Introduction to Woods1
Close up of Vessel & Cell
Introduction to Woods2
Wood Behavior
Introduction to Woods3
Advantages of Wood
Introduction to Woods4
Disadvantages of Woods
Introduction to Woods5
Stresses Applied to Wood
Introduction to Woods6
Stress – Strain Relationship
Introduction to Woods7
Shrinkage & Swelling
Introduction to Woods8
Shrinkage & Swelling
Introduction to Woods9
Stress Grading
Introduction to Woods10
Stress Grading
Introduction to Woods11
Visual Grading
Introduction to Woods12
Machine Grading
Introduction to Woods13
Grading Organizations
Introduction to Woods14
Grading Lumber
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Properties of Wood and Lumber Grades
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Sawn Lumber: Wood members that have been manufactured by cutting a member directly from a log.
Design values for sawn lumber depend onspecies groupgradeLoad durationmoisture contentSize
Grading Structural Lumber
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Lumbers are graded based on size and number of growth (strength-reducing) characteristics they have (knots, checks, shakes, splits,...)
Introduction to Woods18
Grade Types
Grading Structural Lumber
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Visually Graded
Grading Structural LumberVisually Graded WWP: Lumber Grading Agency 12 : Mill Number SEL STR: Lumber Grade DOUG FIR-L: Lumber Species S-GRN: Moisture ContentMachine Stress
Rated 1650:Nominal Bending Stress,
psi 1.5E: Modulus of Elasticity,
million psi20
Sizes Categories
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There are three main size categories of lumber:
Boards: 1 to 1 1/2” thick, 2” and widerDimension lumber: 2 to 4” thick 2” and
widerTimbers: 5” and thicker, 5” and widerNote 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.
Dimension LumberJoists 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 DeckingLight 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)
TimbersBeams 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
Design Values
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An important part of wood design is being able to determine design values for the following mechanical properties:Bending stress, FbTension stress parallel to grain, FtShear stress, FvCompressive stress parallel to grain, FcCompressive stress perpendic. to grain, FcModulus of Elasticity, E
Wet Service Factor (CM) Cont.
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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 CM (CM values are less than one and are given at the beginning of Table 4A).
Cr= Repetitive Member Factor
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A 15% increase in the tabulated Fb 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
lumber2. 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.
Cr= Repetitive Member Factor
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If one member should become overloaded, parallel members come into play.
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