AWC - NDS - Connections - safety factor 1

[taz112358]

I'm looking at pull-though on plywood and starting to question what safety factor has already been applied (if any) to the table on Table 12.2F of the AWC-NDS2018. The table is labeled "design values" so I'm assuming the safety factor is already built in? I know I need to put the adjustment factors in but the number that spits out is ultimate or allowable?

In the connection calculator online the value they produce is "ASD adjusted capacity" which I like a lot more because I know it's been put though a safety factor.

I've looked online and around the NDS and safety factor is not mentioned anywhere.

Any help is greatly appreciated

 

[Eng16080]

First of all, if you were checking a connection based on head pull-through strength, you would take the WH (design value) from Table 12.2F as you mentioned, and then you would calculate the adjusted pull-through strength, WH', based on the factors in Table 11.3.1.

For ASD: WH' = WH x CD x CM x Ct
and for LRFD: WH' = WH x CM x Ct x 3.32 x 0.65 x lambda

At this point, WH' is already accounting for a safety factor, and you would compare this value directly to your factored load from the load combination that you're checking.

Now to try to answer the safety factor question:
I could be wrong, be I think, if you were to calculate the LRFD value, excluding the strength reduction factor, phi, which is 0.65 in the above equation, that would essentially give you the nominal capacity for LRFD design. If then you multiplied that value by the ratio of your factored load per ASD load combination to the associated LRFD load combination, and then further multiplied by the ratio of CD/lambda, I think that would give you something of a safety factor per ASD. If the ratio of your factored load per ASD combo to LRFD is 0.67 and the ratio of CD/lambda is 1.25, then that makes the safety factor 3.32*0.67*1.25 = 2.78. Again, I'm not sure this is exactly right, but it seems logical to me.

Again, for this particular calculation, the safety factor is already accounted for. This is the case for most strength calculations per the NDS code. One exception, though, is wood shear wall design where you take a nominal unit shear capacity and then either divide by a reduction (safety) factor for ASD or multiply by a resistance factor for LRFD. In that particular case, the "safety factor" is 2.0.

And yes, this stuff is all confusing, especially when you're used to steel design, for example, where the safety factor (omega) is explicitly defined. I've caught other engineers not applying the reduction factor for shear wall design. They assumed it was already accounted for like most of the NDS code.

 

[taz112358]

Yes thank you!

The adjustment factors to get the final W'h, I've got that clearly defined in the NDS.

I just didn't want to assume any ultimate pull though without knowing a real safety factor that was used in that Wh table.

I'm looking through the commentary now -- no safety factor really mentioned...
Bradford Douglas - Evaluation of fastener head pull-though, 2018 is the reference...
aaaand it looks like a safety factor of 4 or more for most of the Ptest vs Wh values...

It looks like they've got 7/16" plywood and #14 testing at 498# and Wh = 109#.
I've got 5/8" plywood and #15 screws, Wh = 148#. I'm sure it probably tested around 600# or more.

I'm still not fully comfortable without knowing an exact safety factor on there I'm to the point where I'm going to go out in the shop and do some pull through tests with our machine on a few different thickness of plywood.

 

[dhengr]

Taz112358:
Let us know your test results and procedure, they might be interesting and telling. They could vary by how hard/deep you have set the screw head, crushing the outer ply of the plywood. They will depend on the type, grade, number of plys, etc., of the plywood or sheet goods. And, much of that stuff is getting pretty crappy these days. Let us know how one of the inner plys being a void or a knot changes the results. Then you will start to understand why the FoS is so high in wood design and so hard to pin down exactly. Take a look at some discussion on grading rules and the general philosophy behind wood design and the material’s variability. This might be enlightening. They have not made this basic material and design understanding simpler by adding all the phes, phis, phos phums and omegas incorporated in LRFD. It’s all hidden within the black magic now.

Eng16080's comparative analysis looks about right to me. But it still doesn’t make us better/smarter designers. It just shows that we can do the arithmetic gymnastics.