SRT153 - Building Materials Science Class (On-line)_default (1)

Introduction to Wood and Timber Products

• Overview of class focus on wood and timber products following prior introductory session on building material properties.

Outline of Today's Class

• Examination of wood structures and types.

• Discussion of moisture content as a key property.

• Exploration of defects in wood and timber products.

• Overview of timber members and products.

• Reference materials for visual aid and supplementary information on Australian timber.

Recap of Previous Class

• Covered general properties of materials including:

  • Mechanical properties: compression, tension, shear, ductility, plasticity, elasticity.

  • Acoustic performances and building movements.

  • Key deteriorations in building materials.

  • Environmental properties, sustainability, and toxicity discussed briefly.

Major Building Materials in Australia

• Four primary materials include:

  • Timber

  • Brick

  • Concrete

  • Steel

• Importance of timber as a building material:

  • High strength-to-weight ratio compared to other materials (e.g., timber: 400 kg/m³ vs. stone: 2000 kg/m³).

  • Suitable for bending, compression, and tension forces.

  • Bio-based material that absorbs carbon, beneficial for global warming mitigation.

Mechanical Properties of Timber

• Timber exhibits non-elastic behavior, specifically creep (long-term deformation under stress).

• Strength characteristics:

  • Strongest longitudinally (along growth direction).

  • Weaker in radial and tangential directions.

• Timber is an excellent insulator.

  

Microstructure of Wood

• Composed of longitudinal cells called tracheids and fewer radial cells called rays.

  • Tracheids conduct nutrients; rays store and transfer nutrients.

  • Living bark (conducts nutrients) lies beneath protective bark.

  • Cambium layer generates new cells and growth.

  • Sapwood stores nutrients; heartwood provides structural strength but is non-living.

Types of Wood

• Two broad categories:

  • Softwoods: Simpler microstructure, large tracheids, small rays, derived from coniferous trees.

  • Hardwoods: More complex microstructure with larger number of radial cells (rays) and two types of longitudinal cells (fibers and larger vessels) from broadleaf trees.

• Common softwoods: Radiata pine, cypress pine, spruce.

• Common hardwoods: Oak, mahogany, maple.

Importance of Moisture Content in Timber

• Moisture varies from 30% to 300% relative to oven-dried weight.

• After cutting, free water evaporates first, followed by moisture held in cellulose (fiber saturation point).

• Shrinkage occurs as moisture decreases, affecting stiffness and strength:

  • 10% shrinkage in tangentially oriented wood vs. 5% in radially oriented wood.

Effects of Moisture on Timber

• Moisture influences biological susceptibility (fungal and insect attacks).

• Low moisture in timber promotes durability.

• High moisture leads to increased creep and potential damage to coatings.

Defects and Distortions in Timber

• Common defects include:

  • Knots: Uneven wear, tension, reduce strength.

  • Gum veins: Impact shear strength and appearance.

  • Checks, splits, shakes: Distortions causing structural issues.

  • Geometric distortions: Copying, bowing, springing, and twisting due to uneven moisture shrinkage.

Hazardous Conditions Affecting Timber

• Weathering leads to degradation, resulting in uniform gray color and change in appearance.

• Insect attacks (termite and beetle) and fungal growth require conditions like moisture and warmth.

  • Termites cause significant structural damage.

  • Fungi require moisture (20%) and oxygen.

• Chemical degradation occurs from acidic conditions or metal fastener corrosion due to chemical interactions with timber.

Treatment of Wood

• Treatment is crucial for preventing hazards affecting wood, involving strategies that consider:

  • Intended use of facilities (human or animal).

  • Required structural reliability and aesthetics.

  • Microstructure of wood influencing treatment types.

Statutory Labels for Commercial Timber

• Important information on wood labeling includes:

  • Grade (F-rating for strength).

  • Method of grading (Visual Stress Grading, VSG).

  • Moisture content and species.

  • Producer information and compliance with national standards.

Timber Members in Construction

• Common timber members include lintels, noggings, beams, top/bottom plates.

• Understanding timber's structural efficiency contributes to its continued use in construction.

Engineered Wood Products

• Types include:

  • Cross Laminated Timber (CLT): Sustainable, good fire resistance, suitable for mid-rise buildings.

  • Laminated Veneer Lumber (LVL): Used for straight lengths.

  • Glued Laminated Timber (Glulam): Attractive architectural designs using laminated sections.

Circular Economy in Timber Use

• Potential for wood reuse and remanufacturing to support sustainability goals.

Conclusion

• Wood as a versatile, environmentally friendly material, crucial for construction.

• Importance of understanding wood types, properties, moisture content, and treatment.

• Next class will cover cement, concrete, and related materials.

• Reminder for students to engage with seminars and quizzes.