Section 2.3.2/Load Duration Factor
NDS 2018 Section 2.3.2 and Table 2.3.2 define the load duration factor (CD) that adjusts wood design values based on how long the load acts, a concept unique to wood design.
Wood's strength varies with the duration of loading. Short-duration loads produce higher allowable stresses than long-duration loads. The load duration factor CD from Table 2.3.2 adjusts all reference design values to account for this behavior. For permanent (dead) loads: CD = 0.90. For occupancy live loads (10-year duration): CD = 1.00. For snow loads (2-month duration): CD = 1.15. For construction loads (7-day duration): CD = 1.25. For wind and earthquake loads (10-minute duration): CD = 1.60. For impact loads: CD = 2.00. The load duration factor applies to all reference design values for bending, compression, shear, and tension, but does NOT apply to modulus of elasticity (E or Emin) or to compression perpendicular to grain (Fc_perp).
Why this section exists
Wood is a viscoelastic material that creeps under sustained load. A wood member subjected to a permanent load will continue to deflect over time and will fail at a lower stress than the same member subjected to a brief load. The load duration factor captures this behavior in a simple multiplier. The reference design values in NDS supplement tables assume a 10-year load duration (CD = 1.0), which corresponds to occupancy live loads. Dead load acting alone requires a 10% reduction (CD = 0.90), while wind and earthquake loads acting for only minutes allow a 60% increase (CD = 1.60). When multiple loads act simultaneously, the shortest-duration load governs the CD factor for that load combination.
What plan reviewers look for
Plan reviewers check the structural calculations for the correct CD factor in each load combination. They verify that CD = 0.90 is applied when dead load acts alone (this is the governing case for many long-span wood beams). They check that CD is not applied to modulus of elasticity or compression perpendicular to grain. For connections, they verify CD is applied to the fastener design values per Chapter 12. They confirm that the shortest duration load in the combination sets the CD value (e.g., D + W uses CD = 1.60, not 0.90).