5.4 Timber and Wood Products
5 min read•august 15, 2024
Wood is a versatile and sustainable construction material with unique properties. Its cellular structure and anisotropic nature give it and flexibility, making it ideal for various applications. However, and natural variability affect its performance, requiring careful consideration in engineering design.
Engineered wood products like , , and expand timber's capabilities. These materials offer improved strength, stability, and versatility, allowing for innovative structural solutions in buildings. Timber's environmental benefits, including carbon sequestration and renewability, make it increasingly popular in sustainable construction practices.
The structure and properties of wood as an engineering material
Anisotropic nature and cellular structure
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- Wood exhibits anisotropic properties varying depending on the direction of applied force relative to its grain structure
- Cellular structure comprises cellulose fibers embedded in a matrix of lignin and hemicellulose contributing to unique mechanical properties
- Strength-to-weight ratio compares favorably to steel making wood an efficient structural material (bridges, trusses)
- Moisture content significantly affects mechanical properties, dimensional stability, and durability
- High moisture content reduces strength and stiffness
- Low moisture content increases brittleness
- Viscoelastic behavior causes time-dependent deformation under load affecting long-term performance (creep in beams)
Natural variability and versatile properties
- Properties vary due to species, growth conditions, and defects necessitating grading systems for structural applications
- Visual grading assesses , grain angle, and other visible characteristics
- Machine stress rating measures stiffness to predict strength
- Thermal insulation properties make wood suitable for energy-efficient construction (wall framing, roof sheathing)
- Acoustic performance allows for sound absorption and diffusion (concert halls, recording studios)
- Aesthetic qualities provide versatility for both structural and non-structural uses (exposed beams, furniture)
Various types of engineered wood products and their applications in construction
Plywood and oriented strand board (OSB)
- Plywood consists of cross-layered thin veneers providing improved dimensional stability and strength in multiple directions
- Used for wall sheathing, roof decking, and concrete formwork
- Oriented Strand Board (OSB) manufactured from wood strands arranged in layers
- Cost-effective alternative to plywood for sheathing and subflooring
- Commonly used in residential construction for wall and roof sheathing
Glued laminated timber and cross-laminated timber
- Glued Laminated Timber (Glulam) consists of layers of dimensioned bonded together
- Allows creation of large, curved structural members (arches, long-span beams)
- Used in commercial and industrial buildings for aesthetic and structural purposes
- Cross-Laminated Timber () panels made by layering wood boards at right angles
- Enables construction of tall wood buildings and large-span structures
- Used for floors, walls, and roofs in multi-story buildings
Engineered lumber products
- Laminated Veneer Lumber () produced by bonding thin wood veneers together
- Strong, uniform material for beams and headers in residential and commercial construction
- Parallel Strand Lumber () utilizes long wood strands aligned parallel to the length of the member
- Creates high-strength columns and beams for heavy-load applications (transfer girders, columns)
- combine engineered wood flanges with a web material
- Lightweight and efficient alternative to solid sawn lumber for floor and roof framing
- Allows for longer spans and easier installation of mechanical systems
Environmental impact and sustainability aspects of timber as a construction material
Carbon sequestration and renewable resource
- Timber acts as a when sourced from sustainably managed forests
- Contributes to carbon sequestration by storing carbon dioxide absorbed during tree growth
- Reduces greenhouse gas emissions compared to energy-intensive materials (concrete, steel)
- Production of engineered wood products often utilizes a higher percentage of harvested tree improving resource efficiency
- Reduces waste and maximizes use of available timber
Life cycle assessment and sustainable practices
- Life Cycle Assessment (LCA) studies generally show timber construction has lower embodied energy and carbon footprint than steel or concrete alternatives
- Sustainable forestry practices certified by organizations (Forest Stewardship Council) ensure long-term viability of timber resources
- Protect biodiversity and maintain forest ecosystems
- Timber use in construction contributes to green building certifications (LEED) due to environmental benefits
- Renewable material credits
- Local sourcing potential
Waste management and innovations
- Wood waste from construction and demolition can be recycled or used for bioenergy production
- Enhances material's sustainability profile through circular economy principles
- Innovations in wood preservation techniques reduce environmental impact of treated timber
- Extends service life in challenging environments (marine structures, outdoor applications)
- Develops less toxic preservatives (copper-based treatments)
Structural behavior of timber elements under different loading conditions
Orthotropic properties and failure modes
- Orthotropic nature of wood results in different strength and stiffness properties parallel and perpendicular to grain
- Must be considered in structural design (column buckling, bending)
- Timber elements typically exhibit linear elastic behavior up to proportional limit followed by nonlinear behavior before failure
- Failure modes in timber structures include:
- Bending (tension failure in bottom fibers)
- Compression (crushing of wood fibers)
- Tension (splitting along grain)
- Shear (sliding between wood fibers)
- Each failure mode governed by specific strength properties of wood species and grade
Connections and long-term behavior
- Connections in timber structures are critical design elements
- Often determine overall strength and ductility of system
- Types include nailed, bolted, and glued connections
- Long-term loading effects must be accounted for in design of timber structures
- Creep causes increased deformation over time under sustained loads
- Load duration affects strength (shorter duration loads allow higher design stresses)
Size effect and fire performance
- Size effect in timber elements causes larger members to have lower strength per unit area
- Important consideration in structural design of large timber elements
- Fire performance of timber structures characterized by predictable charring rate
- Allows for design of fire-resistant timber elements and systems
- Char layer acts as insulation protecting inner core
Common timber design codes and standards used in civil engineering practice
North American standards
- National Design Specification (NDS) for Wood Construction serves as primary reference for timber design in United States
- Provides design values and methodologies for various wood products
- International Building Code (IBC) includes provisions for timber construction
- References specific wood design standards for compliance
- American Wood Council (AWC) publishes technical design guides and supplements
- Aids engineers in timber design and construction practices
International and material-specific standards
- Eurocode 5: Design of Timber Structures used in European countries
- Offers comprehensive approach to timber engineering across Europe
- Canadian Standards Association (CSA) O86 standard governs engineering design of wood structures in Canada
- Provides alternative approach to timber design for North American practice
- International provides numerous standards for testing and specifying wood products
- Includes methods for determining mechanical properties and durability
Grading and product-specific standards
- Grading rules for structural lumber essential for determining allowable design values
- Inspection Bureau (SPIB) publishes rules for southern pine species
- Other agencies cover different wood species and regions
- Product-specific standards exist for engineered wood products
- Plywood: PS 1 Structural Plywood
- OSB: PS 2 Performance Standard for Wood-Based Structural-Use Panels
- Glulam: A190.1 Standard for Wood Products - Structural Glued Laminated Timber
Key Terms to Review (24)
ANSI: ANSI stands for the American National Standards Institute, which is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, and systems in the United States. These standards help ensure quality, safety, and interoperability in various industries, including construction and manufacturing. ANSI plays a vital role in promoting global competitiveness and fostering innovation by ensuring that American standards align with international practices.
ASTM: ASTM stands for the American Society for Testing and Materials, an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services. In the context of timber and wood products, ASTM standards are crucial as they provide guidelines for testing and quality assurance to ensure that wood materials meet safety and performance criteria.
Beam: A beam is a structural element that primarily resists loads applied laterally to its axis. It is designed to support and transfer weight from one point to another, typically spanning across supports like walls or columns. Beams play a crucial role in maintaining the integrity and stability of structures, distributing forces effectively while resisting bending, shear, and deflection.
Bolts: Bolts are mechanical fasteners typically used to secure two or more components together. They consist of a cylindrical body with external threads and a head on one end, allowing them to be driven into a pre-drilled hole and tightened with a nut. In the context of timber and wood products, bolts provide strong connections that can withstand tension and shear forces, making them essential for structural integrity.
Certified Wood: Certified wood refers to timber and wood products that have been verified as being sourced from sustainably managed forests. This certification ensures that the wood meets specific environmental and social standards, promoting responsible forestry practices that help protect ecosystems and support local communities.
Chemical treatment: Chemical treatment refers to the use of chemical substances to modify or preserve timber and wood products, enhancing their durability, resistance to pests, and overall performance. This process is crucial in extending the lifespan of wood by preventing decay, insect infestation, and other forms of deterioration that can compromise structural integrity and appearance.
CLT: CLT, or Cross-Laminated Timber, is an engineered wood product made from layers of lumber boards stacked crosswise and glued together. This construction technique enhances the strength and stability of the wood, making it a popular choice for sustainable building projects. CLT combines the natural beauty of wood with the structural performance of traditional building materials, offering a renewable alternative that aligns with modern architectural demands.
Douglas fir: Douglas fir is a species of evergreen tree, known scientifically as Pseudotsuga menziesii, that is native to North America and is widely recognized for its strength, versatility, and durability as a timber resource. Its wood is commonly used in construction, furniture making, and various wood products due to its excellent structural properties and resistance to decay.
Glulam: Glulam, or glued laminated timber, is an engineered wood product made from layers of wood laminations bonded together with durable adhesives. This construction technique allows for the creation of structural components that can achieve long spans and complex shapes, making glulam a popular choice in various construction applications due to its strength and versatility.
I-joists: I-joists are engineered wood products that consist of a thin web of material sandwiched between two flanges, forming an 'I' shape. They are designed for floor and roof framing, providing strength and stability while being lightweight and easy to work with. The unique construction allows for longer spans compared to traditional lumber, making them a popular choice in modern construction.
Knots: Knots are natural imperfections in timber that occur where a branch has grown from the trunk of a tree. These knots can impact the strength and appearance of wood products, making them a crucial consideration in timber construction and use. Understanding knots helps engineers and builders evaluate the quality and structural integrity of wood, as they can affect the load-bearing capacity and overall performance of timber structures.
Lumber: Lumber refers to wood that has been processed into beams and planks, a primary material used in construction and various wood products. It is categorized into softwood and hardwood types, each with unique properties and uses that affect structural integrity and aesthetics in building applications.
LVL: LVL, or Laminated Veneer Lumber, is an engineered wood product made by bonding together thin layers of wood veneers with adhesives under heat and pressure. This process creates a strong and stable material that is often used in structural applications like beams and headers. LVL is designed to have consistent strength and performance characteristics, making it a preferred choice in construction over traditional lumber.
Moisture content: Moisture content is the measure of the amount of water present in a material, typically expressed as a percentage of the material's dry weight. It plays a crucial role in determining the properties and behavior of materials like timber and soil, affecting their strength, durability, and stability. Understanding moisture content is essential for proper material selection, construction practices, and ensuring the longevity of structures.
Nails: Nails are metal fasteners used primarily in construction and woodworking to join materials together. They come in various sizes, types, and materials, making them essential for attaching wooden components in structures and furniture. Proper selection and installation of nails are crucial for ensuring the structural integrity and longevity of wood products.
OSB: OSB, or Oriented Strand Board, is an engineered wood product made from strands of wood that are oriented in specific directions and bonded together with adhesive under heat and pressure. It is commonly used in construction as a structural panel due to its strength, durability, and cost-effectiveness. OSB is often preferred for its uniformity and ability to perform well in various applications, including flooring, roofing, and wall sheathing.
Plywood: Plywood is a type of engineered wood product made from thin layers of wood veneer that are glued together with the grain of each layer oriented in alternating directions. This construction provides plywood with enhanced strength, stability, and resistance to warping, making it a versatile material in construction and design. Its ability to be manufactured in various thicknesses and sizes allows it to be used in a wide range of applications, from flooring and cabinetry to structural components in buildings.
Pressure treatment: Pressure treatment is a process used to enhance the durability and longevity of timber by infusing it with preservatives under high pressure. This method effectively protects wood from decay, insects, and other environmental factors that can cause deterioration, ensuring its suitability for various construction applications. By utilizing pressure treatment, wood products can maintain their structural integrity over time, making them a vital choice in both residential and commercial building projects.
PSL: PSL stands for Parallel Strand Lumber, a type of engineered wood product made from strands of wood that are bonded together with adhesives under heat and pressure. This manufacturing process results in a strong, stable, and uniform material that is commonly used in construction and structural applications, particularly where high strength-to-weight ratios are needed. PSL is recognized for its performance characteristics, making it an essential option for builders and architects when designing timber structures.
Renewable resource: A renewable resource is a natural resource that can be replenished or regenerated naturally over time, allowing it to be used repeatedly without the risk of depletion. This concept is crucial in sustainable development, especially regarding materials like timber and wood products, which can be harvested and renewed through responsible forestry practices and management.
Southern pine: Southern pine refers to a group of pine species native to the southeastern United States, including longleaf, loblolly, slash, and shortleaf pines. These trees are highly valued for their strength, versatility, and abundance, making them significant in timber and wood products industries. Their properties make southern pines a preferred choice for construction, furniture, and other applications due to their durability and workability.
Splits: Splits refer to the separations or fractures that occur in timber and wood products due to various stressors, such as environmental changes, improper handling, or material defects. Understanding splits is essential for assessing the integrity and usability of wood materials, as they can impact strength, aesthetic quality, and overall performance in construction and design applications.
Strength: Strength refers to the ability of a material to withstand an applied force without failure or deformation. In the context of timber and wood products, strength encompasses various properties such as tensile strength, compressive strength, and shear strength, which are essential for determining how wood will perform under different loading conditions. Understanding these properties is critical for engineers and builders to ensure that structures made from wood can safely support the loads they encounter throughout their service life.
Truss: A truss is a structural framework composed of interconnected elements, typically arranged in triangular shapes, that support loads and provide stability. This design effectively distributes forces throughout the structure, making it a crucial component in bridges, roofs, and various other engineering applications. The efficiency of a truss lies in its ability to maintain structural integrity while using less material compared to solid beams, which is vital in both static and dynamic load conditions as well as in the utilization of timber and wood products.