Section 3.3.3/Bending Design Value Adjustment
NDS 2018 Section 3.3 covers adjustment factors for sawn lumber bending including size, repetitive member, and wet service factors.
The adjusted bending design value (Fb') for sawn lumber is the reference design value (Fb) multiplied by all applicable adjustment factors: duration of load (CD), wet service (CM), temperature (Ct), beam stability (CL), size factor (CF), flat use (Cfu), incising (Ci), and repetitive member (Cr). The repetitive member factor (Cr = 1.15) applies when three or more members are spaced not more than 24 inches apart and share the load through sheathing or decking.
Why this section exists
Wood strength varies with conditions. Lumber is stronger when loaded briefly (short duration of load factor) and weaker when wet. Deeper members have lower unit bending strength than shallow members (size effect). Members in repetitive assemblies (floor joists, rafters) share load and are statistically less likely to have critical defects at the same location. The adjustment factors translate published reference values to actual design conditions.
What plan reviewers look for
Plan reviewers check the structural calculations for correct application of adjustment factors. They verify that the lumber species and grade match the reference design values used. They check whether the repetitive member factor is claimed appropriately (3+ members at 24 inches on center or less with load-sharing sheathing). They also verify the duration of load factor matches the controlling load combination.
Common violations
Related NDS requirements
Section 3.4.3 covers compression design value adjustments. Section 4.3.1 covers the column stability factor (Cp) for compression members. NDS Supplement Table 4A provides reference design values for visually graded dimension lumber.