Load Duration Factor (ASD Only) Explained:
Wood is unique in the sense that it has the ability to support higher stresses if the loads are being applied for short periods of time. This effect is quantified by the Load Duration factor, or CD for short. This helps greatly for temporary increases in loading (such as an earthquake, wheel load, wind load, etc.) will have additional strength, compared to permanent loading.
CD in essence will convert wood design from Normal Duration (which is considered to be ten years) where CD is equal to 1.0 to other loading time-frames. The scale used is a semi-logarithmic scale with time being the logarithmic portion of the scale. A diagram showing the CD factor compared to time and a table quantifying these numbers have been provided for your use.
Diagram of the Load Duration Factor:
Figure 1: Plot of Load Duration Factor vs. Time (click to enlarge)
Table of various Load Duration Factors:
| Table 1: Load Duration Factors | ||
| TimeFrame | CD | Example |
| Greater than 10 years | 0.9 | Permanent |
| 10 years | 1.0 | Normal (Floor LL) |
| 1 year | 1.075 | Long Term Temporary Load |
| 2 months | 1.15 | Snow Load |
| 7 days | 1.25 | Roof Live Load |
| 10 minutes | 1.6 | Wind or Seismic Load |
| Less than 2 seconds | 2.0 | Impact Load (e.g. Fall Protection Load) |
As you can see from the above table, the normal load is considered to be a 1-10 year load.
References:
- Breyer, D., Fridley, K., Pollock, D., & Cobeen, K., "Design of Wood Structures - ASD", 2003