Why Are 2x12s Rated Lower Than 2x6s?

Q. Recently I needed structural design values (E, Fb, Fv) for treated southern yellow pine. So I referred to a publication from the Southern Pine Council, which indicated that while E (modulus of elasticity) and Fv (allowable shear stress) remain constant, Fb (bending strength) values grow smaller as the lumber dimension grows larger. The values for Number 2 SYP are listed in the chart, below. Why does the Fb change with the lumber size? Also, does treating lumber with a preservative (such as CCA) change the design values?

A. Frank Woeste, P.E., a professor of wood construction engineering at Virginia Tech, responds: It is true that the allowable Fb value decreases as the depth increases. This phenomenon has been proven by extensive testing. The current span tables — not only for southern pine, but for all lumber species — reflect that testing.

As to "why" this is so, there is no conclusive answer, but several theories have been offered over the past three decades. My own theory is based on probability — the probability that, under ASTM test protocols, more knots and other imperfections are likely to occur in wider pieces of lumber.

In the ASTM test for determining allowable Fb, a wood joist is subjected to stress until it breaks. The test standard requires a span-to-joist ratio of 17. For a 2x6, the test joist is 93.5 inches long (5.5 in. x 17), while a sample 2x12 would be 191.25 inches long (11.25 in. x 17). In the test, only the center third of the span is subjected to the full stress level. For a 2x6, this would be 31.2 inches long; for a 2x12, the middle third is 63.75 inches long.

Knots and other natural characteristics control the strength of lumber; for each grade, there is a maximum allowable knot size. If you look at a piece of pine lumber, you’ll notice that the knots are usually clustered a couple of feet apart. Based on some typical frequency of knots along the length of the lumber, it’s likely that there will be more knots in a 63.75-inch section than in a 31.2-inch section, and more likely that the maximum allowable knot will occur in the longer section. So in the test, a 2x12 is more likely to fail at a relatively lower stress level than a 2x6.

Regarding the effect of preservative pressure treating on design properties, according to the 1997 National Design Specification for Wood Construction, the strength values for dead, live, snow, and wind loads are not affected except that most CCA-treated lumber is not re-dried after treatment. After treatment, the lumber typically has a moisture content greater than 19%, so various reductions in strength and connection values (for nails, bolts, and truss plates) may apply.