Wood & Lumber (Better Version)

Wood & Lumber

Learning Objectives

  1. Difference between wood and timber:
    • Wood: Refers to small clear specimens free of macroscopic defects; it is a material.
    • Timber: Refers to unprocessed tree material that contains many macroscopic defects.
  2. Difference between softwood and hardwood:
    • Softwood: Typically comes from coniferous trees and is fast-growing, plentiful, relatively inexpensive, soft, and less dense.
    • Hardwood: Comes from deciduous trees, is aesthetically appealing with varied grain patterns and colors, hard, dense, and usually more expensive.
  3. Softwood trees in construction lumber:
    • Preferred for construction due to their availability, low cost, ease of use, and durability.
  4. Three main axes of wood:
    • Longitudinal, radial, and tangential; their macrostructure and microstructure relate to mechanical properties.
  5. Structural properties and lumber grading systems:
    • Structural grading assesses strength, durability, and workability of wood materials.
  6. Production steps of sawn lumber:
    • 1. Sawing into desired shapes
    • 2. Seasoning
    • 3. Surfacing
    • 4. Grading
    • 5. Optional preservative treatments
  7. Nominal vs actual dimensions:
    • Nominal dimensions refer to the size when sawn, while actual dimensions are smaller after seasoning and surfacing.
  8. Calculating board feet requirements:
    • Use the formula: ext{Board Feet} = rac{( ext{Nominal Width} imes ext{Nominal Depth})}{12} imes ext{Length (in feet)}

Wood as a Construction Material

  • Wood timber is widely utilized in the construction industry due to:
    1. Availability
    2. Low cost
    3. Ease of use
    4. Durability
  • Important facts about wood:
    • Many species exist
    • It is composite in nature
    • It is a natural material, thus contains flaws and imperfections
    • Wood is an anisotropic material; mechanical and physical properties change depending on the direction of the axis.

Major Wooden Structures

  1. Metropol Parasol, Spain:
    • Recognized as the largest wooden structure.
  2. Tillamook Air Museum, Oregon, USA:
    • Notable structure made from wood.
  3. The 20 Most Impressive Wooden Structures in the World:
    • Reference link for further exploration: https://www.popularmechanics.com/technology/infrastructure/g2679/best-wooden-structures-in-the-world/

Wood Cross Section

  • Illustrates the anisotropic properties where mechanical and physical characteristics change according to axis direction.

Terminology

  • Wood: A small clear specimen free of macroscopic defects.
  • Timber: Unprocessed tree material containing many macroscopic defects.
  • Lumber: Sawn timber that serves as a structural element and contains many macroscopic defects.
    • Sources: NarangbaTimbers.com.au

Types of Wood Material

  • Softwoods: Coniferous, fast-growing, relatively inexpensive and easier to cut and manufacture, typically less dense.
  • Hardwoods: Deciduous, often esthetically pleasing with distinct grain patterns and color differences, harder and denser than softwoods, generally more expensive.

Microstructure of Wood

  • Softwood Structure:
    • Generally simpler cell structure.
    • Example: Pine.
  • Hardwood Structure:
    • More complex cell structure:
      • A - Ring-porous (e.g., red oak);
      • B - Semi-ring-porous (e.g., aspen);
      • C - Diffuse-porous (e.g., yellow birch).

Aesthetic Qualities

  • Softwoods:
    • Generally plain or simple grain structure with uniform coloring.
  • Hardwoods:
    • Characterized by patterned grain structure, diverse coloring, and greater visual appeal.

Material Properties

Microstructure of Wood Cells

  • Tracheid Structure:
    • L: Cell lumen
    • ML: Middle lamella
    • P: Primary lamella
    • S1, S2, S3: Layers of secondary wall

Anisotropic Mechanical Properties

  • Timber engineering properties are unique and relative to the individual axis (grain/wood cell orientation).
  • Mechanical properties vary with load direction; typically, properties along the longitudinal axis are greater than those along the radial and tangential axes.

Loading Types and Effects

  1. Tensile Loading:
    • Failure under longitudinal tensile loading involves fiber breaking.
    • Failure under radial or tangential tensile loading involves fiber de-bonding.
  2. Compressive Loading:
    • Failure in longitudinal compressive loading results in localized buckling of fibers (cell walls).
    • Collapse of hollow fibers occurs under radial or tangential compressive loading.
  3. Flexural Loading:
    • Failure under longitudinal shear loading leads to fiber de-bonding.
    • Flexural loading failure results in fiber breaking.

Loading at Angle to Fiber Orientation

  • When a force is applied at an angle to grain, relative strength varies.

Testing for Mechanical Properties

  • Various test specimens of wood for evaluating properties, including tensile parallel and perpendicular to grain, compression parallel and perpendicular to grain, hardness, and bending parameters.

Structural Grades

Lumber Grade Classification and Design Values

  • Example of design values for Eastern White Pine across different grades including Select, No. 1, No. 2, No. 3, Structural, and Utility classifications.
  • Design values specific to compressive loading, tensile loading, and flexural yielding at maximum moisture content of 19%.

Lumber Production Steps

  1. Sawing:
    • Types include plainsawn (maximum yield but prone to distortion) and quartersawn (more dimensionally stable but less yield).
  2. Seasoning:
    • Processes to reach a moisture content (MC) of 19% at the mill. Importance of proper drying to ensure stability and prevent shrinkage.
  3. Surfacing:
    • Lumber is surfaced for smoothness and dimension precision; common practices involve S4S or S2S surfacing.
  4. Grading:
    • Graded according to strength, durability, and workability. Affected by visual defects and strength characteristics.
  5. Preservative Treatments: Optional treatments to enhance durability.

Moisture Content and Shrinkage

  • Moisture content in growing wood varies from 30% to 200%.
  • Fungi cannot survive when moisture content is below approximately 20%.
  • Shrinkage Behavior:
    • Longitudinal shrinkage is minimal; radial and tangential shrinkage is greater, which can lead to checks or cracks in the wood.

Types of Defects

  • Common defects include knots, shakes, checks and splits, wane, cupping, twisting, bowing, and crooking, assessed for influence on mechanical properties and their prevalence.

Example Defects:

  • Knots: Degrade mechanical strength due to inclusion in wood.
  • Checks: Develop during seasoning due to differential shrinkage along the grain.
  • Wane: Occurs when bark or soft material is left on the edge of the board.

Lumber Pricing

  • Lumber is priced by the board foot based on nominal dimensions. Calculation example: To find board feet in a 2x12 board that is 8 feet long:
    • ext{Board Feet} = rac{(2 imes 12)}{12} imes 8 = 16
    • For 30 members, the calculation:
    • 30 imes rac{(2 imes 12)}{12} imes 8 = 480 board feet.

Practice Problems

  • How many board feet are in:
    A) 30 2”x4” that are 8 feet long?
    B) 50 2”x6” that are 10 feet long?
    C) 25 1”x4” that are 8 feet long?