Wood and Timber
Wood is a natural, highly renewable, and versatile construction material. It
has an excellent strength-to-weight ratio, high insulating properties, and
exceptional aesthetic value. However, unlike engineered materials like steel
or concrete, wood is anisotropic—meaning its physical and mechanical
properties vary drastically depending on the direction of the grain.
Structure of Wood
Understanding the macrostructure of a tree trunk is essential for
understanding how timber will behave once sawn into lumber.
Checklist
- Bark: The outer protective layer of the tree.
- Cambium: A microscopic layer just inside the bark where all active cell growth occurs.
- Sapwood: The lighter-colored outer rings of wood that actively conduct sap (water and nutrients) from the roots to the leaves. It has low natural durability against decay.
- Heartwood: The darker, inactive central core of the tree. The cells are filled with extractives (resins, gums) that make it significantly more durable and resistant to decay than sapwood.
- Pith: The very small, soft core at the exact center of the tree.
Softwood vs. Hardwood
The classification of wood into "softwood" and "hardwood" is purely botanical
and refers to the tree's reproduction method, not the physical density or
hardness of the wood itself (e.g., Balsa wood is technically a hardwood but is
incredibly soft).
Botanical Classification
Softwoods come from conifers (evergreen trees with needles and cones like
pine, spruce, and fir). They grow quickly, are less dense, and provide the
vast majority of structural framing lumber. Hardwoods come from deciduous
broadleaf trees (like oak, maple, and mahogany). They are generally denser,
grow slower, and are used primarily for high-traffic flooring, fine furniture,
and architectural millwork.
Orthotropic Properties
Because wood is composed of long, hollow cells aligned vertically up the
trunk, it has unique, independent mechanical properties in three mutually
perpendicular axes.
Checklist
- Longitudinal (Parallel to Grain): The strongest direction, running vertically up the tree. Wood has exceptionally high tensile and compressive strength when loaded parallel to the grain (e.g., a column or the bottom chord of a truss).
- Radial (Perpendicular to rings): Runs horizontally from the pith (center) outward to the bark, crossing the annual growth rings. Strength is drastically lower here.
- Tangential (Tangent to rings): Runs horizontally but tangent to the annual growth rings. This is typically the weakest direction, highly susceptible to shrinkage and splitting.
Moisture Content and Dimensional Stability
Wood is highly hygroscopic, meaning it naturally absorbs and releases
moisture to reach an equilibrium with the surrounding humidity. Managing
moisture is the most critical aspect of using wood in construction.
Moisture Content (MC)
The weight of water in the wood expressed as a percentage of the completely dry weight of the wood.
Fiber Saturation Point (FSP)
The specific moisture content (typically around 25% to 30%) at which all "free
water" in the cell cavities has evaporated, but the cell walls are still
completely saturated with "bound water".
Note
Crucial Concept: Wood does not shrink or swell as its moisture content
changes above the FSP. Dimensional changes (shrinkage) and strength increases
only begin to occur when the moisture content drops below the Fiber
Saturation Point as water leaves the cell walls.
Cellular Moisture & Shrinkage Simulator
Adjust the moisture content to see how water leaves the wood cells. Notice that dimensional shrinkage only occurs when the bound water leaves the cell walls (below the Fiber Saturation Point).
40.0%
Current State
- Phase:Green Wood (Losing Free Water)
- Free Water (Cavity):10.0% MC
- Bound Water (Walls):30.0% MC
- Tangential Shrinkage:0.00%
Free Water
Simplified Wood Cell Cross-Section
Wood Shrinkage vs. Moisture Content
Wood shrinks dimensionally only when its Moisture Content (MC) drops below the Fiber Saturation Point (FSP), typically around 30%. Shrinkage is highly anisotropic: it is greatest tangentially across the growth rings, about half as much radially, and negligible longitudinally (along the grain).
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Mechanical Properties
Because of its fibrous structure, wood's mechanical properties depend heavily
on the direction of the applied load relative to the grain (the longitudinal
axis of the tree).
Checklist
- Tension Parallel to Grain: Wood is incredibly strong in tension parallel to the grain. However, connections are difficult, so it frequently fails at joints before reaching its ultimate tensile capacity.
- Tension Perpendicular to Grain: Wood is exceptionally weak in this direction. Tensile forces across the grain pull the fibers apart, causing the wood to split instantly. This must be avoided in structural design.
- Compression Parallel to Grain: Wood acts like a bundle of straws and can support heavy loads (e.g., wooden columns and posts) until the fibers begin to buckle locally.
- Compression Perpendicular to Grain: Much weaker than parallel compression. The hollow fibers crush easily. This is a critical design check where a beam rests on a support (bearing stress).
- Shear Parallel to Grain (Horizontal Shear): The tendency for the wood fibers to slide past one another. This is often the controlling failure mode in short, heavily loaded timber beams.
Defects and Structural Grading
Unlike manufactured steel, wood contains natural growth characteristics that
reduce its strength. "Grading" is the process of inspecting lumber and
assigning it an allowable design stress based on the presence of these
defects.
Knots
The portion of a branch that has become incorporated into the main trunk.
Knots disrupt the straightness of the grain, creating severe stress
concentrations that significantly reduce the wood's tensile and flexural
strength.
Shakes
Separations or cracks along the grain that occur between the annual growth
rings, typically occurring while the tree is still standing due to wind
stress.
Checks
Cracks extending across the annual growth rings. They are almost always
caused by uneven drying stresses (seasoning), where the outer surface dries
and shrinks faster than the wet inner core.
Wane
The presence of bark, or the lack of wood, on the edge or corner of a sawn
piece of lumber, reducing its usable cross-section.
Durability and Preservation
Wood is an organic material susceptible to biological degradation. To decay, fungi require four conditions simultaneously: 1) Food (the wood), 2) Oxygen, 3) Favorable Temperatures, and 4) Moisture (MC > 20%). Removing any single condition prevents rot. (e.g., Wood continuously submerged deep underwater will not rot due to a lack of oxygen).
When wood must be used in damp environments or in contact with soil, it must
be chemically treated.
Checklist
- Preservatives: Chemicals like Creosote (used for railroad ties), Pentachlorophenol (utility poles), or waterborne salts like ACQ (Alkaline Copper Quaternary, used for residential decks) are injected deep into the wood cells under high pressure in giant industrial cylinders. These chemicals render the wood inedible to fungi and insects (termites).
Decay Mechanisms and Fungi
Wood decay is caused by fungi that consume the cellulose or lignin. Fungi
require four conditions to survive: food (the wood), oxygen, favorable
temperatures, and moisture (specifically, an MC above 20%). If wood is kept
permanently dry (below 20% MC) or permanently submerged (cutting off oxygen),
it will not rot. Where moisture cannot be controlled (e.g., ground contact),
the wood must be pressure-treated with chemical preservatives (like ACQ or
CCA) that make the wood toxic to fungi and termites.
Fire Resistance of Timber
While light-frame wood construction is highly combustible, heavy timber (large
cross-sections) exhibits excellent fire resistance. When exposed to fire, the
outer surface chars rapidly. This thick char layer acts as an exceptional
thermal insulator, protecting the unburned inner core and allowing the timber
column or beam to maintain its structural load-bearing capacity far longer
than unprotected steel, which rapidly softens and collapses at high
temperatures.
Engineered Wood Products (EWPs)
To overcome the natural limitations of solid sawn lumber (like size
constraints and natural defects), the timber industry manufactures Engineered
Wood Products. These products are made by binding together wood strands,
veneers, or fibers with structural adhesives to create larger, stronger, and
highly predictable structural elements.
Checklist
- Glued Laminated Timber (Glulam): Composed of individual wood laminations (dimension lumber) bonded together with durable, moisture-resistant adhesives. Glulams can be manufactured into massive beams and sweeping arches capable of spanning hundreds of feet.
- Laminated Veneer Lumber (LVL): Manufactured by bonding together thin wood veneers (similar to plywood) under heat and pressure, with the grain of all veneers running parallel to the longitudinal axis. This creates a highly uniform, exceptionally strong beam used for headers and edge-forming.
- Cross-Laminated Timber (CLT): Large, structural panels made by gluing together layers of solid sawn lumber, with each layer oriented perpendicular to the adjacent layer. CLT panels are incredibly strong, dimensionally stable, and provide excellent fire resistance, making them the primary material for modern mid-rise and high-rise "mass timber" buildings.
- Oriented Strand Board (OSB): A structural panel made by compressing and gluing rectangular wood strands arranged in cross-oriented layers. It is the modern, cost-effective replacement for plywood in structural floor, wall, and roof sheathing.
Key Takeaways
- Anisotropy: Wood's structural strength is entirely dependent on load direction relative to the grain—strongest in parallel tension/compression, and critically weak in perpendicular tension (splitting).
- Moisture Management: Wood shrinks, swells, and gains strength only when its moisture content drops below the Fiber Saturation Point (FSP, ~30%).
- Defects: Natural growth characteristics like knots, checks, and shakes disrupt the grain and severely reduce the allowable design strength, necessitating strict visual or mechanical grading.
- Preservation: To prevent fungal decay and insect attack in damp environments (MC > 20%), lumber must be pressure-treated with chemical preservatives.