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 SG2 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
D - diameter of screw (inches)
D - diameter of screw (mm)
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