Civil Engineering Materials Lecture 22 (Wood)

Trees are classified into endogenous and exogenous types. Endogenous trees, like palm trees, have intertwined fibers and are generally not used for engineering due to unpredictable properties. Exogenous trees, whose fibers grow outward in concentric layers (annual rings), offer more predictable engineering properties. Exogenous trees are further divided into deciduous (hardwoods, used for furniture, slow-growing, expensive) and conifers (softwoods, widely used in construction, grow rapidly, economical, renewable).

Wood is a historically significant civil engineering material, appreciated for its availability, cost-effectiveness, ease of use, and durability when properly designed. Even today, wood remains crucial in various applications like bridges, utility poles, floors, roofs, trusses, and piles. Understanding its basic properties and limitations is vital for efficient use.

Concentric layers in exogenous trees are called growth rings or annual rings, which can be counted to determine a tree's age. Each annual ring consists of earlywood (rapid spring growth, hollow thin-walled cells) and latewood (denser, summer growth, thick-walled cells, stronger and darker). Key structural features of a tree stem include the pith (center), heartwood (darker, provides structural support), sapwood (lighter, transports sap), cambium (thin layer where new rings form), and bark (inner and outer).

Unlike many isotropic materials like concrete, wood is anisotropic, meaning its properties vary significantly with direction. It has three main directions: longitudinal (parallel to the tree's axis, strongest, least shrinkage), radial (perpendicular to growth rings), and tangential (tangent to growth rings, weakest, most shrinkage). This directional variation affects strength, modulus of elasticity, thermal expansion, shrinkage, and other properties.

Wood is primarily composed of cellulose (approximately 50% by weight), lignin, and hemicellulose. Moisture content is critical for wood properties and is calculated by comparing wet and dry weights. Wood contains two types of moisture: bound water (held in cell walls, causes volume change/shrinkage when it changes) and free water (in cell cavities, does not cause volume change). The Fiber Saturation Point (FSP) is when cells are saturated with bound water but no free water (21-32% moisture content). Above FSP, moisture changes only affect weight, but below FSP, small changes significantly impact physical and mechanical properties.

Shrinkage is highest in the tangential direction, medium in the radial, and almost zero in the longitudinal. No shrinkage occurs above the FSP. Wood's thermal expansion is 5-10 times greater across the grain than parallel to it. When heated, wood expands, then shrinks due to moisture loss. Wood is a good electrical insulator when dry but becomes a better conductor as moisture content increases.

Common wood products include dimensional lumber (for light framing), heavy timber (for heavy framing), round stock (posts, piles, utility poles), and engineered wood products (manufactured by bonding wood strands, veneers, or fibers, such as plywood).

Understanding wood's mechanical properties is prerequisite for proper design. The modulus of elasticity varies with species, moisture content, specific gravity, and grain direction. Wood's stress-strain diagram shows linear behavior up to a certain limit, then non-linear before failure. Tensile strength is significantly higher in the longitudinal direction (over 20 times) compared to the radial direction, and generally greater than compressive strength in the same direction.

Wood exhibits a unique load duration phenomenon: it can support higher loads for short durations than for sustained loads. For instance, its strength can reduce by half after 10 years of sustained loading. Designers typically account for a 10-year load duration. Wood also possesses excellent damping capacity, being ten times greater than structural metals, meaning it can absorb vibrations and shocks more effectively.

Mechanical testing of wood often involves testing actual structural sizes for more realistic design values, though representative samples are also used. Flexural tests are common, along with compression and tension tests. It's crucial to test properties both parallel and perpendicular to the grain due to wood's anisotropic nature. Wood is susceptible to degradation by fungi (most common, requires food, temperature, moisture, oxygen), insects (beetles, termites), marine organisms (in saltwater), and bacteria (causes wet wood and black heartwood).

To prevent degradation, wood can be treated with petroleum-based solutions (highly effective but environmentally sensitive, used where human contact is limited, e.g., utility poles) or waterborne preservatives (salt-based, clean, paintable, but can leach out when exposed to moisture over time).