Wood And Wood Products.2.pdf

Wood and W oo d Products Definition WOOD - The hard fibrous substance which composes the body of a tree and its branches. Technically, wood is known as Xylem.

Wood is the most common building material because of the following properties: • A strong material • Has durability • Light in weight • Ease of fastening • With artistic and natural beauty

Physical Properties Strength - ability of wood to resist stresses and strain. Different wood varies materially in the following manner: • • • •

Wood is resistant to compression along fibers. Stiffness – ability to resist bending Strength in tension – ability to resist in lengthwise stresses Shearing strength – ability of the fibers to resist rupture along or across the grain.

Moisture • An important factor in the strength of wood. • Strength increases with the degree of seasoning of wood • Moisture content can be calculated using the formula: Moisture content=

X 100

Weight • Quality of heaviness •Also an important consideration. Heavy woods are generally strong, light woods are usually weak.

Cleavability • Resistance of wood to cleavage along the grains • Wood splits much more easily when wet because moisture softens the fibers and reduces adhesion across the grain Flexibility and toughness • Moisture content of wood influences flexibility or its ability to bend • Toughness refers to the combined strength, shock resistance and pliability of wood

Classification of Wood •

Mode of Growth • Exogenous –outward growing trees

which are the most preferred for lumbering. e.g. pine, oak, mango, teak •

Endogenous – inside growing trees; less preferred for lumbering because the center core of its log is soft and brittle in character

2. Density • Quality of the mass and volume of the wood • Softwood – conifers or evergreen b) Hardwood – generally deciduous and has broad leaves

3. Leaves • Characteristics of the tree when it comes its leaves are: • Needle shaped

•

Broad shaped

4. Shades or Color of Wood • White • Yellow • Orange • Red • Brown • Black, etc. 5. Grain • Straight grain • Cross grain • Fine grain • Coarse grain

6. Nature of the surface when sawed • Plain • Grained • Figured or marked

Defects in Wood Defects – irregularities found in wood Common defects: • Abnormal Growth are identified as: •

Heart Shakes – radial cracks originating at the heart of the logs

• Wind Shakes or Cup Shakes – cracks or breaks across the annual rings of timber during its growth caused by excessive bending of the tree due to wind.

•

Knots – usually occurs at the starting point of a limb or branch of the wood.

2. Due to Deterioration caused by:

•

Dry Rot – the presence of moisture in wood caused by fungi in seasoned wood

Seasoning of Lumber Seasoning – drying of lumber • Moisture has to be expelled thoroughly to preserve the lumber from shrinkage or decay • Experiments has proven that timber which are immersed in water immediately after being fallen and squared is less subject to splitting and decay • Soaking timber into liquid is the method of seasoning practiced and adopted by the ancient Roman builders Methods adopted in seasoning of lumber: Natural or Air Season –considered as one of the

best methods although the period involved is relatively longer The process: • Lumber is piled outdoor, its length is sloped at about 10% of its length • Lumber is piled in a well ventilated shed. Each piece is properly and evenly spaced from each other for free circulation of the air around the lumber. The uniform spacing is attained by using wooden or bamboo stick inserted between each pieces of lumber.

2. Artificial Seasoning – lumber is stacked inside a drying kiln and then subjected to steam and hot air under pressure. - adopted for quick drying of wood - considered quite inferior in wood quality as compared to those that are dried by natural seasoning method.

Kiln Drying – lumber is dried in a specially built room or chamber. - temperature, humidity and circulation of air is controlled

Wood Decay and Methods of Preservation Wood does not decay naturally through age nor will it decay if its is kept constantly dry or continuously submerged in water. The common causes of decay in wood are: • Alternate moisture and dryness • Fungi and molds • Insects and worms

•

Heat and confined air

Process of preserving wood: • External Processing – accomplished by coating the wood with preservative chemicals that will penetrate the fibers. • Internal Processes – a chemical compound is impregnated at a specified pressure to permeate the wood thoroughly.

Measuring Wood Board Foot – one square foot of wood one inch thick or 144 cubic inches. The formula for finding a board foot is:

Board Foot = Where: Thickness is in inches Width is in inches Length in Feet

Example: find the total board foot of 5 pieces 2” x 6” x 14’ Solution: Bd. Ft. =

= 70 bd. ft. Note: One cubic meter of wood is equal to 423.67 bd. Ft.

Determining Board Foot in a Round Wood or Log Board Foot = Where: D = The smaller diameter of the log L = Length of the log in feet 4 and 16 are the slab reduction and

allowance formula

which are constant in the

Lumber and Related Products The important wood related products for commercial purposes are: • Veneer and Plywood - plywood is made by bonding together thin layers of wood in a way that the grain of each layer is at right angles to the grain of each adjacent layer. The different types of plywood: • Soft plywood – the most common for structural use. • Hard plywood – are used for paneling and finishing where usually only one face is with

•

hardwood finish. Exterior or Marine plywood – is made for external use.

2. Hardboard – pressed wood made from woodchips which are exploded into fibers with high pressure steam - equally strong in all directions but brittle in character and can resist warping.

• 3. Particle Boards – manufactured from wood chips made from chips of wood or sawdust. It is mixed with resin or glue (both adhesives), then, under pressure and heat, pressed into uniform sheets. Particle board is used in building construction and furniture making.