Time Effect Factor (LRFD Only) Explained:
Used in Sawn Lumber Design, Glu Lam Design, Round Timber/Poles Design?, etc, the time effect factor (λ) is similar to the time duration factor? in principle (which is for ASD only), except this factor instead of being based on time duration (e.g. 7 days, 1 yr, etc.) is based on what load combination is being applied (which one should be able to associate the various load combinations with a time duration if they were so inclined).
Note: By using LRFD in lieu of ASD design you are substituting three variables for one, therefore I normally recommend ASD as being the superior choice for wood design.
Time Effect Factors (λ)[1]
| Table 1: Time Effect Factors (α) | |
| Load Combination | λ |
| 1.4(D+F) | 0.6 |
| 1.2(D+F) + 1.6(H) + 0.5(Lr or S or R) | 0.6 |
| 1.2(D+F) + 1.6(L+H) + 0.5(Lr or S or R) | 0.7 (L is from storage) |
| 0.8 (L is from occupancy) | |
| 1.25 (L is from impact) | |
| 1.2D + 1.6(Lr or S or R) + (L or 0.8W) | 0.8 |
| 1.2D + 1.6W + L + 0.5(Lr or S or R) | 1.0 |
| 1.2D + 1.0E + L + 0.2S | 1.0 |
| 0.9D + 1.6W + 1.6H | 1.0 |
| 0.9D + 1.0E + 1.6H | 1.0 |
Notes:
- Time effect factors exceeding 1.0 are not applicable to: connections, pressure borne structural members, or structural members treated with fire retardant chemicals.
- Load combinations shown above are in accordance with ASCE 7-02. Dependent on your own design codes the above load combinations may change.
Interpretation of Table 1 for simplicity:
Below I've created a table to try and simplify the above ideas (Table 2 is not based on the NDS but rather my interpretations of the above information):
| Table 2: Time Effect Factors based on Duration | ||
| TimeFrame | λ | Example |
| Greater than 10 years | 0.6 | Permanent |
| 10 years | 0.7 | Normal (Floor LL) |
| 2 months | 0.8 | Snow Load |
| 10 minutes | 1.0 | Wind or Seismic Load |
| Less than 2 seconds | 1.25 | Impact Load |
References:
- American Forest and Paper Association, "National Design Specification for Wood Construction", 2005
- Found in Appendix N.