Will,
Thanks again for helping me find out what happened with the formula.
Here's a brief explanation that I came across:
Review of ASTM procedures used to establish allowable shear stresses revealed that shear values were being reduced by two separate factors for effects of splits, checks and/or shakes. One of these adjustments was made to the base value, the other was an adjustment to design values for grade effects. In 2000, ASTM standard D245 was revised to remove one of these adjustments, which resulted in an increase of nearly two for allowable shear design values; however, grade effect adjustments were eliminated.
In the 2001 NDS Supplement, shear design values for sawn lumber are generally 1.95 times higher than values printed in the 1997 edition in response to the change in ASTM D245. With this change, shear-related provisions in the NDS were reevaluated and modified where necessary to provide appropriate designs.
As I interpret it, allowable values were changed due to the elimination of some adjustments, then the formula for calculating the stress was adjusted to bring it in line with that change to allowable. Is that the general gist of it?
