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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