Allowable withdrawal load force.
Allowable withdrawal load or force of a common wood screw into side grain perpendicular to the fibers of seasoned dry wood can be expressed as
F = 2850 SG 2 D (1)
F = allowable withdrawal load or force - lb per inch of penetration (lb/in)
D = diameter of screw (in)
Note the difference between allowable load and failure load. Allowable load includes design and safety factors and ranges typically between 2 and 10. Depending on the case allowable load is normally 10 - 50% of the failure load.
Example - Screw Allowable Withdrawal Load
The specific gravity of Canadian Spruce is 0.45 . The diameter of no. 10 screw is 0.190 inches . The withdrawal force per inch of penetration can be estimated as
F = 2850 (0.45) 2 (0.190 in)
= 109 lb per inch of penetration
Screw Withdrawal Force Calculator