Engineering Structures 31 (2009) 150–157

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Engineering Structures

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Fire design of timber slabs made of hollow core elementsAndrea Frangi ∗, Markus Knobloch, Mario FontanaETH Zurich, Institute of Structural Engineering, 8093 Zurich, Switzerland

a r t i c l e i n f o

Article history:Received 15 January 2008Received in revised form26 June 2008Accepted 5 August 2008Available online 10 September 2008

Keywords:TimberCharring modelCharring rateTimber slabs made of hollow core elementsFire testsISO-fire exposure

a b s t r a c t

The fire design of timber structures usually take into account both the loss in cross-section due to charringof wood and the temperature-dependent reduction of strength and stiffness of the uncharred residualcross-section. The fire behaviour of timber assemblies made of hollow core elements is characterisedby different charring phases. After the fire exposed timber layer is completely charred and the char-layer has fallen off, the thin vertical timber members are exposed to fire on 3 sides, leading to veryirregular residual cross-sections with charring depths much greater than for heavy timber structures.Based on an extensive experimental and parametric study, a simplified calculation model for the fireresistance of timber slabs made of hollow core elements has been developed. The calculation modelbases on the reduced cross-section method and takes into account two different charring phases. Thepaper first describes and discusses the simplified calculation model, and then compares the test resultsto the calculation model.

© 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Prefabricated timber assemblies made of hollow core elementsare often used for slabs in residential and commercial buildings.Besides the advantage of element prefabrication and a highstructural performance, the thermal and acoustic insulation ofthe timber assemblies can be significantly improved by insulatingbatts in the cavities and sound absorbers placed behind theperforated acoustic layer.Timber is a combustible material and thus differs from most

other common structural building materials. When sufficient heatis applied to wood, a process of thermal degradation (pyrolysis)takes place producing combustible gases, accompanied by a loss inmass. A charred layer is then formed on the fire-exposed surfacesand the char layer grows in thickness as the fire progresses,reducing the cross-sectional dimensions of the timber member.The char layer is a good isolator and protects the remainingunburned residual cross-section against heat. Because of the smallsize of the timber members of the hollow core elements, thefire action can lead to very irregular residual cross-sections withcharring depths much greater than for heavy timber structures.For the calculation of the fire resistance it is therefore of primaryimportance to know the development of the charring depth duringthe fire exposure.A comprehensive research project on the fire behaviour of

timber slabs made of hollow core elements has been recently

∗ Corresponding author. Tel.: +41 44 633 2640; fax: +41 44 633 1093.E-mail address: frangi@ibk.baug.ethz.ch (A. Frangi).

0141-0296/$ – see front matter© 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2008.08.002

Fig. 1. Typical timber slab made of hollow core elements.

performed at ETH Zurich [1,2]. The research project was aimed atenlarging the experimental background of timber slabs in fire andpermitting the development of a simplified calculation model forthe fire resistance of timber slabs made of hollow core elements.In addition to a large number of small-scale fire tests, the firebehaviour of the timber slabs was experimentally analysed with3 large-scale fire tests on loaded timber slabs (see Table 1). All firetests were based on ISO-fire exposure and performed at the SwissFederal Laboratories for Materials Testing and Research (EMPA)in Dubendorf. The test specimens were manufactured by theSwiss firm Lignatur,Waldstatt. Lignatur elements consist of hollowcore elements made of spruce (picea abies) with a mean densityof 450 kg/m3. The strength properties of the timber elementscorrespond to the strength class C24 according to EN 338 [3]. Fig. 1shows a typical cross-section of Lignatur timber assemblies madeof hollow core elements. The vertical members have a thicknessof 33 mm.The paper describes the simplified calculationmodel for the fire

resistance of timber slabsmade of hollow core elements. Particularattention is given to the analysis of different strategies used in

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Table 1Overview of fire tests performed with timber assemblies made of hollow core assemblies

Objective Test specimen Furnace size (m) Fire exposure (min)

Fire behaviour of joints EI60 0.8× 1.0 70Influence of cavity insulation EI60-31 0.8× 1.0 60

EI60-40 0.8× 1.0 60Influence of acoustic perforations EI30-AL 0.8× 1.0 30

EI60-AL 0.8× 1.0 60EI60-Silence 0.8× 1.0 60

Fire resistance of loaded timber slabs REI30 3.0× 4.85 40REI60 3.0× 4.85 70REI90 3.0× 4.85 105

Fig. 2. Charring depth dchar,0 for one-dimensional charring and notional charringdepth dchar,n [5].

order to improve the fire behaviour of the timber slabs in fire.The paper first describes and discusses the simplified calculationmodel, and then compares the test results to the calculationmodel.

2. Charring of timber

In order to calculate the resistance of structural timbermembers exposed to fire, the loss in cross-section due to charringas well as the reduction in strength and stiffness near the charredlayer due to elevated temperature have to be considered. Fortimber surfaces unprotected throughout the time of fire exposure,the residual cross-section can be calculated by assuming a charringrate constant with time [4]. As a basic value, the one-dimensionalcharring rate β0 is usually taken as the value observed for one-dimensional heat transfer under ISO-fire exposure in a semi-infinite timber slab. EN 1995-1-2 [5] gives a value of β0 =0.65 mm/min for softwood confirmed by several experimentalstudies [6–8]. In order to take into account the effects of cornerrounding and fissures and to simplify the calculation of cross-sectional properties (area, sectionmodulus and secondmoment ofarea) by assuming an equivalent rectangular residual cross-section,design codes generally define charring rates greater than the one-dimensional charring rate. The charring rate including these effectsis called the notional charring rate βn according to EN 1995-1-2and for solid softwood a value of βn = 0.8 mm/min can beassumed. Fig. 2 shows the definition of charring depth dchar,0 forone-dimensional charring and notional charring depth dchar,n.The temperature-dependent reduction in strength and stiffness

near the charred layer can be considered in different ways. EN1995-1-2, for example, gives two alternative simplified methods:the ‘‘Reduced cross-section method’’ and the ‘‘Reduced propertiesmethod’’ [4]. The ‘‘Reduced cross-section method’’ considers thestrength and stiffness reduction near the charred layer by addingan additional depth k.0d0 to the charred layer dchar,n (see Fig. 3).This method permits the designer to use strength and stiffnessproperties for normal temperature for the resulting effective cross-section. Thus the temperature-dependent reduction factor is takenas kmod,fi = 1.0 for the effective cross section.The ‘‘Reduced properties method’’ takes into account the

influence of the temperature reducing the timber stiffness

Fig. 3. Definition of residual cross-section and effective cross-section [5].

and strength properties of the residual cross-section by atemperature-dependent reduction factor kmod,fi. The reduction ofthe timber strength and stiffness properties were derived usingtest results [9], which do not well reflect the physical behaviourof timber in fire [4]. Further this method was developed forstructural members (beams, columns) exposed to fire on three orfour sides [10] and should not be used for slabs exposed to fire onlyon one side [4].

3. Simplified calculation model

3.1. Fire behaviour of timber slabs made of hollow core elements

The fire tests performed with timber assemblies made ofhollow core assemblies showed that the fire behaviour is mainlycharacterised by two different charring phases, before and afterthe timber layer directly exposed to fire is completely charred.Before the fire-exposed timber layer is completely charred, thetimber assembly is exposed to fire only on one side and a moreor less homogenous regular one-dimensional charring similarto that of a heavy timber slab was observed during fire testson timber assemblies performed within the framework of theresearch project (see Fig. 4).After the fire-exposed timber layer is completely charred and

the char-layer begins falling off, the thin vertical timber membersare exposed to fire on three sides, leading to very irregular residualcross-sections with charring depths much greater than for heavytimber structures where the char-layer performs as an effectiveprotection of the remaining unburned residual cross-section(see Fig. 5).From a fire design point of view it is therefore desirable that the

vertical timbermembers are not exposed to fire on three sides. Thiscan be achieved in two different ways:

• the design of the fire-exposed timber layer has to avoid a firepenetration into the cavities for the whole duration of the fireexposure. The timber slabs REI60 and REI90 were designedaccording to this criterion (see Fig. 4).

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Fig. 4. Residual cross-section of the timber slab REI60 after 70 min ISO-fire exposure; the fire-exposed timber layer was so designed that a fire penetration into the cavitieswas prevented.

Fig. 5. Residual cross-section of the timber slab REI30 after 40 min ISO-fire exposure; the thin vertical timber members have been exposed to fire on three sides, leading tovery irregular residual cross-sections.

Fig. 6. Temperature profiles measured on the side of a vertical member of the timber slab EI60-31 (see Fig. 7 for the position of the thermocouples T05, T15, T25, T35 aswell as T06, T16, T26, T36).

• the cavities are filled with insulation material. After the failureof the fire-exposed timber layer, charring occurs mainly on thenarrow side of the vertical members, while the wide sides aremore or less protected by the insulation.

Two small-scale fire tests (EI60-31 and EI60-40, see Table 1)analysed the behaviour of the hollow core elements filledwith twodifferent common insulation materials: rock fibre batts with meltpoint ≥ 1000 ◦C and density of about 32 kg/m3 and glass fibrebattswithmelt point≈600 ◦C and density of about 18 kg/m3. Fig. 6shows the temperature profiles measured on the side of a verticalmember of the timber slab EI60-31 as a function of time (see Fig. 7for the position of the thermocouples). In Fig. 7 the height of thetimber slab is measured from the fire unexposed side. Fig. 8 showsthe residual cross-section of the timber slab EI60-31 after 60 minISO-fire exposure.From Figs. 6 and 8 it can be seen that rock fibre batts were able

to protect the wide sides of the vertical members from charring, sothat one-dimensional charring can be assumed. On the other hand,cavity insulation made of glass fibre batts melted when exposeddirectly to high temperatures, being incapable of protecting thewide sides of the vertical members. It is important to remark thatthe fire protective function of the cavities filled with insulationwith melt point ≥1000 ◦C can be taken into account only if the

cavity insulation remains in place after failure of the fire exposedtimber layer.

3.2. Calculation model for timber slabs made of hollow core elements

The proposed simplified calculation model is based on thereduced cross-sectionmethod according to EN 1995-1-2 and takesinto account the two different charring phases as shown in Fig. 9and generally discussed in the previous paragraph. For simplicity,linear relationships between charring depth and time are assumedfor each phase. Further it is assumed that the vertical timbermembers are not exposed to fire on three sides during the requiredfire resistance. Thus the fire-exposed timber layer is so designedthat a fire penetration into the cavities is prevented or the cavitiesare filled with insulation material with melt point ≥1000 ◦C thatremains in place after the charred fire-exposed timber layer hasfallen off.The first phase is defined as the time period, during which

the charring depth has not yet reached the thickness of the fire-exposed timber layer (dchar,n ≤ hu). The notional charring rate β1,ndescribes the one-dimensional charring of the timber slab duringthe first phase and takes into account the influence of joints andinsulationmaterial (see Section 4.1). The time t1, when the charring

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Fig. 7. Position of the thermocouples T05, T15, T25, T35 as well as T06, T16, T26, T36 used to measure the temperature on the side of a vertical member of the timber slabEI60-31 as given in Fig. 6.

Fig. 8. Residual cross-section of the timber slab EI60-31 after 60 min ISO-fire exposure with hollow core elements filled with different insulation materials.

Fig. 9. Charring model for the calculation of the residual cross-section of the hollow core elements.

depth has reached the thickness of the fire-exposed timber layer(dchar,n = hu) can be calculated as follows:

t1 =huβ1,n

. (1)

The second phase is characterised by the charring of thevertical timber members after the charring depth has reached thethickness of the fire-exposed timber layer (dchar,n ≥ hu). Thenotional charring rate β2,n during this phase is mainly influencedby the thickness of the vertical members. Because of the smallthickness of the vertical members of only 33 mm in the case ofthe Lignatur hollow core elements, a superposition of the heat fluxfrom the sides and below occurs and an increased charring hasto be considered in comparison to one-dimensional charring (seeSection 4.2).For a required time treq of fire resistance, the notional charring

depth for the vertical members of the hollow core elements can becalculated as follows:

dchar,n = β1,n · treq for 0 ≤ treq ≤ t1 (2)

dchar,n = hu + β2,n ·(treq − t1

)for treq ≥ t1. (3)

The effective cross-section is calculated by reducing the initialcross-section by the effective charring depth def, which iscalculated as follows:

def = dchar,n + k0 · d0. (4)

The additional depth k.0d0 takes into account the temperature-dependent reduction of strength and stiffness in the heat affectedzones of the residual cross-section, permitting the designer to usethe strength and stiffness properties for normal temperature forthe effective cross-section. The temperature-dependent reductionfactor is therefore taken as kmod,fi = 1.0 for the effective cross

Table 2Values of the notional charring rate β1,n and β2,n as well as the factor d0 for thedifferent charring phases

Charring phase Notional charring rate βn (mm/min) Factor d0 (mm)

First charring phase:dchar,n ≤ hu

β1,n = 0.8 7

Second charring phase:dchar,n > hu

β2,n = 1.6 20

section. The factor k0 linearly increases from 0 to 1 with timeduring the first 20 min of fire exposure, as it takes normally about20 min to get stabilized temperature profiles in the heat affectedzones of the residual cross-section [4].Values of βn and d0 for the first and second charring phase are

given in Table 2 and discussed in detail in the following section.The values βn,1 and d0 for the first charring phase have directlybeen determined from the fire tests performed. The notionalcharring rate βn,2 for the second charring phase was estimatedby comparing the results of the fire tests with the simplifiedcalculation model and confirmed by fire tests performed in theframe of a different research project. The fire tests performeddid not permit the determination of the value d0 for the secondcharring phase. Thus this parameter has been determined bycomparing the simplified calculation model with an advancedcalculation model. It is important to remark that the values β2,nand d0 for the second charring phase have been evaluated forhollow core elements with 33 mm thick vertical members anda fire resistance up to 60 min. For thicker vertical members thevalues are conservative. Further the minimal thickness of the fire-exposed layer should be hu = 40 mm for a fire resistance of60 min. This restriction is necessary to respect the range forwhich the charring of the vertical members was validated withfire tests.

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Table 3Measured average charring rates in mm/min for the large-scale fire tests on timber slabs as well as the small-scale fire tests; details of the joints can be seen in Fig. 10

Position of measurement Hollow core elements without insulation With insulationSmall-scale test EI60 Test on slab REI60 Test on slab REI90 Small-scale test EI60-31 Small-scale test EI60-40

Fire-exposed lower layer 0.60 0.68 0.73 0.82 0.81Vertical memberJoint type A 0.70 0.80 0.79 – –Joint type B 0.77 0.76 0.70 – –Joint type C 0.70 0.76 0.77 – –No joint – – – 0.81 0.80

4. Discussion of the parameters of the calculation model

4.1. First charring phase

The fire tests showed that during the first phase the timberassembly is exposed to fire only on one side and a more or lesshomogenous regular one-dimensional charring similar to that ofa heavy timber slab can be assumed (see Figs. 4 and 10). Table 3showsmeasured average charring rates of the fire tests performed.Details of the fire tests are described in [1,2].For the hollow core elements without insulation the charring

rate measured at the fire-exposed lower layer varied between0.60 and 0.73 mm/min. At the vertical members, because of theinfluence of the joints, the measured charring rate was higherand varied between 0.70 and 0.80 mm/min. Details of the jointstested as well as the residual cross-section of test specimen EI60after 70 min fire exposure can be seen in Fig. 10. For the hollowcore elements filled with insulation material the charring ratemeasured at the fire-exposed lower layer as well as at the verticalmemberswas higher than for the elementswithout insulation. Themeasured charring rate varied between 0.80 and 0.82 mm/min.A possible reason is that the insulating batts in the cavitiessignificantly improve the thermal performance, but they also causethe fire-exposed layer to heatmore rapidly, possibly increasing thecharring rate [11].The fire tests showed that for the calculation of the charring

depth during the first phase a notional charring depth β1,n =0.8 mm/min can be assumed giving mostly safe design results.The notional charring rate β1,n takes into account the influenceof the insulation material in the cavities as well as the influenceof the joints. For simplicity, a value d0 = 7 mm is assumedfor the calculation of the effective cross-section during the firstcharring phase. The values β1,n = 0.8 mm/min and d0 = 7 mmcorrespond to the values given in EN1995-1-2 for the calculation ofthe effective cross-section based on the reduced cross-section forsolid timber members exposed to fire on three or four sides. Thevalues βn,1 and d0 have been confirmed by the fire tests performedon loaded timber slabs (see Section 5).

4.2. Second charring phase

Extensive research has been recently conducted on the firebehaviour of light timber frame assemblies permitting thedevelopment of a design model which takes into account differentcharring stages due to the protection provided by fire protectivecladdings [12–14]. The designmodel has been included in EN1995-1-2, Annex C. The assemblies considered consisted of solid timbermembers (joists or studs), claddings of gypsum plasterboards, andcavity insulation made of rock or glass fibre. In a series of fire testswithout cladding on the fire-exposed side, the fire behaviour of thetimber members with rock fibre insulation was studied. The firetests showed that rock fibre insulation was capable of protectingthe wide sides of the timber members from fire, however due totwo-dimensional heat flux in the insulation material close to thetimber members, increased charring occurred in comparison to

Table 4Cross-section factor ks for different member thicknesses according to [14]

Member thickness (mm) 33 38 45–48 60 90Cross-section factor ks 1.5 1.4 1.3 1.1 1.0

one-dimensional charring. This confirms the test results performedwithin the framework of this research project. In order to takeinto account this effect and convert the irregular residual cross-section caused by corner rounding into a notional rectangularcross-section, a cross-section factor ks as well as a conversionfactor kn were derived from the fire tests. Values of ks for differentmember thicknesses are given in Table 4.It can be seen that for wide members where the heat flux

is mainly one-dimensional ks is 1.0. The conversion factor kn isa function of area, section modulus and moment of inertia. Forsimplicity these three values can be replaced conservatively by asingle value of 1.5. Starting from the one-dimensional charring β0the notional charring rate βn can be finally calculated as following:

βn = kn · ks · β0. (5)

Thus for timber members with a thickness of 33 mm a charringrate of about 1.5 mm/min can be calculated according to Eq. (5)(βn = 1.5 × 1.5 × 0.65 ≈ 1.5 mm/min, where β0 =0.65 mm/min according to EN 1995-1-2 for softwood). There is agood agreement between this value and the notional charring rateβ2,n = 1.6 mm/min assumed for the calculation of the charringdepth of the vertical members of the hollow core elements duringthe second phase.Although the insulation material is able to protect the wide

sides of the vertical members, the fire tests showed that becauseof the small size of the vertical members the temperaturesmeasured in the vertical members are higher than those in heavytimber cross-sections. Therefore, the influence of the temperature-dependent reduction in strength and stiffness in the heat affectedzones of the vertical members during the second charring phaseis expected to be higher than for the first charring phase. Thesimplified calculation model based on the reduced cross-sectionmethod considers this effect by assuming the factor d0.As the fire tests performed did not permit the determination of

the value d0 for the second charring phase, an advanced calculationmodel as shown in Fig. 11 has been used. The cross-section ofthe timber assembly is divided into finite elements with differentstiffness and strength properties as a function of the averagetemperature Θi(t) measured in the fire tests. Unfortunatelytemperature-dependent properties of wood reported in theliterature exhibit a large scatter and are partially in contradictionto each other [14–19]. The main reason is that the test results arehighly influenced by differences in the testmethods used to collectthose data [14].EN 1995-1-2 gives temperature-dependent timber properties

for FE-calculations. Therefore the reduction values are relatedto tensile and compressive strength and stiffness (see Fig. 12).Bending strength is a derived quantity that is used in the simplifiedcalculation model. The temperature-dependent reduction values

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Fig. 10. Different joints of the hollow core elements tested under ISO-fire exposure; fig. top shows the test specimen EI60 before the fire test; fig. bottom shows the testspecimen EI60 after 70 min fire exposure.

Fig. 11. Calculation model: (a) cross-section composed of different layers, (b) temperature gradient of the cross-section, (c) resulting strains εres .

given in EN 1995-1-2 were derived from many results of large-scale fire tests on loaded timber frame members in bending [13]and were considered as appropriate to model the structuralbehaviour of the timber assemblies made of hollow core elementsin fire. As the fire-exposed side of the timber slabs are mostlysubjected to tension, the stiffness reduction in tension given in EN1995-1-2 was assumed in the advanced calculation model for themodulus of elasticity.Temperatures, material properties, strains and stresses are

assumed in the centre of gravity of each element and are constantwith regard to thickness of the element. Further it is assumed thatBernoulli’s hypothesis of plane sections is valid also in fire. Theresulting time-dependent strains εres,i(t) can be calculated takinginto account the strain of the upper layer ε0(t) and the curvatureχ(t) of the cross-section as follows:

εres,i(t) = ε0(t)+ χ(t) · zi. (6)

The effect of thermal expansion of timber can lead to residualthermal stresses. However the residual thermal stresses are smallin comparison to the stresses due to external mechanical loads andtherefore can beneglected in the calculation of the fire resistance ofstructural timber members [20]. Under the assumption of a linearelastic material behaviour and neglecting the influence of thermalexpansion the resulting time-dependent stresses σres,i(t) can becalculated as follows:

σres,i(t) = Ei(Θi) · [ε0(t)+ χ(t) · zi] . (7)

The strain of the upper layer ε0(t) and the curvature χ(t) of thecross-section can be found from static equilibrium requiring thatthe internal bending moment MΘ and the internal axial forceNΘ are in equilibrium with the external mechanical loads. For

Fig. 12. Temperature dependent reduction factor for local values of strength andmodulus of elasticity (MOE) in tension and compression according to EN 1995-1-2to be used for FE-calculations.

a simply supported member which is subjected only to externalbending momentM (external axial force N = 0) the conditions ofequilibrium can be written as follows:

NΘ(t) =n∑i=1

σres,i(t) · Ai = 0 (8)

156 A. Frangi et al. / Engineering Structures 31 (2009) 150–157

Fig. 13. Comparison of the fire resistance after 60 min fire exposure as a functionof the height of the cross-section according to the advanced calculation model andthe simplified calculation model assuming different values of d0 .

MΘ(t) =n∑i=1

σres,i(t) · Ai · zi = M. (9)

The solutions of the above equation system are:

ε0(t) =−(∑EAi · zi

)·M(∑

EAi)·(∑EAi · z2i

)−(∑EAi · zi

)2 (10)

χ(t) =

(∑EAi)·M(∑

EAi)·(∑EAi · z2i

)−(∑EAi · zi

)2 . (11)

The bending resistance MR of the cross-section can finally becalculated using following failure criteria:

σres,i(MR) = fd,fi,i (12)

where fd,fi,i is the strength taking into account the strengthreduction due to the influence of the temperature according toFig. 12.The advanced calculation model was used in order to calculate

the bending resistance of timber slabs made of hollow coreelements with different heights varying between 150 and 300mmsubjected to 60 min ISO-fire. For the calculation it was assumedthat the fire-exposed timber layer has a thickness of 40mm. Fig. 13compares the results of the advanced calculation model to thesimplified calculation model based on the reduced cross-sectionmethod. In the figure, the ratioMR,simplified/MR,advanced as a functionof different assumptions for the factor d0 is given.It can be seen that under the assumption of a factor of d0 =

7 mm used for the first charring phase the simplified calculationmodel leads to bending resistances which are 20 up to 40% higherthan in comparison to the advanced model. Thus the simplifiedcalculation model leads in this case to unsafe results. The mainreason is that because of the small size, the vertical membersare more affected by the heat in comparison to heavy timbersections. On the other hand under the assumption of a factor ofd0 = 20 mm a good agreement between the advanced and thesimplified calculationmodel is observed. Thus it is proposed to used0 = 20 mm for the simplified calculation model.

Fig. 14. Comparison between measured and calculated charring depths accordingthe simplified calculation method.

5. Comparison to fire tests

The charring behaviour of the hollow core elements wasanalysed with small-scale fire tests conducted with unloadedspecimens. A series of fire tests looked at the influence of differentinsulation materials and type of joints (see Figs. 8 and 10).Another series of small-scale fire tests studied the influence ofacoustic perforations on the charring behaviour of the hollowcore elements. Details of all fire tests are summarised in [1,2].Fig. 14 compares the measured charring depths with the charringdepths calculated according to the simplified calculation model.For the calculation of the charring depth the values given in Table 2were used. There is a good agreement between the simplifiedcalculationmodel and the results of the fire tests. The average ratiodchar,model/dchar,test is 1.08.The global structural behaviour of the timber slabs made of

hollow core elements was analysed with fire tests on loaded slabsperformed in EMPA’s horizontal furnace (3.0×4.85m). The timberslabs were made of 0.2 m high hollow elements of spruce witha mean density of 450 kg/m3 and moisture content of 10%± 2%.Two different slabs with three different joints between the timberelements were designed for a fire resistance of 60 and 90 min.The slabs called in the following REI60 and REI90 were designedso that a fire penetration into the cavities was prevented. Thefire-exposed timber layer had a thickness of 64 mm for the slabREI60 and 97 mm for the slab REI90. The load level during the firetests was set in such a way that the maximum bending momentcorresponded to that in a slab of about 7.10 m span, with apermanent load of 1.5 kN/m2 and a reduced accompanying liveload of 1.5 kN/m2. In the fire tests the temperatures in selectedlocations, the vertical deflections and the horizontal deformationswere measured. Details of the fire tests on loaded slabs aredescribed in [1].The fire tests showed a fire resistance of more than 60 min and

90 min respectively. No relevant smoke or flame penetration wasobserved through the three different joints between the timberelements during the fire tests. For security reasons the tests werenot conducted until failure of the slab. As a failure criterion, arate of deflection of about 1.0 cm/min as specified in the testingprotocol [21]was used for estimating the fire resistance of the slab.The fire test on the slab REI60 was stopped after 70 min. Afterthe test, the slab was loaded until failure which occurred under

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Fig. 15. Measured deflections during the large-scale fire tests on the slabs REI60and REI90.

Table 5Comparison between test results and simplified calculation model

Fire test ttest (min) tR,test (min) tR,model (min) tR,model/tR,test (–)

Slab REI60 70 77 85 1.10Slab REI90 105 107 109 1.02

1.08 times the load applied during the fire test. For this slab a fireresistance of 77 min was estimated based on the increase of thedeformations measured during the fire test (see Fig. 15). The firetest on the slab REI90 was stopped after 105 min. For this slaba fire resistance of 107 min was estimated based on the increaseof the deformations measured during the fire test (see Fig. 15).The fire resistance of the slabs was calculated using the simplifiedcalculation model based on the reduced cross-section method asdescribed in Section 3.2. For the calculation of the fire resistancethe values given in Table 2 were used. It can be seen that thesimplified calculation model was able to predict the fire resistancequite well (see Table 5).

6. Conclusions

The fire resistance of timber slabsmade of hollow core elementscan be calculated with the simplified calculation model presentedin this paper. The calculation model based on the reduced cross-section method according to EN 1995-1-2 takes into account twodifferent charring phases, before and after the fire-exposed layeris completely charred. For simplicity linear relationships betweencharring depth and time are assumed for each phase. Further itis assumed that the vertical timber members are not exposed tofire on three sides. This can be achieved in two different ways: thedesign of the fire-exposed timber layer prevents a fire penetrationinto the cavities, or the cavities are filled with insulation materialwith melt point ≥ 1000 ◦C. It is important to remark that theinsulation must remain in place after failure of the fire-exposedtimber layer in order to protect the wide sides of the verticalmembers of the hollow core elements from charring.The fire tests showed that before the fire-exposed timber layer

is completely charred (i.e. during the first charring phase) the

timber assembly is exposed to fire only on one side and a moreor less homogenous regular one-dimensional charring similar tothat of a heavy timber slab can be assumed. For the calculation ofthe effective cross-section a notional charring rate of 0.8 mm/minand a factor d0 = 7 mm can be assumed giving safe results. Afterthe fire-exposed timber layer is completely charred and has fallenoff (i.e. during the second charring phase), an increased charringrate of the vertical members of the hollow core elements has tobe considered due to two-dimensional heat flux. For the hollowcore elements tested with a thickness of the vertical members of33 mm a notional charring rate of 1.6 mm/min can be assumed asconfirmed by the results of fire tests. Because of the small size ofthe vertical members, the temperatures measured in the verticalmembers are higher than in comparison to heavy timber cross-sections. For this reason during the second charring phase thefactor d0, which takes into account the temperature-dependentreduction in strength and stiffness in the heat affected zones ofthe vertical members, has to be increased to 20 mm. Additionalexperimental investigations are planned in order to confirm thisvalue, calculated using an advanced calculation model.

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