CSLB General Building (B)Framing and CarpentryMedium
A framer must choose lumber for a long-span floor joist carrying a heavy point load, comparing Douglas fir-larch Select Structural grade to No. 2 grade of the same size. What is the primary reason Select Structural allows for a longer allowable span under the same load?
- ASelect Structural is treated with fire-retardant chemicals that increase load capacity
- BSelect Structural lumber is manufactured in larger nominal dimensions than No. 2
- CSelect Structural lumber is kiln-dried to a lower moisture content than No. 2
- DSelect Structural has fewer growth defects, resulting in a higher allowable bending stress (Fb)
Show answer & explanationAnswer & explanation
Correct answer: D. Select Structural has fewer growth defects, resulting in a higher allowable bending stress (Fb)
Lumber grades are based on visual grading rules that limit knots, slope of grain, and other defects. Select Structural grade has the fewest allowable defects, giving it a higher allowable bending stress (Fb) value in span tables than lower grades like No. 2, allowing longer spans or heavier loads for the same size member.
Why the other options are wrong
- A. Grading is unrelated to fire-retardant treatment, which is a separate specification.
- B. Grade does not change nominal dimensions; a 2x10 is a 2x10 regardless of grade.
- C. Moisture content designation (like S-DRY) is separate from grade and does not define grade differences.
Lumber Grade & Allowable Stress
Visual grading rules limit defects (knots, grain slope) in lumber; fewer defects result in higher allowable bending stress (Fb) values used in span tables.
- Select Structural > No.1 > No.2 > No.3 in strength
- Fb value determines span table capacity
- Grade stamp indicates species, grade, and mill
Memory trick: Fewer flaws, more force—cleaner lumber carries more load.