LESSON 9: TIMBER

• Hardwood comes from a deciduous tree which loses its leaves annually
• Hardwoods tend to be slower growing, and are therefore usually more dense.
• A hardwood is an angiosperm, a plant that produces seeds with some sort of covering such as a shell or a fruit.
• Angiosperms usually form flowers to reproduce.
• Softwood comes from a conifer, which usually remains evergreen.
• Softwood trees are known as a gymnosperm.
• Gymnosperms reproduce by forming cones which emit pollen to be spread by the wind to other trees.
• Evergreens do tend to be less dense than deciduous trees, and therefore easier to cut, while most hardwoods tend to be more dense, and therefore sturdier.
• In practical terms, this denseness also means that the wood will split if you pound a nail into it.
• Green Wood is wood that has been recently cut and therefore has not had an opportunity to season (dry) by evaporation of the internal moisture.
• Green wood is considered to have 100% moisture content relative to air-dried.
• Knots: that portion of a branch that has become incorporated in the bole of a tree.
• Reaction Woods: abnormal woody tissue is frequently associated with leaning boles and crooked limbs of both conifers and hardwoods
• Juvenile Wood: the wood produced near the pith of the tree and has considerably different physical and anatomical properties than that of mature wood
• Pitch Pocket: a well-defined opening that contains free resin.
• The pitch pocket extends parallel to the annual rings; it is almost flat on the pith side and curved on the bark side.
• Bird Peck: damage caused by woodpeckers, most of the time small holes around the wood
• Wane: the presence of bark or absence of wood on the corner of along the length of a lumber
• Warp: distortion in wood due to shrinkage and swelling
• Check: A crack in the wood structure of a piece, usually running lengthwise, usually restricted to the end of a board and do not penetrate as far as the opposite side of a piece of sawn timber.
• Shake: separation of grain between the growth rings, often extending along the boards face and sometimes below its surface
• Split: longitudinal separation of the fibers which extends to the opposite face of the lumber
• Stain: discoloration that penetrates the wood fiber, can be any color other than the natural color of wood (blue or brown)
• Spalt: any form of discoloration caused by fungi (found in dead trees)
• Dry Rot: fungus breaks down wood fibers and renders the wood weak and brittle
• Wormholes: small holes caused by insects (beetles)
• Tensile strength to the grain is much higher (3x) than compressive strength.
• Limiting factor for tension members is compression or shear at the point of concentration.
• Tensile strength perpendicular to the grain is (1/3) of the strength // to the grain.
• Compressive strength parallel to the grain is 3-4x than compressive strength when load is perpendicular to the grain.
• Failure when load is perpendicular to the grain: crushing of wood fiber.
• Failure when load is parallel to the grain: bending or buckling of wood fibers.
• shear strength is very low but if wood is free of defect, initial failure will be compressive.
• compared to other building materials, wood has high modulus of elasticity relative to its compressive strength; wood is considered to have good elastic properties.
• wood has no defined yield point; proportional limit is used to determine the elastic modulus.
• direction of wood fibers (strength depends heavily of the direction of load with respect to the grain).
• moisture (drying beyond the FSP (Fiber saturation point), leads to increase in strength).
• Rate of Growth - greater number of annual rings per unit length gives higher strength.