Nails and Screws under Shear

Do nails and screws react to shear force differently? I was removing some hurricane boarding and applied a twisting motion to two boards and a screw connecting them broke. The situation was two 2"x4"s connected by a 4 inch screw, one 2"x4" driven into the ground. This was used as a brace for the hurricane boards. I’m trying to build my knowledge of these things since I am working in the roof installing some hurricane braces and straps. Should I be using nails always instead of screws are does it depend on how the force will be applied to the area? I admit this is being badly worded so if someone knows of such things and needs clarification, please let me know.

They “behave” the same under shear, however the ultimate strength will depend on the cross-sectional area under shear and the strength of the material.

When I was installing plywood shearwalling in my garage (seismic upgrades here in California) the code states that you must use certain size nails…comparing the framing nails to the drywall screws that I’ve got onhand I’d say that the nails are made of stronger steel and have a larger cross-sectional area and so will have a higher shearing strength.

Screws will probably have a higher tearout strength (load required to pull them straight out of the wood) but that’s due to the gripping action of the threads. There are undoubtedly screws every bit as strong as framing nails (just look for thicker screws, perhaps made of stainless steel), how much do you want to pay?

So what to use depends on the application. If you are concerned about the fasteners being pulled out, go for good strong screws. If you are concerned with shearing loads, go with framing nails. If both are an issue I suppose you could break out the lag screws which are enormous thick screws.

You’re on the right track Valgard. Drywall screws not only have a thinner shank than do nails, they are also hardened to a higher degree. While common nails will accept a shear load without fracture, drywall screws are not so forgiving.
The means of installation also is a factor. It is a realistic possibility that I can overtorque a threaded fastener to the point just below that of failure, and not know it. Unless I bend a nail when driving it, I have not altered it’s physical properties, and therefore it’s structural contribution to the building assembly.

Building codes call out nailing schedules for framing-screws aren’t presented as an option in any code that I can recall reading-steel framing excepted.

Well, due to limited space, screws were considered because it’s easier to put them in with a flexible shaft bit holder than hammering in nails. Ring shank nails will be the prefered method for attaching the braces but I was wondering about the screws. I sure don’t want to drive in screws and get a false sense of security only to have them snap when a shear force is applied because they were not designed to withstand the load. I have some interior screws which specifically state, “Not designed for shear loading” but it was only on one package and one type of screw.

Given the variety of building codes, I strongly urge you to consult the local Authority Having Jurisdiction or AHJ to see what code applies in your Parish. LA could be using a statewide code-I don’t know. Engineered connectors, such as hurricane strapping must be installed in complete accordance with the manufacturer directive, including type, length, and number of fasteners, or the connection does not meet spec. My last statement is true wherever you are working.

Most of the sites I tried on the web don’t recomment drywall screws for structural members as they are deficient in shear strength compared to other fasteners. Regular wood screws might do but could have fatigue failures under vibration because of the many stress raisers in the form of sharp bends at the root of the thread.

I’ve watched TV pictures of people nailing plywood protectors and often wondered why they don’t use duplex-head nails (scaffold nails) so that they would be easy to pull out withour messing up the wood.

I’ve always wondered why people don’t install threaded barrels around the window so that plywood panels can be fastened down with appropriately sized bolts (not dinky drywall screws), given that you seem to have to do this repeatedly.

This was all prompted because I started doing heavy research into the topic of why the roof on a home usually fails during a hurricane. In Dade county after Andrew, it was found that most nails missed in the sheathing missed the truss. I’m going to do some heavy inspecting so figured while I am up there, additional hurricane braces can’t hurt. Would doing this put my house out of compliance with some local building code?

Valgard, your suggestion is actually how I install the hurricane boards on the front of my house. Works very well.

What you’ve observed is one of the drawbacks of pneumatic nailers. If I’m nailing off sheathing by hand and miss the stud or rafter, I can tell it right away. No such feedback with my framing nailer. Sheathing manufacturers now have OSB with gridlines on it to help stay on target, but the diligence of the carpenter makes or breaks that equation.

Adding hurricane strapping wouldn’t make your home noncompliant, and you’re correct in that it certainly can’t hurt, given your location. I’d still investigate to see if local or state codes for new construction require them, and if so, follow their nailoff detail. Absent a code requirement, I’d fall back to whatever the manufacturer specifies-after all, they did the engineering work regarding resistance to uplift, etc.