New Southern Pine Design Values

On June 1st, new design values for southern pine lumber came into effect. These results are based on full-scale testing of various lumber sizes, and supersede the interim results that went into effect last year, which only affected 2″-4″ sized lumber. The kick in the teeth is that the new values show a sizable decrease in capacity for compression, bending, and tension, with reductions ranging from 10-30%. More information can be found at the SPIB site.

Allowable design values vs. actual strength at failure.
Allowable design values vs. actual strength at failure.

The changes are the result of the large-scale destructive testing of thousands of pieces of southern pine lumber. Wood is a highly variable material, and so requires a large number of samples to reliably establish safe design values. This sort of testing first began in the late 1800’s, and is conducted every so often by lumber testing organizations. Testing standards have changed over the years, but currently must follow ASTM D 1990. Testing organizations must be certified by the American Lumber Standards Committee. There are currently seven organizations, which are responsible for various regions and wood varieties. Southern pine lumber is covered by the Southern Pine Inspection Bureau.

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SPIB didn’t investigate the reason for the reduction in wood strength, only speculating that “a change in the timber resource mix” may be responsible. One likely explanation is the average age of modern pines at harvest. Over time, tree plantations have become more and more efficient – modern plantations yield nearly six times the volume of lumber than those in the 1940’s. Part of this efficiency comes from a reduction in the rotation age, the time it takes for a tree to be harvested after planting. These younger trees have a higher proportion of juvenile wood, which has reduced strength. This is merely speculation, though the strength reduction of juvenile wood does seem to match that observed in the new design values.

Screen Shot 2013-06-17 at 8.16.43 PM

The reductions in lumber strength obviously mean that more, larger members will be required to carry a given load. Pine is an immensely popular building material, but there’s been speculation as to how much the inevitable cost increase will result in a switch to Douglas Fir, or other woods. It speaks to the integrity of those involved that the SPIB, not an unbiased organization, would so willingly promulgate a change that will harm it’s interests.

Building materials, of course, have never had static, unchanging properties. Looking through any old building code or textbook, and it’s obvious how much stronger modern steels and cements are. It’s interesting to see that wood, which we might think of as natural and thus unchanging, isn’t immune to this.

Formation and Properties of Juvenile Wood in Southern Pines: A Synopsis. Forest Products Laboratory, 2001.
The Development of Pine Plantation Silviculture in the Southern United States. Fox et al, 2006.
Southern Pine Design Value Forum: Findings and Recommendations. SPIB, 2011.
Influence of Juvenile Wood Content on Shear Parallel, Compression, and Tension Transverse to Grain Strength and Mode I Fracture Toughness for Loblolly Pine. Forest Products Laboratory, 2008.
SPIB Proposed Design Values and Analysis. SFPA, 2012.
Southern Pine Reference Design Values. SFPA, 2013.
Voluntary Product Standard PS 20-10. NIST, 2010.
ASTM D1990
National Design Specification for Wood Construction. AWC, 2008.

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