Today is the day. June 1, 2012 is the effective date for the new design values for No. 2 Dense and lower grades of visually graded Southern Pine lumber. The changes are for lumber that is 2"-4" thick and 2"-4" wide. The Addendum to the National Design Specification for Wood Construction by the American Wood Council can be found here. Please note that this is only for visually graded lumber and does not affect machine stress rated lumber.
For engineers, the severity of the change in design values for 2x4's will have less impact than what may be expected. (Not true for truss manufacturers.) We typically only use 2x4's as wall studs; and for many projects we are using 2x6 wall studs as directed by an Architect. This results in 2x4's being a small portion of a typical project.
Moving forward, is it only a matter of time before the other grades and sizes of Southern Pine have reduced design values. If you want to see what those other design values may look like, click here for information from the Southern Pine Inspection Bureau.
I have already been asked the question of changing from Southern Pine to Spruce Pine since the new design values have changed. If you allow the change, you will need to make adjustments to all of the connection designs to account for the lower specific gravity of Spruce Pine. This makes a change much more complicated than just checking the stud strength.