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
Grading Structural Lumber 17 Lumbers are graded based on size and number of growth (strength-reducing) characteristics they have (knots, checks, shakes, splits,...)
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
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.
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 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
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).
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.
C r = Repetitive Member Factor 27 If one member should become overloaded, parallel members come into play.