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Old 08-21-2012, 01:11 AM   #1
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Nail Shear Strength Test


Recently there was a thread regarding the virtues of nails vs screws....with the common theme being 'shear strenght'.

Me, being the engineering technogeek I am, I'm not one to accept the a blanket statement of "Nails are better" without something to back it up...

So...I did a test.

I took 2 2x4's and nailed them together with 16d nails. (edit...ok....wrong pic...this is the screws....but the nails were in the same spot)



I left the other ends about 12" apart. The intent is to take this end, set it on a scale and measure how much force it takes to bring the ends together.



So I rotated the test assembly and put the end on the scale....



The weight of the stud was enough to make it drop down to the position shown. The only measurement on the scale was the raw weight of the wood.

The nails did not give....the wood did.

So...I repeated the test using #8 3" screws.

Basically....alsmost exactly the same results.....

Initial conclusion....all the talk about shear strength....bogus....the softness of the wood negates the shear strength of the nails or screws..I don't think the average DF1 stud is strong enough to put a nail or screw into shear failure.....

To add to that...when I was taking the test pieces apart...the nailed pieces came apart easy...just lift one board, pulled apart with no problem....the screws? Damage to the wood...and significant more effort to pull apart....at least 3-4x more force.

With that said......

I can not imagine trying to do a framing job using just screws......the time to use screws would be an easy 5-10x longer over an air nailer...

Given the way typical framing is done....I see no advantage to screws over nails....few of the forces are in tension vs shear....in tension, screws would be significantly better....but that is why we have so many Simpson hardware choices....
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Old 08-21-2012, 06:17 AM   #2
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Nail Shear Strength Test


That’s not much of a test. How about taking that same screw and a 16d nail and drive them in a board about half of their length and start bending them back and forth and see what you get.
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Old 08-21-2012, 07:06 AM   #3
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Nail Shear Strength Test


While the wood gave and softened the stress on the nails and screws, adding the Simpson ties adds a new dimension of stress, the metal and the wood now have a different stress factor other than just the wood and will produce much more shear force.
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Old 08-21-2012, 07:10 AM   #4
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Nail Shear Strength Test


Its not a very good test sorry to say...wood wil split every time.
Its the reason they dont let you use screws in joist hangers, screws snap and nails bend.
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Old 08-21-2012, 08:03 AM   #5
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Nail Shear Strength Test


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Originally Posted by kwikfishron View Post
Thatís not much of a test. How about taking that same screw and a 16d nail and drive them in a board about half of their length and start bending them back and forth and see what you get.
THIS is how the test should be done!

HOWEVER you also need to test it over time, those gold screws will hold up ok when new but after 5 years they'll snap with barely a tap. I've demo'd countless decks and most of the decking is screwed down with those gold screws or the newer decks have the coated deck screws, you can snap them easily with barely a swing from the hammer.
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Old 08-21-2012, 08:22 AM   #6
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Nail Shear Strength Test


the shear test that is best is to drive a srew into a piece of wood leaving some of it sticking out, do the same with a nail... next,with a hammer from the sides tap on the screw from two different directions. the screw will maybe bend just a little bit and then snap off. Now do that with a nail. you will have to get that nail really hot with friction and movement before it will snap off.

what you are showing with the soft wood giving before the screw should always be considered when using joist hangers on a structure, especially anything up to a triple hanger and over. We have had to add structural plywood in some cases because the wood was to soft to attach the hanger to effectively.
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Old 08-21-2012, 08:26 AM   #7
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Nail Shear Strength Test


Of course no disrespect to DDawg...Thanks for doing the write up!
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Old 08-21-2012, 09:55 AM   #8
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Nail Shear Strength Test


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the shear test that is best is to drive a srew into a piece of wood leaving some of it sticking out, do the same with a nail... next,with a hammer from the sides tap on the screw from two different directions. the screw will maybe bend just a little bit and then snap off. Now do that with a nail. you will have to get that nail really hot with friction and movement before it will snap off.

what you are showing with the soft wood giving before the screw should always be considered when using joist hangers on a structure, especially anything up to a triple hanger and over. We have had to add structural plywood in some cases because the wood was to soft to attach the hanger to effectively.

What you are describing is not shear....that is bending.

By nature, a screw is a much stiffer metal....and takes more force to bend than a nail...while nails are a soft ductal metal by comparison.....

That one YouTube video where a guy is smacking the nail and screw from the side bending it and 'trying' to demonstrate shear....is not demonstrating shear....that is bending...and screws will break before a nail does....

The intent of the test was to demonstrate shear......and I think it shows that the wood is more of a factor than the fastner.

I'm not saying to use screws....as already noted...nails are a lot cheaper and faster...and given the application...work fine....

Kwikfish....I don't see the difference in your test vs mine other than a shorter stud....can you post a skectch of what you have in mind? I'll try it.

With that said....I will be pulling up a bunch of nails soon.....

When I put down my floor on the 2nd story I used 10d nails thru the 1 1/8" T&G plywood per the drawings....along with glue....I have a bunch of them working their way out....those that are...I'll replace with screws....which won't come out....(as easy)
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Old 08-21-2012, 12:19 PM   #9
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Nail Shear Strength Test


I don't think this test is really measuring sheer either. Since you are rotating the board around an axis point (actually 2 different points), you are measuring sheer and torsion.

For pure sheer, you would need to secure a nail or screw into a board which is secured to the floor, and then pull the other board horizontally until the fastener reaches its breaking point (the wood would probably break before the fastener). That would be pure sheer.
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Old 08-21-2012, 12:33 PM   #10
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Nail Shear Strength Test


Oh, it's shear....one nail has shear forces in one direction...the other nail, the opposite direction. The rotation is only changing the direction of the shear....which is actually a pretty small change.

If you draw it....it will be a bit more obvious.....but it is shear.....all I'm doing is using the mechanical advantage of the stud vs setting up the test sample in a press along wth a strain gauge.....not something I keep around the house....
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Old 08-21-2012, 04:00 PM   #11
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Nail Shear Strength Test


AGE and WEATHER play a MAJOR factor into this and your tests wont cover that.
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Old 08-21-2012, 04:03 PM   #12
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Nail Shear Strength Test


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AGE and WEATHER play a MAJOR factor into this and your tests wont cover that.
I agree.....I'm tempted to repeat the test with some old DF wood I have that came out of my house.
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Old 08-21-2012, 05:06 PM   #13
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Nail Shear Strength Test


Its a great idea its just by using the ends of the board its going to split as soon as you apply the side pressure.

Try using a 2x6 and make the axis about 3 ft up the board you will get a better reading.
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Old 08-21-2012, 05:10 PM   #14
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Nail Shear Strength Test


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Its a great idea its just by using the ends of the board its going to split as soon as you apply the side pressure.

Try using a 2x6 and make the axis about 3 ft up the board you will get a better reading.
Good point....

I think I'll try it again....one test at the end....another 12" inboard.....I think anything over 12" is not going to show any real difference....

I also assume that the moisture content of the wood would be a significant variable....

"wood would" Now that that 10 times....
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Old 08-21-2012, 05:27 PM   #15
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Nail Shear Strength Test


Pull some deck screws out of a 10 year old deck and test those as well, you might be surprised.
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