Materials - Wood

Softwoods

• Key species: Douglas fir, West Coast Hemlock, Redwood, Pines, Cedars, Spruce
• Used as structural & framing lumber, sheathing, roofing, subflooring, exterior siding, flooring, trim, interior paneling

Materials - Woods

Hardwoods

Key species: maple, oak, poplar, birch, beech, cherry, black walnut, basswood, hickory, gum, ash

Lumber used for flooring, cabinets, furniture

Materials - Wood

Moisture Content

• amount of water contained in wood (expressed as % relative to weight of oven-dried wood)
• unseasoned (green/fresh cut) = very high MC
• seasoning reduces MC significantly

Materials - Wood

Moisture Content & Seasoning

• Seasoning lumber increases strength; reduces shrinkage, checking, warping in service; reduces susceptibility to fungus rot in storage; improves capacity to accept pressure preservative treatment

Materials - Wood

Air Drying

• 12-15% moisture content
• dried by exposure to air over period of several months
• usually used for dimension or lower grades of lumber
• complete air drying of larger, structural timbers usually impractical

Materials - Wood

Kiln Drying

• 6 - 19% moisture content
• dried in kiln under controlled temp. to achieve desired MC
• hastens drying time and kills stain, decay fungi, insects
• used primarily for appearance grades or finished lumber where dimensional stability and appearance are important
• for hardwoods, air drying in yards often combined with kiln drying

Materials - Wood

Equilibrium Moisture Content

• Point of equilibrium between moisture in lumber and surrounding air
• To ensure wood will experience only minor dimensional changes during use, specification of lumber stock should be as close as possible to MC the lumber will reach in use
• Consider temp. & humidity of location
• Especially important for doors, etc. that require proper fit between wood elements

Materials - Wood

Strength

Ability of wood to resist loading depends on strength, orientation in the structure, and cross-sectional dimension of the member

Materials - Wood

Tensile Strength

• Ability to resist stretching or stresses imposed parallel to grain
• Reduced by knots, splits, checks, other defects
• Tensile strength of wood perpendicular to grain is very low and very high parallel to grain

Material - Wood

Compressive Strength

• ability to resist loads that are crushing to members
• cs parallel to grain 2-5x greater than cs perpendicular to grain
• important relative to vertical or horizontal load-bearing members

Materials - Wood

Fiber Stress in Bending

• ability of horizontal load-bearing member to carry max. load in bending without causing fibers at extreme top (most compression) and bottom (most tension) to fail

Materials - Wood

Modulus of Elasticity (E)

• measure of stiffness
• ability of member to resist deflection
• ratio of the amount by which material will deflect in proportion to applied load
• used in load-bearing calculations to determine size of horizontal members

Materials - Wood

Horizontal Shear Stress

• ability of wood fibers at top half of bending member (in compression) to resist shearing horizontally from fibers in bottom half (in tension)
• horizontal shearing stresses largest at each end of a beam along the central (neutral) axis
• avoid installing bolts along this plane

Materials - Wood

Grain

• direction of fibers in lumber as consequence of natural growth or sawing
• close grain: tightly spaced annual growth rings with small pores
• coarse grain: widely spaced rings with large pores
• coarse-grained woods accept stains preservatives, paints, surface sealers better than close-grained woods
• cross grain: slope of grain defect from aberrant growth or improper sawing; weakens lumber
• straight grained lumber is strongest; fibers oriented parallel to longitudinal axis of the piece

Materials - Wood

Insect & Decay Resistance

• Termite-resistant species: redwood, bald cypress (tidewater red), Eastern red cedar,
• Fungi/decay-resistanc species: bald cypress (tidewater red), cedars, redwood, black locust, black walnut

Materials - Wood

Decay

• Fungi: molds, stain, dry rot, soft rot
• Most decay in wood occurs when moisture content >30%
• Fungal decay does not occur in permanently submerged water (fungi need oxygen)
• Pressure-treated wood should be used where a member will be embedded in moist ground

Materials - Wood

Preservative Treatment

Three major classes:

• Water-borne
• Oil borne
• Creosote

American Wood Preservers Assoc (AWPA): standards and certifications

Materials - Wood

Water-Borne Preservatives

• Used to pressure treat lumber
• leave wood clean, odorless, easy to paint
• present less environmental hazard than other preservatives

Materials - Wood

Water-Borne Preservatives

• ACA - ammoniacal copper arsenate: ok for ground or water contact
• CCA - chromated copper arsenate: ok for ground or water contact; limited marine application
• ACC - acid copper chromate: ok for ground or water contact, resistant to marine borer

Materials - Wood

Oil-Borne Preservatives

• Can be used any application except contact with salt water
• often prohibited for use where it will come in contact with people
• Penta designated toxic environ. hazard

Materials - Wood

Creosote Preservative

• used where protection against wood destroying orgs high priority, painting not req'd, and odor not a concern
• potential hazard for  human exposure
• creosote-tar mixtures: marine/saltwater apps; pilings for shore dwellings
• creosote-petroleum mixtures: economical but not used for marine installations

Materials - Wood

Lumber Classification

• American Lumber Standards Committee (PS20-70 American Softwood Lumber Standards) establishes basic principles for grading lumber
• Grade: slope of grain; natural & manufacturing defects; number, tightness & location of knots
• Common grade: suitable for general construction
• Select grade: higher quality surface for painting or natural finishes

Materials - Wood

Lumber Classification: Uses

• Yard lumber: ordinary construction, general building purposes
• Structural lumber: 2 in. or more nominal thickness; aka stress graded lumber

Materials - Wood

Lumber Classification: Size

• Lumber sized & spec'd as nominal (size before shrinkage and planing) Dressed dims smaller - except length (shrinkage longitudinal to grain is negligible)
• Boards: <2 in. thick, 2 in. or more wide; sheathing, flooring, roofing, finish, trim, siding, paneling
• Dimension lumber: 2-4 in. thick, 2 in wide or more; used principally for structural elements
• Timbers: >5 in x 5 in: beams, stringers, posts, caps, sills, girders, purlins, other members
• Most dimension lumber & timbers are stress graded