Wood and Timber

Wood and Timber

Wood is a natural, renewable, and versatile construction material. It is anisotropic, meaning its properties vary depending on the direction of the grain.

Structure of Wood

• Macrostructure: Bark, cambium, sapwood (active), heartwood (inactive, durable), pith.
• Microstructure: Cellulose fibers (reinforcement) embedded in a matrix of lignin (binder).

Softwood vs. Hardwood

• Softwood: Conifers (pine, spruce, fir). Needles/cones. Generally less dense.
• Hardwood: Deciduous (oak, maple, birch). Broad leaves. Generally denser.

Physical Properties

Below FSP, wood shrinks as it dries. Above FSP, dimension changes are negligible.

$$MC = \frac{W_{wet} - W_{dry}}{W_{dry}} \times 100$$

• Green Wood: Freshly cut, MC > FSP.
• Seasoned Wood: Dried to equilibrium moisture content (EMC).
• Kiln-Dried: Artificially dried to specific MC (typically 19% or 15%).

Mechanical Properties

Wood is strongest in tension parallel to the grain and weakest in tension perpendicular to the grain (splitting).

• Modulus of Rupture (MOR): Flexural strength.
• Modulus of Elasticity (MOE): Stiffness.
• Compression Parallel to Grain: High strength (columns).
• Compression Perpendicular to Grain: Low strength (bearing).
• Shear Parallel to Grain: Important for connections.

Defects and Grading

• Knots: Where branches grew from the stem. Reduce strength.
• Shakes: Separations along the grain.
• Checks: Cracks across growth rings due to drying.
• Wane: Bark or lack of wood on the edge.

Grading: Visual (ASTM D245) or Machine Stress Rated (MSR).

Preservation

• Decay: Caused by fungi (requires food, oxygen, moisture, favorable temp).
• Insects: Termites, beetles.
• Preservatives: Creosote, Pentachlorophenol, CCA (Chromated Copper Arsenate - restricted), ACQ (Alkaline Copper Quaternary).