Wood: Properties, Production, and Application
Chapter 3: Wood
Qualities of Wood
Strong & Stiff: Wood exhibits high tensile and compressive strength, making it ideal for construction.
Light: Compared to many synthetic building materials, wood is lightweight, which facilitates handling and installation.
Easily Worked/Shaped: Wood can be easily cut, shaped, and finished to suit various applications.
Fastened Quickly & Economically: Wood can be connected using various fastening methods that are both efficient and cost-effective.
Recyclable: Wood is a renewable resource that can be repurposed or recycled at the end of its life cycle.
Biodegradable: It breaks down naturally, thus minimizing environmental impact.
Renewable Resource: Harvesting wood from sustainably managed forests supports ongoing supply and environmental health.
Undesirable Characteristics of Wood
Not Perfectly Straight nor Precise: Natural variations can lead to inconsistencies in dimensions.
Affected by Moisture: Moisture absorption or loss can lead to changes in size and shape.
Contains Growth Defects: Growth patterns can lead to knots and other imperfections, affecting structural integrity.
Can Split & Warp: Structural weakening can occur due to improper drying or excessive moisture.
Burns Easily: Wood is flammable, which can pose fire hazards if not treated appropriately.
Decays: Exposure to moisture can lead to fungal growth and deterioration.
Susceptible to Insect Damage: Pests, such as termites, can compromise wood structure significantly.
Tree Composition
Bark: The outer protective layer of the tree, which plays a vital role in protecting the inner structures.
Cambium: A thin layer of growth tissue that produces new bark and wood.
Sapwood: The younger, outer part of the wood that conducts water and nutrients.
Heartwood: The older, denser wood at the center that provides support and strength.
Pith: The central core of the tree that may not have significant structural value as the tree matures.
Tree Cells
Primarily Hollow, Cylindrical Cells: The structure of wood cells is designed for strength and efficiency. - Axis Running Parallel to the Tree: Orientation of the cells follows the grain direction, contributing to the strength. - Tough Cellulose Bound by Lignin: Cellulose provides tensile strength, while lignin contributes rigidity and resistance to decay.
Impacts the Properties of Wood: The composition and arrangement of these cells significantly influence wood's characteristics and applications.
Tree Growth
Springwood (Earlywood): - Represents faster growth periods. - Cells produced are larger and less dense compared to summerwood.
Summerwood (Latewood): - Characterizes slower growth conditions. - Cells produced are smaller and denser, lending to strength and durability.
Lumber Production - Sawing
Plain Sawing: - Maximum Yield: Produces a variety of grain patterns. - Common Use: Generally used for framing lumber.
Quarter Sawing: - Perpendicular to Annual Rings: Yield is less but results in a more consistent grain pattern. - Improved Wearing Quality: Less distortion makes it preferable for high-quality applications.
Lumber Drying
Drying Methods: - Air Drying: Natural method involving ambient air. - Kiln Drying: Uses controlled heat to reduce moisture content effectively.
Effects of Drying: - Shrinkage: Reduction in size occurs due to moisture loss. - Reduced Weight: Dry lumber is lighter and easier to handle. - Increase of Strength and Stiffness: As moisture decreases, physical properties improve. - More Dimensionally Stable: Lower moisture content promotes stability under varying environmental conditions.
Lumber Defects
Growth Defects: - Knots & Knot Holes: These occur naturally and can weaken lumber. - Decay and/or Insect Damage: Biological factors adversely affect structural integrity.
Manufacturing Defects: - Splits & Checks: Result from poor handling or drying processes. - Crook, Bow, Cup, and/or Twist: Warp patterns that result from improper drying or storage.
Lumber Grade & Species
Graded by: - Strength & Stiffness (for structural lumber) or Appearance (for finish lumber).
Sold by Species and Grade: Specific species and grades correlate to pricing; higher quality means a higher price due to scarcity or desirability.
Design / Selection Criteria
Primary Considerations: - Expected Load/Stress: Understanding the demands placed on the lumber. - Required Appearance: Aesthetic requirements for visible surfaces. - Local Availability and Price: Economic factors influencing material choices.
Species and Grade Selection: Based on intended use, load-bearing capacity, and visual attributes.
Lumber Thickness and Width
Nominal Dimensions: - Thickness: - Less than 2 inches for boards. - 2 to 4 inches for dimension lumber. - 5 inches or more for timbers. - Widths: Commonly found in 2, 4, 6, 8, 10, and 12 inches for framing; typically 1 inch for finish lumber.
Actual Dimensions: Differences due to drying and surfacing processes often lead to: - Up to 6 inches - actual sizes can be 1/2 inch less. - Greater than 6 inches - actual sizes can be 3/4 inch less.
Lumber Lengths
Typically measured in two-foot increments: Standard sizes include: - 8’, 10’, 12’, 14’, and 16’ (greater lengths available on special order).
Lumber Pricing
Unit Measure - Board Feet: One board foot is defined as: - 1 inch x 12 inches x 12 inches.
Calculation: - Total board feet can be calculated as:
Lumber Pricing Factors
Species: Different woods have varying costs linked to availability and demand.
Grade (& Drying Process): The quality and processing techniques affect pricing.
Lumber Size: Dimensions and thickness specifications contribute to overall cost estimates.
Laminated Wood (Glulam)
Lamination Process: A method that combines layers of wood products with adhesives to create larger spans or specific shapes.
Why Laminate?: - To create sizes not readily available naturally. - To create unique shapes for architectural or structural needs. - Improve overall quality and performance of the material.
Wood Panel Products
Why Panelize?: - Allows more controlled product characteristics. - Promotes efficient use of forest resources. - Increases labor productivity through standardized products.
Types of Wood Panels: - Plywood panels. - Composite panels. - Non-veneered panels.
Veneered Panels - Plywood
Construction: Thin layers of veneer glued together.
Composition: Typically made with an odd number of veneers that alternate direction for stability.
Size/Thickness: Commonly available in dimensions of 4’x8’, with thickness ranging from ¼” to 1”.
Composite Panels
Composition: - Use parallel outside face veneers with a core of reconstituted wood fibers for strength and efficiency.
Veneer Grades
Grading System: Veneers rated based on smoothness and structural integrity. Common grades include A, B, C (plugged), C, D.
Price Variances: Higher grades result in higher costs based on demand and usage.
Non-Veneered Panels
Types: Consist of: - Oriented Strand Board (OSB): - Made with long strand-like wood particles; compressed and glued with alternating grain orientation for strength. - Waferboard: - Composed of large wafer-like particles, ideal for low-moisture areas. - Particleboard: - Constructed from smaller wood particles, also suited for low-moisture environments. - Fiberboard: Used in a variety of wood product applications.
Plywood vs. OSB
In 1994, the American Plywood Association shifted to "Engineered Wood Products".
Production Trends: - Plywood production has declined to 19 billion square feet, while OSB has increased to approximately 10 billion square feet, overtaking plywood's market share by 1999.
Panel Standards & Grading
Established by the American Plywood Association (APA): - Standards assess structural adequacy, dimensional stability, and durability of adhesives used for construction.
Structural Ratings
Generally referred to by thickness or span ratings provided by grade stamps. - Long dimensions are rated perpendicular to the support for optimal strength.
Exposure Durability Classifications
Exterior Applications: Siding or areas with continuous exposure to moisture.
Exposure I: Utilizes waterproof glue but lower quality veneers suitable for less demanding applications.
Exposure II: Rated for protected environments with minimized risk of wetting.
Chemical Treatment
Concerns: Addressing issues of combustibility, decay, and insect resistance.
Pressure Treated: Lumber products like 2x6x8 are often pressure-treated to enhance durability and longevity in various applications.
Wood Polymer Composite Planks
Advantages: - Enhanced decay resistance, ensuring longevity in exterior environments. - Easy workability enabling simple installation and interaction with standard woodworking tools.
Wood Fasteners
Types of Fasteners Used in Wood Construction: - Nails (with various types like common, finish, brad, etc.). - Lag screws for heavy-duty applications. - Bolts for structural connections. - Toothed plates and sheet metal devices for rigid connections. - Machine-driven fasteners like staples, improved for productivity in installations.
Nails
Common Types: - Common Nails: For general construction. - Finish Nails: Used for detailed and cosmetic applications. - Specific Types: Include casing nails, brad nails, and specialized nails for roofing.
Sizes: Measured in pennies; various sizes include 2d through 60d.
Coatings: Finish types can include bright uncoated steel or corrosion-resistant resin-coated options.
Anchorage Techniques
Nailing Methods: - Face, end, or toe nailing: Various techniques determined by structural needs and design requirements for connections.
Machine Driven Nails & Staples
Nail Guns and Staple Guns: Description of battery-operated, electric, or pneumatic tools that efficiently deliver collated nails in high productivity scenarios.
Wood Screws
Size Requirements: Sizing determined by gauge and length; installation plays a critical role in holding power.
Types of Heads: Including square, hex, etc., based on application needs.
Lag & Drywall Screws
Lag Screws: Heavy-duty applications requiring installation with a wrench.
Drywall Screws: Specialized for attaching drywall materials securely.
Bolts
Types: Available in machine bolts and carriage bolts, crucial for heavy structural connections.
Washers: Enhancing the performance of bolts in load applications.
Toothed Plates
Used in Roof & Floor Trusses: Sheet metal plates with teeth for effective fastening techniques.
Sheet Metal Framing Devices
Types of Devices: - Joist hangers, framing anchors, angle anchors, rafter anchors designed for light and heavy timber applications.
Adhesives in Wood Construction
Applications: Widely implemented in the manufacture of wood products including panels and cabinetry for enhanced durability and reliability on site.
Wood Manufactured Components
Common Components: Utilization of trusses, wood I-joists, beams, and panel components in modern wood construction practices.
Trusses
Types: - Roof and floor trusses optimized for specific load and span requirements.
Construction Techniques: - Majority being factory built for standardized quality, typically 2X4 or 2X6 lumber utilized with toothed fasteners.
Sustainability in Wood Construction
Importance: Wood is the only major renewable structural material in construction processes.
Key Practices: - Sustainable forestry practices are essential, encompassing clearcutting and replanting strategies to ensure a balance in forest health.
Mill Practices: Efficient lumber recovery and considerations during transportation help to lessen the environmental impact of wood harvesting and processing.
Construction Process: Focus on minimizing waste and enhancing indoor air quality (IAQ) during building processes.