Southern Pine lumber will get new engineering strength values and new span tables with reduced spans for visually graded lumber, if the American Lumber Standards Committee (ALSC) accepts a proposed set of revised strength values offered by the Southern Pine Inspection Bureau (SPIB), the agency tasked with monitoring the strength of Southern Pine wood. An ALSC hearing on the proposed new values is set for October 18 in Washington, D.C.; assuming ALSC approves the change, new span tables could take effect soon thereafter.
Design values for Southern Pine #2 2x4 lumber have already been reduced, following a partial testing program conducted by SPIB which revealed a decrease in strength of the timber resource. SPIB's proposal to lower the design values for other sizes and grades based on a model projection, however, was rejected by ALSC; instead the Standards Committee instructed the Inspection Bureau to test other sizes and grades before changing any of the other published values.
That testing program is now complete, and SPIB's revised values have been submitted to the ALSC. ALSC then sent the submittal to the Forest Products Laboratory for review by government engineers specializing in forest products; that review will be taken into account as ALSC makes its decision.
It appears that testing of wider-dimension lumber has shown a decline in strength in 2x6, 2x8, 2x10, and 2x12 lumber — but not quite as sharp a decline as had been feared following the 2x4-only tests. Still, the changes in span tables will be significant. The Southern Forest Products Association (SFPA) has produced provisional span tables based on the SPIB submittal to ALSC. Those tables, which you can download from the SFPA website, www.southernpine.com ("Proposed Maximum Spans"), show spans shortened by something on the order of two feet for typical applications. For example, a 2003 span table from SFPA lists the span limit for visually graded #2 2x10 Southern Pine, used as a floor joist at 16" o.c. with a 10 psf dead load, a 40 psf live load, and L/360 deflection limit as 16'1". After the change, that same 2x10 joist under the same load and deflection conditions will have an allowable span of only 14 feet. You could get back to a 16'2" span by framing the floor at 12" o.c.; or, you could get a 16'6" span by stepping up to a 2x12 joist depth. But either of those adjustments adds cost; so for framers looking for an optimal design, the task will be a little harder when and if these proposed new values become official.