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Component Manufacturing dverti$ dverti $ er
Don’t Forget! You Saw it in the
April 2021 #13261 Page #86
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Wood Structure Design Basics—Lumber Grading
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By Frank Woeste and Linda Brown his series of articles, Wood Structure Design Basics, is intended to serve truss designers (TDs) and others who are new to the wood construction industry and/or may not have the opportunity to study the basics of wood design in a formal setting such as a university. The mix of basics will include an introduction to lumber grading and design properties, published design values and applicable adjustments to design values, how simple elements such as floor joists, rafters, columns, and wall columns are analyzed for maximum stresses and deflection, and how the design stresses are aligned with sizes and grades of available lumber so your selection will yield a design solution based on the provisions of the AWC National Design Specification® (NDS®) for Wood Construction. In addition, the design of connections made with nails, lag screws, and bolts will be demonstrated.
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Lumber Characteristics As a natural biological material, lumber is ideal for construction because it is both renewable and sustainable. While the “renewable” aspect is attractive, the “sustainable” part is as good as it gets. Learning about and using lumber to meet society’s needs is an ideal career path, because the current level of public interest in sustainability and the environment is unprecedented. Being a natural material derived from trees, solid-sawn lumber has characteristics that impact the lumber grade. Some of the characteristics are: • Knots (especially edge knots) • Slope of grain (which can severely impact strength) • Density • Wane • Splits, shakes, checks • Warp (twist, bow, crook, cup). Of the characteristics listed, knots and slope of grain have the greatest impact on lumber strength. Wane (or absence of wood) has a minimal (or no) impact on strength, but it can interfere with the installation or use of the product, such as shown in Figure 1. Continued next page PHONE: 800-289-5627
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