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%.