Structural fire engineering design:materials behaviour – timber

J Bregulla Dipl-Ing PhDV Enjily BSc PhD CEng FIStructE FICE MIMechE FIWScBRE Centre for Timber Technology and Construction

Digest 487Part 4

This Digest is part of a suite of related documents containing guidance for the construction industry on structural fire engineering design. Theintention is to produce performance based guidance that brings together fire engineering and structural engineering providing aframework within which designers are free to develop site specificsolutions based on real performance criteria. The Digests containinformation complementary to the existing and emerging fire engineering codes and standards. Each Digest may be used in isolation or as part of the full integrated suite.This Part provides a general guide on the response of structural timbermembers to fire and discusses current design methods available, including those described in European and British standards. The reader is recommended to refer to specific publications provided at the end of this Digest for more detailed information and design guidance.

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Fire is an emotive subject and one that is closelyassociated with timber from its use as a source ofheat and energy throughout the history ofmankind. In spite of this emotional link we haveused timber in the construction of dwellings forcenturies and found that it can perform as well asother structural materials with a greaterperformance achieved for large-section timber.Ease of combustion is largely a function of thesurface-area-to-volume ratio; this is why woodshavings will burn readily, and large-sectiontimber beams can be left unprotected and will notsupport fire growth unless there is an additionalheat source to maintain burning. Solid timberignites in the presence of a pilot flame at surfacetemperatures between 300 to 365 °C, dependingon timber properties such as density and moisturecontent; these surface temperatures can be reachedupon short to medium term exposure to about 400to 500 °C. Panel products such as plywood orparticleboard have similar ignition properties tosolid wood.

The fire properties of timber can be modifiedusing surface treatments such as intumescentcoatings and impregnated inorganic salts. Thesetreatments improve the fire resistance of timber.

The rate at which timber combusts is influencedby a number of properties including density,timber species, moisture content and the surface-area-to-volume ratio. The rate of combustion canbe reduced with fire retardant treatments. Being an organic and anisotropic material, timber alsohas a highly complex reaction to fire with many ofthe thermal and chemical properties differing inthe three orthogonal directions. Properties alsochange with temperature and time. The oneproperty remaining relatively stable is the charring rate. Charred timber is an extremely good insulator, with the conductivity of charcoalbeing about one sixth of solid timber. Behind thelayer of charcoal is the pyrolysis zone where thechemical composition of the timber graduallychanges to release combustible gases[1]. Becausecharcoal is such a good insulator and timber is a

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natural insulator with good thermal diffusivity the thicknessof the advancing pyrolytic layer is notionally only 5 mm,leaving the remaining timber section unaffected by theeffects of the fire.

Both the predictable rate of charring and unaffectedproperties of the residual section ensure that the structuralresponse of timber to fire can be easily predicted bycalculation. This forms the basis for fire engineering designof timber structures. When the cross-section of timbermembers is insufficient to resist the applied structural load,protection can be provided by a combination of timberpanelling, mineral board and rock fibre insulating materials.

Scope of applications

Following changes in the Building Regulations 1991, timberframe buildings could be constructed to a height of 20 m inEngland and Wales, typically eight storeys. At that timeBuilding Standards in Scotland only permitted combustiblematerials in separating wall construction to a height of 11 m.Since then, fire tests conducted on the TF2000 building atBRE Cardington helped to harmonise standards across theUK, establishing the limit for UK timber frame building at18 m height. This applies to the structural use of timber inseparating and external walls although there may still belimitations on the use of timber in other parts of the buildingstructure. The main limitations are summarised in Table 1. A more detailed account is contained in BR 454.

Fire exposed timber structures

Timber can perform exceptionally well in a fire scenario,particularly large-section timber. The formation of char helpsto protect the timber member that can still remain at ambienttemperature at a depth of about 30 mm beneath the fire-exposed surface. Smaller-section timber typically used forplatform frame type construction will need protection fromfire by linings. Experience of real fires has demonstrated theability of timber to maintain structural integrity during thecourse of a fire for a specified period.

Designing with exposed timber to achieve fire resistancehas been studied by numerous researchers over the years witha great accumulation of data supporting the current designprocedures. These procedures are relatively simple, based onthe rate of charring as already described. Both BS 5268-4.1and prEN 1995-1-2 (hereafter referred to as Eurocode 5)adopt charring rates as the basis for calculations, and useassumptions of effective cross-section and reduced strengthand stiffness to determine the residual load bearing capacityof exposed structural members. Arris rounding is ofparticular importance for both codes since this will occur atall discontinuities and corners. The radius of arris roundingequals the depth of charring for fire exposures greater than30 minutes. Exposure less than 30 minutes when the residualsection has no dimensions less than 50 mm will generallyresult in very little arris rounding.

Timber members may be wholly or partially exposed to afire depending on the proximity of other building elementssuch as walls and ceilings. If an exposed beam or column hasmore fire resistance than an adjoining lining then the scenarioof complete fire exposure of the structural element should beconsidered for the remainder of the fire design period.

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Table 1 Limitations on the use of timber in buildingsTimber building components Limitations on useas, or as part of ...

Stairs, landings and lobbies There is no limitation for timber stairs provided the maximum height of the building is no more than 11 m above ground level and does not connect to a covered car park. Buildings of 3 to 7 storeys may have timber stairs provided there is more than one escape route†. Scotland does not permit timber to be used for escape stairs, landings, protected lobbies, floors or stairwell construction

Linings No limitations for rooms with a plan area of up to 4 m2. Linings for larger rooms should achieve a Class 1 rating over 50% or more of the room. Timber requires treatment to achieve a Class 1 rating. Timber linings are not permitted in protected and common areas

Loadbearing walls Separating and external walls permitted up to a height of 18 m from ground level to top floor levelWaste chutes and service shafts Waste chutes must be made from non-combustible material. Compartmentation must be maintained around

all protected services, shafts and waste chutesCavity barriers Timber cavity barriers are not permitted for buildings higher than 18 m† Timber stairs for single escape routes may be considered case-by-case based on the results of the Cardington TF2000 stair tests. For further information consult

BR 454.

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Calculation of charring ratesCharring rates for hardwoods and softwoods listed inAppendix A of BS 5268-2 are provided in BS 5268-4.1.These rates are expressed as charring depths over fireexposure periods of 30 and 60 minutes; they can be comparedin Table 2 with the currently proposed charring rates inEurocode 5.

The charring rates are based on a one-dimensionalscenario which does not take account of arris rounding,charring at glue lines or discontinuities, fissures, timbershakes and other defects. Alternative notional charring ratesare also provided by Eurocode 5 that take these effects intoaccount. Charring rates are provided in Eurocode 5 forspecific characteristic densities which may be modified fortimber panels to take account of other characteristic densities.

Effective cross-section methodCalculations to BS 5268-4.1 assume that the effective cross-section is the residual non-charred section after exposure tofire. The residual strength of this section for determining loadcapacity is taken as twice the long term dry stresses forsections with breadth less than 70 mm, or 2.25 times greaterfor a breadth of 70 mm or more.

A less simplified method based on the depth of charringand depth of thermal degradation in the underlying pyrolysiszone is provided by Eurocode 5. Using the assumptions madein Eurocode 5, the depth of the pyrolysis zone may be takenas about 25 mm. By integration of the temperature profile

across the pyrolysis zone, it can be shown that the averagetemperature in this zone is about 80 ºC assuming an ambienttemperature of 20 ºC. At this temperature the averagestrength is 70% of the ambient strength. Relating this to thedepth of pyrolysis zone yields a net depth, d0, of ineffectivetimber in the pyrolysis zone of 7 mm. This value may beadjusted for different fire exposure cases and durations. Theeffective cross-section is calculated by reducing the initialcross-section by the effective charring depth, def :

def = dchar + k0 d0

where:dchar = βn.t

k0 = adjustment factor for duration and surface protectiond0 = 7 mm

Reduced strength and stiffness method (for t ≥ 20 minutes)This second method adopted by Eurocode 5 is also derivedfrom integration of the temperature profiles in the pyrolysiszone. The modified timber properties, effective across theuncharred timber section, are established using:

fd,fi = kmod,fi

f20

γM,fi

where:fd,fi = design strength in fire (N/mm2)f20 = 20% fractile of a strength property at

normal temperature (N/mm2)γM,fi = partial safety factor for timber in fire

with, as shown in Figure 2 (page 4):kmod,fi = 1 – 1/125 x p/Ar Compressive strengthkmod,fi = 1 – 1/200 x p/Ar Bending strengthkmod,fi = 1 – 1/330 x p/Ar Tensile strength

where:p = perimeter of fire exposed residual cross-section (m)Ar = area of residual cross-section (m2)

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Table 2 Comparison of charring rates for British and European standardsStandard Material class Charring rate (mm/min) Depth of charring after fire exposure (mm)

(β) t = 30 mins t = 60 mins

BS 5268-4.1 Softwood – 20 40Hardwood, density ρ ≥ 650 kg/m3

(18% moisture content) – 15 30

One- Notional One- Notional One- Notionaldimensional dimensional dimensional

(β0) (βn)

prEN 1995-1-2 Solid softwood and beech 0.65 0.7 19.5 21 39 42(Eurocode 5) Glue laminated softwood and beech 0.65 0.8 19.5 24 39 48

Solid or glue laminated hardwood, density ρ ≥ 290 kg/m3 0.65 0.7 19.5 21 39 42Solid or glue laminated hardwood, ρ ≥ 450 kg/m3 0.5 0.55 15 16.5 30 33

Figure 1 Depths of charring of a timber column at differentheights: left to right, top to bottom of column

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Fire resistance of connections

Timber members are usually connected with fasteners suchas nails, bolts, screws and dowels. These fasteners can be alsoused in combination with jointing members such as steel joisthangers or embedded steel plates (flitch beams). The stiffnessand strength of steel reduce considerably upon heat exposureand it is common practice to encase a steel jointing memberfor protection. This protection can be either provided by thetimber member itself or by additional sacrificial linings (egwood-based panels or gypsum plasterboard). Prescriptiveconnection solutions are provided in BS 5268-4.1; andcalculation procedures and guidelines for a variety ofconnection types, including unprotected steel plates andglued-in timber plugs, are given in Eurocode 5.

Fire protected timber structures

Where parts of a timber structure do not have sufficient fireresistance, they must be protected using claddings or otherprotective materials such as rock fibre batts[2]. The two codes,Eurocode 5 and BS 5268-4.1 adopt different strategies forensuring sufficient protection is offered to structuralmembers. The British Standard has very prescriptive rulesthat, if followed, will result in a safe design for the requiredfire resistance. A more deterministic approach is adopted byEurocode 5 that considers both the time delay to the onset ofcharring, tch, and the time for failure of the protectivecladding, tf . Different charring rates are considered for whenthe timber is first exposed to the developed fire. After a charlayer of 25 mm has been achieved, the charring rates areassumed to revert to the initial, non-protected charring rate.

Both codes provide guidance for achieving firecompartmentation with timber frame constructions. Firecompartmentation ensures that, in the case of ignition, thefire does not spread to other parts of the building but remainswithin the compartment for the required duration; this appliesto multi-occupancy and multi-use buildings, and to buildingshaving party walls with other buildings. Compartmentationrules also apply to roofs where fire may spread via the eavesand roof voids to adjoining buildings; in these casesfirebreaks in the roofs should provide the necessaryresistance between properties. This requirement may beavoided by designing the ceiling of an uppermost storey asfully fire resisting.

Careful attention should also be paid to the overlaps andfixing of linings at corners and junctions, and to detailing[3].In particular:● lining joints should be staggered and exposed joints taped

and jointed (Figure 3);● ceiling boards should be trapped by wallboards

(Figure 4a);● wallboard lay-up in corners should be stepped so that outer

boards trap inner boards (Figure 4b);● in floating floor construction, wallboards should be

trapped by the floating floorboards (Figure 4c);● the specified fixings and fixing intervals must be applied to

each layer of wall and ceiling boards to achieve the overallrequired fire resistance.

Alternative lining materials to plasterboard may also be used;these include fibreboard, tongued-and-grooved timberboards, and cement bonded particleboard, although theremay be limitations on their application. Fire treatments mayextend their scope of application as described in the nextsection.

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Figure 4 Overlaps for two layers of wall linings in corners

4b Plan forwall/wall

joint

4c Elevation forwall/floor joint

4a Elevation forceiling/wall joint

Figure 3 Plan for staggered joints and fixings for wallboards inlinear construction

Floating floor

600 mm

Figure 2 Modification factor kmod,fi for compressive, tensile andbending strength (Eurocode 5)

0 20 40 60 80 100p/Ar (m–1)

Compressive

Bending

Tensile

Mod

ifica

tion

fact

or (k

mod

,fi)

1

0.8

0.6

0.4

0.2

0

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Fire retardants

Fire retardants can be applied to timber to provide increasedresistance to flame spread and reduce the active participationof timber in a fire, especially at the early stages. Thesetreatments can improve the performance of timber (fromClass 3 to Class 1) but cannot make timber non-combustible.

The purpose of fire retardant systems is to reduce the heatsupplied to the substrate so that the pyrolysis rate is reducedand temperatures remain too low for sustained flaming. This can be achieved by chemical modification of the timberor formation of a physical barrier (IP 24/79). These twoprocesses are generally called, respectively, impregnationtreatments and intumescent coatings. The basic principles of a fire retardant may be based on one or more of the following modes.

Mode A: endothermic decomposition. Creation of a heat-sink by using a compound that decomposes in a highlyendothermic process giving non-combustible volatileproducts (eg aluminium hydroxide or magnesium hydroxide)which perform a blanketing action in the flame.

Mode B: radical scavenging. Flame poisoning by evolution of chemical species that scavenge H and OHradicals – these radicals are the most active in propagatingthermo-oxidation in flames (eg hydrogen halides, metalhalides and P-containing compounds).

Mode C: charred layer formation. Limitation of heat andmass transfer across the phase boundary by creation of aninsulating charred layer on the surface.

Mode D: dilution of combustible mass. Reduction in theconcentration of combustibles in both the solid and gaseousphases of combustion, therefore reducing the heat ofcombustion and potential for flaming and self-sustainedcombustion.

Inorganic flame retardantsThese are water soluble salts that are pressure impregnatedinto the timber which is then dried. They are generallyeffective in the starting phase of fire propagation due to theirlow decomposition temperature of around 200 to 300 ºC. Themechanisms for fire suppression are mainly due to mode Aalthough boric acid has additional char formation (mode C).Other salts (eg ammonium polyphosphates and redphosphorus) also produce mode C reactions.

Halogen containing flame retardantsThese may be composed of bromine and chlorine containingcompounds that can be incorporated in to resin compounds.Decomposition at temperatures of 200 to 400 ºC producelarge quantities of radical scavengers (mode B) that inhibitthe chain reaction in the flame. However, in fire, halogenatedcompounds generate corrosive and physiologicallyhazardous acid gases. There may also be some corrosiveeffect on fixings and electrical cabling. Hydrogen halides arebelieved to be less toxic than the carbon monoxide that can be given off in a fire.

Phosphorus containing flame retardantsThese compounds (usually phosphoric and phosphonic acidesters) act by forming a protective layer of char (mode C).The compounds may be halogenated to provide a secondaryfire retardant mechanism (mode B) although theaccompanying generation of acidic gases may make thisundesirable. Low volatility must also be ensured as somecompounds may evaporate from the substrate.

Intumescent coatingsIntumescent coatings can be built up to provide a porouslayer of foam that acts as thermal insulation and expelsoxygen by the presence of non-flammable gases in the poresof the foam. In the heat of a fire these coatings can expand upto 100 times their initial volume. Charring of the foam layeralso helps to provide protection by mode C. These coatingswill need to be built up to sufficient thickness for achievingthe required fire resistance. The layers often will need to bemuch thicker layer than those adopted for normal coatingssuch as paints.

Advanced analytical methods

Advanced calculation models may be used to predict thebehaviour of individual timber members, parts of a timberstructure or entire structures subjected to fire exposure.Eurocode 5 gives guidance on the change of thermal andmechanical properties of timber with temperature, takingaccount of non-linear material reactions. The effect oftemperature on the following timber properties is provided:● thermal conductivity for wood and char layer;● specific heat for wood and charcoal;● density;● strength (compression, tension and shear);● modulus of elasticity (parallel to grain of softwood).

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References

[1] White R H. Analytical methods for determining fire resistance of timber members. SFPE handbook of fireprotection engineering (2nd edn), pp 4–217 to 4–229. Quincy (MA), National Fire Protection Association, 1995.

[2] British Gypsum. The white book. Loughborough, British Gypsum Ltd, 2001.

[3] Trada Technology. Timber frame construction (3rd edn). High Wycombe, Trada Technology Ltd, 2001.

BREBR 454 Multi-storey timber frame buildings: a design guideIP 24/79 The effect of flame-retardant treatments on some mechanical properties of wood

British Standards InstitutionBS 5268-4.1:1978 Structural use of timber. Fire resistance of timber structures. Recommendations for

calculating fire resistance of timber membersBS 5268-4.2:1990 Structural use of timber. Fire resistance of timber structures. Recommendations for

calculating fire resistance of timber stud walls and joisted floor constructions

Structural EurocodeprEN 1995-1-2:2003 Eurocode 5. Design of timber structures. General. Structural fire design

Further readingSociety of Fire Protection Engineers and National Fire Protection Association. Timber engineering STEP 1, Basis of design, material properties, structural components and joints. Centrum Hout, Almere (The Netherlands), 1995.

Society of Fire Protection Engineers and National Fire Protection Association. Timber engineering STEP 2, Design – details and structural systems. Centrum Hout, Almere (The Netherlands), 1995.

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Acknowledgements

This Digest has been produced with the support of the Office of theDeputy Prime Minister.The authors also thank R Grantham (formerly of BRE) and T Lennon(of FRS) for their contributions to this Digest

Other BRE and FBE publications concerned with fire engineering and fire designDigest 462 Steel structures supporting composite floor slabs: design for fireBR 128 Guidelines for the construction of fire-resisting structural elementsBR 135 Fire performance of external thermal insulation for walls of multi-storey buildings (2nd edition)BR 368 Design methodologies for smoke and heat exhaust ventilationBR 459 Fire safety engineering. A reference guideFB 5 New fire design method for steel frames with composite floor slabs

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