Effective Length For Wood Bending Members

Effective length Description

When solving for the Beam Stability Factor (C_L) the effective length of the wood must be used (in lieu of the actual length). The effective length is determined by the length of the beam, the dimensions of the beam, and the loading conditions (distributed load vs point load, etc.).

Note: For the following l_u can be taken as the beams unbraced length.

Effective Length, l_e for bending members^[1]:

Note: Additional Loading combinations can be found in Table 3.3.3 of the NDS^[1]

Members not found in the above tables or in Table 3.3.3 of the NDS:

For members not found in either the Table above, or the expanded NDS table, the following can be applied for either single span or cantilever members.

• l_e = 2.06*l_u (when l_u/d < 7)
• l_e = 1.63*l_u + 3d (when 7< l_u/d < 14.3)
• l_e = 1.84*l_u (when l_u/d > 14.3)

Note: These number are the conservative maximums for the effective length. And therefore if the above tables can be applied you would get a lower effective length (and therefore a higher allowable bending stress). Therefore it is recommended to use the above tables if possible, otherwise the above three scenarios can be used.

References:

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