechanical loading and to the same thermal loading (ISO834), as it was mentioned previously (see table 1). Their sections are respectively, for Mu 20: 20470cm2 and for MuCH (12) 20: 20350 cm2. We
note that the fire resistances of these two types of walls are considerably higher than preceding walls (Figure 6). We observe also that MuCH (12) behaves better than Mu 20. Because its section is lower than Mu 20 section, furthermore the reinforcement of the first is Ø12, but the second is reinforced with Ø10, so that, MuCH (12) has a better strength [OTM 13].
The results show that the great span wall leads a reduction in the fire resistance. In addition, this analysis justifies well the significance of firewalls, with reinforced concrete walls, since, the obtained resistances are considerable (620min =10 hours).
Figure6. Fire resistance of walls « Mu 20» and «MuCH (12)20 » depending on the load
5. Conclusions
The temperature rises considerably in the wall Mu20, by comparing with Mu(12)20, after the same period and the same type of heating. Because the coating of the first is lower than the coating of the second one. We conclude that the coating has a significant effect on the development of the temperature. It is due to the reinforcing steel which has an elevated thermal conductivity. When it is small; this is leading to diminution of the fire resistance.
A remarkable advantage of reinforced concrete, it has great resistance to fire, it loses its structural integrity much more slowly than the others materials when subjected to high temperature. For these reasons, the considered reinforced concrete walls have in general, a good fire behavior;however, the sizing has a significant effect on fire resistance, as well, Mu 20 with a section of 20x470 cm 2 is less resistant than Mu (12) 20 having a section : 20x350 cm2.
The fire resistance of the wall with a little span is considerably higher than the fire resistance wall with a great span. We conclude that a significant span, more than 3.5 m is unfavorable for the firewall, since it leads to a reduction in fire resistance.
In other hand, the reinforcing steel of Mu20 is ϕ10 but for Mu(12)20 we use ϕ12. This is another reason which justifies the good fire behavior of wall “Mu (12) 20”. In fact, the amount of reinforcing steel has an important effect on fire resistance. Among the studied walls, Mu (12) 20 has the best mechanical behavior and the best fire resistance, Indeed his good rigidity allows him to behave in quite ductile manner.
Mechanical loading has a considerable effect on the walls under fire, this is close with experimental results of numerous researches of studied structures, the fire acts indirectly on the structures by hurting the materials. The fire destroys the materials mechanical properties, so that, the walls become unable to bear the mechanical load.
After a period of heating, the displacements occur in the reinforced concrete walls, and keep increasing due to the reduced bearing capacity. Concerning the two similar walls “Mu 20” and “MuF 20”, the latter has a bad mechanical behavior because it has an important thermal loading; as a result its displacements are the lowest.
6. Bibliographie
[EC2 05]Eurocode 2., Calcul des structures en béton et Document d'Application Nationale Partie 1-2 : Règles générales, Calcul du comportement au feu, (2005).
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[EC3 05]EUROCODE 3 : Calcul des structures en acier –Partie 1-2 : Règles générales – Calcul du comportement au feu, (2005).
[ENV 91] ENV 1991 1-2, Eurocode 1, Actions sur les structures – Partie 1-2 : Actions générales - Actions sur les structures exposées au feu (2002),
[FRA 05] Franssen J-M. (2005), SAFIR, A Thermal/Structural Program Modeling Structures under Fire, Engineering Journal, A.I.S.C., Vol 42, No. 3, 143-158.
[MEM 10]Mémoire de fin d’étude, ingéniorat dirigé par Dr Hacène chaouch A, Etude d’un bâtiment à usage d’habitation, RDC + 9 étages, (2010).
[MUR 97] murs coupe-feu en béton, ( DTU feu béton, Eurocode 2, DAN partie 1-2, 1997)iut-tice.ujf-grenoble.fr/tice- espaces/GC/materiaux/mtx3/.../8.5.pdf
[MOS 09] Moss P.J., Dhakal R.P., Bongb M.W., Buchanan A.H., “Design of steel portal frame buildings for fire safety”
Journal of Constructional Steel Research 65 1216_1224, (2009).
[NAD 06]Nadjai A, O’Gara M, Faris. A and Jurgen R,.Compartment Masonry Walls in Fire Situations, Belfast, (2006).
[OTM 13] Otmani-Benmehidi N., Arar M., Chine I., Modeling the Behavior of Reinforced Concrete Walls under Fire, Considering the Impact of the Span on Firewalls, The Proceeding of International Conference on Soft Computing and Software Engineering 2013 [SCSE’13], San Francisco State University, CA, U.S.A.(2013).
[REN 12] Cheer-Germ Go, Jun-Ren Tang, Jen-Hao Chi , Cheng-Tung Chen, Yue-Lin Huang, “Fire-resistance property of reinforced lightweight aggregate concrete wall” Construction and Building Materials 30, 725–733, (2012).
[SCH 09] Schneider U., Schneider M., Franssen J.M., “ Numerical evaluation of load induced thermal strain in restraint structures compared with an experimental study on reinforced concrete columns, Fire And Material, (2009).
[VAS 04]Vassart O, Cajot LG, O'Connor M, Shenkai Y, Fraud C, Zhao, et al. 3D simulation of Industrial Hall in case of fire. In: Benchmark between ABAQUS, ANSYS and SAFIR. Proceedings of the 10th international conference, vol. 2.
London, UK: Interscience Communications Limited; (2004).
Table 3. Nomenclature
Mu 20: Reinforced concrete wall with a thickness equal to 20cm and a span equal to 470cm (reinforcement :Ø10), this wall was exposed to fire on one side.
MuF 20: Reinforced concrete wall with a thickness equal to 20cm and a span equal to 470cm (reinforcement :Ø10), this wall was exposed to fire on two sides.
Mu (12) 20: Reinforced concrete wall with a thickness equal to 20cm and a span equal to 350cm (reinforcement :Ø12), this wall was exposed to fire on one side.
MuCH (12) 20: Reinforced concrete wall with a thickness equal to 20cm and a span equal to 350cm (reinforcement: Ø12), this wall was exposed to fire on one side. In this case, we use the ultimate mechanical loading of wall Mu 20.
Firewall: Reinforced concrete wall intended to limit the spread of fire from a structure to another nearby.
N-ult: Ultimate compressive load.
M-ult: Ultimate moment.
Rf: Fire resistance.
H: Height.
h: Hour.
L: Span of wall.
e: Thickness of wall.
N: Compressive load.
Ø: Diameter of used steel.
