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1
CE113
Introduction to Properties of Wood
2
Material Properties
Microstructure & macrostructureSoftwoods & hardwoodsDefects & stress gradingMoisture & load duration effectsStress grading
© 2002 R. J. Schmidt
3
Wood Chemical Composition
Cellulose (40-50%)Lignin (18-25%)Hemicellulose (20-30%)ExtractivesAsh-forming minerals
(2-30%)
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Wood Chemical Composition
Cellulose – Carbohydrate of long chain polymers organized into microfibrils
(Note: Cotton is pure cellulose)Lignin – Organic compound the binds the microfibrils together in the secondary walls and middle lamella
( )nOHC 5106
5
Wood Chemical Composition
Hemicellulose – A cellulose-like material in the wood cell wall that is easily decomposed by dilute acid to yield simple sugars Extractives – Organic material deposited during heartwood formation (gums, resins, oils, alkaloids)
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Micrograph – Red Pine
ew = early woodlw = late woodrc = resin canalr = ray
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Macrostructure – Red Oak
A = cambium layerB = inner barkC = outer barkD = sap woodE = heart woodF = pithG = rays
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Macrostructure – Douglas Fir
ob = outer barkib = inner barkcz = cambium zonegi = growth incrementp = pithx = wood
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Softwoods
Naked seeds in conesNeedle-like leavesUsually evergreenSimple cell structureSome are very high strengthn Douglas firn Southern Pine
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Hardwoods
Seeds are enclosed in shellsBroad leavesDeciduousComplex cell structureSome are very low strengthn Aspenn Cottonwood
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Defects
KnotsChecks, shakes, splitsReaction woodWane, skipSlope of grainWarp
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Knots
Intergrownn Tree trunk grows
around and with a living branch
Encasedn Tree trunk grows
around a dead branch
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Checks, shakes, splits
Checks – radial cracks due to differential shrinkage (ST > SR)Shakes – tangential cracks due to shock loading or growth defectSplit – any thru-thickness crack
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Reaction Wood
Growth defectAbnormal cell growth while under bending stressSoftwoods – Compression woodHardwoods – Tension woodBigger problem for lumber than for timber
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Reaction Wood
Compression Wood Tension Wood
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Compression Wood in Lumber
Longitudinalshrinkagecracking
Warp fromdifferentialshrinkage
Compressionwood
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Wane & Skip
Cutting defectsWane – Incomplete rectangular cross section due to cutting too close to the barkSkip – Saw gouge, chip, or other recessed area not removed by planing at the mill.
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Slope of Grain
Growth or cutting defectWood cell fibers not parallel to member axisCommon around knotsCauses failure by shear and tension perp.
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Warp
Bow – Curvature about minor axisCrook – Curvature about major axisTwist – board does not lie flatKink – Sharp bend at a knotCup – Curvature about longitudinal axis
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Warp
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Moisture Content
MC = (Weight of H2O)÷(Weight of Solids) MC > 100% in a living treeat 700F & 65%RH, MC ˜ 12%MC > 19% defined as “wet service”
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Shrinkage
Decrease in dimension due to loss of moisturen St ˜ 2xSr
Softwoods – Doug Fir: St = 7-8%Hardwoods – Red Oak: St = 9-13%
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Shrinkage Deformation
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Moisture Affects
StiffnessStrength
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Duration of Load EffectsMadison Curve
wind
snowroof live
livedead
impact
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Stress Grading
Method of classifying lumber to identify its structural qualitiesVisual or Machine methodsAgencies that write grading rulesn WWPA – Western Wood Products Assn.n NELMA – Northeast Lumber Manufacturers Assn.n NLGA – National Lumber Grades Authority
(Canada)n … several others
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Stress Grading
Visual gradingn Visual examination of each piecen Identification of defect types and effectsn High variability in resultn For dimension lumber and timbers
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Visual GradingRules
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Lumber and Timber
Lumber:
Timber:
n Posts and Timbers
n Beams and Stringers
5 ,5 ≥≥ db
in 2+≤ bd
in 2+> bd
in 4in 2 << bb
d
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In-Grade Test Program
For dimension lumber onlyTest each stress grade individuallyAccount for statistical variationUse “lower 5% exclusion limit”n 5% of all pieces will be below the limit
Reduce by factor of 2.1n Factor of safetyn Time effect factor
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Test Data Variation
0.0E+00
1.0E-04
2.0E-04
3.0E-04
0 2500 5000 7500 10000
Breaking Strength (psi)
Fre
qu
ency Mean = 5000 psi
COV = 0.3
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Test Statistics
Mean of all tests:
Standard deviation:
Coefficient of variation:
5% exclusion limit: σ64.1%5 −= FF
F
σ
FCOV σ=
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Sample Design Value, Fb
0.0E+00
1.0E-04
2.0E-04
3.0E-04
0 2500 5000 7500 10000
Breaking Strength (psi)
Fre
qu
ency
5%
5%
b
F 5000psiCOV 0.3
1500psi
F F 1.64
F 2540psiFS 2.1F 1210psi
==
σ =
= − σ
===
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Timber Design Values
For timbers onlySmall, clear specimens testedUse “lower 5% exclusion limit”Adjust for knots, size effect, etc.Reduce by factor of 2.1
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Stress Grading
Machine gradingn Nondestructive evaluation of each piecen Measurement of stiffness or densityn Statistical correlation to strengthn For dimension lumber only (currently)n Most machine graded lumber goes into
nail-plate trusses
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E vs Fb Correlation, Eqn. Form
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E vs Fb Correlation, Real Data
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Grade Stamps
Visual Grade Machine Grade
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Design Methods
Allowable stress designn ANSI/AF&PA NDS-1997 (NDS-2001)
Load and resistance factor designn AF&PA/ASCE Standard 16-95
Both are recognized by building codes
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Allowable stress design (ASD)
aka: Working stress designUses service level loadsAssumes elastic behaviorUnknown reliability (probability of failure)
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Allowable Stress Design
Design inequality:
… is usually written in terms of stress:
FSR
Q ni ≤∑
,'bb Ff ≤FS
MORFb =',
x
xb S
Mf =
(in psi)
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Load and Resistance Factor Design (LRFD)
aka: Limit states designaka: Ultimate strength design
(like concrete)Uses factored loadsRecognizes inelastic behavior (if any)Well defined reliability (probability of failure)
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Load and Resistance Factor Design
Design inequality:
… is usually written in terms of force or moment:
nii RQ φγ ≤∑
'MM bu λφ≤ LDu MMM 6.12.1 +=
time effect factor (only used in wood)
Factor up the moments
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ASD vs LRFD
A structure does not care which design method was usedBehavior is independent of design rulesASD has worked fine for yearsFor inelastic (ductile) materials, LRFD more closely expresses behavior
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ASD vs LRFD
ASD advantagesn Comfort and familiarity – not trivial
considerations!LRFD advantagesn More consistent reliability, esp. w.r.t. loadsn Adapts more easily to new developments:w Engineered wood productsw Connection systemswMixed material systems
48
NDS Overview
SpecificationAllowable stress tablesCommentaryOther supplements
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Specification
Chapter 1 – Boiler plate & notationn fi à Actual stress, an analysis quantityn Fi à Allowable stress or material strengthn Ci à Adjustment factorn Fi
’ à Adjusted allowable stress
50
Specification
Chapter 2 - Equation format for allowable stresses
[ ]fcrifuVFLtMDbb CCCCCCCCCCCFF ='
adjustment factors
tabulated design value (allowable stress)
53
Design Values for Wood Construction (DVWC)
Materials coveredn Dimension lumberw Visually graded
n Western woodsn Southern pine
wMachine graded
n Timbers – softwood and hardwoodw Visually graded
54
Design Values for Wood Construction (DVWC)
Properties listedn Fb à Bendingn Ft à Tensionn Fv à Shearn Fc à Compression perpendicularn Fc à Compression paralleln E à Modulus of Elasticity (MOE)
⊥
55
Allowable Stresses