If there’s an existing structure where the moisture content (MC) is less than 19% for southern pine timbers, could the designer back out the wet service adjustment from the NDS published design values in Table 4D?

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No. The published Southern Pine timber values are for wet and dry use. The Southern Pine grade rules, section 164.4 state:

“Where lumber is over 4″ thick, the stress ratings apply without regard to seasoning, and stress ratings as herein indicated for a moisture content over 19% apply also to seasoned lumber if over 4″ thick.”