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Introduction to Woods 1 Close up of Vessel & Cell

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Introduction to Woods 2 Wood Behavior

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Introduction to Woods 3 Advantages of Wood

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Introduction to Woods 4 Disadvantages of Woods

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Introduction to Woods 5 Stresses Applied to Wood

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Introduction to Woods 6 Stress – Strain Relationship

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Introduction to Woods 7 Shrinkage & Swelling

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Introduction to Woods 8 Shrinkage & Swelling

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Introduction to Woods 9 Stress Grading

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Introduction to Woods 10 Stress Grading

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Introduction to Woods 11 Visual Grading

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Introduction to Woods 12 Machine Grading

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Introduction to Woods 13 Grading Organizations

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Introduction to Woods 14 Grading Lumber

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Properties of Wood and Lumber Grades 16 Sawn Lumber: Wood members that have been manufactured by cutting a member directly from a log. Design values for sawn lumber depend on species group grade Load duration moisture content Size

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Grading Structural Lumber 17 Lumbers are graded based on size and number of growth (strength-reducing) characteristics they have (knots, checks, shakes, splits,...)

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Introduction to Woods 18 Grade Types

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Grading Structural Lumber 19 Visually Graded

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Grading Structural Lumber Visually Graded WWP: Lumber Grading Agency 12 : Mill Number SEL STR: Lumber Grade DOUG FIR-L: Lumber Species S-GRN: Moisture Content Machine Stress Rated 1650:Nominal Bending Stress, psi 1.5E: Modulus of Elasticity, million psi 20

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Sizes Categories 21 There are three main size categories of lumber: Boards: 1 to 1 1/2” thick, 2” and wider Dimension lumber: 2 to 4” thick 2” and wider Timbers: 5” and thicker, 5” and wider Note 1: Thickness is the smaller cross sectional dimension and width is the larger dimension. Note 2: Dressed dimensions (S4S, Surfaced four Sides) are less than nominal dimensions (1.5”x3.5” for a 2x4). For stress calculations, dressed dimensions are used.

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Dimension Lumber Joists and Planks Joists: 2-4 in thick and at least 6 in wide (graded based on bending strength on narrow edge) Planks: 2-4 in thick and at least 6 in wide (graded based on bending strength on wider dimension) Light Framing and Decking Light Framing: 2-4 in thick and 2-4 in wide (studs, joists and rafters) Decking: 2-4 in thick and 4 in or wider (used on their wider dimension)

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Timbers Beams and Stringers: at least 4”thick and at least 2” wider than they are thick; installed horizontally and ranked based on bending stress when loaded on the narrower dimension Posts and Timbers: have a width that is no more then 2” greater than thickness (square or nearly square); installed vertically and ranked based on compression parallel to the grain

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Design Values 24 An important part of wood design is being able to determine design values for the following mechanical properties: Bending stress, F b Tension stress parallel to grain, F t Shear stress, F v Compressive stress parallel to grain, F c Compressive stress perpendic. to grain, F c Modulus of Elasticity, E

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Wet Service Factor (C M ) Cont. 25 The tabulated values for sawn lumber apply to members with EMC of 19% or less. If MC in service will exceed 19% for an extended period of time, the tabulated values are to be multiplied by C M (C M values are less than one and are given at the beginning of Table 4A).

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C r = Repetitive Member Factor 26 A 15% increase in the tabulated F b for repetitive-member systems is recognized in the NDS. A repetitive-member system is defined as one that has: 1. Three or more parallel members of dimension lumber 2. Members spaced not more than 24 in. o.c. 3. Members connected together by a load-distributing element such as roof, floor, or wall sheathing.

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C r = Repetitive Member Factor 27 If one member should become overloaded, parallel members come into play.

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WORKSHEET 5 PROPERTIES OF SECTIONS AND DEFLECTIONS

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