Adjustment Factors Description (sawn lumber)

Wood design uses adjustment factors to determine the allowable design values. Design of wood members can be based on moisture, duration, temperature, along with many other variables that will be described in more detail below. The factors will most often be of the form of a capital C with a subscript. It is important to pay attention to the subscript, and especially the subscripts case. For example Cf is different than CF when being used in wood design.

Most factors will end up defaulting to 1.0 therefore it will not change the allowable design value. With practice, you will get used to which factors will need to be checked, and which can be ignored. For example, if the wood is to remain dry while in use, then CM will often be able to be taken as 1.0.

You will start with the allowable bending stress, which must be taken from the NDS[1]. This stress will then be multiplied by the adjustment factors shown below to find the allowable design value, which will be designated with an apostrophe (').

Adjustment Factors Applicability Table (for Sawn Lumber)

Table 1: Adjustment Factors Applicability (for Sawn Lumber)
ASD Only ASD & LRFD LRFD Only
Conversion CD CM Ct CL CF Cfu Ci Cr CP CT Cb KF var λ
F 'b = Fb x x x x x x x x x x x
F 't = Ft x x x x x x x x
F 'v = Fv x x x x x x x
F 'c-p = Fc-p x x x x x x x
F 'c = Fc x x x x x x x x x
E' = E x x x
E 'min = Emin x x x x x x

Note: An 'x' indicates the Adjustment factor should be used when calculating the corresponding stress.


where:

Fb = Bending Stress
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]
Emin = Minimum Modulus of Elasticity [2]

Factor Descriptions:


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