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Jonathan Ochshorn's Structural Elements for Architects and Builders, Third Edition
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Chapter 3 — Wood: Sectional properties

Within each species, lumber is further classified by its size. Various grades of lumber are then identified for each size classification. The actual ("dressed") sizes of lumber, which are currently 1/2 in., 3/4 in., or 1 in. smaller than their nominal dimensions, are shown in Appendix Table A-3.12, together with some important cross-sectional properties.

Following are the current rules (but be aware that older — even currently available — lumber may follow older rules) that establish the actual cross-sectional dimensions of lumber: (1) subtract 1/2 in. from any nominal dimension of 6 in. or less; (2) subtract 3/4 in. from any nominal dimension greater than 6 in. and less than 16 in. (for dimension lumber only, subtract 3/4 in. from a nominal dimension of 16 in.); (3) for timbers only, subtract 1 in. from any nominal dimension of 16 in. or greater. Thus, a 2 × 4 is really 1-1/2 in. × 31/2 in.; a 4 × 10 is really 3-1/2 in. × 9-1/4 in.; and a 12 × 20 is really 11-1/4 in. × 19 in. Because so much older lumber is still in circulation, the prior rules governing lumber sizes may well be encountered: (1) subtract 1/2 in. from all nominal dimensions except, for dimension lumber only, subtract 3/4 in. where the nominal dimension is greater than 6 in. It is possible, even likely, that you will encounter even older dimensioning rules if you are dealing with renovations of wooden structures from the 1960s or earlier.

Although the 2012 National Design Specification for Wood Construction (NDS) reduced the dry dressed sizes of certain timbers by subtracting 3/4 in. or 1 in. from nominal dimensions, the green minimum sizes remained the same, with only 1/2 in. subtracted from the nominal dimensions. For such timbers, all structural calculations use the green sizes, even when the dry minimum dressed dimensions are smaller. The appendix sizes show the appropriate dimensions that are to be used for structural calculations.

When describing wooden elements, the standard nomenclature used in timber design can be quite confusing: the smaller dimension, or thickness, is what we ordinarily call "width"; the longer dimension, or width, is what we usually call depth. Thus, the section modulus of a timber beam, described later in this chapter, is not "width" times "depth" squared, divided by 6 (as it would be in a strength of materials text); rather, it is thickness times width squared, divided by 6. Got that? Standard glulam posts and beams come in depths that are multiples of the lamination size; and in an assortment of widths whose finished dimensions are different from those of dimension lumber. Some typical cross-sectional dimensions are shown in Appendix Table A-3.13.