I’ve read around some of the past posts regarding screws vs nails on this website and others, and I have a question:
Would a partially threaded screw (the unthreaded portion being in the area where the two pieces of wood meet) work as well or close to as well as a comparable nail where shear is a factor? Assume the two have the same shank diameter, and the screw is one that has a design where the shank diameter and the minor diameter are the same (if you look at it from the side, its profile is more like a nail that has threads rather than a traditional wood screw.)
If brittleness is still an issue (I assume it probably is) would it make any difference if the screw was stainless steel (since those cannot be hardened)? I understand that stainless steel is softer than regular steel; given that, would it be better to use a softer, less brittle screw or a harder, more brittle screw in an application where shear is a factor?
I’m sorry if this ends up causing a hullaballoo like some of the other threads I’ve seen, that was not my intention.
Replies
That's news to me. I thought that ordinary screws are more brittle than nails, and that stainless steel screws are more brittle than ordinary screws--that's certainly the experience I've had in all the work I've done on my house this past two years. It's been much easier to bugger up the slots in the SS Phillips screws than in ordinary screws. Then again, there are a number of different recipes for SS, and maybe I've just been getting the "brittle" formulation.
Yes, ordinary screws are more brittle because they're hardened while usually nails are not.
The SS screws I was thinking of using were SPAX, and I've not had issues with the heads stripping.
I actually just went and did some testing after posting the first reply. Using the highly scientific method of "hitting the heads of the screws while 3/4 of the screw is in a board" I came back with the following results:
Out of 3 vinyl coated deck screws from fastenal tested, all of them had their heads break off when hit, but I did have to hit them harder than the others to get anything to happen.
Out of 3 Spax, all 3 bent down against the board they were in but did not break.
Out of 3 no name stainless steel screws I had lying around (I think deckmate but not entirely sure since they weren't in the original box--I tend to steal those clamshell boxes to use on other things) all 3 broke like the vinyl coated deck screws. They were much easier to do this to than the others--I barely had to tap them.
Also did 3 nails, all of them bent down. It was harder to get them to do this than it was the Spax.
The reason that "hitting the
The reason that "hitting the screw with a hammer" test isn't valid is that you are doing an apples/orange test.
Instead, put the fastener FULLY into the wood with a joist hanger (if that's your application), put a block of wood in that hanger and hit that. I'll bet you get dramaticly different results. Go back and run the same test with a nail. If you are testing a wood on wood connection, run it that way instead.
The reason your original test isn't valid is that the hardend steel of the screw actually distributes the force more evenly thoughout the bearing area of the screw. The wood is much softer than either the screw or nail. The nail - being softer metal - tends to bend and round the hole near the surface of the wood. Eventually, the nail will slide right out.
This happenes well past what an engineer would consider a failure of course, which I think is movement of 1/4" in the fastener.
Screw/Nails
Ammos,
Frequently asked question. Some inspectors will let screws slide on hanger/bracket installations, most will not. However, an engineer/inspector can spec something different. Your are correct about the critical point being the un-threaded shaft below the head, the grade and hardness of the fastener is the deciding factor.
See this documentation for testing on Screw Products, Inc. fasteners, compare them with nails and see what the inspector says.
http://www.screw-products.com/specs.htm
Stainless steel is "softer" than most steel, so the shank of the screw will break or the head strip-out, as noted above with the phillips drive screw. A good star-drive (torx) will help in this case.
Good Luck!!
Thanks for the replies. So it sounds like while a screw as I described would work for getting close to the shear strength, a stainless steel screw made such would not. (The stainless I were looking at using were SPAX, I've used them quite often before and had good results)
The reason I was asking was because I would, if possible, prefer to use all stainless steel hardware and if that's not possible coated hardware. (Something just doesn't sit right with me putting in Galv. since I know it will eventually rust) I think now I'll just go with stainless lag bolts or carriage bolts.
Thanks for the link gbaune; while I'm not sure if I'll use it this go around I have it linked and may order from them for future projects. It looks like a good bunch of screws.
Looked up the RSS screws you mentioned, is this the one you were thinking of? And have you used it before? I see that the PHEinox stainless version has less shear strength--but could I just use more screws (obviously a limit to this though) to make up the difference?
http://www.grkfasteners.com/en/RSS_1_2_information.htm
Can't find a rating for the SS "power lag" spax (which is what I was thinking of using) but I emailed them so I'll probably get that soon enough. I did find a rating for the SS deck screws though, and it's 1000#P/320#S, which is well under the RSS screw loads.