SBCA Reports on July 18-19 SPIB Meeting - Important Design Value Change Information

Scott Ward, of Southern Components and the incoming President of SBCA, and Kirk Grundahl, P.E., Executive Director of SBCA, attended the recent SPIB Technical Committee meeting on the southern pine testing process and further proposed revisions to southern pine design values.

A Technical Committee report on the meeting was prepared by SPIB and can be found here. As noted and made very clear at the meeting, the SPIB Technical Committee met solely to review the findings of the current testing and analysis and will next be making recommendations and providing advice on design values for Southern Pine dimension lumber to the SPIB Board of Governors, which information they will incorporate into their decision making process. Therefore, all information coming out of the SPIB meeting is subject to change as the final phases of the evaluation process are completed.

As SBCA has consistently stated, CMs, when they are purchasing lumber for their trusses and components, are essentially purchasing lumber design values and related properties. Having consistent properties in turn create more reliable engineered designs. As the engineering programs, that all CMs use, become more sophisticated, the demands on the raw material (lumber, OSB, LVL, etc.) to accommodate such sophistication will increase as greater engineering precision will bring about more economical and innovative building design. Given the billions of board feet of lumber design values and related wood product properties CMs purchase each year, safe resistance to loads is predicated on the forest product industry’s grade process being accurate. This reality demonstrates the importance of SPIB’s work in the realm of the southern pine lumber resource.

Based on their participation at the recent SPIB Technical Committee,  Scott Ward and Kirk Grundahl each have the following observations with respect to the Southern Pine design value changes:

1. The specific gravity (SG) will remain the same with a coefficient of variation of about 12 to 13%.
   
   1. This variation is less than the general strength variation of visually graded lumber of 25% so this is both a good and interesting finding.
   2. Generally, it is expected that SG and strength are positively correlated so a strength reduction on the one hand with SG gravity staying the same on the other hand, will be surprising for many.
   3. The reason that this is the case is the strength reduction in Southern Pine seems to be more correlated to knots and grain structure performance than the raw wood fiber density.
   4. Given that NDS fastener performance is tied to SG, SBCA will be reviewing the data to see what affects the low-end SG values have on connection performance and may have a few more questions in the future for the lumber industry.
2. There was discussion on the grade model used. This discussion included if the grade model SPIB was using to classify grade types should change and also whether or not to re-create the dense classification. The motion was to use the grade model that SPIB has developed and re-create all pre-August 2011 Southern Pine lumber grade types including dense.
3. The final motion was that the SPIB Technical Committee will provide a recommendation to the SPIB Board of Governors Board that the design value changes that were presented, using the test data created will form the basis of the new southern pine design values. As stated above, there may be changes yet to occur as this SPIB Board of Governors review and approval process takes place, so all observations are preliminary in nature.

The question everyone has been asking of Ward and Grundahl is what does all this rather mundane reporting mean to the typical CM. Here is a bit more CM focused information, which is consistent with the general reporting that has been and will continue to be provided by others:

1. Clearly, no change to Southern Pine specific gravity values benefits current truss plate design values and truss plate sizes.
2. ASTM D1990, the applicable lumber design value testing standard, does not evaluate shear properties so that are no changes to the 175 psi Fv design value.
3. Compression perpendicular to grain values are not grade dependent and will remain the same.
4. Currently these values are as follows and are based on the dense classification:
   
   1. Fcperp is 660 psi for Dense SS, #1 Dense and so forth.
   2. Fcperp is 565 psi for regular SS, #1, and so forth.
   3. Fcperp is 480 psi for NonDense SS, #1 NonDense, and so forth.

Obviously most CMs are interested in what is going to happen to the 2x4s that they purchase. Here are a few very general observations based on the recent SPIB Technical Committee discussion and our best guess of the general trends the new testing has shown. All comparisons are based on the original 2011 southern pine design values and NOT the June 1^st design values:

1. Dense SS will have a decrease in bending strength in the 15% range, an increase in tension in the 10% range, a decrease in compression in the 10% range and MOE will remain the same.
2. SS will have a decrease in bending strength in the 15% range, no material increase in tension, a decrease in compression in the 15% range and MOE will remain the same.
3. #1 Dense will have a decrease in bending strength in the 15% range, no material increase in tension, a decrease in compression in the 15% range and MOE will remain the same.
4. #1 will have a decrease in bending strength in the 15% range, no material increase in tension, a decrease in compression in the 15% range and MOE will be roughly 5% less.
5. #2 Dense will have a decrease in bending strength in the 30% range, no material increase in tension, a decrease in compression in the 15% range and MOE will be roughly 5% less.

i.      Other than compression increasing by roughly 30%, there is essentially no change in the values  from those values that went into effect on June 1, 2012.

6. #2 will have a decrease in bending strength in the 30% range, a decrease in tension in the 25% range, a decrease in compression in the 10% range and MOE will be roughly 5% less.

i.      Other than compression increasing by roughly 30%, there is essentially no change in the values from those values that went into effect on June 1, 2012.

6. 2x6 material follows roughly the same general trends as 2x4.
7. What is most interesting to CMs that compete with stick framing, 2x8s 2x10s and 2x12s are seeing the following design value changes in the key bending and MOE properties:
   
   1. SS will have a decrease in bending strength in the 15% range and MOE will remain the same.
   2. #1 will have a decrease in bending strength in the 20% range and MOE will be roughly 5% less.
   3. #2 will have a decrease in bending strength in the 25% range and MOE will be roughly 5% less.

Here are the  general CM-oriented conclusions that Ward and Grundahl drew from the meeting:

1. Floor trusses and I-joists should have an improved competitive position in the market.
2. As stated above, given that CMs purchase several billion board feet of design values as defined by the lumber grade stamp each year, the more reliable the Southern Pine properties, the more building design will be economical and innovative through the use of the SBC Industry’s sophisticated software. 
3. The full In-grade testing process produced results that appear to be much better than SPIB originally postulated in the fall of 2011 where SPIB proposed an across the board change in design values for all sizes and grades stating: “The attached supplement was approved on 26 August, 2011 by the Board of Governors of the Southern Pine Inspection Bureau to serve as interim design values, based upon a reasonable estimate projected from testing of No.2 2x4 lumber, until the minimum testing on three sizes and two grades is completed. The Board of Governors deemed it necessary to put forward these values based upon testing of a combined tension and bending of over 800 2x4 No.2 specimens. This testing indicated a reduction in tension and bending values of approximately 25% and 30%.”

We suspect that a great deal will be learned as the entire industry looks back at this process as implemented by SPIB and ALSC.