Southern Pine grading greatly complicates the task of making lumber substitutions. The progression of the structural values of Machine Evaluated Lumber (MEL) is nothing like the progression of values of either Visually Graded or Machine Stress Rated (MSR) lumber, which both ascend like rungs on a ladder. This wide disparity is charted below. For example M42 performs 6% better than DSS in bending, but not as well as SS in Tension (1600 vs 1650), and therefore can’t be interchanged with either DSS or SS.
M42 would be expected to perform better than DSS on 2x4 roof truss top chords, which are controlled by bending (2850 vs 2700) and compression (2100 vs 2050) but 16% less efficiently on bottom chords, which are controlled mainly by tension. On shallow or long span floor trusses the tension value is crucial, and so is deflection, which is controlled largely by the Modulus of Elasticity, the “E” value. Note that the “E” value of M42 is 5% lower than DSS.
To provide better purchasing flexibility, consider using “Hybrid Lumber” in your truss designs. This lumber, created within design software, combines the lowest values of two different grades and/or species, listing both on design drawings as is shown here. Note that there is somewhat of a price to be paid for using the lowest values of two grades. If, for instance, combining the properties of 2400 2.0E and Select Structural into a Hybrid Lumber, the 2400 would be disadvantaged by about 17% on bottom chords (see the inset to chart above).
In summary, Southern Pine’s grading anomalies should be recognized ahead of the purchasing decision to insure adequate supply and to determine if alternatives exist, including other species. Note in the Hybrid Lumber output example above that Southern Pine (SP) and Spruce (SPF) are used interchangeably, and, as will be shown next month, without much sacrifice.
Next Month:
What Lumber Truss Manufacturers are Buying