By Richard Wallace – Southern Forest Products Association
The Southern Pine Inspection Bureau has submitted new design values for visually graded southern pine dimension lumber for review by the American Lumber Standard Committee. The new values will ensure that southern pine lumber will continue to be strong and reliable when used properly in the construction of residential and commercial buildings.
SPIB is the first rules-writing agency to submit new values. Agencies responsible for other species are in different stages for evaluating design values.
The last major change for visually graded dimension lumber occurred in 1991 when design values for southern pine and other North American species were published based on in-grade testing of full-size samples of commercially produced lumber. Since 1994, SPIB has conducted an annual resource monitoring program developed in collaboration with the U.S. Forest Products Laboratory. Although the level established to trigger additional testing was never reached, overall trends in the annual test data suggested a possible shift in the resource mix. These trends, along with anecdotal external information, prompted SPIB to conduct a year-long program of testing and data review.
SPIB and Timber Products Inspec-tion selected full-size No. 2 2×4 specimens, following a sampling plan approved by ALSC. SPIB and TPI conducted destructive tests in bending and tension, plus gathered stiffness and property data, all in accordance with established ASTM standards. SPIB also performed the data analysis in conformance with ASTM standards and then submitted the results to ALSC. SPIB’s data analysis is currently being reviewed by FPL.
Moving forward, the southern pine industry will fill out the full in-grade testing matrix, sampling Select Struc-tural 2x4s, No. 2 and Select Structural 2x8s, and No. 2 and Select Structural 2x10s. SPIB and TPI will conduct destructive tests in bending, tension and compression, plus gather stiffness and property data. Testing should be completed by spring 2012. Following this in-grade re-assessment, southern pine will continue to be monitored on an annual basis with destructive tests.
ALSC’s board of review could approve new design values as soon as their next meeting, to be held Jan. 5. Or, ALSC may decide it needs additional information, such as the results from SPIB’s full in-grade matrix test results, before granting final approval. SPIB will publish new design values for visually graded southern pine upon receiving final approval from ALSC.
Southern pine remains strong, reliable
Southern pine’s strength remains comparable to other softwood species used in residential and commercial construction. “The strength and superior treatability of southern pine lumber against decay and termites continues to provide a great value for manufacturers, designers, builders and consumers,” said Adrian Blocker, president of the Southern Forest Products Association. “The extensive testing protocols used assure that everyone involved in the specification, sale and use of our products has the most current information about how southern pine lumber can be used confidently and economically.”
He added, “Component manufacturers, treaters and design-build professionals have many southern pine product options available to them. These customers can achieve the product performance they need by identifying a particular design value, choosing from a number of visual grade selections, or specifying their choice of mechanically graded lumber.”
Proposed changes
Design values are assigned to six basic lumber properties. These values are used in designing light-frame construction and provide guidance for designers in calculating the performance of a structural system. Design values for four of the properties-bending, tension, compression and stiffness-are based on data from testing full-size, commercially produced lumber specimens. Proposed new design values for these properties may be reduced approximately 25-30%. New design values are proposed to change for all grades and all sizes of visually graded southern pine dimension lumber 2″ to 4″ thick, 2″ and wider. “All grades” includes Select Structural, No. 1, No. 2, No. 3, Stud, Construction, Standard and Utility. “All sizes” includes 2x2s through 4x12s (and wider).
Design values for shear and compression perpendicular-to-grain are based on specific gravity. The average specific gravity observed from the latest testing of No. 2 2x4s was not found to be different enough from the published southern pine specific gravity value of 0.55 to warrant a proposed change at this time. Because specific gravity is not proposed to change, the design values for shear and compression perpendicular are also not proposed to change.
What about Dense lumber?
Test results did not indicate enough of a difference between Dense, unclassified and NonDense lumber to continue publishing three different design values for each lumber grade. The result will be a simplified table with design values assigned by lumber grade without density classifications. However, it is important to note that SPIB Grading Rules will not change. Dense lumber remains an option for southern pine users requiring Dense material, such as proprietary grades for laminated beams. If Dense lumber is specified, it will have the same new design values as the corresponding unclassified grade. For example, if No. 2 Dense is specified, it will have the same design values as No. 2.
