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Screw FOS

Screw FOS

Screw FOS

I have a client who is requesting that I give them the design wind pressure on a structure so they can space their screws appropriately. Simple enough. However, the table they provided from the manufacturer (attached) requires the designer to "apply appropriate safety factor from the design method used to calculate design load". So I need to also provide them with the appropriate FOS to use as well I am going to be going out on an adventure to find this FOS (hoping the mfr will provide more information), but in case I can't find anything what FOS should be used?

RE: Screw FOS

When I first read the title of your post I thought you were going rouge and saying the hell with FOS.

I've seen FOS = 3.0 referenced in ESR reports, like this one. Check out the notes at the bottom of the allowable load tables.

RE: Screw FOS

LOL, I guess I should have come up with a better name

I found an ESR report, but they don't list any valuable information on what I am looking for.

I also have a call into GP.

RE: Screw FOS

Agreed with CANPRO on both counts.

just call me Lo.

RE: Screw FOS

what FoS would you apply if you were doing the work, ie if you were designing the screw spacing ?

is 3 general practice ?

do different loads (live, dead, snow, seismic, …) have different FoS ?? if so, is it unreasonable to choose the most conservative one ??

another day in paradise, or is paradise one day closer ?

RE: Screw FOS

Another reference could be AAMA (American Architectural Manufacturers Association) TIR-A9-14 - Design Guide for Metal Cladding Fasteners, which uses a safety factor of 3 for fasteners with a 1/4" diameter or less, based on allowable stress design. Useful document for those in the cladding industry.

RE: Screw FOS

You can check with louvre manufacturers - they typically use TEK screws or similar to fasten the louvre to structural members. Wind loading is typically what they design for.
Might be able to get an idea of the FOS they use.

RE: Screw FOS

I recently went down this rabbit hole and didn't come back with much satisfactory. I was trying to reverse engineer the actual screw rupture/failure loads from the NDS design formula for withdrawal.

The best I found was the NDS commentary which seems to indicate that withdrawal design values were 1/5 the rupture test values (50s era testing). The commentary on more recent NDS editions doesn't seem to clearly indicate if this factor was maintained.

A safety factor of 5 seems to be pretty common for fasteners and structural connectors I think. I'm pretty sure Simpson connectors, for example, are tested/calculated at FOS of 5.

RE: Screw FOS

Looking at this from a higher level -

you touch it, you own it. if you are the structural engineer for the building, I wouldnt start spec'ing engineering factors of safety for the exterior gypsum board, unless you are going to sign off on the entirety of the installation of that board (and be compensated for that).

this kind of stuff should be handled and overseen by a building envelope engineer, and if there is none on the project, then i would take it on entirely, rather than dipping half of my foot in.

RE: Screw FOS

You could use the factor of safety in the North American Cold Formed Steel Specification. Page 101

RE: Screw FOS

APA’s Technical Note E830, “Fasteners Loads for Plywood - Screws” says a traditional factor of safety is 5. The document is a free download after registering on APAs website

RE: Screw FOS

Along the same lines as Scratch3r's comment - SSMA catalog uses a safety factor of 3 for fastening to light gauge.


RE: Screw FOS

I'm going to come at this from a different perspective. While I agree with the factors of safety that others have listed for fasteners that attach to wood and cold-formed steel framing, it appears to me that the table you've referred us to in the original post were average ultimate failure loads of the Denselement board based on different fastener spacing configurations and stud spacings.

If you look at the pullout load on a single fastener into 43 mil (Fy=33ksi) material you can see that the tension load on the fastener based off of tributary area doesn't calculate out properly:
16" Stud spacing, 8" screw center to center spacing, 131 psf average ultimate load
Max Pullout/Tension of fastener = (131psf) x ((16 in x 8 in)/144 = 116.4 lb
Allowable Pullout of #6 = Pn/Omega = ((0.85)(0.0451 in)(0.138in)(45ksi))/3.0 = 79.3 lb

Based off of that information I would think that the safety factor that the manufacturer is referencing is for the Denselement board only and that the attachment fasteners should be governed by the applicable codes. Material safety factors are generally quite a bit lower than fastener safety factors and could provide you with a higher allowable load for the sheathing; however, I would definitely defer to the manufacturer since they do not explain what type of failure mode was observed (rupture, yielding, etc). I work in the cold-formed steel framing industry and have see others use a factor of safety of 1.67 but haven't ever received confirmation if this is correct or not from the manufacturer.

If you receive an answer from GP please post, it would be great information to have moving forward.

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