Nature of Wood Notes

Nature of Wood

Sustainability of Wood

• Wood is truly sustainable if forests are managed responsibly.
• This means that the volume of wood used is less than the volume grown.

Composition of Wood

• 50% of wood is carbon.
• A tree is a living organism that reacts to its environment, needing nutrients, water, sunshine, and wind.

Photosynthesis

• The process of photosynthesis is represented by the following equation:
  $2CO<em>2 + 2H</em>2O {Light} {C<em>6H</em>{12}O<em>6} + O</em>2$

Softwoods and Hardwoods

• Softwoods come from coniferous, evergreen forests.
• Hardwoods come from deciduous, broad-leaved forests.

Biological Role of Wood

• Compression Wood
  • Forms on the compression side of branches or stems.
  • Common in conifers (softwood species).
  • High in lignin content, making it darker in color.
• Tension Wood
  • Forms on the tension side of branches or stems.
  • Common in hardwood species.
  • Contains highly crystalline cellulose aligned along the tensile direction.

New Zealand Native Species

• Their supply is not abundant enough to sustain the timber construction industry.
• They are considered specialty products.

Usage of Hardwoods and Softwoods

• Hardwoods are used, but in limited cases.
• Softwoods generally grow faster than hardwoods.
• The timber industry worldwide relies on softwood species, which provide the bulk of timber used in timber construction.
• Softwoods are generally cheaper.

Radiata Pine

• Mentioned as a plantation forest example (RED STAGⓇ WOOD SOLUTIONS).

Cross Section of a Tree

• A - Outer bark (dead)
• B - Inner bark (living)
• C - Cambium
• D - Sapwood
• E - Heartwood
• F - Pith
• G - Wood rays

Sapwood vs. Heartwood

• Sapwood
  • Participates in sap conduction.
  • Lighter in color.
  • Not resistant to fungi.
• Heartwood
  • Does not participate in sap conduction.
  • More resistant to fungi.
  • Difficult to treat.
  • More stable in changing moisture conditions.

Structural Properties

• There are no differences in structural properties between sapwood and heartwood.

Earlywood and Latewood

• Light bands are earlywood.
• Dark bands are latewood.

Examples of Wood

• Radiata pine
• Douglas fir

Cell Structure

• Wood is composed of cells.
• Summerwood = Latewood
• Springwood = Earlywood
• One annual ring consists of springwood and summerwood.

Cell Components

• Cells are composed of three components: cellulose, hemicellulose, and lignin.
• Cells are responsible for the strength of wood.
• Cells are organic components and serve as food for bugs.

Tracheids

• Cells along the main axis of the tree are called tracheids.
• They are about 100 times longer than their diameter and range from 2 to 6 mm.

Wall Layers

• P: Random, Approximate thickness 3%, Angle to longitudinal axis 50-70°.
• S1: 85°, Approximate thickness 10%, Angle to longitudinal axis 10-30°.
• S2: 2°, Approximate thickness 60%, Angle to longitudinal axis 60-90°.
• S3.

Earlywood & Latewood

• Comparison of earlywood and latewood.

Wood Anatomy

• Tangential Section
• Transverse Section
• Radial Section
• A - Tracheid
• B - Rays (narrow)
• C - Resin canal
• D - Fusiform ray

Density of Tracheids

• Latewood tracheids are about 3 times as dense as earlywood tracheids.
• The slower the growth, the denser the wood.

Timber Types

1. Round wood (Piles, posts, and poles)
2. Dimensional Lumber/Timber (Sawn into usually rectangular sections for beams, planks, joists, etc.)
3. Composite and engineered wood products

Wood Characteristics

• Relationship between Elastic Modulus in Longitudinal (EL), Radial (ER), and Tangential (ET) directions:
  $EL : ER : ET ≈ 20 : 1.6 : 1$

Flatsawn vs. Quartersawn

• T - Flatsawn
• R - Quartersawn

Different Ways to Saw a Log

• Flatgrain
• Riftsawn
• Quartersawn
• Combination

Log Information

• Example log information including log number, pruning status, and dimensions mentioned.

Density of Wood

• Density (weight per unit volume) is the single most important indicator of strength in wood.
• It is related to characteristics like hardness, ease of machining, and nailing resistance.

Specific Gravity

• Specific gravity is the ratio of the density of wood to the density of water.
• It is customary to use oven-dry weight and current volume.
• South African ironwood has a specific gravity of 1.49.
• Examples of specific gravity for different woods:
  • Softwoods: Radiata pine (0.5), Douglas fir, Canadian Western red cedar (0.3), Balsa (0.1).
  • Hardwoods: Lignum vitae (1.2), White oak (0.7).

Strength of Wood

• Measured in MPa or N/mm².
• Relationship between relative density and:
  • Modulus of elasticity
  • Modulus of rupture
  • Compression parallel to grain
  • Compression perpendicular to grain

Compressive Strength vs. Density

• The relationship between compressive strength and density is shown by the equation:
  $y = 0.1134x - 9.935$
  $R^2 = 0.6968$

Tensile Strength vs. Density

• The relationship between tensile strength and density is shown by the equation:
  $y = 0.1928x - 13.137$
  $R^2 = 0.2576$

Maximum Compression Strength vs. Specific Gravity

• Graph showing the relationship between maximum compression strength and specific gravity for air-dried and green wood.

Thermal Expansion Coefficient

• Expansion/contraction due to changes in moisture content is more significant.
• Parallel-to-grain (independent of species and density):
  $α = 1.7 – 2.5 x 10^{-6} / °F$
• Perpendicular-to-grain (depends on specific gravity):
  • Radial direction: $α_r = (18G + 5.5) x 10^{-6} / °F$
  • Tangential direction: $α_t = (18G + 10.2) x 10^{-6} / °F$

Properties of Wood Variability

• Wood is extremely variable because of its natural characteristics.
• Different species have different responses to the surrounding environment and imposed loads.
• Due to this variability, different treatments may be needed.