Lesson 1 showed that wood is a porous material made up of cells of various kinds. Depending on the nature of these cells, some woods have more or less solid wood substance for a given sized piece. If you think of a brick of Swiss cheese (with all its holes) and an identical-size brick of cheddar, you can guess that the cheddar brick contains more cheese. So with wood, the fewer holes (cells), the more wood substance.
It has been shown how rate of growth affects the mechanical properties of wood. But how does growth rate affect density? There are no hard and fast rules applying to all species, on how growth affects density. Individual species, or groups of species, must be considered to get some idea how rate of growth affects density, and in turn, strength.
Timber showing the greatest proportion of latewood - with thick-walled cells - has the highest density and in turn strength. Both growth rate and percentage of latewood are used in certain grading rules for some species. When safety factors are especially important such as for scaffolding or bridge work, inspectors visually check the ends of timbers for percentage latewood.
Water exists in wood as either free water or bound water. Free water occurs within a cell cavity as a liquid. It is the easiest and first to be removed during drying. This free water moves toward the end surfaces through connecting cells, and laterally through the pits of neighbouring cells. It is evaporated from the wood faces as well as the ends. The point at which all free water is removed from the cell cavity is known as the fibre saturation point (fsp) and is reached at around 30 percent moisture content.
|Figure 9 - Anatomy of longitudinal cells, in relation to moisture loss.|
The loss of free water, down to the fsp, has no effect on the strength of wood. However, when bound water begins to be removed, most strength properties increase. The increase in strength is directly related to the amount of moisture removed. Thus, other things being equal, a spruce 2 x 4 (stud) is stronger at 18 percent moisture content than one green from the saw.
Other Properties Affecting Strength
This refers to a deviation of the line of longitudinal cells, to a straight line parallel to the sides of the piece of lumber. It may be caused by an abnormal growth pattern in the tree, or how the log was sawn. It is usually expressed as a ratio; for example, 1 in 12 (finch of slope in 12-inches length). A slope of grain of 1 in 6 results in a 60 percent reduction in bending strength (strength of a horizontal beam, such as a floor joist, for example). A 1 in 16 slope causes only a 20 percent reduction (see Figure 7). Most lumber grading rules specify the maximum slope of grain permitted in the grade.
Knots, common in sawn products, are caused by limbs on the tree stem. When a saw cuts through a limb (or its stub) a knot remains. Depending on the angle of both the limb and the saw, a round knot, an oval knot, or a spike knot (longitudinal) will result. For strength purposes knots are classified by size, number, form, and quality. The first two classes are self-evident. Knot form and quality are described as: tight, loose, intergrown, firm and rotten. Most grading rules take these factors into account.
|Figure 10 - Shakes and Checks|
These are separations occurring in the wood (see Figure 10). Shakes parallel to the annual rings are called ring shake and those in the heart of the tree and perpendicular to the annual rings are called star shake. In living trees, both forms of shake are caused by wounds, but not all wounds result in shake. Factors that may extend the formation of shake are, internal growth stresses, bending of the tree by wind and the freezing of free water within the cells. Checks are generally produced in the rays of sawn-wood products during drying. Depending on their severity, checks and shakes have a very great affect in reducing the strength of wood.
Agents Causing Wood Decay
|Figure 11 - Tree stem showing fungi conks, indicating very advanced decay.|
Decay, or rot; is not permitted in wood used for structural purposes. Recognizing decay in wood is very important. Most decay in wood is caused by fungi. Some of these mushroom like organisms (see Figure 11) attack the wood, eventually destroying its cellular structure. Fungi may originate in the growing tree or the wood may become infected after it is in use. Most fungi originating in the growing tree do not continue to degrade the wood during use.
Stains and molds in wood are not as serious as decay and are usually accepted in structural timber in local markets. Stain may be caused by fungi and is often an indication of worse things to come. Usually, molds and stain fungi merely give an unpleasant colour to the wood. One common example is the sap staining fungi causing blue-stain in green wood. Bacteria, another type of organism that attacks wood, and was once thought to result in little damage is now known to produce enzymes, which cause shakes in red oak. During kiln drying of red oak affected by these bacteria, honeycombing and serious checking may often develop.
To develop and cause wood damage, fungi requires food, air, moisture, and warmth. If any one of these conditions is removed, the fungi will die or remain dormant. One of the most common misconceptions and misused terms is `dry-rot'; often used to describe the brown cubical rot mentioned earlier. The dry condition may be due to the time one observes the damage - long after the rot has taken place and perhaps on a dry day. Decay will not proceed unless there is sufficient moisture available. The critical moisture content below which fungi cannot function is 20 percent.
Effect of Silviculture on Wood Properties
Wood Properties For Selected Products
Pulp & Paper
- Newsprint - mechanical and groundwood pulps. Most softwood species are acceptable, but those with considerable latewood are not favoured (eg. larch, pine). Length of longitudinal cells; ie. fibres, is important. Spruces, particularly black spruce, produce the highest quality newsprint largely due to cell length. Thermo-mechanical pulp (TMP), now mainly used in newsprint installations, requires the same properties.
- Chemical Pulps - Sulphite and Kraft. Sulphite pulp is usually mixed with groundwood pulp for newsprint. Kraft pulp is used in many applications, particularly for linerboard. Almost any species and quality of wood may be used for kraft. For sulphite, woods with very heavy latewood are not usable. Both produce strong pulps, and strength is related to longitudinal cell length. Black spruce best demonstrates these qualities.
- Softwood - Softwoods are used primarily for structural and construction lumber, but also find their way into products such as trusses and laminated timbers. These must contain pieces that have properties favouring high strength, such as: moderate growth rate; high proportion of summerwood; straight grain, no compression wood; low moisture content; no decay. For less demanding uses, including construction lumber, most of these properties are not as important.
- Hardwood - Since hardwood is used extensively in decorative applications such as mouldings, furniture and cabinets, its appearance is most important. Properties that enhance appearance and performance include: moderate growth rate; well-defined annual rings, rays and pores to produce a pleasing `grain'; absence of shakes or checks in finished product; all heartwood or all sapwood to feature colour integrity; very exact moisture content with little or no variation within or between pieces so that shrinkage does not take place in use.
Pieces should be strong, straight-grained, with a moderate growth rate, no tension wood, no knots, shakes, checks, or decay, high proportion of late wood.
There is a range of panel products. One type is made from veneer logs, other types from low-density woods, and others from mill residue. Plywood is made from veneer, oriented strand board (OSB) from aligned wafers, particle board from small particles, and fibreboard from fibres and fibre bundles.
There are many other wood products. Some of them have specialized properties. Recently developed products include laminated veneer lumber (LVL), finger jointed lumber and edged glued panels. These are generally made from lower grade material (with corresponding lower level properties) to make a highly usable, high strength product.
Selected Strength Values
- Regard the values as relative to each other to get a good impression of strength.
- Rupture is a measure of the ultimate strength of wood at the breaking point.
- Elasticity is deflection in response to load. Even though a piece of wood will not break under a load (weight) it may deflect to such an extent that it cannot be used. This applies to such applications as floor joists, rafters, etc.
- Mullins, E.J. and T.S. McKnight, 1981.
Canadian Woods - Their Properties and Uses Third Edition, Supply and Services Canada, Ottawa.
- Jessome, A.P., 1977. Strength and Related Properties of Woods grown in Canada, Forintek Canada Corp., Ottawa
- Bodig, Joseph and Benjamin Jayne, 1982. Mechanics of Wood and Wood Composites, Van Nostrand Reinhold, Toronto
- Cech, M.Y. and F. Pfaff, 1977. Kiln Operator's Manual For Eastern Canada, Forintek Canada Corp., Ottawa
- Calvert, WW. and F.J. Petro, 1993. Grading_Standing Hardwood Trees in Nova Scotia, N. S. Dept Natural Resources, Halifax
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