Wood as a Construction Material

Introduction to Wood

• Wood is the earliest construction material used by mankind
• It is still widely used today for building frames, bridges, utility poles, floors, roofs, trusses, and piles
• Wood is easy to use, durable, has high strength, low weight, is widely available, and has a low cost

Classification of Trees

• Trees can be classified into two types: Endogenous and Exogenous
• Endogenous trees have intertwined growth, are very strong and lightweight, but not generally used for engineering applications in the US
• Exogenous trees have outward growth, with fibers growing from the center outward, forming concentric layers (annual rings) that give predictable engineering properties
• Deciduous trees are hardwoods, expensive, and slow-growing, while Coniferous trees are softwoods

Structure of Wood

• Each annual ring of an exogenous tree is composed of earlywood and latewood
• Earlywood is the light ring formed during rapid spring growth, with hollow, thin-walled cells
• Latewood is the dark ring formed during dense summer growth, with thick-walled cells that are stronger and harder

Wood Properties

• Wood is anisotropic, meaning its properties change with direction
• Longitudinal, radial, and tangential directions influence strength, modulus, thermal expansion, conductivity, shrinkage, and other properties
• Wood is strongest and least prone to shrinkage in the longitudinal direction (parallel to the grain)

Wood Production

• The production process involves harvesting, sawing, seasoning (drying), surfing (planing), grading, and preservative treating (optional)

Wood Products for Construction

• Dimensional lumber is used for light framing, such as studs, joists, beams, rafters, trusses, and decking
• Heavy timber is used for heavy framing, railroad ties, and landscaping
• Round stock is used for posts, poles, and utility poles
• Specialty items include handrails, spindles, radius edge decking, and lattice
• Engineered wood products are created by bonding wood strands, veneers, lumber, or other wood fibers to form a large, integral composite unit

Lumber Defects

• Defects can affect both appearance and mechanical properties
• Causes of defects include natural wood growth, seasoning too fast, wood diseases, animal parasites, and faulty processing
• Knots are a common defect that can degrade mechanical properties

Mechanical Properties of Wood

• Wood is extremely anisotropic
• Modulus of Elasticity depends on species variation, moisture content, specific gravity, and direction of grain
• Strength properties vary widely due to anisotropy, moisture content, defects, and other factors
• Tensile strength is greater than compressive strength, and tensile strength parallel to the grain is 20x greater than perpendicular
• Load duration affects the mechanical properties of wood, with higher loads supported for short durations

Design Considerations

• Strength properties must be adjusted for factors such as load duration, temperature, size, flat use, beam stability, column stability, and bearing area
• Design values assume 10-year loading and/or 90% of full maximum load throughout the life of the structure

Ferrous Metals

• Ferrous metals are classified as those with iron as the major constituent
• Examples include pig iron, cast iron, and steel

Iron

• Iron is extracted from iron ores such as Hematite, Magnetite, Siderite, and Limonite
• Commercial iron ores contain 25-70% metallic iron, with impurities such as sulfur, phosphorus, silica, and clay

Pig Iron

• Pig iron is found to be combined with high carbon, silicon, manganese, sulfur, and phosphorus
• The characteristics of pig iron are determined by its carbon content
• Pig iron is not malleable at any temperature

Cast Iron

• Cast iron is produced by remelting pig iron to eliminate impurities and achieve uniformity
• The molten iron is cast into desired shapes
• Cast iron has a carbon content of 1.5-6% depending on the pig iron used

Properties of Cast Iron

• Cast iron is hard and brittle, with a high compressive strength and low tensile strength
• The stress-strain curve of cast iron has no straight-line portion

Uses of Cast Iron

• Cast iron is used for pipes, machinery parts, and other applications where tensile stresses are low and dynamic loading is not present
• Gray cast iron is used for pipes and machinery parts, while malleable cast iron is used for small parts requiring ductility
• White cast iron is used where abrasion resistance is needed

Steel

• Steel is produced from pig iron by removing impurities, decreasing the carbon content, and adding alloying elements
• The excess carbon is removed as CO2 gas, and the oxides of other impurities form a slag on top of the molten steel
• Steel is an alloy of iron and carbon with a carbon content of less than 1.5%