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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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Species and Species Groups
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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Species and Species Groups
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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Species and Species Groups
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
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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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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
Wood canadien
Council du bois
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Cellular MakeupCanadian Conseil
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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Moisture ContentCanadian Conseil
Wood canadien
Council du bois
C C
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ShrinkageCanadian Conseil
Wood canadien
Council du bois
C di C il
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ShrinkageCanadian Conseil
Wood canadien
Council du bois
C di C il
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ShrinkageCanadian Conseil
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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ShrinkageCanadian Conseil
Wood canadien
Council du bois
Canadian Conseil
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Moisture ContentCanadian Conseil
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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Estimating ShrinkageExample, contd.
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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Estimating ShrinkageExample, contd.
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
Sizes of Structural Lumber
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Sizes of Structural Lumber
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
Sizes of Structural Lumber
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Sizes of Structural Lumber
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
Grading Structural Lumber
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Grading Structural Lumber
Majority of sawn lumber is visually graded
Grading Structural Lumber
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Grading Structural Lumber
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
Grading Structural Lumber
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Grading Structural Lumber
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
Recommended