Lecture 3-Basic Wood Characteristics

Lecture 3-Basic Wood Characteristics - Lecture Three Basic...

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Lecture Three Basic Wood Characteristics Basic constituents of wood: 1) Cellulose (70%) the primary constituent of wood 2) Lignin (~25%) this is the constituent which gives the wood strength 3) Extractive: consisting of tannin, starch, oils, resins, fats, and waxes 4) Ash: forming minerals
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Two basic types of wood
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SOFTWOODS: Softwoods are woods extracted from coniferous or evergreen tress. Pine, spruce, fir, cedar, and redwood are all softwoods but they have different properties and prices and are therefore used in different ways in building construction. The general pattern of softwood use in building construction: 1) Framing: Pine, spruce 2) Beams: Fir, cedar, reswood 3) Siding: Cedar, reswood 4) Shingles: Cedar HARDWOODS: Because the trees that produce hardwoods take much longer to mature than those that produce softwoods, hardwoods generally command much higher prices, hardwoods are generally used only for finishing.
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Relationship between tree growth and wood strength: · Wood grows in an annual fashion, with concentric rings from the center (in cross section) marking the beginning and end of each year’s growth. · We speak of these rings as the grain of the wood. When referring to wood we speak of direction as being parallel to, or across the grain; meaning respectively: vertically and horizontally with respect to the original standing tree. Wood is two to five times stronger parallel to the grain. When speaking of beams it is common to speak of stresses with respect to their direction: 1) Compression is caused by pushing the ends of the beam together. 2) Tension is caused by pulling the ends of the beam apart. 3) Shear is caused by sideways force, i.e. force not along the long axis of the beam.
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Wood is 30% stronger in compression than in tension. Wood is stronger in resisting shear across the grain than it is parallel to the grain. Moisture content and seasoning: New wood, that is wood recently cut down, contains a large amount of moisture (this is known as green lumber). Over time, the moisture evaporates, and this drying causes the wood to shrink, warp, and twist. In general, hardwoods shrink more than softwoods.
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Lecture 3-Basic Wood Characteristics - Lecture Three Basic...

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