Wet Service Factor Explained:
Fig. 1: Example of when the CM factor should be used.
The Wet Service Factor (or CM for short) is used to signify wood that will not be used in a dry condition (e.g. covered structures).
Note that wood that cycles between wet/dry conditions will degrade in condition faster than wood that is subject to continuous wet or dry conditions. For example, a column supporting a wood pier under water will retain it's strength longer than the part of the column that is stuck between the tide (as tide goes up it will be wet, and as the tide goes down it will then dry). This is because the transition between wet and dry will deteriorate the wood faster than a constantly wet, or constantly dry member.
Note: If a piece of wood is considered to be in a dry condition (therefore it doesn't have greater than the moisture contents described below), you can take CM to be equal to unity (1.0).
For Sawn Lumber:
If the moisture content while in service will exceed 19% for an extended period of time, then the following CM factors must be used.
| Table 1: Wet Service Factor (Sawn Lumber) | |
| Design Value | CM |
| Fb | 1.00 |
| Ft | 1.00 |
| Fv | 1.00 |
| Fc-p | 0.67 |
| Fc | 0.91 |
| E & Emin | 1.00 |
where:
Ft = Tension Stress
Fv = Shear Stress
Fc-p = Compression Stress Perpindicular to the grain
Fc = Compression Stress Parallel to the grain
E = Modulus of Elasticity [2]
For GluLams:
If the moisture content while in service will exceed 16% for an extended period of time, then the following CM factors must be used.
| Table 2: Wet Service Factor (Glu-Lam) | |
| Design Value | CM |
| Fb | 0.8 |
| Ft | 0.8 |
| Fv | 0.875 |
| Fc-p | 0.53 |
| Fc | 0.73 |
| E & Emin | 0.833 |
For Connection Design?:
Connections must be checked 2 ways: the moisture content at the time of fabrication and the moisture content in service must both be used to evaluate the strength of the connection. Proper Wet service factors for connections can be found in Table 10.3.3 of the NDS [1].
References:
- American Forest and Paper Association, "National Design Specification for Wood Construction", 2005