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January 2021 #13258 Page #78
“Hybrid Lumber” Grade Stamps Require Special Attention
By Frank Woeste and Don Bender
umber properties are influenced by species and growing region. Species with similar design properties are combined into species groups, as shown in Tables 4A, 4B, and 4F of the NDS Supplement1. For example, a common species group in the southeastern U.S. is Southern Pine (SP), which includes Loblolly, Longleaf, Shortleaf, and Slash Pine. The same species group can have different design values depending on where it is grown, such as Douglas Fir-Larch which is grown in the U.S. and Canada. In this case, the NDS Supplement lists DF-L and DF-L (North), the latter being grown in Canada.
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Lumber mills sometimes process logs from different regions, and this has given rise to a different type of grade stamp. The purpose of this article is to highlight lumber stamps with multiple species/regions, hereafter referred to as “hybrid lumber” stamps. Hybrid lumber stamps are unlike the traditional stamp that component manufacturers (CMs) have relied upon when designing and manufacturing wood trusses and prefabricated wall panels. The hybrid lumber stamp identifies a designated mixture of lumber products (two or more species and/or species groups). We will explain how to recognize a hybrid stamp, to determine the proper design values for the truss chords and webs, and explain how the design values impact member design and the four metal plate “gripping values.” In addition, we will show how the “diaphragm and shear wall” design values for prefabricated wall panels are impacted by the specific gravity of the framing lumber.
Traditional Grade Mark As an example, a southern pine grade mark from a 2x8 is shown in Photo 1.
NDS Supplement to the National Design Specification. 2018. American Wood Council. https://awc.org/codesstandards/publications/nds-2018.
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