Wood has been a fundamental material in construction for centuries, valued for its unique blend of strength, workability, and renewability. Its structure, from microscopic cellular components to macroscopic grain patterns, dictates how it performs. This inherent makeup allows wood to serve as a versatile building material.
The Natural Composition of Wood
Wood’s fundamental nature stems from its cellular structure, primarily composed of three organic polymers: cellulose, hemicellulose, and lignin. Cellulose, a long-chain polymer, forms the wood’s structural skeleton, contributing significantly to its strength. Hemicellulose, a branched polymer, and lignin, an amorphous polymer, act as binders, holding the cellulose fibers together and providing rigidity. Lignin is often referred to as the “cementing agent” that binds individual cells.
Trees grow by adding layers of wood annually, forming distinct growth rings visible in a cross-section. These rings indicate the tree’s age and reveal growth patterns based on environmental conditions. Variations in cell size and density within these rings contribute to wood grain appearance. Wood also consists of sapwood and heartwood. Sapwood, the outer living portion, transports water and nutrients. Heartwood, the inactive inner core, provides structural support and often contains extractives that give it darker color and improved decay resistance.
Understanding Wood’s Structural Behavior
Wood exhibits distinct mechanical properties, including strength and stiffness, important for its use in construction. Strength refers to its ability to withstand forces like tension (pulling apart), compression (pushing together), and shear (sliding forces) before failure. Stiffness, measured by the modulus of elasticity (MOE), indicates how much a material will deform under a given load. A higher MOE means less deflection under load.
Wood’s anisotropic nature means its mechanical properties vary significantly depending on the direction of the applied force relative to its grain. Wood is strongest when forces are applied parallel to the grain, such as in columns or beams. Its strength is considerably lower when forces are applied perpendicular to the grain, which influences how connections and fasteners perform. This directional dependency is a key consideration in wood design.
Moisture content significantly influences wood’s structural behavior. Wood is a hygroscopic material, meaning it absorbs and releases moisture from its environment. As moisture content changes, wood undergoes dimensional changes, shrinking when it dries and swelling when it absorbs water. High moisture content can reduce wood’s strength and stiffness, making it more susceptible to decay and insect attack. Maintaining appropriate moisture levels is important for the longevity and performance of wood structures.
Types and Forms of Wood for Construction
Wood used in construction is broadly categorized into hardwoods and softwoods, a distinction based on botanical classification rather than actual hardness. Hardwoods originate from deciduous trees, which lose their leaves seasonally, and are denser and more durable, often used for flooring, furniture, and decorative applications. Examples include oak, maple, and walnut. Softwoods come from coniferous trees, which have needles and cones, grow faster, and are lighter and less dense, making them common for framing, sheathing, and general construction. Pine, cedar, and spruce are common softwoods.
Lumber grading is a standardized process that assesses wood quality, strength, and appearance, ensuring its suitability for specific structural applications. This grading considers factors like knots, grain patterns, and other defects, which can impact a piece of lumber’s load-bearing capacity. Softwood lumber is stress-graded for structural uses, indicating its allowable load resistance.
Engineered wood products have significantly improved wood construction by overcoming some natural limitations of solid lumber, offering improved consistency, strength, and larger sizes. These include:
Plywood: Made by gluing together thin layers of wood veneer with alternating grain directions, creating a strong and stable panel used for sheathing and subflooring.
Oriented Strand Board (OSB): Manufactured from compressed layers of wood strands bonded with adhesive, used for wall and roof sheathing due to its strength and stability.
Glued-laminated timber (glulam): Consists of multiple layers of lumber glued together, forming large, strong beams suitable for long spans and heavy loads.
Laminated Veneer Lumber (LVL): Similar to glulam but uses thin wood veneers bonded together, creating strong and consistent structural members like headers and beams.
Designing and Maintaining Wood Structures
Wood is used for framing residential and light commercial buildings, forming the skeletal structure of floors, walls, and roofs. For floor systems, wood joists or engineered wood products like I-joists provide support, transferring loads to walls and foundations. Wall framing involves vertical studs and horizontal plates, creating a framework for exterior and interior finishes. Roof structures utilize rafters or trusses to create sloped or flat surfaces, supporting roofing materials and resisting environmental forces.
Maintaining wood structures involves ensuring their long-term durability and resistance to decay and insect infestations. Moisture management is important, as prolonged exposure to water can lead to fungal decay and mold growth. Proper design details, such as generous roof overhangs, effective drainage systems, and adequate ventilation, help prevent water intrusion and allow wood to dry. Raising wood elements off the ground, such as with foundations, also reduces moisture exposure and improves ventilation.
For applications where wood is susceptible to moisture or insect exposure, such as sill plates or exterior timbers, using naturally durable wood species or preservative-treated wood is beneficial. Preservative treatments enhance wood’s resistance to biological agents like fungi and termites, extending its service life. Regular inspections and prompt repair of any damage also contribute to the stability and longevity of wood structures.