Wood Fundamentals
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Transcript Wood Fundamentals
Close up of Vessel & Cell
1
Introduction to Woods
Wood Behavior
2
Introduction to Woods
Advantages of Wood
3
Introduction to Woods
Disadvantages of Woods
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Introduction to Woods
Stresses Applied to Wood
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Introduction to Woods
Stress – Strain Relationship
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Introduction to Woods
Shrinkage & Swelling
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Introduction to Woods
Shrinkage & Swelling
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Introduction to Woods
Stress Grading
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Introduction to Woods
Stress Grading
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Introduction to Woods
Visual Grading
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Introduction to Woods
Machine Grading
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Introduction to Woods
Grading Organizations
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Introduction to Woods
Grading Lumber
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Introduction to Woods
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Properties of Wood and Lumber
Grades
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
Lumbers are graded based on size and number of growth
(strength-reducing) characteristics they have (knots, checks,
shakes, splits,...)
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Grade Types
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Introduction to Woods
Grading Structural Lumber
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
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Sizes Categories
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)
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
Design Values
An important part of wood design is being able to determine
design values for the following mechanical properties:
Bending stress, Fb
Tension stress parallel to grain, Ft
Shear stress, Fv
Compressive stress parallel to grain, Fc
Compressive stress perpendic. to grain, Fc
Modulus of Elasticity, E
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Wet Service Factor (CM) Cont.
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).
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Cr= Repetitive Member Factor
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 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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Cr= Repetitive Member Factor
If one member should become overloaded, parallel members
come into play.
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