Integrated Buildings & Structures

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Wood Shinkage

  • 1.  Wood Shinkage

    Posted 11-20-2020 08:33 PM

    This afternoon, I was pursuing a live webinar about the design of wood connections through my ASCE membership; the instructor mentions the Wood Shrinkage and how important it is for engineers to understand and be familiar with causes, how, and how much wood shrink? So, I thought it's a good idea to talk about this topic, might some of you are familiar with it but it's good to review the information anyway! And please feel free to add any additional information.

    Wood Shrinkage starts when the moisture content dips below the fiber saturation point (when the bound water saturates the cell, but free water dissipates).

    With time, the moisture content of wood will always come into equilibrium with the relative humidity of the air surrounding it; in other words, bound water escapes until the equilibrium moisture content (EMC) is reached. The type range of EMC, depending on humidity, 10-15%, could be 5% or 25%. 

    How much does wood shrink? Usually, the lumber supplied with a moisture content (MC) range of 15%-19%, but if it's not been protected on-site, lumber will have higher MC. For example, the depth of 2x12  (cross-section of the grain) wood piece lessens by  3/8" if MC drops from 30% to 15% and by 1⁄4" - 3/8" if MC drops from 15% to 8%.

    Which way does wood shrinks? The Directions matter! Wood shrinks most in the radial and tangential directions. Minimal shrinkage occurs in the longitudinal direction. In detail, the Radial and the Tangential direction: the dimension changes approximately 0.1% and 0.2%, respectively, for every 1% change in moisture content between 0% and 30%, However, the Longitudinal direction: the dimension does not change with different moisture contents. The Western Wood Product Association (WWPA) suggest longitudinal shrinkage coefficient = 0.005%.



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    Meena Al Chqmaqchi EIT, A.M.ASCE
    Junior Engineer
    Greater Sacramento CA
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  • 2.  RE: Wood Shinkage

    Posted 11-21-2020 01:01 PM
    There are a number of shrinkage calculators available online (not that the calculation is that complex).  If you are designing a wood-framed building with masonry stair towers or elevator shafts, determining the shrinkage is critical!

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    Elwin Robison Ph.D., M.ASCE
    Principal
    Kent OH
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  • 3.  RE: Wood Shinkage

    Posted 11-22-2020 09:08 AM
    Thanks for your comment and it would be great if you could provide us with your recommendation of the available online shrinkage calculator.


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    Meena Al Chqmaqchi EIT,A.M.ASCE
    Civil Engineer
    Citrus Heights CA
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  • 4.  RE: Wood Shinkage

    Posted 11-22-2020 12:39 PM
    I like the Simpson Strong Tie calculator.  It is easy to use, and I specify Simpson products in my work.  My students have used it (and when they run hand calculations to verify the calculator it is always accurate).

    Wood Shrinkage Calculator. The Wood Shrinkage Calculator is a quick and easy tool to estimate the amount of shrinkage the structure may experience as the wood member loses moisture content after it is framed and in service.
    www2.strongtie.com