The American Lumber Standards Committee (ALSC) has voted to accept new design values for Southern Pine lumber, including all sizes and grades of dimensional lumber. The move caps a long-running process of re-assessment for Southern Pine, starting with reports of surprising weakness in 2x4 lumber being graded and tested at sawmills, and ending with an extensive testing program of sawn lumber pulled at random from sawmills throughout the Southern Pine growing region.
In order to allow time for the industry to adjust, the new design values will not take effect until June 1, 2013. But that’s a formality: the fact is that the wood has changed over the years, and the new values reflect the measured strength of lumber being produced in the industry right now. Even so, there’s no reason to be hasty: the design values reflect the strength of the weakest 5% of samples tested, and so 95% of all the wood in the marketplace is likely to be stronger than the published minimums. So structures already permitted under the old guidelines can be built as designed without undue risk to the buyers. Still, engineers are advised to start using the new values immediately for all structures now being planned.
Here’s the background: Design values for #2 2x4 Southern Pine were lowered last year after preliminary testing of just that size and grade. Now, full tests have been conducted for 2x8 and 2x10 lumber as well, including #1, #2, and Select Structural grades. The results have been extrapolated to cover 2x6 and 2x12 wood also, and result in significantly lower values for the modulus of elasticity (MOE) and modulus of rupture of the lumber, which represent key structural engineering attributes of the wood.
For truss makers who use Southern Pine, the result will require a redesign of their product — a process many manufacturers are already implementing. For deck builders and house framers, the change will work its way through as a change in the span tables for the 2x8, 2x10, and 2x12 lumber they use to design floor systems.
The Southern Forest Products Association (SFPA) has already developed new span tables for Southern Pine, and published them at this link: (“Maximum Spans for Southern Pine Effective June 1, 2013”).
And the Structural Building Components Association (SBCA), an industry association representing truss manufacturers and other component makers, maintains a complete timeline with documents relating to the strength downgrade at this link: (“Design Values Communication - Oct 2011 - Jan 2013”).