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Let's say I have screws with 1,000 lbs shear strength. If I add more screws, does the total shear strength of the joint increase? If I use 3 screws, then put a 2,000 lbs shear load on the joint, will all the fasteners fail?

I guess I should have been more clear, as most answers seem to be overly focused on the materials being fastened. The point of this question was to focus on the shear strength of the screws themselves. You can assume the materials being fastened are stronger than the screws, the holes are perfectly spaced, and are properly sized for the fasteners. For purposes of this question, the fastened materials will never fail before the screws.

  • See my comment on ArchonOSX's answer. If you install the screws properly and then apply the load, they will share the load and support it. If you install the screws in a way or under conditions where the screws don't all share the load equally and simultaneously, it would not have the combined shear strength of three screws. Several of the answers also get into the ramifications of the material you're fastening, and the fastener locations. – fixer1234 Jul 15 '17 at 22:09
  • So, the material will never fail, (i.e.: around the screw hole or buckle from overloading,) and you want to know if you add more fasteners will it increase the shear value....? (of the materials being fastened together?) Of course it will... Hmmm...that was easy, but do you mean increase "proportionately"? If so, then it depends...depends on shape, loading, etc.... This was the easiest answer I've ever given, or I'm lost... – Lee Sam Jul 16 '17 at 20:31
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The question as clarified can be viewed as the amount of metal in the cross sections of the fasteners. The total combined cross sectional area of all the fasteners can be divided up however you want. One big screw or a dozen small screws with the same total cross sectional area will have approximately the same combined shear strength.

It isn't exactly the same because the metal isn't uniform throughout and there is variability between screws, but that is essentially the model. In this downloadable spec sheet, there is some discussion, and charts where you can compare shear strength to cross section and see the relationship. Additional discussion can be found in this downloadable reference.

It's slightly more complicated than that, though. There was a study of the shear strength of a connection for various numbers of fasteners of different types, which can be downloaded here. It looked at some established formulas for predicting shear strength based on fastener material.

It concluded that while shear strength is generally proportional to the number of screws, the formulas overestimated capacity by a small amount when the number of fasteners for the connection exceeded seven screws in low and normal ductility steels. Apparently, beyond seven fasteners, the cumulative effect of extraneous factors and statistical variations become significant. They calculated a correction factor: the estimated composite shear strength should be multiplied by a factor of 0.85 for connections with more than seven screws.

So in your example, three screws each with 1,000 lb. shear strength will have a combined shear strength of 3,000 lbs. and support a 2,000 lb. load.

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If you have a single screw with that level of shear strength, it will also have minimum distance specifications for the material in which it is used. As an example, a fastener of 12 mm diameter should be placed no closer than 6 mm from the edge of the material.

Regarding spacing relative to one another, the material used is a factor as well as many other characteristics such as fastener diameter and material.

I found an interesting brief in PDF form here:

fastener spacing which opens directly into the PDF or a download window. It references spacing specific to wood trusses and fasteners and also takes into consideration wood grain.

There's an astonishing calculator located fastener calculator which uses a formula far beyond my comprehension:

fastener forumla part 1

fastener formula part 2

In the PDF link, it is suggested for the reader to enlist the services of an engineer. If you are using loads of one to two thousand pounds, that might be a good idea.

On the other hand, if you are building something without an inherent catastrophe in case of failure, build it and learn. I wouldn't construct a bridge across which I would drive the family vehicle, but I might consider a bridge across a gully for a golf cart or similar transport.

Generally, if you have the spacing for three bolts, it will be stronger than one, if there is no slop or play or similar poor construction. A single bolt in a tight hole with two others in loose or misaligned holes will not be optimum and could fail prematurely.

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Maybe I am missing the point to your question based on the other answers.

My understanding of fasteners is shear strength is perpendicular to the axis of the fastener. E.g. A wall anchor holding a panel on the wall. The load is perpendicular to the fastener and is attempting to shear it in half.

Whereas tensile strenth is parallel to the fastener attempting to pull it out.

So, I would say the shear strength adds up. If you have 3 screws good for 1000 lbs each and they are distributing the 2000 lb load evenly then the point load on each fastener will be less than its rating. So, the load stays in place.

I don't have a formula for this though. They said there wouldn't be any math on this test. 😉

  • Just to add a thought--distributing the load is a key requirement. That means all of the fasteners share the load at the same time. If you have a condition where a fastener is in a loose hole, or one fastener is already under heavy load when you put in the next one, the fasteners may not be subject to an equal share of the load at the same time. One fastener might not be under any load until other fasteners fail. If the situation is that the load falls on the fasteners sequentially instead of simultaneously, worst theoretical case is that the capacity is roughly that of a single fastener. – fixer1234 Jul 15 '17 at 22:00
  • @fixer1234 Failure is based in WOOD failing not STEEL, except when fastening steel material together. The fastener(s) will "tear" through (around) the wood long before the fastener snaps. – Lee Sam Jul 15 '17 at 23:34
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    @LeeSam, good point, if you're talking about wood. The question doesn't mention the material, though. BTW, I've seen steel screws snap from sheer in wood. Of course, it's possible they were defective screws. Maybe we should VTC Tester101's question as unclear or too broad because it leaves out the critical information of the material being fastened. :-) – fixer1234 Jul 16 '17 at 3:20
  • @fixer1234 Hmmm...well, in general "more is better" (to a point) regardless the material. – Lee Sam Jul 16 '17 at 5:48
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Wow, Fred's answer is complicated and I'm not sure I understand it...and I'm suppose to...

Yes, the more the better...up to a point. In general for bolts, nails and screws we worry about 1) size of fastener, 2) edge distance, 3) spacing between fasteners, 4) embedment, 5) size (thickness) and type (species) of material being fastened, and 6) If the material is in single shear or double shear.

In order to "develop full strength" I'd use the following: A.) Bolts: a) edge distance perpendicular to grain loading = 4 times bolt diameter, b) edge distance parallel to grain = 1.5 times bolt diameter. c) center to center spacing perpendicular and parallel to grain = 4 times bolt diameter.

B.) Screws and nails: The code allows spacing down to 2" apart for SHEAR on plywood, (which is your question, right?) (See IBC Table 2306.3.2.) However, I've seen 2x material split at that spacing, then you have no value. That's why we use 3x material around garage door openings and stagger the nails.

Where I live, we use a lot of Doug. Fir-Larch for structural material. Pine, Hemlock and other softer materials will have the same edge distances, spacing, etc. but carry less. I'd use local Building Code tables (Chapter 23) for maximum loading capabilities.

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