Design Values for Wood Members

Last updated June 27, 2022
By Ian Story

The typical procedure used to determine design values for allowable stresses in wood members is as follows:

Test a representative sample of wood members for the desired property (i.e., Ft, Fb, Fv), recording the stress values at failure. These ultimate stresses will typically follow a roughly normal distribution, similar to the chart below. From this distribution, take the characteristic value, typically defined as the 5th percentile of failure (in other words, pick the stress level that causes failure in only 5% of the specimens tested).

Shear stress at failure for Structural Composite Lumber beams (source: Siller, T.S. 2000. Evaluation of the torsion test as a method to determine the shear strength of structural composite lumber)

Apply a factor of safety to the characteristic stress to determine the allowable design stress. This factor of safety varies, but is typically in the range of 1.67 to 2.5 for wood members.