CE479_properties12

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CE 479: DESIGN OF BUILDING

COMPONENTS AND SYSTEMSFALL 2012J. LIU

Wood: Intro, Properties, Grades


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Introduction to WoodProperties

Design Specifications

Sizes, Grading

OUTLINE


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Wood Members

Species and Species Groups

Introduction to Wood


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Wood Members

Sawn lumber or solid sawn lumber

Wood members manufactured by cutting a member

directly from a log

Glued laminated timbers

a.k.a. glulams

Laminated stock, glued and laid up to form largerwood members


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Wood Members

Wood poles/timber piles

Manufactured products

Plywood

Oriented strand board (OSB) Structural composite lumber (laminated veneer or

parallel strand lumber)

Fabricated components

TrussesWood I-joists

Box beams


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Sawn lumberBasic size classifications

Dimension lumber

Smaller (thinner) sizes of structural lumber

Ranges from 2x2 through 4x16

Any material with nominal thickness of 2 to 4 inches

Timbers

Larger sizes

5 inch minimum nominal dimension Practically speaking, smallest timber size is a 6x6


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Structural designer uses lumber from a commercial

species group rather than a specific species

Same grading rules, reference design values, grade

stamps are applied to all species in a species group


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Note: some groups have similar names; each is

separate and distinctdifferent sets of reference

design values

Douglas Fir-Larch and Douglas Fir-Larch (N) Hem-Fir and Hem-Fir (N)

Spruce-Pine-Fir and Spruce-Pine-Fir (S)

(N) indicates a Canadian species group; (S) indicatesUSA species


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Hardwoods and Softwoods

Hardwoods - broadleafed deciduous trees

Softwoodsnarrow, needle-like leaves, generally

evergreen, also known as conifers C is for Conifers http://www.youtube.com/watch?v=FijQbZeTGNc

Large majority comes from Softwoods

Note: Douglas Fir-Larch and Southern Pine are

classified as softwoods, but are relatively dense and

have structural properties exceeding those of many

hardwoods
http://www.youtube.com/watch?v=FijQbZeTGNchttp://www.youtube.com/watch?v=FijQbZeTGNc


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Typical Commercial

Hardwoods

Maples

Oaks

Birches

Elms

Walnut

Canadian Conseil

Wood canadien

Council du bois


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Typical Commercial

Softwoods

Spruces

Pines

Firs

Cedars

Hemlocks

Larches

Canadian Conseil

Wood canadien

Council du bois


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Cellular Makeup

Growth Characteristics (+ Guest Lecture R. Kristie)Moisture Content

Shrinkage

Specific Gravity

StrengthOther Properties, Decay (+ Guest Lecture R. Kristie)

Properties


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Interior of a Tree

Age

Conditions of growth

Structures

Some properties

Canadian Conseil

Wood canadien

Council du bois


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Cellular MakeupCanadian Conseil

Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Wood canadien

Council du bois


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Growth Characteristics

Include density, moisture content, knots, checks,

shakes, splits, slope of grain, reaction wood, decay

Affect strength of lumber

Limits on size and number of defects permitted in agiven stress grade


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Knots

Portion of a branch or limb that hasbeen incorporated into the mainbody of the tree

Displace clear wood, cause slope ofgrain to deviate around them,therefore decreasing mechanicalproperties

Can cause stress concentrationsand/or checking

Effect on tension and compression;greater effect on tension


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Checks, Shakes, Splits

Separations of wood fibers

(A) Checks = radial cracks

(C) Shakes = separation parallel to annual rings

(B) Splits = complete separation of wood fibers


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Slope of Grain

Deviation of wood fibers from a line that is parallel

to edge of piece of lumber

Expressed as a ratio (e.g., 1:8, 1:15, etc.)

Measured over sufficient area to be representativeof general slope of fibers; local deviations around

knots disregarded


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Reaction Wood

Known as compression wood in softwood species

Abnormal wood that forms on underside of leaning

and crooked trees

Hard and brittle

Unbalanced structure in wood

Not permitted in stress grades of lumber


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Moisture ContentCanadian Conseil

Wood canadien

Council du bois


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Moisture Content

Moisture content in living trees comes from sap

(water and dissolved mineral salts)

Can be as high as 200% in sapwood of some tress

May be 30% in heartwood of others

Held in wood in two ways:

Free water in the cell cavity

First to be driven off as wood dries Bound water in the cell walls


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Moisture Content

Moisture content of lumber in service is much less

than that of a living tree (can be 200 percent)

Equilibrium moisture content (EMC)

Average moisture content in service

Ranges between 714%

MC at time of construction will be higher than EMC of abuilding (perhaps 2 times higher)


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Moisture Content

Fiber Saturation Point (FSP)

Moisture content that corresponds to complete loss of

free water

100% of bound water remaining No loss of bound water occurs above FSP

No volume changes or other changes in structural

properties associated with change in MC above FSP


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Moisture Content


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Fiber Saturation Point (FSP)


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Above FSPCanadian Conseil

Wood canadien

Council du bois


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Below FSPCanadian Conseil

Wood canadien

Council du bois


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ShrinkageCanadian Conseil

Wood canadien

Council du bois


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Wood canadien

Council du bois

C C


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Wood canadien

Council du bois

C di C il


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Wood canadien

Council du bois

C di C il


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Wood canadien

Council du bois


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Shrinkage

Shrinkage causes reduction insection properties, butreduction in MC increasesstructural properties

Drying of lumber in order toincrease structural properties isknown as Seasoning

Seasoning usually refers to acontrolled drying process suchas air or kiln drying


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Shrinkage


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Seasoning Checks

Canadian Conseil


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Wood canadien

Council du bois

Canadian Conseil


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Wood canadien

Council du bois

Canadian Conseil


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Kiln DryingCanadian Conseil

Wood canadien

Council du bois

Canadian Conseil


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Rate of DryingCanadian Conseil

Wood canadien

Council du bois


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Equilibrium Moisture Content


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Estimating Shrinkage

Wood Handbook (Forest Products Laboratory) providesvalues of tangential, radial, and volumetric shrinkagefrom clearwood samples, for different species Values given from 0 at nominal FSP to full shrinkage at zero

MC; intermediate values are interpolated

Other methods exist, but a simpler methodrecommended for following reasons: Shrinkage is a variable property

Orientation of annual rings in a real piece of lumber

unknown Designer will probably only know species group, not

individual species


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Estimating Shrinkage

Simple method (Rummelhart and Fantozzi, 1992) Constant shrinkage of 6 percent used for both width

and thickness of a member

Shrinkage taken as 0 at an FSP of 30 percent, and thefull 6 percent shrinkage assumed to occur at an MC ofzero.

Linear interpolation used for MC values between 30and 0.

Method based on western species lumber, butmethod shown to give reasonable estimates for mostspecies


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Estimating Shrinkage - Example

Estimate the shrinkage that will occur in a four-

story wood-frame wall that uses Hem-Fir

lumber. Consider a decrease in moisture from

15 to 8 percent.


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Estimating ShrinkageExample, contd.

A shrinkage of 6 percent is assumed to occur between

MC=30% and MC=0%. Use linear interpolation.

Shrinkage value SV = 6/30 = 0.2% per 1 % change in MC= 0.002 in/in per 1% change in MC

Shrinkage S that occurs in the dimension, d, of a piece:

Shrinkage S = SV x dx MC = 0.002 x dx MC


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Shrinkage in depth of 2x12 floor

joist:Sfloor = 0.002 in/in x 11.25 in x (15-8) = 0.158 in

Shrinkage in thickness of one 2x wall plate:

Splate= 0.002 in/in x 1.5 in x (15-8) = 0.021 in

Shrinkage in length of a stud; longitudinal

shrinkage is small:

Sstud 0 in


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Total S = 3 Sfloor + 12 Splate

Total S = 3 (0.158 in) + 12 (0.021 in)

Total S = 0.725 in in


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Moisture Content and Lumber Sizes

Moisture content of lumber affects cross-sectionaldimensions

No need to adjust section properties to account forinitial MC and EMC and resulting shrinkage

Grading practices for dimension lumber haveestablished the dry size (MC19 percent) of a

member as basis for structural calculations

Manufacturing adjusted to MC of wood at time ofmanufacturer (i.e., lumber from green wood islarger at time of manufacture)

Canadian Conseil


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Specific Gravity Wood canadienCouncil du bois


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Specific Gravity and Strength

Canadian Conseil


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Strength Wood canadienCouncil du bois

Canadian Conseil


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Compressive Strength Wood canadienCouncil du bois

Canadian Conseil


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Compressive Strength Wood canadienCouncil du bois

Canadian Conseil


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Strength Wood canadienCouncil du bois

SCanadian Conseil


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Tensile Strength Wood canadienCouncil du bois

l S hCanadian Conseil


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Tensile Strength Wood canadienCouncil du bois

R d l S C d M b


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Radial Stress in Curved Members

B diCanadian Conseil

W d di


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Bending Wood canadienCouncil du bois

L i di l ShCanadian Conseil

W d di


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Longitudinal Shear Wood canadienCouncil du bois

F i L diCanadian Conseil

W d di


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Fatigue Loading Wood canadienCouncil du bois

TCanadian Conseil

Wood canadien


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Temperature Wood canadienCouncil du bois

Preservative Processes, Fire-retardant


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,

Chemicals

Th l E iCanadian Conseil

Wood canadien


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Thermal Expansion Wood canadienCouncil du bois

I l ti A tiCanadian Conseil

Wood canadien


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Insulation, Acoustics Wood canadienCouncil du bois

P T tiCanadian Conseil

Wood canadien


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Pressure-Treating Wood canadienCouncil du bois

P T ti


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Pressure-Treating

Chemical does not saturate the complete cross section;minimize field cutting and drilling of holes

Many species (e.g. southern pines) readily accept

treatment Others require incising (small cuts or incisions on all four

sides)

Modification of modulus of elasticity and bending, tension

and compression parallel to grain must be made No modification required for pressure-treated lumber

without incising


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NDS for Wood Construction

Design Specifications

NDS f W d C t ti


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NDS for Wood Construction

2012 National Design Specification (NDS) forWood Construction

All or part of NDS usually incorporated into the

International Building Code (IBC) Integration of new Load and Resistance Factor Design

(LRFD) and traditional Allowable Stress Design (ASD)

provisions

NDS Supplement Contains numerical values of design stresses


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Size Categories

Commercial Grades

Grading Structural Lumber

Grade Marks

Machine Grading

Basic Design Values

Sizes, Grading

Si es of Str ct ral L mber


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Sizes of Structural Lumber

Dressed lumber

Surfaced to standard net size

Net size is less than nominal size

Most structural lumber is dressed Dressed on a planing machine for smooth surfaces and

uniform sizes

Typically surfaced four sides (S4S)

Other finishes include S2S1Esurfaced 2 sides 1 edge



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Rough Sawn Large timbers are commonly rough sawn

Dimensions close to standard net sizes

Textured surface Approximately 1/8 in larger than standard net sizes

Full Sawn

Less common

Actual size of lumber same as the specified size



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Consider nominal 8 x 12 member (8 in x 12 in)

DRESSED ROUGH

SAWN

FULL

SAWN7 x 11 in7-5/8 x 11-5/8 in 8 x 12 in

Standard Dressed Size

Nominal Size Actual Size

Dressed Lumber


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Dressed Lumber


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NDS 2012 Supplement Chapter 3 Section Properties

Size Categories Nominal Size Ranges


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Size CategoriesNominal Size Ranges

Boards to 1-1/2 in thick

2 in and wider

Dimension Lumber 2 to 4 in thick

2 in and wider

Timbers 5 in and thicker

5 in and wider

Size Categories Subdivisions


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Size CategoriesSubdivisions

Boards Stress-Rated Board (SRB)

Dimension Lumber

Structural Light Framing (SLF) Light Framing

Studs

Structural Joists and Planks (SJ&P)

Decking Timbers

Beams and Stringers (B&S)

Posts and Timbers (P&T)

Size Categories


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Size Categories

Name Nominal

Thickness

Nominal Width Examples of Sizes

Light Framing (LF)

and Structural

Light Framing (SLF)

2 to 4 in 2 to 4 in 2 x 2, 2 x 4, 4x4

Structural Joist and

Plank (SJ&P)

2 to 4 in 5 in and wider 2 x 6, 2 x 14,

4 x 10

Stud 2 to 4 in 2 in and wider 2 x 4, 2 x 6, 4 x 6

(lengths 10 ft and

shorter)

Decking* 2 to 4 in 4 in and wider 2 x 4, 2 x 8

*stressed about its minor axis

Size Categories


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Size Categories

Name Nominal

Thickness

Nominal Width Examples of Sizes

Beams and

Stringers (B&S)

5 in and thicker More than 2 in

greater than

thickness

6 x 10, 6 x 14,

12 x 16

Posts and Timbers

(P&T)

5 in and thicker Not more than 2 in

greater than

thickness

6 x 6, 6 x 8,

12 x 14

NDS 2012 Section 4.1.3

Commercial Grades


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Commercial Grades

Vary within various size and use categories Different design values apply to same grade name

in different size categories

For example, Select Structural is available in SLF,SJ&P, B&S, and P&T

Lumber grading rules reflect anticipated use ofwood member based on size, but no restriction on

actual use Reference design values given for tension, compression

and bending for ALL size categories

Commercial Grades Examples


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Commercial GradesExamples

Structural Light Framing (SLF) Select Structural, No. 1 and Better, No. 1, No. 2, No. 3

Light Framing (LF)

Construction, Standard, Utility

Stud

Stud

Decking

Select Decking, Commercial Decking Beams & Stringers

Dense Select Structural, Select Structural, Dense No. 1, No.1, Dense No. 2, No. 2



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Majority of sawn lumber is visually graded



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Grade stamp includes:Grade

Species or species group

Other pertinent information Stress grade

If lumber grade has recognized mechanical properties

for use in structural design, referred to as a stress

grade



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More than one set of grading rules can be used tograde some commercial species groups

For example, Douglas Fir-Larch can be graded under

Western Wood Products Association (WWPA) rules or

under West Coast Lumber Inspection Bureau (WCLIB)

rules

Tables in NDS supplement clearly identify grading rules

(e.g. WWPA and/or WCLIB)

Grade Marks


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Grade Marks

Lumber Grading Agency

(e.g. Western Wood Products Association (WWPA))

Mill number

Grade Marks


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Grade Marks

Moisture content at time of surfacing, or condition of

seasoning

Lumber Grade

Grade Marks


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Grade Marks

S-DRY = Surface Dry

S-GRN = Surface

Green KD = Kiln Dried

MC = Moisture

Content

Grade Marks Moisture Content


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Grade Marks, Moisture Content

S-GRN (MC greater than 19 percent at timemanufacture)

Assumed to have 19 percent initial moisture content

S-DRY or KD (MC of 19percent or less at time ofmanufacture) Assumed to have 15 percent initial moisture content

These assumptions appropriate for relatively thin

material (i.e., 2 x floor joists and wall plates) Final moisture content can be taken as equilibrium

moisture content (EMC)between 7 to 14 percent

Grade Marks


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Grade Marks

Commercial lumber species (Douglas Fir)

Grade Marks


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Grade Marks

Grade Marks


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Grade Marks

Grade Marks


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Grade Marks

HTheat-treated Sometimes heat-treated to kill insects for international

shipments

Not the same as KD

kiln dried Relatively high temperatures for relatively short times

Grade Marks


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Grade Marks

Machine Grading


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Machine Grading

Machine evaluation

Lumber moves through a machine that non-

destructively tests for a given property of the

lumber such as density; other structural propertiesmeasured or derived

Typically only used on lumber for which very

accurate structural properties needed

Also visually checked

Machine Grading


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Machine Grading

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CE479_properties12