SBCA Releases SP Design Value Comparison Tables and Analysis

Originally published by: SBCAJanuary 10, 2013

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As we reported earlier, the Southern Pine Inspection Bureau (SPIB) has resubmitted recommended design value changes to all sizes of Southern Yellow Pine (SYP) to the American Lumber Standards Committee (ALSC) Board of Review. The U.S. Forest Products Lab (FPL) verified SPIB’s design values, and the ALSC is now scheduled to hold a hearing on those recommendations on January 30, 2013.

SBCA has put together a series of tables comparing changes to SYP design values, which can be viewed by clicking on the pdf link below. There are separate tabs for each size (2x3, 2x4, 2x5, 2x6, 2x8, 2x10, and 2x12), and each page lists applicable design values:

  • “SPIB 2011” were the published SYP design values prior to June 30, 2012;
  • “SPIB 2011 with June 2012” were approved and published SYP design values for 2x4 and less that went into effect after June 30, 2012;
  • “SPIB September 2012” were the proposed SYP design values SPIB initially sent to ALSC for all sizes and grades; and,
  • “SPIB December 2012” are the proposed SYP design values SPIB most recently sent to ALSC for all sizes and grades.

In comparing the “SPIB December 2012” values to the original “SPIB 2011” values, SBCA makes the following observations:

  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.
  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.                      

In addition, for component manufacturers 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.

In looking back, it is quite amazing to have a 15% to 30% change in the structural resistance properties of an engineering material take well over two years to be correct so that engineering can be performed with accurate and reliable design properties.

As a result of these design value changes, SBCA observes that floor trusses and I-joists should have an improved competitive position in the market. Having more reliable Southern Pine design value properties will also mean building design can now become more economical and innovative through the use of the SBC Industry’s sophisticated software.

In the end, 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.

SBCA is currently finalizing updated floor span tables that component manufacturers can use to compare 2x10 and 2x12 joists and 2x3 and 2x4 floor truss sing most North American grades (MSR and visual) and species.