Close up of Vessel & Cell
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Introduction to Woods
Wood Behavior
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Introduction to Woods
Advantages of Wood
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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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