Wood Notes

Wood

Wood is the fibrous substance of a tree, providing support, growth, and stability. It includes sawn and processed wood used in construction, cladding, decking, and furniture.

Timber and Lumber refer to trees or wooded areas, whether or not they are harvested. Timber is less processed than lumber.

Internal Parts of Wood

  • Heartwood

  • Sapwood

  • Growth rings

  • Inner bark

  • Pith

  • Radial surface

  • Transverse surface

  • Outer bark

  • Tangential surface

Wood Processing

This refers to the various methods and stages involved in converting raw wood into usable products.

Advantages of Wood

  • Economical: Generally readily available.

  • High effective properties: High strength-to-weight and stiffness-to-weight ratios.

  • Low maintenance: Requires less upkeep compared to other materials.

  • Manufacturability: Can be easily shaped and formed.

  • Renewable resource: Sustainable when managed properly.

  • High variability in properties: Offers diverse options for different applications.

Disadvantages of Wood

  • Natural defects: Susceptible to flaws that can affect integrity.

  • Moisture effects: Causes dimensional instability (shrinkage/expansion) and decay (poor durability).

  • Poor fire resistance: Highly combustible unless treated.

Natural Defects

Defects are structural faults that impair strength, resilience, or durability, not surface blemishes.

Defects Due to Natural Forces

  1. Burls:

    • Formed by injury or shock in a tree's early life.

    • Growth becomes upset, leading to irregular projections.

  2. Shakes: Splits along the length of the plank where the cells or fibers separate.

    • Caused by harsh weather conditions.

    • Types include:

      • Radial shakes (splits inwards)

        • Heart shake

        • Star shake

        • Frost shake (winter wood separates from summer wood)

      • Tangential shakes

        • Cup shake

        • Ring shake

  3. Chemical Stains: Discoloration due to chemical action by external agents.

  4. Coarse Grain: Wide annual rings due to rapid tree growth, resulting in lower strength.

  5. Dead Wood: Timber from dead standing trees, indicated by light weight and reddish color.

  6. Knots:

    • Remains of outgrowing branches.

    • Grain runs at an angle, distorting fibers and reducing strength.

Defects Due to Conversion

Occur during the conversion of timber to commercial form.

  • Chip mark: Marks or signs from chips on the finished surface.

  • Diagonal grain: Improper sawing of timber.

  • Torn grain: Small depressions due to tools falling on the surface.

  • Wane: Presence of the original rounded surface on the finished surface.

Defects Due to Fungi

Fungi attack timber when:

  • Moisture content is above 25%.

  • Environment is warm enough.

  • There is sufficient air.

Wood remains free of fungi for centuries below 25% moisture or when submerged in water due to lack of air.

Defects Due to Insects

Various wood-boring insects attack timber in different conditions.

  • Insects responsible for decay:

    • Beetles:

      • Small insects that form pinholes and tunnels in all directions.

      • Convert timber into fine powder.

    • Marine borers:

      • Found in marine water; dig holes for shelter, not feeding.

      • Cause wood to lose color and strength.

    • Termites:

      • Known as white ants, found in tropical areas.

      • Feed on wood inside out, forming tunnels and living in colonies.

      • Do not disturb the outer shell.

      • Good timbers like teak and sal are not attacked.

Decay in Timber

Causes for early decay include:

  • Alternate dry and wet conditions.

  • Bad storage or stacking.

  • Fungi.

  • Improper seasoning.

  • Insects.

  • Contact with damp walls or earth.

  • Shocks or impacts in early life.

  • Use of timber with sapwood.

  • Using seasoned timber without preservatives.

  • Using unseasoned wood with protective coatings.

Wood Seasoning

Process to reduce moisture content to a required level, developing strength, elasticity, and durability. Well-seasoned timber has about 15% moisture content.

Methods of Seasoning Timber

  1. Natural Seasoning:

    • Subjecting timber to natural elements (air or water).

    • Gives good results but takes more time.

  2. Artificial Seasoning:

    • Drying lumber using man-made devices like kilns.

    • Faster, taking only 4-5 days.

Natural Seasoning Types

  • Water Seasoning: Immersing timber in flowing water to remove sap, taking 2-4 weeks, followed by drying.

  • Air Seasoning:

    • Arranging timber logs in layers in a shed with gaps.

    • Platform built 300mm above ground for air circulation.

    • Slow process but produces well-seasoned timber.

Artificial Seasoning Types

  • Seasoning by Boiling:

    • Boiling timber in water for 3-4 hours, then drying.

    • Hot steam can be passed through timber logs in enclosed rooms.

    • Develops strength and elasticity but is costly.

  • Chemical Seasoning:

    • Storing timber in suitable salt solution.

    • Salt absorbs water, removing moisture, then timber is dried.

    • Affects the strength of the timber.

  • Kiln Seasoning:

    • Subjecting timber to hot air in an airtight chamber.

    • Hot air circulates, reducing moisture content.

    • Temperature is raised using heating coils.

    • Costly but gives good strength results.

  • Electrical Seasoning:

    • Subjecting timber to high-frequency alternating currents.

    • Resistance is measured; seasoning stops when required resistance is reached.

    • Also called rapid seasoning, but uneconomical.

Wood Products

  • Plywood:

    • Manufactured wood panel from thin sheets of wood veneer.

    • Flexible, inexpensive, workable, reusable, and locally manufactured.

    • Resistant to cracking, shrinkage, splitting, and warping, with high strength.

  • Veneer: Thin slices of wood (less than 3mm) glued onto core panels (wood, particle board, or medium-density fiberboard) for flat panels like doors and furniture.

  • Particle Board: Composite product of wood particles (chips, shavings, sawdust) and a synthetic resin or binder.

  • Laminate: Printed surface that looks like real wood, made of plastic bonded to a composite base. Stronger, heat- and scratch-resistant, making it easier to care for than real wood or veneer.

Properties of Wood

  • Tensile Strength:

    • Very strong in tension parallel to the grain.

    • Defects like knots greatly reduce this strength.

    • Timber should never be loaded perpendicular to the grain.

    • Tensile strength perpendicular to the grain is about one-third of the shear strength parallel to the grain.

  • Compressive Strength: Wood is very strong in compression parallel to the grain because the wood cells act as slender columns bonded together, giving and receiving support from each other.

  • Flexural Strength: Lumber in bending experiences flexural and shear stresses. Wood is very strong in bending, characterized by the modulus of rupture. Shallow beams have greater resistance to bending than deeper beams.

Laboratory Testing for Timber

Tensile Strength Parallel to the Grain Test (ASTM D143)

  • Uses a 2-in gauge length extensometer.

  • Specimen:

    • Length = 18 in

    • Double curvature gradual transition fillet of a 17.5-in radius to a net cross-section of 3/8 by 3/16 in

    • 2.5-in long zone of uniform cross-section.

Procedure:

  1. Measure cross-sectional dimensions and calculate the area.

  2. Place specimen in a testing machine and slowly extend it until failure at a rate of 0.005 in/s (1 mm/s).

  3. Record applied force and deformation of the gauge section.

  4. Process data to obtain stress (load divided by original area) and strain (elongation divided by original gauge length of 2.0 in).

  5. Graph data to obtain a stress-strain diagram; the slope is the modulus of elasticity.

  6. Note fracture pattern and record maximum load.

  7. Calculate tensile strength by dividing the maximum load by the original area.

  8. Determine moisture content (MC) of a small section near the failure.

Computations

Example calculations of ultimate tensile strength (f<em>t1=P/A=P/(tw)f<em>{t1} = P/A = P/(tw)) for different specimens are provided, ranging from 15460 psi to 21400 psi. The average tensile strength (f</em>tf</em>t) is calculated as 18020 psi.

Compressive Strength Parallel to the Grain Test (ASTM D143)

  • Standard specimen size: 2 in by 2 in by 8 in.

Procedure:

  1. Measure dimensions and calculate cross-sectional area. Lightly sand the end grain.

  2. Apply load until well past the proportional limit at a rate of 0.003 in/in of nominal specimen length per minute.

  3. Record applied force and deformation (to the nearest 0.0001-in) of the 6-inch central section.

  4. Process data to obtain stress and strain. Graph data to obtain a stress-strain diagram; the slope is the modulus of elasticity.

  5. Note the failure pattern of the specimen and record the maximum load.

  6. Calculate the tensile strength by dividing the maximum load applied on the specimen by the original cross-sectional area.

  7. Determine the weight of the specimen just before testing; and determine the MC of a small section, about 1.0 in in length, near the failure.

Failure Types

  • Crushing

  • Wedge splitting

  • Shearing

  • Splitting

  • Crushing and splitting

  • Brooming or end rolling (unacceptable)

Flexural Strength Parallel to the Grain Test (ASTM D143)

  • Standard specimen size: 2 in by 2 in by 30 in

Procedure:

  1. Measure dimensions of a straight grain material.

  2. Place the specimen in a testing machine with the direction of growth rings parallel to the direction of loading. Apply load at the center of a 28-in span using a bearing block of hardwood until well pass the proportional limit. The rate of movement of the crosshead should be 0.10 in/min. Loading should continue until the deflection reaches 6.0 in or specimen fails to support a load of 200 lb.

  3. Record applied force and deflection. Calculate flexure strength using the modulus of rupture Equation 5.4.

  4. Note the failure pattern of the specimen, appearance of the fractured surface, and how failure developed.

  5. Determine the weight of the specimen just before testing; and determine the MC of a small section, about 1.0 in in length, near the failure.

Failure Patterns

  • Brash (abrupt failure)

  • Fibrous (fracture showing splinters)

  • Compression

  • Horizontal shear

Laboratory Test for Wood Moisture Content of Wood

  • Moisture Content (MC) is the weight of water contained in wood, expressed as a percentage of the weight of oven-dry wood weight.

  • Standard procedure: ASTM D4442, Method A—primary oven-drying.

  • Use eight specimens processed using Guide ASTM D4933.

  • After taking the initial weight, the drying time can take 24 h for a one- to two-inch sample in an oven at a temperature of 103 ±2°C103 \,\pm 2°C. The specimens are weighed every three hours; when the change in weight is less than twice the sensitivity of the scale (i.e., 0.2-g change in weight of a 100-g sample), the sample is considered dry; and its weight is the oven-dry weight.

Example Calculation

A wood sample initially weighs 205 g, but decreases to 110 g after oven drying to a constant weight. What is its percent moisture content?

Solution: Missing from transcript.

Density and Specific Gravity

  • Density is the ratio of mass to volume, with units of lb/ft^3 (U.S. customary) or g/cm^3 (SI).

  • Specific Gravity is the dimensionless ratio of the density of a material to the density of water at 4°C4°C, when water is densest, γ\gamma (62.43 lb/ft^3 or 1.00 g/cm^3).

  • Standard procedure: ASTM D2395.

  • Volume is determined from dimensions (length, width, and thickness).

  • Mass is determined before and after oven drying.

Example Calculation

A red oak sample initially weighs 173.2 g, but decreases to 152.3 g after drying to a constant weight. The average dimensions (three points each) after drying are 0.75 in by 3.50 in by 5.46 in. Compute the percent MC and the specific gravity for the specimen.

Solution: Missing from transcript.

Philippine Native Trees

Table 619.1-1 provides groupings of species for determining allowable loads for timber joints based on relative density, categorized into Groups I, II, III, and IV. Species listed include Malabayabas, Makaasim, Malugai, and others.

Bolt Design Values

Table 619.1-2 (cont'd) provides reference values on one bolt (double shear) in seasoned wood loaded at both ends. It details design values (P and Q in kN) for bolts loaded parallel (0°) and perpendicular (90°) to grain, based on diameter (d), length of bolt in the main member (L), and species group.

Split Ring Connector Design Values

Table 619.1-3 lists split ring connector unit reference design values based on bolt diameter, split ring diameter, number of faces of member with connectors on the same bolt, net thickness of member, and species group.

Shear Plate Connector Design Values

Table 619.1-4 lists shear plate connector unit reference design values based on shear plate diameter, bolt diameter, number of faces of member with connectors on the same bolt, net thickness of the member and species group.