Timber Grading

8/18/2019 Timber Grading

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Production rocess ‐ in the mill rior to ackin and shipping

Sorting of products into groups with similar characteristics and properties

ruc ura gra eruc ura gra e

-- visual appearancevisual appearance

-- strength & stiffnessstrength & stiffness

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a specific grade


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Gradin is sim l sortin a roduction run into rou s that haveGradin is sim l sortin a roduction run into rou s that have

similar properties. The grouping of the properties can be any

mixture of appearance and structural properties. In order to give

similar properties. The grouping of the properties can be any

mixture of appearance and structural properties. In order to give

some uniformity across the industry there are standards for the

sorting of timber products. Many of these are Australian Standards,

some uniformity across the industry there are standards for the

sorting of timber products. Many of these are Australian Standards,

but some are industry-based standards.

Grading standards make it easier for a designer to communicate

but some are industry-based standards.

Grading standards make it easier for a designer to communicate

what is required in a piece of timber to the supplier. A grade

designation refers to a full suite of structural, utility or appearance

what is required in a piece of timber to the supplier. A grade

designation refers to a full suite of structural, utility or appearance

properties. Thus reference to a grade designator will mean that the

timber supplied should have the properties that enables it to meet

properties. Thus reference to a grade designator will mean that the

timber supplied should have the properties that enables it to meet

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ose gra ng requ remen s.ose gra ng requ remen s.


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‐ • Aust. Standards AS2796, 1810 etc

•

– knot size & frequency (location unimportant) – splits, cracks, checks (size and frequency) – , – utility ‐ want, wane, cup, bow,

spring, twist

Feature grademakes a featureof natural

Select gradeclear, uniform wood.Free of natural characteristics

c arac er s cseg knots

eg no no s, gum ve ns e c.

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Criterion for sort is estimatedstructural properties of timber

Lower stiffness

Higher stiffness

• Used for classification of timber with defined structural properties ‐ includes framing for housing

• Each grade associated with a suite of structural properties

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Structural gradingStructural grading is the process of sorting the timber on the basis of estimatesis the process of sorting the timber on the basis of estimates

of the structural properties of the timber. The only way in which we can know theof the structural properties of the timber. The only way in which we can know the

strength of timber for sure is to break each and every piece. The process ofstrength of timber for sure is to break each and every piece. The process of

structural grading is sorting by some characteristics of the timber that arestructural grading is sorting by some characteristics of the timber that are

reasonably well correlated with all of the structural properties.reasonably well correlated with all of the structural properties.

Structural properties awarded to each grouping are generally as follows:Structural properties awarded to each grouping are generally as follows:

-- --

typicallytypically

this is taken as a characteristic strength based on the lower 5th %this is taken as a characteristic strength based on the lower 5th %ile ile..

StiffnessStiffness -- a characteristic stiffness based on the mean stiffness is used.a characteristic stiffness based on the mean stiffness is used.

ThisThis

is appropriate for use in floors, frames and systems where there is loadis appropriate for use in floors, frames and systems where there is loadsharing between the parallel elements. It is also quite appropriate for thesharing between the parallel elements. It is also quite appropriate for the

prediction of the deflected appearance of structures.prediction of the deflected appearance of structures.

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principals of Structural Grading.principals of Structural Grading.


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Structural grading

Timber Stress GradesStructurall raded

TABLE 2.4STRUCTURAL DESIGN PROPERTIES

FOR F-GRADES

products need to be assigned properties for

Characteristic strength, MPaTension parallel to

grain( f 't)

StressGrade

Bending

( f ' b) Hardwood Softwood

Shear in

beam

( f 's)

Compression

parallel tograin( f 'c)

Characteristic shortduration average

modulus of elasticity parallel to grain, Mpa

(E)F34 100 60 50 7.2 75 21 500 .F14 40 25 21 3.7 30 12 000F11 35 20 17 3.1 25 10 500F8 25 15 13 2.5 20 9 100F7 20 12 10 2.1 15 7 900F5 16 9.7 8.2 1.8 12 6 900F4 13 7.7 6.5 1.5 9.7 6 100

Stress Grade StructuralStructural

gives proper esDeflection

BendingTension

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Strength StiffnessCompressionShear


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The strength and stiffness of timber varies from species toThe strength and stiffness of timber varies from species tospecies . In Australia, there are thousands of species ofspecies . In Australia, there are thousands of species oftimber and certainl hundreds of commerciall lo edtimber and certainl hundreds of commerciall lo edspecies. It would be a very daunting task to evaluate eachspecies. It would be a very daunting task to evaluate eachgrade of each and every species individuallygrade of each and every species individually..

Instead, for most of our species the strength properties areInstead, for most of our species the strength properties areassigned using a system of “assigned using a system of “ Stress gradesStress grades ”. The structural”. The structuralproperties that correspond to each Stress grade are givenproperties that correspond to each Stress grade are givenn . e g er e num er o en . e g er e num er o e - -gra e, egra e, ehigher the strength assigned to timber of that grade.higher the strength assigned to timber of that grade.Different strengths are used for bending, tension, shear andDifferent strengths are used for bending, tension, shear and

..If testing or evaluation has confirmed that the properties areIf testing or evaluation has confirmed that the properties arebetween two Fbetween two F- -grades, the lower one would be assignedgrades, the lower one would be assigned

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Structural grading

Timber Stress Grades• Limited number of grade descriptions

– F‐ grade system ‐ general visual graded timber – MGP grades ‐ machine graded softwoods – GL grades ‐ glued laminated timber – A ‐ grades ‐ visually graded Victorian Ash

Used by producers Used by designers

Design properties• F- grades AS 1720.1

Grading method• F- grades (visual, machine)

.• GL grades AS 1720.1• A - grades AS 1720.1

• GL grades (manufacture)• A - grades (visual)

88Grading is link between producer & designer


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Structural grading

ress ra es

– Stress grade is assigned to a package of timber – Stress rade ives structural ro erties

• Each piece in a package can be taken to have those properties• In most cases, timber has significantly greater strength than the

stress grade (5th%ile)• Stiffness is fre uentl close to the stiffness assi ned to thestress grade (mean)

• Each piece stamped with Stress Grade atgrading

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– o oure mar s mac ne s ressgrading) indicate F grades


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some easily measured parameter to correlate withsome easily measured parameter to correlate withstrength and stiffness properties. The moststrength and stiffness properties. The mostcommon are:common are:

VisualVisual gradinggrading -- grade indicator is thegrade indicator is thepresence of visually discerniblepresence of visually discernible

featuresfeatures

Machine stressMachine stress gradinggrading -- grade indicator isgrade indicator is

Grading by nonGrading by non--destructivedestructive scanningscanning of theof thetimbertimber -- grade indicator isgrade indicator is

scann ng ou pu sscann ng ou pu sProof gradingProof grading -- grade indicator is lower limitgrade indicator is lower limit

of major axis flexural strength.of major axis flexural strength.

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Visual Stress Grading

3 Step Process

sorts intoStructural GradesStep 1

#2#2 #3#3 #4#4

For each species ,

Tables in rules assi n

F?F? F?F? F?F? F?F?F?F?

an F-grade to each ofthe structural grades .

Step 3

-

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be stamped

on each piece.


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gradinggrading -- Grades are structural No. 1 to structural No.Grades are structural No. 1 to structural No.5. Within a species, it can be said that structural No. 15. Within a species, it can be said that structural No. 1s ou per orm e er an s ruc ura o . acs ou per orm e er an s ruc ura o . ac

species has an Fspecies has an F- -grade assigned for each structuralgrade assigned for each structuralgrade number. For example:grade number. For example:Seasoned mountain ash (Seasoned mountain ash ( euceuc.. regnansregnans ) has F27) has F27

assigned to structural No. 1 grade. In the production ofassigned to structural No. 1 grade. In the production of..to the best pieces, and these would be stamped withto the best pieces, and these would be stamped withF27.F27.

easone oop p ne as ass gne o s ruc ura o.easone oop p ne as ass gne o s ruc ura o.1 grade. This is because as a species, hoop pine has1 grade. This is because as a species, hoop pine hasmuch lower strength than mountain ash.much lower strength than mountain ash.

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The timber is specified by stress grade only.The timber is specified by stress grade only.


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Visual Stress Grading

Example ‐ seasoned tallowwoodTallowwood Australian Hardwood

Step 1

sorts intoStructural Grades

#1#1#2#2 #3#3 #4#4

#5#5 AS 2082

For each species ,

Tables in rules assi n

F22F22 F14F14 F11F11F17F17

Step 3an F-grade to each of the structural grades .

-

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be stamped

on each piece.


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•

(F5)Grade Method

Grading standard

(Visual Stress Grading)

(AS2858)

Mill / producer Moisture condition(Seasoned)

1515(radiata pine)


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Timber Grading

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