Wood Size Factor (C_F)

Size Factor Explained:

Wood is not a homogeneous material (like steel), and one of the factors that takes this non-homogeneity into account is the Wood Size Factor (C_F ). Not only does the size of the wood beam change the value of the size factor, but also what grade the wood is (No. 1, No. 2, Construction, etc.). In general, the larger a wooden member becomes, the lower your Size factor will get. This in turn will lower your allowable design stresses.

The size factor is used for Sawn Lumber Design to calculate bending stress (F_b), tension stress (F_t), and compression stress (F_c).

Where to find various size factors ^[1]:

Size factors (for members 2"-4" wide) can be found in the NDS^[1], Tables 4A, 4B, 4D, 4E, & 4F of the supplement dependent on what type of wooden member is being used. In most cases it will be conservative to take unity as 1.0. For construction grade sawn lumber, a C_F of 1.0 is always used (as long as the lumber is between 2" and 4" in width, and less than 12" in depth).

For members thicker than 5" that exceed 12" in depth, the following equation can be used:

where:

Note: d can also be taken as diameter for circular members.

References:

1. American Forest and Paper Association, "National Design Specification for Wood Construction", 2005