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Materials for Civil and Construction Engineers

CHAPTER 10Wood

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Wood is the earliest construction material used by mankind. easy to usedurablehigh strengthlow weightwidely availablelow cost

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Still very widely used today for:building framesbridgesutility polesfloorsroofstrussespiles, etc.

Introduction

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Classification of Trees Endogenous (intertwined growth): e.g., palm treesvery strong and lightweightnot generally used for engineering applications in U.S.

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• Exogenous (outward growth): e.g., most other trees Fibers grow from the center outward by adding concentric

layers (annual rings) which gives more predictable engineering properties.

Deciduous (broad leaf) = hardwood (ash, oak, maple, walnut, etc.) – expensive slow growing

Coniferous (cone bearing, evergreens) = softwood (Douglas fir, pine, spruce, cedar, etc.)

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Each annual ring of exogenous tree is composed of:Earlywood (light ring): rapid spring growth of hollow

thin-walled cellsLatewood (dark ring): dense summer growth of thick-

walled cells which are much harder & stronger

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10.1 Structure of Wood

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Main Structural Features of Tree Stem

From center axis outwards:Pith – center stemHeartwood (darker) –

provides structural supportSapwood (lighter) – transports

the sapCambium (very thin layer) –

location of wood growthInner barkOuter bark

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Wood is Anisotropic Longitudinal

parallel to the long axis (grain)strongest and least shrinkage

Radialperpendicular to the

growth rings (out from center) Tangential

tangent to the growth ringsweakest and most shrinkage

directions influence strength, modulus, thermal expansion, conductivity, shrinkage, etc.

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– properties change with direction:

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10.2 Chemical Composition Cellulose

50% by weightpolymer that forms

strands (fibrils) that make up cell walls (wood fibers)

Gcellulose = 1.5High density indicates

higher strength

Lignin23-33% of softwood 16-25% of hardwood by

weightglue

Hemicellulose15-20% of softwood 20-30% of hardwood

Extractives5-30% by weighttannins, coloring matters,

essential oils, fats, resins, waxes, starches

Ash-forming (minerals)0.1-3% by weightcalcium, potassium,

phosphate, silica

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10.3 Moisture Content shrinkage, strength, & weight depend on moisture content depends on air temperature and humidity:

slow changing so it tends to adjust near the averageEquilibrium Moisture Content (EMC)

moisture content for average atmospheric conditions

1% when hot & dry >130oF & 5% humidity20% when warm & humid <80oF & 90% humidity

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100d

dmc W

WWM

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Fiber Saturation Point (FSP) moisture content when cells are completely saturated with

bound water but no free water inside cell cavities

FSP = 21-32%Above FSP

changes affect only wet weight Below FSP

small changes strongly affect all physical and mechanical properties

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held tightly in cell cavities, wood shrinks on removal

water inside cell cavities doesn't affect shrinkage

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Shrinkagelargest shrinkage is in the tangential directionsmallest shrinkage is in the longitudinal directionzero shrinkage above FSP regardless of direction

For glulam (varying growth ring orientations)assume 6% shrinkage in 30% change in m/c below FSP (or

1% shrinkage per 5% change in m/c)

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Fiber SaturationPoint

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10.4 Wood Production

Production Steps:

1. Harvesting

2. Sawing

3. Seasoning (drying)

4. Surfacing (Planing) (optional)

5. Grading

6. Preservative Treating (optional)

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Wood Products for ConstructionDimensional lumber – 2” to 5" thick – 2x4,s etc.

used for light framing – studs, joists, beams, rafters, trusses, decking Heavy timber – 4x6, 6x6, 8x8 and larger

usually rough sawn (actual sizes)used for heavy framing, railroad ties, landscaping

Round stockposts and poles – used for marine piling, utility poles, etc.

Specialty itemshandrails, spindles, radius edge decking, turned posts, lattice, etc.

Engineered wood productsbonding wood strands, veneers, lumber or other wood fibers large integral composite unit

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Step 1. Harvestingminimal sapconcerns of fire hazardother plant growth and underbrush is minimal

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Live (plain) sawing – most rapid and economicQuarter sawing – maximum amount of prime (vertical)

cutsCombination – most typical

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Live (Plain) Sawing Quarter Sawing Combination

Step 2. Sawing

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Three types of board cutFlat-sawn (grain is <45o from flat side)

worst quality, most problems and defectsRift-sawn (45o-80o)Quarter-sawn (vertical- or edge-sawn) (80o-90o)

best quality, least shrinkage problems

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Flat-Sawn Rift-Sawn Quarter-Sawn

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Green wood has 30-200% moisture content ~15% when it leaves the mill Methods of Seasoning

air drying (cheap & slow)kiln drying (fast & expensive)usually a combination

Uneven shrinkage in different directions during seasoning causes warping, checks, shakes, etc.

Type of cut controls these problems (vertical is the best)

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Step 3. Seasoning (Drying)

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Surfacing takes 1/4" (or more) from each side

S4S = surfaced 4 sides = “dressed”

Nominal sizes refer to the rough-sawn (unsurfaced)

dimensions of the lumber in inches

For example, the actual dimensions of a 2 x 4 are 1 ½ in. x 3

½ in.

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Step 4. Surfacing (Planing)

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Several agencies for different regions and species Graded according to number of defects that affect strength

& durability (knots, checks, pitch pockets, shakes, stains) Visual (appearance) grading Stress (structural) grading – Table 10.3 Hardwood grades – visual (also stress) grading Softwood grades – visual & machine stress grading For civil engineering applications, appearance grades are

less important than structural grades

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10.5 Lumber Grades (Step 5)

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Affect both appearance & mechanical properties Caused by:

natural wood growthseasoning too fastwood diseasesanimal parasitesfaulty processing

Knots – branch base that degrades mechanical propertiessound, tight knots may be good in compression but don’t

count on it

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10.6 Lumber Defects

USC21Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

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Shakes – wood separations between annual ringsWane – bark or other soft wood left on the edge of the board• Sap Streak – colored streak of sap accumulated in wood

fibers• Reaction Wood – extra dense woody tissue that can cause

warping and cracking• Pitch pockets – opening between annual rings that contain

resin• Bark Pockets – small patches of bark embedded in the wood• Checks – ruptures along the grain from drying

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• Splits – lengthwise separations caused by mishandling or seasoning

• Warping – (several types) from uneven drying of internal tree stressBowing – lengthwise curvature from end to endCrooking – lengthwise curvature from side to sideCupping – edges roll upTwisting – one corner lifts

• Raised, Loosened, or Fuzzy Grain• Chipped or Torn Grain• Machine Burn – from worn saw blades

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1. Specific Gravity & Density

Specific gravity of the cell walls (cellulose) = 1.5 regardless of species

excellent indicator of the amount of material (and properties) in dry wood

closer to 1.5 means more cell walls which is denser & stronger

Dry density = usually 20-45 lb/ft3 (300-700 kg/m3)

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10.7 Physical Properties of Wood

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2. Thermal Properties Thermal conductivityThe rate that heat flows through (inverse of thermal

resistance R value)Good R value (R = 1 / conductivity)

much better than metalsslightly worse than insulationreduces loss of heat and colddelays fire

• Specific HeatRatio of the quantity of heat required to raise the temp. of

the material 1o to that required to raise the temp. of an equal mass of water 1o

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• Thermal Diffusivity

Rate that material absorbs heat from surroundings

Much better (lower) than most other building materials• Thermal Expansion

Anisotropic: 5-10x greater across grain than parallel to it

Applying heat to wood:

first expands the wood from thermal expansion

then it shrinks from moisture loss (when below FSP)

3. Electrical Properties• Good electrical insulator which decreases with moisture

content – more water is a better electrical conductor

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Wood is extremely anisotropic1. Modulus of Elasticity 1-2 x 106 psi – for compression parallel to the grain linear up to proportional limit, then small non-linear curve

Depends on: species variation moisture content specific gravity direction of grain

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10.8 Mechanical Properties of Wood

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2. Strength Properties

Vary widely because of anisotropy, moisture content, defects, etc.

Tensile strength is greater than compressive strength

Tensile strength parallel to grain is 20x greater than perpendicular

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3. Load Duration Wood can support higher loads of short duration than

sustained loads Under sustained loads wood continues to deform Design values assume 10 year loading and/or 90% of full

maximum load throughout life of the structure Multiply design values by load duration factors for short-

duration loads

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Load Duration Factors

Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

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4. Damping Capacity

Vibration damping (like shock absorbers) increases with moisture content up to FSP

10x greater damping than structural metals

wood structures dampen vibrations much better than metal

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strength of wood structures is usually controlled by the joints and connections, which is the main concern of structural wood design classeswe have lots of experience with smaller structures

(residential, light commercial) so design is usually empirical

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Wood is tested to predict performance two main techniques

testing of timbers of structural sizes (ASTM D 198) testing of representative, small, clear specimens (ASTM

D 143) Testing of structural-size members is more important –more

applicable to design values Tests include flexure, compression, tension, etc. Flexure test is more commonly used than the other tests Two-point, third-point, or center-point loading

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10.9 Mechanical Testing

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Third-point bending test on a 4 x 6 wood lumber

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Compression parallel to grain

Tension perpendicular

to grain

Compressionperpendicular

to grain

Tension parallelto grain

Hardness perpendicular

to grain

Hardness parallel to grain

Bending

Testing Representative, Small, Clear Specimens

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10.10 Design Considerations For design of wood structures, strength properties (Tables 10.3

&10.4) must be adjusted for the following factors

Load duration Wet service

Temperature Beam stability

Size Volume (glulam only)

Flat use Curvature (glulam only)

Column stability Bearing area

Repetitive member (lumber only)

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10.11 Organisms that Degrade Wood

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Fungi caused dry rot

Spruce Ips Beetle

Bacteria damageblack heartwood Termite damage

Marine-borer damage to a buried pile

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10.12 Wood Preservation1. Petroleum-based Solutions

2. Waterborne Preservatives (Salts)

Application Techniques

• Superficial treatment: generally not effective

• Liquid penetration (pressure treating at high temp., heat, & moisture)

Structural members need to be fabricated as much as possible before treatment in order not to expose untreated wood by cutting, drilling holes, etc.

If not possible, treat cuts and holes with a liberal application of field applied preservative

Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

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Made by bonding together wood strands, veneers,

lumber, or other forms of wood fibers to produce

large units

engineered to produce specific and consistent

mechanical properties that are better than natural

large pieces

very difficult and expensive to find high quality

large natural pieces

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10.13 Engineered Wood Products

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Plywoodthin sheets (plies) glued

together with the grain at right angles to each other so it has the same properties in both directions

veneer is peeled from a soaked log on a giant lathe

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Particle & strand boardglue together wood scraps with resin to form sheets:

particle board = sawdust sized particleschip board = randomly oriented wood chipsOSB = wood chips & strands oriented in specific

direction

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Floor joistsmade with two 2x4s or 2x6s

as flanges and an OSB web

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Glue-Laminated Timbers (Glulam)lumber glued together with the parallel grainused for structural members, furniture, sports equipment,

and decorative wood finishespreferred because:

ease of manufacturing large

members from standard commercial lumbercan vary the cross section along

the lengthspecial architectural designscan use lower wood grade in less stressed areasminimizes shrinkage defects

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