Effective June 1, 2012, the American Lumber Standard Committee (ALSC) Board of Review has approved changes to design values for 2- to 4-inch thick, 2- to 4-inch wide, Southern Pine lumber. The size of lumber affected – 2×4’s – are used extensively in load-bearing stud walls and by the pre-engineered manufactured truss industry, as well as engineered lumber systems (think Weyerhauser’s iLevel family of products, including the ubiquitous Trus Joists (TJI’s) and LVL’s).
Let’s look at a No. 2 stud for example. The No. 2 grade is widely used as the ‘typical’ lumber grade when engineering wood structures using Southern Pine lumber. The modulus of elasticity (E, a measure of the stiffness of the lumber) has been reduced by 12.5%. The Compression Parallel to Grain value (Fc – a measure of the strength of the 2×4 when used as a post or column) has been reduced by 33%. The bending strength value (Fb a measure of the strength of the stud when used as a joist or beam) has been reduced by 30%.. The combination of these three reductions in design values will reduce the allowable capacity of a 2×4 stud, 8 foot tall, from 608 lbs to 529 lbs – a 13% reduction. This will have, and likely already has had, a big impact on the pre-engineered truss industry (like gang-nailed roof and floor trusses), and the engineered lumber industry. Structural engineers designing wood bearing walls, especially when those walls are subjected to wind or other lateral loads, will also need to ‘recalibrate’ their wall designs to accommodate these new, reduced, design values.
The National Design Specification® for Wood Construction (NDS®) Supplement: Design Values for Wood Construction publishes wood structural design values. They have published an Addendum detailing the value reductions that applies to the 2001, 2005, and 2012 Editions of the NDS. Here’s a link to the Addendum.