FIRE RESISTANCE OF CELLULAR WOODEN SLABS WITH … · David Couto 1, Elza Fonseca 2(*), Paulo Piloto 2, Jorge Meireles 3, Luísa Barreira 3, Débora Ferreira 2 1Master student in Industrial

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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PAPER REF: 5599

FIRE RESISTANCE OF CELLULAR WOODEN SLABS WITH

RECTANGULAR AND CIRCULAR PERFORATIONS

David Couto1, Elza Fonseca

2(*), Paulo Piloto

2, Jorge Meireles

3, Luísa Barreira

3, Débora Ferreira

2

1Master student in Industrial Engineering, Polytechnic Institute of Bragança, Portugal 2Department of Applied Mechanics, Polytechnic Institute of Bragança, Portugal 3Laboratory of Mechanical Technology, Polytechnic Institute of Bragança, Portugal (*)Email: [email protected]

ABSTRACT

This work presents a numerical and experimental approach in order to predict the thermal

behaviour and the performance of typical cellular wooden slabs with rectangular and circular

perforations when submitted to fire conditions. For this purpose a 3D numerical model was

validated with different experimental tests at real scale obtained in laboratory in four cellular

wooden slabs. This study was conducted in accordance with European standard (EN 1365-2,

1999) and using a fire resistance furnace which complies the requirements of European

standard (EN 1363-1, 1999). The numerical temperatures show good agreement with

experimental results. The numerical model can easily be adjusted for other constructive

solutions, to facilitate fire safety tests, in buildings with wooden slabs used in floors or in

ceilings applications.

Keywords: Cellular wooden slab, perforation, fire.

INTRODUCTION

Wood is a natural and organic cellular solid material. It is highly anisotropic due to the wood

cells shapes and the cell walls orientation. Commercial wood is obtained from two categories

of plants, as hardwoods (angiosperms) and softwoods (gymnosperms) (TEMTIS, 2008).

There are a wide range of wood applications that demonstrate the strength and stability of the

material. Wood offers the innovative and modern design, reliable structures, in a variety of

common outdoor and indoor building spaces. Considering the behavior of wood when

subjected to the developing fire, wood-based materials will burn and are rated as combustible.

But, wood material exhibits many desirable characteristics, whilst the exposed surfaces will

ignite when the heat flux becomes great enough, and initially burn fairly vigorously it soon

builds up a layer of insulating charcoal. As wood is a poor conductor of heat there is a very

low transmission of heat into remaining unburned material, and this has many benefits

(TEMTIS, 2008). Wood material when exposed to fire produces a surrounding charring depth

layer, with no mechanical resistance, resulting a reduced residual section. In perforated

cellular wooden slabs, the perforations size could influence the fire effect over the slab

thickness.

In wooden slab with perforations, the shape and the size of perforation can limit the use of

these constructive elements in terms of fire resistance. The wooden slab with perforations are

typical and very common engineering solutions, used in the ceiling plates to improve the

acoustic absorption of compartments. The constructive elements should be design in

accordance, to prevent and delay the fire damage effect, allowing that the slab could remain in

Symposium_29

Safety in Wood Materials

service during more time. The p

facilitating the penetration of flames and heat flow.

In this work, the main objecti

tests to predict the time-temperature

method with appropriate material properties and boundary conditions;

constructive solutions of wooden slabs with rectangular

the fire resistance in such way that contributes for a safe design in typical perforated wooden

slab. The subject of this work is a study in progress according others investigatio

by the authors of this work (Fonseca et al.).

METHODOLOGY AND MATERIALS

In this work, four wooden slab

(perforated side) will be submitted to a fire action. The

considers three different cellular zones

no perforation), Fig. 1.

Fig

Slab 1 and 2 present in the

(R1=250x20) mm in cell 3, and (R2=5

perforations with a diameter equal to (d1=20) mm in cell 3, and (d2=10) mm in cell 1

represented in Fig. 2.

Fig. 2 - Geometry of wooden slabs

Fig. 3 represents the construction of the four cellular wooden slabs, according the defined

dimensions.

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The perforations increase the slab surface exposed to the fire action,

facilitating the penetration of flames and heat flow.

In this work, the main objectives are: present a numerical model validated with experimental

temperature evolution during a fire scenario using a finite element

method with appropriate material properties and boundary conditions;

wooden slabs with rectangular and circular perforations; determine


The subject of this work is a study in progress according others investigatio

by the authors of this work (Fonseca et al.).

METHODOLOGY AND MATERIALS

wooden slabs were considered for analysis. The bottom surface of the slab

(perforated side) will be submitted to a fire action. The geometric model

considers three different cellular zones (two cells with different perforations and one cell with

Fig. 1 - Wooden slab with cellular zones.

in the exposed surface, two types of rectangul

(R1=250x20) mm in cell 3, and (R2=50x20) mm in cell 1. Slab 3 and slab 4 present circular

perforations with a diameter equal to (d1=20) mm in cell 3, and (d2=10) mm in cell 1

(a)

Geometry of wooden slabs: a) slab_1_2 rectangular perforations, b) slab_3_4 circular perforations

3 represents the construction of the four cellular wooden slabs, according the defined

surface exposed to the fire action,

ves are: present a numerical model validated with experimental

evolution during a fire scenario using a finite element

method with appropriate material properties and boundary conditions; use different

perforations; determine


The subject of this work is a study in progress according others investigations realized

were considered for analysis. The bottom surface of the slab

model of each slab

(two cells with different perforations and one cell with

rectangular perforations

in cell 1. Slab 3 and slab 4 present circular

perforations with a diameter equal to (d1=20) mm in cell 3, and (d2=10) mm in cell 1, as

(b)

: a) slab_1_2 rectangular perforations, b) slab_3_4 circular perforations.

3 represents the construction of the four cellular wooden slabs, according the defined


Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores

Slab_1_2.

Fig. 3 - Cellular

INSTRUMENTATION OF THE WOODEN SLABS

In the experimental tests, thermocouples were installed to measure the temperature in

different locations and located in the same positions in all slabs, as

Fig. 4

Fig. 5 shows the instrumentation of the wooden slabs, based on the criteria of (EN 1365

1999) with interest to measuring the temperature in different positions (ceiling plate, beams,

metal connectors and cellular zones). Three types of thermocouples wer

spot measurements Tij, copper discs protected with plasterboard for measuring the

temperature in the unexposed side, thermocouple wire welded to the connectors TCi and plate

thermocouples TPi for measuring the temperature within the

signal of the thermocouples was made with a data acquisition systems HBM (MGC Plus and

Spider 8).

Fig. 5



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Slab_3_4.

Cellular wooden slabs with rectangular and circular perforations.

INSTRUMENTATION OF THE WOODEN SLABS


different locations and located in the same positions in all slabs, as defined in Fig. 4.

4 - Thermocouples positions in wooden slabs.

Fig. 5 shows the instrumentation of the wooden slabs, based on the criteria of (EN 1365


metal connectors and cellular zones). Three types of thermocouples were used: single wire for



thermocouples TPi for measuring the temperature within the cellular zones. The acquisition


Fig. 5 - Thermocouples installation in wooden slabs.

wooden slabs with rectangular and circular perforations.


defined in Fig. 4.

Fig. 5 shows the instrumentation of the wooden slabs, based on the criteria of (EN 1365-2,


e used: single wire for



cellular zones. The acquisition


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The slabs were tested on fire resistance furnace at Polytec

This furnace is equipped with 4 burners running with natural gas, with a total power of

360kW, a working volume of 1m

Fig. 6 - Test slab in the fire resistance furnace, before, during and after the test.

NUMERICAL MODEL OF THE WOODEN SLABS

The finite element method was used for thermal transient analysis running with Ansys

program. Each wooden slab was exposed to fire, at the bo

rectangular and circular perforations,

cavities the temperature follows real heating curves obtained

convection coefficient is taken equal to 25W

the exposed surface. At the unexposed

dependent of slab) and the value of convection is equal to 4

taken constant and equal to 1,0 for exposed side and internal

Fig. 7 shows the mesh and the boundary conditions used in all numerical simulations.

Fig.

Each wooden slab was exposed to fire, at th

and circular perforations, respectively.

furnace heating curves (Tforno

The furnace was switch off between 450

temperatures of the internal cavities

test by mean of plate thermocouples (TPi

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The slabs were tested on fire resistance furnace at Polytechnic Institute of Bragança, Fig. 6.


360kW, a working volume of 1m3, prepared to work with any standard fire curve.

Test slab in the fire resistance furnace, before, during and after the test.

NUMERICAL MODEL OF THE WOODEN SLABS


Each wooden slab was exposed to fire, at the bottom surface, during 900s

rectangular and circular perforations, respectively. At the exposed surface

the temperature follows real heating curves obtained during experimental tests

convection coefficient is taken equal to 25W/m2K (EN1991-1-2, 2002) inside cavities and in

. At the unexposed surface the room temperature is applied

) and the value of convection is equal to 4 W/m2K. The surface emissivity is

equal to 1,0 for exposed side and internal cavities (EN1991


Fig. 7 - Numerical model for each wooden slab.

Each wooden slab was exposed to fire, at the bottom surface, during 900s for both rectangular

respectively. Temperature in the exposed surface

forno for Slab_1_2 and Slab_3_4), as represented in Fig. 7 and 8

between 450-500s and the front door opened near of 900s. The

temperatures of the internal cavities follow curves based on data obtained during

by mean of plate thermocouples (TPi, i=1,3 for Slab_1_2 and Slab_3_4

hnic Institute of Bragança, Fig. 6.


, prepared to work with any standard fire curve.

Test slab in the fire resistance furnace, before, during and after the test.


urface, during 900s for

surface and internal

during experimental tests. The

inside cavities and in

applied (16ºC or 20ºC,

K. The surface emissivity is

EN1991-1-2, 2002).


for both rectangular

Temperature in the exposed surface follows the real

for Slab_1_2 and Slab_3_4), as represented in Fig. 7 and 8.

500s and the front door opened near of 900s. The

follow curves based on data obtained during each slab

, i=1,3 for Slab_1_2 and Slab_3_4).



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The total area of each perforation type is compare for cell 1 and 3, as represented in table 1.

Cell 1 has a similar open space between slabs with rectangular or circular perforations. Cell 3

with circular perforations represents 50% open space in comparison to the rectangular

perforations. The temperature measurements of plate thermocouple TPi within the cellular

zones was affected in proportion to with the open space.

The non-linearity due to the thermal material properties dependence will be taken into account

in the numerical simulation. The density of the ceiling wood is equal to 450kg/m3 and the

density of the floor board and beams equals 480kg/m3. The thermal properties of wood vary

considerably with temperature and should be applied according Eurocode 5 (EN1995-1-2,

2003). This standard code provides the wood design values for density ratio, thermal

conductivity and specific heat.

Fig. 8 - Heating curves.

Table 1: Open space in each cell.

Slab_1_2

(Rectangular

perforations)

Slab_3_4

(Circular

perforations)

Cell 3,

mm2

30000 (R1) 15072 (d1)

(d1≅50%R1)

Cell 1,

mm2

18000 (R2)

(R2≅60%R1)

16328 (d2)

(d2≅91%R2)

Total,

mm2

48000 31400

(≅65%)

DISCUSSION OF RESULTS

During the real fire test exposure, the insulation criteria were verified, in both wooden slabs,

since the temperature rise on the unexposed surface did not exceed 180ºC on any of the disc

thermocouples or 140ºC in average with respect to the initial average temperature, defined

according to the European standard for fire resistance tests (EN 1365-2, 1999). The integrity

criteria was also verified during experimental tests using the cotton ignition test, where no

flame appearance occurred during the wooden slabs testing (EN 1365-2, 1999).

The figures below (Fig. 9 a) to e)) show the time-temperature evolution from experimental

real fire exposure (Ti_exp) and the corresponding numerical results (Ti_num) for each typical

slab (slab_1_2 and slab_3_4). As regards, the time-temperature numerical simulation results

are in agreement and with the same behaviour with the experimental results. Some

discrepancies between experimental and numerical results may be justified due to different

level of moisture in slabs and also to the relative position of thermocouples, slightly located

away with respect to the numerical nodal positions. In general, good agreement was found

between both results.

The nodal positions in the border of the rectangular and circular slots (T33 and T34, T35 and

T36, cell 3 and cell 1, respectively) have higher values of temperatures when compared with

all inside nodal positions which remain at lower temperatures (T24 and T25, cell 3 and cell 1,

respectively).

Comparing cell 3 and cell 1, the rectangular perforations enable a faster heating process with

higher temperatures compared to the circular perforations. Attending to the table 1, the

circular perforation type for cell 1 or 3 has almost the same open space in slab_3_4, which

permits to conclude a similar temperature evolution (T24 and T25) inside the cell. In slab 1_2

the temperature inside cell 3 is higher when compared with cell 1, which agrees with the

0

200

400

600

800

1000

1200

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]

Slab_3_4 Slab_3_4_TP1 Slab_3_4_TP2 Slab_3_4_TP3

Slab_1_2 Slab_1_2_TP1 Slab_1_2_TP2 Slab_1_2_TP3

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calculated open space between R1 and R2. In cell 3 and 1 the temperature increases until

switch off the furnace (450-500s) and decreases after this time.

The other results in cell 2, on beam and unexposed surface have similar behaviour between

the wooden slabs. During all test time the temperatures are below 100ºC, with an almost

constant behaviour and slight increase.

a)

b)

c)

d)

e)

Fig. 9 - Time-temperature history for each wooden slab (slab_1_2 and slab_3_4): a) on cell 3, b) on cell 2, c) on

cell 1, d) on beams, e) at unexposed surface.

0

100

200

300

400

500

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]

T33-num T34-num T24-num T33-exp T34-exp T24-exp

0

100

200

300

400

500

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]


0

50

100

150

200

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]

T17-num T37-num T17-exp T37-exp

0

50

100

150

200

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]


0

100

200

300

400

500

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]


0

100

200

300

400

500

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]


0

50

100

150

200

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]


0

50

100

150

200

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]


0

50

100

150

200

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]

T11-num T21-num T31-num T12-num T32-num T22-num

T11-exp T21-exp T31-exp T12-exp T32-exp T22-exp

0

50

100

150

200

0 100 200 300 400 500 600 700 800 900

T [ºC]

t [seg]

T11-num T21-num T31-num T12-num T32-num T22-num

T11-exp T21-exp T31-exp T12-exp T32-exp T22-exp



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CONCLUSIONS

Wood material when exposed to fire presents a thermal physical degradation. The evolution

of the temperature inside the cellular zones was characterized and the shape and size of

perforations could be compared with the unperforated cell. The size of the perforation is

responsible for different temperature evolution. The damage effect of fire is higher in the slab

with larger perforations, as expected. The perforated wooden slabs reach a temperature almost

twice the unperforated, justified by the temperature recorded within the cavities. To the same

time test, the temperature of the unperforated cellular zone did not exceed 100°C, while in the

cavities with openings this value is higher. This study allows to verify the evolution of the

temperature throughout a fire resistance test of a wooden slab. This type of analysis and tests

are important in safety and structural design because it determines how quickly the cross-

section size decreases to a critical level at high temperatures.

ACKNOWLEDGMENTS

The authors gratefully acknowledge to Jular company, who provided technical support for the

wood slabs construction.

REFERENCES

[1]-CEN, EN1365-2 - Fire resistance tests for loadbearing elements. Part 2: Floors and roofs,

Brussels, 1999.

[2]-CEN, EN 1363-1 - Fire resistance tests - Part 1, General requirements, Brussels, 1999.

[3]-CEN, EN1991-1-2 - Eurocode 1: Actions on Structures, Part 1-2: General actions -

Actions on Structures Exposed to Fire, Brussels, 2002.

[4]-CEN, EN1995-1-2 - Eurocode 5: Design of timber structures - Part 1-2: Structural fire

design, Brussels, 2003.

[5]-Fonseca E.M.M., Barreira L. Experimental and Numerical Method for Determining Wood

Char-Layer at High Temperatures due an Anaerobic Heating, International Journal of Safety

and Security Engineering, 2011, 1(1), pp. 65-76.

[6]-Fonseca E.M.M., Coelho D.C.S., Barreira L.M.S. Structural Safety in Wooden Beams

under Thermal and Mechanical Loading Conditions, International Journal of Safety and

Security Engineering, 2012, 2(3), pp. 242-255.

[7]-Fonseca E.M.M., Couto D., Piloto P.A.G. Fire safety in perforated wooden slabs: a

numerical approach, 5 Int. Conf. Safety and Security Eng., 2013, 134, pp.577-584.

[8]-Fonseca E.M.M., Barreira L.M.S., Meireles J.M., Piloto P.A.G. Numerical Model to

Assess the Fire Behaviour of Cellular Wood Slabs with Drillings, 4th International

Conference on Integrity, Reliability & Failure, S.Gomes et al (Eds.), Edições INEGI,

Proceedings IRF’2013, CD-ROM; Funchal, 2013.

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[9]-Leonardo da Vinci Pilot Project. Handbook 1 – Timber Structures, CZ/06/B/FPP/168007,

Education Materials for Designing and Testing of Timber Structures – TEMTIS, 2008.

[10]-Schaffer E.L. Structural Fire Design: Wood, Forest Products Laboratory, 1984, FPL

450:16p.

Documents

FIRE RESISTANCE OF CELLULAR WOODEN SLABS WITH … · David Couto 1, Elza Fonseca 2(*), Paulo Piloto 2, Jorge Meireles 3, Luísa Barreira 3, Débora Ferreira 2 1Master student in Industrial