Lecture Three
Basic Wood Characteristics
Basic constituents of wood:
1) Cellulose (70%) the primary constituent of wood
2) Lignin (~25%) this is the constituent which gives the wood strength
3) Extractive: consisting of tannin, starch, oils, resins, fats, and waxes
4) Ash: forming minerals

Two basic types of wood
SOFTWOODS:
 Softwoods are woods extracted from coniferous or evergreen tress. Pine, spruce, fir, cedar, and redwood are all softwoods but they have different properties and prices and are therefore used in different ways in building construction.
The general pattern of softwood use in building construction:
1) Framing: Pine, spruce
2) Beams: Fir, cedar, reswood
3) Siding: Cedar, reswood
4) Shingles: Cedar
HARDWOODS:
Because the trees that produce hardwoods take much longer to mature than those that produce softwoods, hardwoods generally command much higher prices, hardwoods are generally used only for finishing.
Relationship between tree growth and wood strength:
· Wood grows in an annual fashion, with concentric rings from the center (in cross section) marking the beginning and end of each year’s growth.

· We speak of these rings as the grain of the wood. When referring to wood we speak of direction as being parallel to, or across the grain; meaning respectively: vertically and horizontally with respect to the original standing tree.

Wood is two to five times stronger parallel to the grain.
When speaking of beams it is common to speak of stresses with respect to their direction:
1) Compression is caused by pushing the ends of the beam together.
2) Tension is caused by pulling the ends of the beam apart.
3) Shear is caused by sideways force, i.e. force not along the long axis of the beam.
Wood is 30% stronger in compression than in tension.
Wood is stronger in resisting shear across the grain than it is parallel to the grain.
Moisture content and seasoning:
 New wood, that is wood recently cut down, contains a large amount of moisture (this is known as green lumber). Over time, the moisture evaporates, and this drying causes the wood to shrink, warp, and twist. In general, hardwoods shrink more than softwoods.
Moisture content and seasoning:
Moist wood is subject to rot, and therefore, decay. In order to resist decay, the moisture content in wood must be below 20%. Commonly, wood is dried to about 8% moisture content for interior uses, and 12% moisture content for framing purposes.
Seasoning:
Seasoning is the process of preparing wood to resist decay. The methods of seasoning are as follows:
1) Air drying (for two to six monthes)
2) Kiln drying
3) Pressure treatment: in which the wood is impregnated with chemicals that prevent decay.
Sawing of trunks:
1) Plain sawing: this method results in the greatest yield.
2) Quarter sawing: more expensive than plain sawing, this method leaves the grain exposed, and results in less warping of wood.
3) Riftsawing: radial cutting, used for expensive woods.
Characteristics of commercial lumber
Units of measure, sizes:
Bulk lumber is measured in BOARD FEET. One board foot is determined as 1” thick X 12”X 12” in volume. One thousand board feet is commonly abbreviated with a capital “M”. Board measure is abbreviated “b.m.”
Standard inch measured cross sections:
1) Nominal size is the size of a piece of wood before it is planed at the mill. This is the size by which the wood is sold. That is, a 2” x 4” is two inches by four inches in cross section BEFORE it is planed.
2) Dressed size is the actual size of the purchased lumber. It is generally ½”  smaller in every dimension than the nominal size. Thus a 2” x 4”  is really only 1 ½”  x 3 ½”.
Standardization of sizes:
The nominal sizes of commercially available lumber are standardized both for length, and for cross section. The result is lower costs per board foot than for wood cut at “off sizes”.
1) Lengths are standardized in feet at 2’, 8’, 10’, 12’, 14’, & 16’.
2) Cross section is standardized in increments of 2 inches:
a. 1”x2”, 4”, 6” 8”, 10”, & 12”
b. 2”x2”, 4”, 6”, 8”, & 10”
c. 4”x4”, 6”, 8”, 10”, *12”
Grading of lumber
Lumber is graded for strength, imperfections (such as knots), and appearance.

Canadian lumber is graded by one of the following organizations:

1) BC Lumber Manufacturer’s Association
2) West Coast Lumber Inspection Bureau
3) Western Wood Products Association
4) Eastern Spruce Grading Committee
5) Eastern Pine Grading Committee
6) Canadian Lumberman’s Association

General grading for wood construction:
1) Utility- poorest, not generally used for constuction
2) Standard, construction- commonly used for building
3) Select structural- best quality, used for larger structural members
 
Different schemes exist for woods with specific uses:
1) Decking wood- Select, Commercial grades
2) Softwood siding- A,B,C,D
3) Hardwood siding- Firsts, Seconds, Select, No. 1 Common
4) Cedar shingles- 1, 2, 3
 

Man-Made Wood Products
Plywood:
Veneers of wood are thin layers of wood obtained by rotary cutting. Plywood sheets are composed of several layers of wood veneer, glued together.
1) Each layer of plywood has its grain offset 90 degrees to the adjoining layers to prevent warping.
2) Plywood is constitutes of facce plies and core plies.
 
Plywood:
Each face ply is graded according to appearance and durability on a scale of A to D in descending quality, as follows:
a. “A”= smooth and paintable
b. “ B”=contains repair plugs, and/ or tight knots
c. “C”= contains knots, splits, and/or plugs
d. “D”= not suitable to be exposed, inner only
e. “N”= is a special category where a finish veneer (such as walnut) is applied to the face; it indicates freedom from defects.
Plywood is commonly graded with two letters; one for each face ply. Some common grades of plywood and their common uses are as follows:
a. NN, NA, NB- used for furniture, cabinetwork
b. AA- used where both sides will be visible
c. AB- used to cover unfinished framing
d. AC- used for roof decking
e. BC- used for general utility
f. BB- used for formwork
g. CC- used for rough/temmporary situations
 
Plywood is available in Interior and Exterior Grades, which are determined by whether or not moisture-resistant adhesives are used (Exterior Grade is characterized by moisture resistance).
Hardboard
· Hardboard describes a material made from wood fibers which are bonded under heat and pressure.
· Its two forms are basic and prefinished (which is known as masonite). Masonite can be waterproofed if necessary.
· Common uses include underfloors, cabinet work, and ceilings.

Particle board:
· Particle board differs from hardboard in that the chips or wood pieces are glued together with resins and other binders.
· It comes in interior and exterior grades. Aspenite is a kind of particle board with rather large constituent fibres. It is commonly used for sheathing as a substitute for plywood.
Composite board:
· Consists of a homogeneous layers of refined wood product (card paper). Impregnated with asphaltic materials. Because it is waterproof, it is used as light construction sheathing.
· Good acoustic qualities, it is often used for ceilings and interior wall finishes.
Pulp and Paper products:
1) Building paper is felt paper impregnated with asphalt, sometimes reinforced with fibreglass. It is waterproof, used as sheathing paper, and roofing felts (acts as vapor barrier).
2) Asphalt Shingles are rags of wood and pulp, which are saturated with asphalt. Granules of marble are affixed to the exposed surface.
3) Concrete forms: round concrete columns are often poured into a sonotube: a cylinder of paper coated with wax, which is peeled away.
4) Acoustic fiberboards (basically the same as composite board)
Laminated timber beams
· Conventional beams are built up from smaller members by nailing or screwing.
· Laminated timber beams are a commercially engineered alternative.
· Characteristics:
i. Built up from 2” members
ii. Boned with waterproof glues
iii. Sized from 4”x9” to 6”x36”
Advantages of laminated timber beams over conventionally built up beams:
1) The best lumber can be selectively placed in the highest stressed area of the beam.
2) More dimensionally stable
3) Better appearance
4) Smaller size beam is necessary for the same span (for example in a 20’ span you need an 8”x14” conventional beam, but only a 4”x16” laminated beam).