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SITE LAYOUT, FOOTINGS, AND FOUNDATIONS

Lumber Sizes Wood members are referred to by nominal sizes (e.g., 2×4); however, true dimensions are somewhat less. The difference occurs during the dressing stage of the lumber process, when each surface of the member is planed to its final dressed dimension after shrinkage has occurred as a result of the drying or “seasoning” process. Generally, there is a 1/4- to 3/4-inch difference between the nominal and dressed sizes of “dry” sawn lumber (refer to NDS-S Table 1B for specific dimensions). For example, a 2×4 is actually 1.5 inches by 3.5 inches, a 2×10 is 1.5 5-4 Residential Structural Design Guide Chapter 5 – Design of Light-Wood Framing inches by 9.25 inches, and a 1×4 is 3/4-inch by 3.5 inches. This guide uses nominal member size, but it is important to note that the designer must apply the actual dimensions of the lumber when analyzing structural performance or detailing construction dimensions. Based on the expected application, the tabulated values in the NDS are classified by the species of wood as well as by the nominal size of a member. Typical NDS classifications follow: • Boards are less than 2 inches thick. • Dimension lumber is a minimum of 2 inches wide and 2 to 4 inches thick. • Beams and stringers are a minimum of 5 inches thick, with the width at least 2 inches greater than the thickness dimension. • Posts and timbers are a minimum of 5 inches thick, and the width does not exceed the thickness by more than 2 inches. • Decking is 2 to 4 inches thick and loaded in the weak axis of bending for a roof, floor, or wall surface. Most wood used in light-frame residential construction takes the form of dimension lumber. Lumber Grades Lumber is graded in accordance with standardized grading rules that consider the effect of natural growth characteristics and “defects,” such as knots and angle of grain, on the member’s structural properties. Growth characteristics reduce the overall strength of the member relative to a “perfect,” clear-grained member without any natural defects. Most lumber is visually graded, although it can also be machine stress-rated or machine evaluated. Visually graded lumber is graded by an individual who examines the wood member at the mill in accordance with an approved agency’s grading rules. The grader separates wood members into the appropriate grade classes. Typical visual grading classes in order of decreasing strength properties are Select Structural, No. 1, No. 2, Stud, etc. Refer to the NDS Supplement (NDS-S) for more information on grades of different species of lumber. The designer should consult a lumber supplier or contractor regarding locally available lumber species and grades. Machine stress rated (MSR) and machine evaluated lumber (MEL) is subjected to nondestructive testing of each piece. The wood member is then marked with the appropriate grade stamp, which includes the allowable bending stress (Fb) and the modulus of elasticity (E). This grading method yields lumber with more consistent structural properties than visual grading onl