Truss Design Drawing Notes Address Unreliable Lumber Design Values

Originally published by: SBCAOctober 17, 2013

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As noted in the following three articles:

  1. A Transparent Review of SPIB's Supplement 13 & Appendix A
  2. SBCA Approves its Design Value Policy
  3. SBCA's Public Letter to ALSC, AWC, NAHB, SFPA and ICC

design values are very important to the engineering value that the structural building component industry provides to the framing market.

The operative words of the Southern Pine Inspection Bureau's (SPIB) Supplement No. 13, Appendix A are the following:

Wood is a natural product subject to variations in geography, climate, specific site characteristics, silvacultural practices, and harvesting decisions. Its’ strength properties are not only anisotropic (vary by principal axis) but also can vary with proximity to the center of the tree. These characteristics complicate the assignment of individual pieces into design value groups based on the visual appearance...This value is then used to establish the design value.

Each piece or lot of visually graded lumber is not mechanically tested to verify strength properties. Since the stress ratings are representative of the entire producing region, lots from a specific location may have physical properties at the extremes of the property range or statistical distribution representing that range of strength values.

As noted in the article above, “A Transparent Review of SPIB's Supplement 13 & Appendix A” the language removed from the final SPIB Supplement #13 version  though a motion made by Professor Don Bender of Washington State University during the January 2013 ALSC meeting, follows:

Designers of wood structures are cautioned to take into consideration the variability of wood within a species and grade grouping.

This language seems to be critical in that it is quite instructive as to the intentions of SPIB with respect to who is responsible for lumber design values in the context of how lumber design values are derived.

To this end, SBCA has created a series of truss design drawing notes that should be reviewed and considered for use as appropriate (please note: the above hyperlink will require a CM-only member login to access).

If you have any questions please contact SBCA.