Screw Withdrawal Value

[PT99]

In the NDS equation, W=2850 G^2 D, for a wood screw, is the D, the thread dia of the screw or minor (min shank) dia. For example, #10 screw has 0.19 for thread and 0.146 for shank (min),
NDS identifies D as Dowel Type Fastener, so not really clear.

 

[skeletron]

We (CSA O86) use the outside thread diameter as the nominal screw diameter.

 

[phamENG]

For the NDS, it should be pretty easy to work out.

...W...shall be determined from Table 12.2B or Equation 12.2-2...

The equation calls for a diameter, the table uses screw number. Solve the equation for D using values from the table and compare it to outside thread and root diameters and see which one it is.

My guess is that it's the outside for two reasons:
1) the NDS and CSA 086 tend to align closely on a lot of things (if you ignore Limit States Design, that is)
2) In the same section of the NDS, it talks about limiting the tensile strength to the strength of the screw at the root diameter, D_r. So if they feel the need to define a different variable for rood diameter, there's a pretty good chance 'D' is not talking about root diameter.

 

[PT99]

Thanks,
I saw that table after I posted, so D is the nominal diameter.