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Component Manufacturing dverti$er
Don’t Forget! You Saw it in the
Adverti$er
Truss Lumber: Checking the Moisture Content
October 2018 #10231 Page #80
By FrankTruss Woeste
Lumber: Checking the Moisture Content
A
LL T
OD
The lack of attention to the moisture content (MC) of lumber used in truss manufacturing can cause or contribute to in-service performance issues. Mr. Glenn Traylor’s fine article in the September issue of The Advertiser, “Does Your Lumber Have a Shelf Life?�, was greatly appreciated, as Glenn once he lack of attention to the moisture content (MC) again provided practical educational contributions. In this article, Glenn offered 18 items that CMs should of lumber used in truss manufacturing can cause consider for the management of truss lumber and the secondorrecommendation caught performance my attention:issues. Mr. contribute to in-service
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Glenn Traylor’s fine article in the September issue of The Advertiser, “Does Your Lumber Have a Shelf Life?�, was greatly appreciated, as Glenn once again provided “2. Lumber shouldcontributions. be inspected In forthis moisture practical educational article,content.� Glenn offered 18 items that CMs should consider for the management of truss lumber and the second recommendation caught my attention:
For me, the suggestion begs the question—how should lumber MC be inspected?
“2. Lumber should be inspected for moisture content.�
For me,often the suggestion the question—how should lumber inspected? I’ve been told begs by various construction-related partiesMC thatbethey “check the moisture� by
lifting it. While this been practice for lumber freshly cut from a live is not appropriate forbytruss I’ve often told works by various construction-related parties thattree, theyit“check the moisture� lifting it. this practice cut from livefollowing tree, it is not appropriate for truss lumber. Moisture lumber.While Moisture contentworks (MC)forinlumber percentfreshly is defined by athe equation: content (MC) in percent is defined by the following equation:
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So, with this definition Green Lumberin mind, we can consider how to evaluate MC and how it affects lumber in use.
At the time lumber is sawn from logs, the MC is typically 150% up to 200%. For the more common MC GreenofLumber 150%, the weight of water in the wood is 1.5 times its dry weight. Hence, freshly cut firewood will indeed feel heavy when lifted. To a lesser extent, commonly available pressure treated (PT) lumber will feel heavier when it istime liftedlumber as the treating addsthe a water lumber.For the more At the is sawn process from logs, MC ischemical typicallysolution 150% to upthe to 200%.
common MC 150%,Lumber the weight of water in the wood is 1.5 times its dry weight. Hence, freshly cut MC ofof Truss Background firewood will indeed feel heavy when lifted. To a lesser extent, commonly available pressure treated (PT) Dry lumber, by definition, is lumber having a MC of less than or equal to 19% (MC≤19%). Tabulated design lumbervalues will feel heavier when it is lifted as the treating process adds a water chemical solution to the for most truss lumber in the U.S. are based on dry lumber. Lumber having a MC > 19% is partially lumber.seasoned and requires a downward adjustment of tabulated design values based on factors in the National Design Specification for Wood Construction (NDS) Supplement.
MC of Truss The Lumber impact of Background lumber MC between 19–30% is substantial in terms of design stresses, shrinkage, and
creep deflection in-service. At 30%, the lumber is assumed to be “saturated�, thus higher moisture contents only make the lumber heavier and contribute to additional creep deflection in-service. MC values between Dry lumber, by definition, is lumber having a MC of less than or equal to 19% (MC≤19%). 30–200% do not impact shrinkage potential. Continued next page
Tabulated values for most truss lumber inonline the U.S. are based on dry lumber. Lumber having FAX: a 800-524-4982 PHONE: design 800-289-5627 Read/Subscribe at www.componentadvertiser.com