CGN3501C Test 2 Study Guide: Just Wood

exogenous

grows outward by adding new cells in a layer between the existing wood and the bark (softwoods and hardwoods)

softwoods

conifers (needles) or cone-bearing, grows year-round (and therefore faster)

Ex: pine, spruce, fir, hemlock, cedar, cypress, redwood

Used in construction, less expensive

hardwoods

broad-leafed, mostly deciduous (sheds its leaves in the wintertime)

Ex: oak, maple, ash, walnut, hickory, poplar, gum, birch

Takes ~20 years to grow → more expensive → used to make furniture, cabinets

endogenous

grows inward by adding new cells to the old

Ex: bamboo, palm

wood structure

Cellulose & hemicellulose (55-80%) provides tensile axial strength & elastic property of wood

Lignin (15-30%) cements the cellulose together to provide compressive strength

cross section of a tree

Cambium Layer: layer of new cells beneath the bark (growth region of the tree)

Pith: innermost ring

Heartwood: inactive inner portion, relatively darker in color (more resistant to insects & decay as compared with sapwood)

Sapwood: active outer portion, relatively lighter in color

Medullary Ray: group of cells in the radial direction, adding strength to the radial direction

early wood (springwood)

Inner light colored layer, which grows in the spring and grows relatively faster

latewood (summerwood)

outer darker layer, grows in the summer and relatively slower

density of wood

Solid wood substance (cellulose) specific gravity = 1.5

Wood (with air-filled cavities) specific gravity = 0.3-0.9

shrinkage of wood

Shrinkage increases with density of wood and moisture content. Caused by lateral contraction of the cell walls as they dry out. Ranges of shrinkage between fiber-saturation point and oven-dry condition: Volumetric 7-21%, longitudinal 0.1-0.3%, radial 2-8%, tangential 4-14%

axial strength parallel to the grain

Strength is greatest in this direction (wood fibers run mostly in this direction) - 5000 psi

Tensile strength in this direction is about 2-3 times the compressive strength - 10,000 psi

axial strength perpendicular to the grain

Tensile strength perpendicular to the grain is less than 1/10 of tensile strength parallel to grain - 800 psi

Compressive strength perpendicular to the grain is about 1/4-1/3 of compressive strength parallel to grain - 1200 psi

shear strength parallel to the grain

About the same as the compressive strength perpendicular to grain - 1200 psi

electric conductivity

increases with moisture content

defects in wood

Knots: formed at the base of the branches extending into the wood of the tree. Cause stress concentrations (if it will break, will break at the knot)

Shakes: cracks along the grain, originating in the growth of the tree

Checks: longitudinal splits across the growth rings resulting from uneven drying.

Waynes: areas where the lumber has been cut too close to the edge of the log and there is bark on the boards

Pitch Pockets: accumulations of resins in openings between the annual rings

Compression Wood: formed on the lower side of branches

Darker than normal wood. High lignin content. Higher specific gravity, greater longitudinal shrinkage. Not as tough as normal wood

Warping: caused by unequal shrinkage

Decay: caused by insect attack

plywood

Laminated wood usually made of an odd number of thin veneers (thin sheets of wood) bonded with synthetic resin. The grain of one ply is at right angles to the next

laminated veneer lumber (LVL)

A board product made by gluing pieces of thin lumber or veneer together to make a large member

Grains of all pieces are oriented along the long axis of the panel

Used in structural applications

particle board

a flat board made from wood flakes mixed with an adhesive and formed under pressure

Not used for structural purposes

Usually used in making furniture and associated products

oriented strand board (OSB)

a board made from large wood flakes mixed with an waterproof adhesive and arranged in layers at right angles to one another

Cheaper alternative to plywood

Used in roof sheathing and floor sheathing

laminated strand lumber (LSL)

Lumber made by wood strands mixed with a water-proof adhesive

Available in sizes larger than sawn lumber and tend to be significantly stronger than lumber of equal size (due to minimization of defects)

Used in construction where high strength and large size are required