Structural Materials - Chapter 5: Wood

what is the specific gravity of all wood substance?

1.55

what is the modulus of elasticity in the longitudinal direction of wood?

1000 (softwood) - 2000 (hardwood) ksi

what moisture content does lumber contain at the time of manufacture?

50%

how does the modulus of elasticity change with moisture content?

MOE decreases 1-3% for every 1% increase in moisture content

how does the modulus of elasticity in each direction compare?

MOE in the radial direction is 10% of that in the longitudinal direction; MOE in tangential direction is 1/2 of that in the radial direction

split

a complete separation of wood fibers; usually at the ends, throughout the thickness of lumber & parallel to the fiber direction; affects durability & allows moisture into timber which will eventually lead to decay

knot

a cross-section or longitudinal section of a branch that was cut with the lumber; force the grains to deviate which affects mechanical properties; allow stress concentrations to occur; harder, denser, and possess different shrinkage characteristics

checks

a lengthwise separation of wood occurring across or through the annual rings; usually occur as a result of seasoning; may occur anywhere on a piece of lumber; affects load carrying capacity

shake

a lengthwise separation of wood occurring between and parallel to annual rings

hygroscopic

a term describing compounds that remove moisture from the air

moisture content varies with

age of the tree, size of the tree, location

what are the 2 methods of seasoning lumber?

air drying & kiln drying

which 3 directions does shrinkage occur in?

along its axis, along a radius (across annual rings), along a tangent to the radius (along annual rings)

what does decay look like in wood?

appears as light colored sections; fungi uses cell walls & lignin as food; makes the lumber brittle & weak

wood grain

arrangement of wood fibers

hardwoods - examples

aspen, cherry, cottonwood, elm, hickory, maple, oak, sycamore, walnut

kiln drying

can't lose more than 2% moisture content a day; if it does, the temperature is too high & the properties are affected; used for drying lumber used in furniture or flooring

cellulose

carbohydrate of high molecular weight, accounts for 50% of wood substance; the main building block of cellulose is sugar (glucose); provides axial strength through strong bonds within & between fibers

softwoods - examples

cedar, douglas fir, fir, hemlock, larch, pine, redwood, spruce, tamarack

lumen

cell cavity

shrinkage varies with

cell wall thickness, species, arrangement of cells, etc

chemical composition of dry wood

cellulose, hemicellulose, lignin, extractives

air drying

cheaper than kiln drying; lumber is stacked in open sided sheds; slow drying lumber stacked in the winter (can't rely on it in winter); lumber will not be affected by summer heat until its MC is relatively low; softwood - 6 wks, hardwood - 8-10 wks

how does compressive strength vary with moisture content?

compressive strength increases an average 4-6% for every 1% decrease in moisture content

how does compressive strength vary with direction?

compressive strength perpendicular to the grain varies between 12-18% of that parallel to the grain; lumber possesses significantly higher compressive strength when it is dry

softwoods

conifers - generally evergreens; easier to grow; most structural lumber

why is only smaller sized lumber seasoned in the US?

costs are high

types of defects

cracks (checks, shakes, splits) & knots (due to branches)

when water is dried out of the cell walls...

decrease in dimensions, shrinkage of the lumber

what does orthotropic mean?

different directions have different properties

which is more dense - dry sapwood or dry heartwood?

dry heartwood

benefits of wood

fabrication, weight, reusability, insulating

plantation trees' rings

fast growing trees with wide annual rings; 2-4 growth rings per inch

defects

faults that may affect strength and durability

what 3 categories is lumber classified into?

framing lumber (joists, posts, trusses), appearance lumber (strength is not the primary concern), industrial lumber (mining, scaffolding, & foundations)

hardwoods (deciduous)

generally shed their leaves at the end of a growing season; used in furniture and decorative veneers; because they are expensive, they have limited use in construction

specific gravity is normally determined in 3 conditions

green (MC > 19%) air-dry (MC = 12%) oven dry

which is more dense - green lumber or dry lumber?

green lumber

which is more dense - green sapwood or green heartwood

green sapwood

comparing density

hardwood is denser than softwood

which is more durable? hardwoods or softwoods? heartwood or sapwood?

hardwood is more durable than softwood; heartwood is more durable than sapwood

which shrinks more - hardwoods or softwoods?

hardwoods (less lignin)

hemicellulose

has several sugars (different than cellulose) tied in its cellular structure; works with lignin as a binding agent; hardwood is 20-30% hemicellulose, softwood is 15-20%

palm & coconut trees

have no growth rings

fibrils

highly ordered strands of linear cellulose molecules; form the large structural elements that make up the cell walls of wood fibers

what is durability?

how long the wood can last; the long-term performance of the material

spring wood

inner layer of each growth ring; more rapid growth, lighter in color

middle lamella

intercellular layer between wood cells composed of lignin

how do you prevent attack from decay in wood?

keep moisture out

what is pressure treatment?

lumber exposed to the ground requires chemical treatment to prevent destruction from fungus; pressure treatment applies vacuum & pressure to force preservatives into the wood to give it lasting structural protection

how does the modulus of elasticity change with moisture content?

modulus of elasticity increases with decreasing moisture content (dry wood can carry more load than wet wood)

2 most important properties that affect the strength & durability of clear wood

moisture content & specific gravity

shrinkage occurs...

moisture evaporates more rapidly from the ends than sides

how does the moment of inertia change with moisture content?

moment of inertia decreases with decreasing moisture content (reduction in cross sectional area)

slow growing trees' rings

narrower growth rings; fine grained; 7-12 growth rings per inch

radial axis

normal to the annual rings and perpendicular to the grain direction

cracks

occur in various parts of the tree & are given names such as checks, shakes, & splits; most are the result of shrinkage from drying the lumber

summer wood

outer layer of each growth ring; heavier, harder, and stronger

specific gravity of wood

oven dry weight of the wood / density of water (volume of green wood)

wood density

oven-dry weight / volume of green wood

longitudinal axis

parallel to the grain

what are the main chemicals in wood treatment?

pentachlorophenol (for poles, posts, beams, & bridges); creosote (coal tar carried in an oil base, used for piles, poles, railroad ties); inorganic arsenicals (waterborne preservatives - chromated copper arsenate)

tangential axis

perpendicular to the grain but tangent to the annual rings

macro structure of wood

pith annual rings (heartwood > sapwood) cambium layer bark

types of sawn wood

plain sawn, quarter sawn, rift sawn

lumber

processed wood

what conditions are good for fungal growth?

proper temperature; moisture content over 19%, oxygen, food (wood fiber)

specific gravity

ratio between the density of a material and the density of water; s.g. = 0.5 >>> 31.2 lb/ft3

density of wood

ratio of cell wall to lumen

types of wood

refers to the botanical origin of the species, not the relative strength or density; most of the time, hardwoods are stronger than softwoods

which shrinks more - sapwood or heartwood?

sapwood (higher MC)

moisture content within a tree

sapwood generally has a higher MC than heartwood

what do the beginning stages of attack look like?

significant reduction in mechanical properties; no change in volume; no change in appearance (eventual slight green deposit)

the density of lumber depends on

species, growth of the tree, moisture content, part of the tree

how is stiffness affected by changes in moisture content?

stiffness is not affected by moisture content, as it is a function of inertia and modulus of elasticity

how does the strength and stiffness of wood react with s.g.?

strength & stiffness of wood increases with specific gravity

in which direction is the highest shrinkage?

tangential

what is the average value of tangential shrinkage per decrease in moisture content?

tangential shrinkage can be assumed as 1% per every 4% decrease in moisture content

what 3 factors affect the drying process?

temperature, relative humidity, air circulation

lignin

the glue that holds tubular cells together (longitudinal shear strength is limited by strength of lignin); 23-33% of softwood, 16-25% hardwood by weight; provides rigidity and compressive strength to the cell walls

density

the mass or weight per unit volume; directly related to porosity

equilibrium moisture content

the moisture content at which wood neither gains nor loses moisture relative to the surrounding air; below the FSP; dependent on RH, ambient temperature, and drying conditions

fiber saturation point

the point at which all free water is lost; cavities contain only air, cell walls are saturated, approximately a MC of 25-30%; above FSP, volume is constant, below FSP, cross-sectional dimensions decrease

what is seasoning?

the process of controlled drying of lumber to improve its structural properties; increases strength, lowers shrinkage in service, improves decay resistance, reduces weight, prepares the lumber for preservatives

how is the radial direction affected by load?

the radial direction is somewhat stronger than the tangential direction due to the reinforcing action of the rays

how is the longitudinal direction affected by load?

the strongest direction is the longitudinal direction; it has the least amount of dimensional change

growth rings

the wood produced in one growing season makes up a single growth ring; visible due to differences in cells formed in early & late part of growing season

water loss

there is no change in the shape of the cell wall when water is lost from the cavity, they shrink when water is lost from the cell wall

timber

unprocessed wood

extractives

up to 10% of wood substance; chemicals which provide color, odor, taste, & decay resistance to wood; essential oils, fats, resins, waxes, gums, starches; amount of extractives depends on the species, growth conditions, & the time of year the tree is harvested; some are toxic to organisms that attack

shrinkage

water evaporates out of cell cavities; no change in cross-sectional dimensions

free water

water in the cavities - first water to be lost

bound or absorbed water

water in the cell walls; if there is water in the cavity, the walls are saturated

moisture content

weight of water / oven dry weight (100%)

when does wood have the greatest of all strength characteristics?

when it is subjected to tensile forces parallel to the grain (2-4x the compressive strength parallel to the grain)

green lumber

when logs are split into lumber; MC > 30%, moisture in cell walls & cavities; changes in MC will affect cross-sectional properties

fibers

wood cells; considerably elongated (1 mm for hardwood, 6 mm for softwood)

what is unique about wood?

wood is the only naturally renewable material

what is clear wood?

wood with no knots

which shrinks more - young trees or mature trees?

young trees (more MC)