timber structures - 10

8/18/2019 Timber Structures - 10

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Winter 2003 Design of Wood Structures 8-1

TIMBER STRUCTURES


OVERVIEW

• This section will: – describe the fire behaviour of timber construction – give design methods for heavy timber structural

members exposed to fire – briefly discuss fire behaviour of connections in timber

structures


DESCRIPTION OF TIMBERCONSTRUCTION

• Timber structures are divided into two categories: – heavy timber structures – light timber/wood frame construction

• Heavy timber construction describes all uses oflarge-dimension timber framing in buildings

• Heavy timber structures are principal structuralelements (beams, columns, decks or truss)

• Light timber frame construction uses smaller sizesof wood framing (studs in walls, joists in floors)


Glulam

• 'Glue laminated timber' (glulam) are membersmade from several laminations glued together

• Fire tests have shown that glulam membersexposed to fires behave in the same way as solidsawn-timber members of the same cross section


Fire Behaviour of TimberStructures

• Heavy timber members have good fire resistance• When large timber members are exposed to fires

the wood surface initially burns rapidly• The burned wood becomes a layer of char which

insulates the solid wood below and slow downthe burning rate

• The char layer does not usually burn• Above 100°C, moisture in the wood evaporates• Some of this moisture travels out to the burning

face, but some travels into the woodWinter 2003 Design of Wood Structures 8-6

Fire-retardant Treatments

• Fire-retardant chemicals are available for treatingwood to reduce its combustibility

• The purpose of the chemical treatments is toreduce the rate of flame spread

• Chemical pressure impregnation is effective• Impregnation can have some negative effects

– loss of wood strength – corrosion of fasteners

• Fire-retardant chemicals do not significantlyimprove the fire resistance of timber members


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FIRE-RESISTANCE RATINGS

• Design process for fire-resistance requires that:provided fire-resistance > design fire severity

• Verification is usually in time or strength domain• Temp. domain is not used for timber structures

(no critical temp for fire-exposed timber)• Usually, fire design of heavy timber structures is

by calculation methods• Some countries have generic fire-resistance

ratings for heavy timber construction• There are very few proprietary ratings


WOOD TEMPERATURES

• When heavy timber members are exposed tosevere fires, the outer layer of wood chars

• Boundary between the char layer and remainingwood corresponds to about 300°C temp.

• Below the char layer there is a layer of heatedwood about 35 mm thick

• Layer above 200°C is the pyrolysis zone (thermaldecomposition to gases, see Figure below)

• Moisture evaporates in the wood above 100°C• Structural design of heavy timber members is

based on the rate of charring of the wood surface


WOOD TEMPERATURES

Char layer and pyrolysis zone in a timber beam


Temperatures Below the Char

• Temp. in wood below char layer was measured• For semi-infinite solid wood, temp. T ( oC) below

the char layer is given by:T = T i + (T p - Ti)(1 - x/a) 2

• Ti is the wood initial temp. ( oC), T p is the temp. atwhich charring starts (300°C), x is the distancebelow the char layer (mm), and a is the thicknessof the heat-affected layer (40 mm)

• Janssens and White (1994) show that a better fitto experimental data is obtained with a = 35 mm


Thermal Properties of Wood

• Temp. inside fire-exposed timber members canbe calculated using FEM

• Thermal properties are not well defined(especially at 100°C and over 300°C)

• Wood density varies greatly between species• After 100°C, density drops to ∼90% of its original

and to ∼20% of its original value above 300°C• Thermal conductivity varies greatly between

authors (see Figure below as an example)• Figure below shows specific heat variation with

temp. (spike means moisture evaporation)Winter 2003 Design of Wood Structures 8-12


Variation of thermal conductivity of wood withtemperature


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Variation of specific heat of wood with temperature


MECHANICAL PROPERTIES OF WOOD

• Wood is greatly different from other materials – wood strength is very variable

– mechanical properties are different in differentdirections

– strength and ductility are different in tensionand compression

– failure stresses depend on the specimens size – strength reduces under long duration loads

• Figure below shows different ways of loading ofwood with different failure modes


MECHANICAL PROPERTIES OF WOOD

Loading of wood in different directions


Mechanical Properties of Wood atNormal Temperatures

• Tension and compression behaviour • Bending behaviour • Design values


Tens ion and com press ion behav iour

• Figure below shows typical stress-strain curvesfor wood specimens with no defects

• Parallel to grain vs. Perpendicular to grain• Compression vs. Tension

• The wood is ductile in compression


Ten s i o n a n d c o m p r e s s i o n b e h av i o u r

Stress--strain relationships for clear wood


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B e n di n g b e ha v io u r

• Bending behaviour is a combination of tensionand compression

• Some ductility is available in timber beams whenthe material is stronger in tension than incompression


Des ign va lues

• Structural design calculations require values ofthe design strength of the wood material

• For limit states design, design stress is 5thpercentile failure stress under short-durationloading

• Due to variations, characteristic stresses areusually obtained from in-grade tests of largenumbers of representative samples

• The 5th percentile value for design in normaltemp. conditions, may be modified to 20thpercentile strength value for fire design


Des ign va lues

• Design strength of timber depends on duration ofthe applied load as a duration-of-load factor

• In limit states design, duration-of-load factor is1.0 for short-duration loads and 0.8 or 0.6 formedium- and long-duration loads

• In working stress design, duration-of-load factoris 1.0 for long-duration loads and 1.25 or 1.6 formedium- and short-duration loads

• The duration-of-load factor for fire design shouldbe the appropriate value for short-duration loads


Mechanical Properties of Wood atElevated Temperatures

• Sources• Effect of moisture content• Plasticity• Parallel to the grain properties• Perpendicular to the grain properties• Shear • Derived results


Sources and g eneral behav iour

• Review on the effect of moisture content (MC)and temperature on the mechanical properties ofwood is given by Gerhards (1982)

• Wood properties are affected by steam at 100°C,wood begins to pyrolyse at about 200°C andturns into char by 300°C

• The range of interest for fire design is thereforefrom room temperature to 300°C


Effec t o f mo is tu re con ten t

• When testing timber at elevated temp., MC issensitive to the test method and specimen size

• Some test specimens are maintained at constantMC throughout the test

• Some tests specimen are at a certain MC beforethe test and allowed to dry out when heated

• If wood is heated to a temperature above 100°Cin dry air, all moisture will evaporate after sometime


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Paral lel to the grain pro pert ies -Modulus of elasticity

• Figures below show the modulus of elasticity ofwood at elevated temperatures

• The effect of temp. on modulus of elasticityparallel to the grain is roughly linear up to 200°C

• There is a scatter over 200°C• Figure below is another example of results

derived by Konig and Walleij (2000) from tests of145 x 45 mm timber studs in insulated walls,exposed to ISO 834 fire while loaded in bending


Paral lel to the grain pro pert ies - Modulus of elasticity

Modulus of elasticity of wood parallel to the grainversus temperature


Paral lel to the grain pro pert ies - Modulus of elasticity

Modulus of elasticity of wood parallel to the grainversus temperature


Paral lel to the grain pro pert ies - Tensile strength

• Below is a Figure showing stress-strain curvesfor temp. of 25°C and 90°C at low and high MCfor samples tested by Ostman (1985)

• Failure stress at 90°C and 29.5% moisturecontent is about 60% of that of dry cool wood

• Another Figure shows a comparison among testdata as derived by different researchers



Stress-strain relationships for wood in tensionparallel to the grain



Tensile strength parallel to the grain versustemperature


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Paral lel to the grain pro pert ies - Compressive strength

• Figure below shows temp. effect on compressivestrength parallel to the grain

• These results are for dry wood except themarked shaded region (MC > 12%)

• The Figure also shows the relationship derivedby Konig and Walleij (2000)


Paral lel to the grain pro pert ies - Compressive strength

Compression strength parallel to the grain versustemperature


Paral lel to the grain pro pert ies - Bending strength

• Figure below shows limited bending test resultscollected by Gerhards (1982)

• The wood shows different slopes for different testresults


Paral lel to the grain pro pert ies - Bending strength

Bending strength of wood versus temperature


Perpendicu la r to the g rain p roper t i es - Modulus of elasticity

• For modulus of elasticity perpendicular to thegrain, Gerhards (1982) reports eight studies asshown in Figure below for temp. up to 100°C

• The dependence on temperature tends to begreater for moisture content above 12%, butthere is a lot of overlap between the studies


Perpendicu la r to the g ra in p roper t ie s - Modulus of elasticity

Modulus of elasticity perpendicular to grain versustemperature


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Perpendicu la r to the g rain p roper t i es - Tensile strength

• Temp. effect on tensile strength perpendicular tothe grain is shown in the Figure below

• There is a wide range of results for different MC,but a trend of a greater strength reduction as themoisture content increases

• There are no results of tests over 100°C


Perpendicu la r to the g ra in p roper t ie s - Tensile strength

Effect of temperature on tensile strengthperpendicular to the grain


Perpendicu la r to the g rain p roper t i es - Compressive strength

• Figure below shows temp. effect on strength incompression perpendicular to the grain

• This shows data from five studies reported byGerhards (1982) (overlap and scatter)


Perpendicu la r to the g ra in p roper t i es - Compressive strength

Effect of temperature on compression strength ofwood perpendicular to the grain


D er i v ed r e s u lt s - Reduction factors

• Figure (a), temp. effect on mechanical properties – modulus of elasticity is assumed to drop linearly to

50% of its normal temperature value at 300°C – tension strength follows the same relationship to

200°C, then drops to zero at 300°C (wet or dry)

– compression strength for dry wood drops linearly tozero at 300°C – compression strength for wet wood drops to 50% at

100°C and remain constant until it reaches 160°C,after which it follows the relationship for dry wood

• Figure (b) shows temp. profile below char layer • Figure (c) shows drop in wood strength below

char layer (significant reduction below 25 mm)Winter 2003 Design of Wood Structures 8-42

D er i v ed r es u l t s - Reduction factors

(a) Effect of temp. onmechanicalproperties of wood

(b) Temp. profile

below char layer (c) Reduction instrength of woodbelow char layer


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D er i v ed r es u l t s - Stress-strain relationship

• Figure below shows stress-strain curves derivedby Konig and Walleij from computer modelling

• The curves are idealized in a simple way• In the tension region, linear elastic behaviour has

been assumed until failure• In the compression region, elasto-plastic

behaviour has been assumed• The curves include the creep effects


D er i v ed r e s u lt s - Stress-strain relationship

Derived stress-strain relationships for wood atelevated temperatures


DESIGN CONCEPTS FOR HEAVYTIMBER EXPOSED TO FIRE

• Large timber members have good fire-resistance• Fire-resistance can be calculated if charring rate

is predicted on surfaces exposed to standard fire• Figure below shows common fire exposures

(three-/four-sided of rectangular members)• The original cross section b x d is reduced to

residual cross section b f x d f after charring• The depth to the char front c (mm) is given by:

c = β x t• β is the charring rate (mm/min), and t is the fire

exposure time (min)Winter 2003 Design of Wood Structures 8-46


Design concepts for large timber members



• Dimensions of the residual cross section are:b f = b - 2cd f = d - c (three-sided exposure)d

f = d - 2c (four-sided exposure)

• Char temperature is about 300°C• There is a layer of heated wood about 35 mm

thick below the char layer • Structural design of timber members is based on

the strength and stiffness of the residual member


Verification

• Verification of strength during fire exposure:U*fire ≤ R fire

• U*fire is the design force and R fire is the loadcapacity

• The design force U *fire may be axial force N *fire ,bending moment M *fire or shear force V *fire

• The load capacity is calculated as axial force N f ,bending moment M f or shear force V f


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Verification - S i m p l y s u p p o r te d b ea m s

• For a beam under a bending moment:M*fire ≤ Mf

• M*fire is the bending moment and M f is the designflexural capacity under fire conditions given by:

Mf = Z f f f • f f is the design strength of wood in fire conditions

(MPa) and Z f is elastic section modulus (mm 3)• The value of f f should always be the strength

under short-duration loads• For rectangular sections with no corner rounding:

Zf = b f d f 2 / 6Winter 2003 Design of Wood Structures 8-50

Charring rate

• Rate of charring (under standard fires) dependson the density and moisture content of the wood

• Many codes specify a constant charring rate of: – 0.60 - 0.75 mm/min for softwoods – about 0.5 mm/min for hardwoods

• The effect of density, ρ (kg/m 3), and MC on thecharring rate is shown in Figure below, given bythe equation below for charring rate β (mm/min):

β = 0.4 + (280 / ρ)2• Table below shows recommended charring rates


Charring rate

Charring rate as affected by density and MC


Charring rate

Charring rates for design

• β for actual cross sections with rounded corners• β1 (10% larger notional charring rate) is for no

corner rounding

Char rateMaterial Minimum density

(mg/m3)β

(mm/minute)β1

(mm/minute)Glue-laminated softwood timber Solid or glue-laminated hardwood timber

290450

0.640.50

0.700.55

Softwood panel products (plywood, particle board) minimum thickness 20 mm

450 0.9


Charring rate

• In North America, recommendations for charringrate are given by AFPA (White, 1988)

• The proposed charring rate β is the averagecharring rate (mm/min) given by:

β = 2.58 βn / t0.187

• βn is a nominal charring rate ( βn=0.635 mm/min)and t is the time (min)

• The resulting char layer thickness c (mm) is:c = β t = 2.58 βn t0.813

• Figure below shows the resulting depth of charduring 4 hours of standard fire exposure


Charring rate

Depth of char from North Americanrecommendations


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Corner Rounding

• All fire tests of large rectangular timber sectionsshow some rounding of the corners

• Figure below shows a typical charred cross section• Most design codes assume the radius of the

rounding as equal to the depth of the charred layer • If corner rounding is considered in beams exposed

to fire on 3 sides, the section modulus Z f,r of thereduced cross section is given by:

Zf = b f d f 2 / 6 - 0.215 r 2 d f • b f is the beam residual width, d f is beam residual

depth, and r is the radius of the charred corner Winter 2003 Design of Wood Structures 8-56

Corner Rounding

Residual cross section of timber beam exposed tofire


Effect of Heated Wood Below the CharLine

• There are several alternative design methods toallow for heated wood below the char line

• Some codes ignore any reduction of woodstrength below the char, which can lead tounsafe results for small cross sections

• There are two methods to account for variabletemperature inside the unaffected region: – the effective cross section method – the reduced properties method


Effec t ive c ross sec t ion m ethod

• The effective cross section method accounts forheated wood below char by removing a nominallayer of zero strength from the cross section

• Wood in the effective cross section is assumedto have normal temperature properties

• The flexural capacity, M f = Z f f f , is calculated withno corner rounding, with Z f for 3-sided exposure:

Zf,z = (b f - 2z)(d f - z) 2 / 6• z is the thickness of zero-strength layer (mm)• The design strength of wood is the strength at

normal temp. f b

(MPa) so that f f = f

b


Effec t ive c ross s ec t ion method

• In Eurocode, the thickness of zero-strength layer: – z = 7 mm for more than 20 min fire exposure – 0 < z < 7 mm for less than 20 min fire exposure

(reduced proportionately)• The AFPA North American design method

increases the nominal charring rate by 20% toallow for the heated wood below the char line

• Using the effective cross section method inaccordance with Eurocode, the charring rate β1should be used (earlier Table)


Reduced proper t i e s method

• The reduced properties method (Eurocode) isbased on a strength reduction factor k f applied toall of the wood below the char layer

• The flexural capacity is M f = Z f f f , with Z f and f f as

Zf = b f d f 2

/ 6 - 0.215 r 2

d f f f = k f f b• Zf includes corner rounding ( β from earlier Table)


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Reduced proper t i e s method

• kf is strength reduction factor for residualsections approximated by: – k f = 0.8 for NA design equations (Lie,1977) – k f = 1.0 - 1/g (A r / p) for Eurocode design equations,

where p is the perimeter of the f ire-exposed residualcross section (m), A r is the area of the residual crosssection (m 2), g is a factor (m -1), with the value of 200for bending, 125 for compression and 330 for tensilestrength and modulus of elasticity


Charac te r i s t i c s t reng th o f wo od

• For normal temp. design, characteristic designstrength is taken as the 5 th percentile value

• In most limit states design, 5 th percentile strengthvalue f 0.05 , obtained from tests, is listed in codes

• For fire design, most codes use 5 th percentilestrength value f 0.05 so that f b = f 0.05

• Some codes modify 5 th percentile strength f 0.05 to20 th percentile for fire design so that designstrength f b for fire conditions is f b = k 20 f 0.05

• k20 is a correction factor to convert 5 th percentileto 20 th percentile values (1.25 for solid timber,1.15 for glulam in Eurocode)


Charac te r i s t i c s t reng th o f woo d

• The AFPA method uses the mean value of woodstrength for fire design (working stress design)

• In the method, the allowable stress in the code f ais modified to give an allowable stress in fireconditions f a,f using

f a,f = kmean f a• f a is the code allowable stress (MPa), k mean is a

correction factor to convert allowable stresses tomean values (2.85 for tension and bending, 2.58for compression, 2.03 for buckling failures)


WORKED EXAMPLE 1

Consider a softwood glulam beam, 130 mm wideby 720 mm deep, spanning 7.5 m with a deadload G = 4.0 kN/m (including self weight) and liveload Q = 7.0 kN/m. The beam is laterallyrestrained with timber decking nailed to the topedge. Check the design for normal conditionsand for 60 minutes fire-resistance rating,exposed to fire on three sides. Use the Eurocodemethod with the charring rates from Table 8-52and the factor k 20 = 1.15.


WORKED EXAMPLE 1The characteristic flexural strength is f b = 17.7 MPa.

The strength reduction factor is Φ = 0.8 for normaldesign and Φf = 1.0 for fire design. The duration-of-load factor is k d = 0.8 for cold design and k d =1.0 for fire design.

Check design for normal conditions• Design load

wc = 1.2G+1.6Q = 1.2x4.0+1.6x7.0 = 16.0 kN/m• Bending moment

M* = w cL2/8 = 16.0x7.5 2/8 = 112 kNm• Section modulus:

Z = bd 2/6 = 130x720 2/6 = 11.2x10 6 mm 3


WORKED EXAMPLE 1

• Nominal strengthMn = k d f 0.05 Z = 0.8x17.7x11.2 = 159 kNm

• Design strengthΦ Mn = 0.8x159 = 127 kNmM* ≤ Φ Mn so design is OK.

Loads for fire conditions• Design load

wf = 1.0G+0.4Q = 1.0x4.0+0.4x7.0 = 6.8 kN/m• Bending moment

M*fire = w f L2/8 = 6.8 x 7.52/8 = 47.8 kNm


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WORKED EXAMPLE 3

Calculate the time to failure for the beam in WorkedExample 1 using NA empirical design equation.

• Design bending moment: M * = 112 kNm• Design strength: Φ Mn = Φ k1 f b Z = 127 kNm• Load ratio: R a = M* / Φ Mn = 112/127 = 0.882• z factor: z = 0.7 +0.3/R a = 0.7+0.3/0.882 = 1.04• Time to failure:

tf = 0.1 z b (4-b/d)tf = 0.1x1.04x130 (4-130/720) = 50.1 min

• Time to failure is less than 60 minutes, so thebeam fails in the fire.


DESIGN OF HEAVY TIMBERMEMBERS EXPOSED TO FIRE

• Beams

• Tension Members• Columns• Beam-columns• Decking• Timber-concrete Composite Structures


Beams

• Beams can be designed using the same designequations as for normal temperature conditions,with modifications for strength and cross section

• It is important to determine which surfaces of thebeam are exposed to fire (see Figure below)

• In addition to flexural strength calculations,lateral torsional buckling must also be checked

• Shear stresses are not a concern for rectangularbeams, but should be considered for I-beams

• Deflections are not usually of concern


Beams

Three-/four-sided beam exposure


Tension Members

• Tension members are not affected by thepossibility of buckling

• The tensile load capacity of a fire-reduced crosssection can be calculated using one of the

design methods – effective cross section – Reduced properties


Columns

• Short columns strength depends on materialcrushing strength and reduced cross section

• Long columns strength (buckling increases withtime) depends on moment of inertia and modulusof elasticity of reduced cross section

• Lateral stability is very important for columns• Columns built into walls may have better fire

resistance (less charring and lateral restraint)• Tests on 16 columns (Malhotra et al. 1970)

achieved fire-resistance ratings between 30 and90 min, depending on load and slenderness ratio


15/17


Beam-columns

• A 'beam-column' is a member subjected tocombined bending and axial loading

• The design approach is to check the generalinteraction formula including both flexuralstrength and axial load capacity, such as:

(N/N u)2 + M/M u ≤ 1• N = applied axial load (kN), N u = axial load

capacity with buckling effects (kN), M = appliedbending moment (kN-m), and M u = flexuralcapacity with lateral buckling effects (kN-m)


Decking

• Assessment of fire resistance of decking mustconsider all three possible failure criteria ofstability, integrity and insulation

• Solid wood decking includes solid timber or glulam timber planks laid flat and butted togetherwith tongue and groove edges, and timberplanks set on edge and nailed together (seedetails in textbook)


Decking - S t ab i l i ty

• The stability criterion can be assessed in thesame way as for beams and columns

• Janssens (1997) proposed an empirical designformula for structural performance of solid decks(based on a temperature and charring model)

• The time to structural failure t sf (min) is given by:tsf = 1.25 d (1 - √0.4R a) - 11.3

• d is the thickness of the deck (mm), and R a is theratio of the applied load to the allowable designload


Decking - I n te g r it y

• The integrity criterion may be the most difficult tosatisfy for wood deck systems

• The difficulties arise at the junctions between theplanks, which may increase in width due toshrinkage of wood which often occurs during thelife of a building

• Tongue and groove joints between the planksare the best solution


Decking - In s u l a ti o n

• If the integrity and stability criteria are satisfied,there will be no problem meeting the insulationcriterion, because the thickness of remainingwood required to carry applied loads will begreater than that required to prevent excessivetemperature rise on the top surface


WORKED EXAMPLE 4

A solid timber deck consists of 150 mm thickplanks joined with central splines as shown inFigure 10.35(c). The deck spans 5 m with asuperimposed dead load of 1.25 kN/m 2 and liveload 5.0 kN/m 2. Calculate the failure time usingJanssen's formula. Use the Eurocode reducedproperties method to calculate if the deck has a90 minute fire-resistance rating.


16/17


WORKED EXAMPLE 4

The characteristic flexural strength of the deckingtimber is f b = 25.0 MPa. The density of the wood

is 5.0 kN/m3

. The strength reduction factor Φ is0.8 for normal design and Φf = 1.0 for fire design.The duration of load factor is k d = 0.8 for colddesign and k d = 1.0 for fire design. The factor k f is 1.15 for fire design.


WORKED EXAMPLE 4

Check for normal conditions• Thickness of deck: d = 150 mm• Self weight of deck: w

s= ρd =5x0.15=0.75 kN/m 2

• Total dead load: G = 0.75+1.25 = 2.0 kN/m 2

• Design load:wc = 1.2G+1.6Q = 1.2x2.0+1.6x5.0 = 10.4 kN/m 2

• Design a strip 1 m wide. Uniformly distributedload = 1.0 x 10.4 = 10.4 kN/m

• Bending moment:M* = w cL2/8 = 10.4x5 2/8 = 32.5 kNm


WORKED EXAMPLE 4

• Section modulus:Z = bd 2/6 = 1000x150 2/6 = 3.75x10 6 mm 3

• Design strength:Φ Mn = Φ k1 f b Z = 0.8x0.8x25x3.75 = 60.0 kNmM* ≤ Φ Mn so design is OK.

Janssen's formula• Load ratio: R a = M* / Φ Mn = 32.5/60 = 0.54• Time to failure:

tsf = 1.25 d (1- √(0.4R a))-11.3tsf = 1.25x150 (1- √(0.4 X 0.54))-11.3 = 89 min


WORKED EXAMPLE 4

Eurocode reduced properties method• Design load:

wc = 1.0G+0.4Q = 1.0x2.0+0.4x5.0 = 4 kN/m 2

• Design a strip 1 m wide. Uniformly distributedload = 1.0 x 4.0 = 4.0 kN/m

• Bending moment:M*fire = w c L2/8 = 4x5 2/8 = 12.5 kNm

• Rate of charring: β = 0.64 mm/min• Depth of char: c = 90 x 0.64 = 57.6 mm


WORKED EXAMPLE 4

• Reduced depth: d f = 150-57.6 = 92.4 mm• Section modulus:

Zf = bd f 2/6 = 1000x92.4 2/6 = 1.42 x 10 6 mm 3

• Section area: A = b d

f = 1000x92.4/10 6 = 0.00924 m 2

• Exposed perimeter: p = b = 1.0 m• Reduction factor:

kf = 1-p/200A = 1-1.0/(200x0.00924) = 0.46• Design strength: M f = k f kd k20 f 0.05 Zf

Mf = 0.46x1.0x1.15x25x1.42 = 18.8 kNmM*fire ≤ Mf so design is OK.


BEHAVIOUR OF TIMBERCONNECTIONS IN FIRE

• The ability of a structure to carry loads dependson the strength and stiffness of the structuralmembers and connections between members

• Under fire, both members and connections mustperform throughout the fire exposure• Most connections are either metal fasteners or

adhesives (very different fire performance)• Little research has been done on performance of

connections in timber structures exposed to fire


17/17


Metal Fasteners

• The behaviour of metal fasteners depends on thetemperature of the metal because: – it affects the strength of the fastener itself – high temperatures lead to charring or loss of strength

of wood in contact with the metal

• Geometry and protection of metal fasteners areexplained in more details in the textbook


Nails and Screws

• Nails are one of the best types of connection intimber structures because they penetrate wood

and do not weaken the wood with drilled holes• Screws have many of the advantages of nailsincluding better gripping capacity than nails

• Noren tested nailed splice joints in tensionexposed to the ISO 834 standard test fire

• Time to failure was inversely proportional toapplied load, varying from 6 to 21 min


Bolted Connections

• Bolted connections are widely used in timberstructures with excellent results

• Fire behaviour of bolted connections depends onthe amount of heat able to enter the woodthrough the bolts

• The theory for nails could be applied to boltedconnections but no comprehensive studies havebeen published


Truss Plates

• Truss plates have been shown to have a poorreputation for fire resistance

• Tests (White et al. 1994) on truss plates under ASTM E-119 standard fire exposure up to 300°C

• In the tests, unprotected plates failed in less than6 min compared with ∼13 min for solid timberwith no connection

• Various combinations of protection increased thefire resistance, the best gave over 30 min fireresistance when all 4 sides of the member were

protected with 13 mm Type X gypsum plaster


Glued Connections

• Many timber structures and timber members areconnected with adhesive

• When exposed to fire, glued wood membersgenerally behave in the same way as solid woodprovided that thermosetting adhesives are used

• Some adhesive such as epoxies are sensitive toelevated temperatures and should not be reliedon in fire conditions

timber structures - 10

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