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Some assembly instructions for a project I am doing requires that I use fasteners (screws) with a minimum of 120 lbs pull out or shear strength. What I plan to use is #8 2 1/2" coarse thread wood screws. I've noticed that none of the boxes of screws I've ever bought in the past two years indicate any such thing anywhere on the box. Is this pretty much a standard weight threshold with all wood screws or are some brands just "cheap" and don't make the cut?

Update: I'll mark the only answer as the answer because it answers my question. However, if you're not sure about the strength of your screws and you absolutely need to be, buy a brand new box that has the shear and pull-out strength clearly labeled on the box. This is what I ultimately did.

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The pull-out strength of a wood screw is going to be more dependent on the wood into which it is installed than on the material of the screw. The geometry of the screw thread also plays a role. "Cheap" screws therefore aren't what you need to worry about, instead, focus on the screw size.

Pull-out happens when you have a shear failure of the wood. A couple of formulas have been proposed to make an estimate of holding power

The Forest Products Lab of the US Department of Agriculture proposes the following:

p=15700*G^2*D*L

where p is the pull out force in pounds; G is the specific gravity of the wood (density of the wood divided by the density of water, see table 5-3 here); D is the shank diameter of the screw in inches; L is the penetration of the threaded part of the screw in inches. (Assumes reasonable pilot hole, screw into side-grain, etc...I haven't communicated all the nuance here. If your application is important enough to be calculating the pullout load, read the whole relevant section of the source document.)

Others have proposed a simple formula (stated incorrectly after the link, corrected here by me) related directly to the shear strength of the wood:

p=pi*D*L*S

where p is again the pullout strength; pi=3.14; D is the diameter of the screw, L is the length of engaged threads, and S is the shear strength of the wood. (make sure your shear strength is in units compatible with your length, e.g. if your length is in inches, your shear strength should be in PSI).

This simpler formula idealizes things and assumes that all the load is carried by a cylindrical region of the wood that is the diameter and length of the embedded screw threads (the pi*D*L above is the area of this surface). If you use this formula, be sure to reduce the actual load by a generous safety factor of 2 or more.

Personally, I'd read the FPL report and use the first formula.

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So the writers of these instructions for my project expect people to read page-long reports to find out what they mean by "pull-out" or "shear" stength? There has got to be a simpler answer. –  oscilatingcretin Nov 1 '12 at 22:12
    
I think the writers of your instructions "punted." I assume the step you're talking about connects something they provided, like a bracket, to something you provided, like a wall. Therefore there's no one right answer. –  mac Nov 1 '12 at 22:16
    
You're spot-on. Their RSIC and my studs. I think I am just going to wing it with some good, name-brand wood screws like your average, not-so-worrysome DIY'er. –  oscilatingcretin Nov 1 '12 at 22:21
    
Ignorance is bliss. I'll still stand on the other side of the room when I come over to your house, though. –  mac Nov 1 '12 at 22:28
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"So the writers of these instructions for my project expect..." likely, no. Rather, this is text the lawyers insisted be added. –  DA01 Nov 1 '12 at 23:33

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