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CE113

Introduction to Properties of Wood

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Material Properties

Microstructure & macrostructureSoftwoods & hardwoodsDefects & stress gradingMoisture & load duration effectsStress grading

© 2002 R. J. Schmidt

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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

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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

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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

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Specification

Chapter 2 - Equation format for allowable stresses

[ ]fcrifuVFLtMDbb CCCCCCCCCCCFF ='

adjustment factors

tabulated design value (allowable stress)

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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

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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)

⊥

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Allowable Stresses