Adjustment Factors Description (sawn lumber)

Glued laminated design uses adjustment factors to determine the allowable design values. Design of glulam 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 (').

Note: Normal glulams will have an allowable bending of 1600 psi, 2000 psi, or 2400 psi (although 2600 psi, 2800 psi, & 3000 psi may also occur).


Adjustment Factors Applicability Table (for Glued Laminated Timber)

Table 1: Adjustment Factors Applicability (for Glued Laminated Timber)
ASD Only ASD & LRFD LRFD Only
Conversion CD CM Ct CL CV Cfu Cc CP Cb KF øvar λ
F 'b = Fb x x x x x x x x x x
F 't = Ft x x x x x x
F 'v = Fv x x x x x x
F 'c-p = Fc-p x x x x x x
F 'c = Fc x x x x x x x
F 'rt = Frt x x x x x x
E' = E x x
E 'min = Emin 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
Frt = Radial tension perpendicular to the grain
E = Modulus of Elasticity [2]
Emin = Minimum Modulus of Elasticity [2]

Factor Descriptions:

Main

Construction

Environmental

General Engineering

Geotechnical

Structural

Transportation

Water Resources

Miscellaneous

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