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# Wood Screws - Withdrawal 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)

where

F = allowable withdrawal load or force - lb per inch of penetration (lb/in)

SG = specific gravity of oven dry wood

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

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## Citation

• The Engineering ToolBox (2012). Wood Screws - Withdrawal Force. [online] Available at: https://www.engineeringtoolbox.com/wood-screws-allowable-withdrawal-load-d_1815.html [Accessed Day Month Year].

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