2021 SDPWS - Shear Wall Safety Factors 2

[phamENG]

Looks like the good folks at AWC are looking to make my life a lot harder. 2021 IBC goes into effect in Virginia in a month (with a one year grace period), so I'm getting acquainted with some of the new referenced standards. Not loving what I'm seeing.

Looks like, in the name of 'simplification', they've removed the extra capacity for shear walls resisting wind. In 2015 and earlier editions, there were different capacities for wind and seismic and you applied a 2.0 safety factor. Now, it's just the 'old' wind capacity, but everyone has to use a safety factor of 2.8 which puts the allowable in line with the old allowable for seismic. AND, for 10d common nail shear walls (the only kind I spec), there's a new 8% reduction in capacity if you build it the normal way with a hold down on the face of the end studs facing toward the shear wall. This is based on performance under cyclic loading.

I get that we need to be checking things for seismic and "wind controls" as a general statement is bad practice...but when my LFRS base shear for wind is 4 or 5 times my seismic loading, I don't worry too much about the capacity of individual elements in that load path for seismic performance. It works. But now I'm getting cut off at the knees, and my typical shear wall now has 45% less capacity than it did yesterday? Whiskey Tango Foxtrot, AWC?

I guess the steel guys in my area will be happy...lots more moment frames!

I can't read. Sorry. See below if you're curious how everyone set me straight.

 

[Deker]

Check SDPWS Section 4.1.4, ASD reduction factors are different for seismic and wind.

 

[phamENG]

Whelp. Let me crawl back under my rock and take some remedial reading lessons, then...

Thanks, Deker.

 

[ChorasDen]

Deker beat me to it, your capacities are essentially the same, just new nominal values for reporting in the tables. However, yes, we are taking adjustments for the eccentric hold downs if located on the inside of stud pack in 2021 SDPWS (don't worry, that will very likely change or be eliminated in the next code cycle, we are working on submittal and revisions for committee review, it will simply require a change in how sheathing is nailed to boundary stud pack, but you are still stuck with it for a 6 year code cycle). I think AWC and Phil Line had a webinar a couple months ago that discussed, maybe there is a recording out there, I can find the name if you're curious.

 

[Celt83]

There was a pretty good webinar by either SEU or Woodworks that went over the changes, if I remember correctly seismic capacity got hit pretty hard for some assemblies. I'll see if I can dig up a link.

 

[Eng16080]

One meaningful difference that I came across between SDPWS 2021 and 2015 is in Footnote 10 of Table 4.3A which applies a 0.92 multiplier for shear walls with 10d nails with "...hold-down attached to the inside face of the end post..."

Ok, ChorasDen beat me to it. Seems to be the trend of this thread.

 

[phamENG]

Yeah. The 0.92 was what I was referring to by the 8% reduction. I just completely misread 4.1.4 when I pulled it up. The result was my quasi-rant of a post...

ChoarsDen - I'm all for more learnin'. I was watching the webinar on short vs long nails in shear walls based on the 2021 SDPWS which is what clued me into it initially. They were saying this is based on cyclic load tests. I'm curious - did this not show up in the old data? Or was it overlooked?

 

[Eng16080]

The 0.92 was what I was referring to by the 8% reduction.

Yup, somehow I read your comment and completely missed that. (2024 will need to be the year I actually learn to read.)

 

[ChorasDen]

I was watching the webinar on short vs long nails in shear walls based on the 2021 SDPWS which is what clued me into it initially.

Yep, that's the one I was referencing, the one presented by Phil Line in Sept 2023. Believe it was titled "Wood-Frame SHear Walls & SPWDS". Slide 44 is the section I am referencing that is in committee to modify the 0.92 adjustment in the next SDPWS, I'll include a photo below. Testing indicates assembly type C & D provide sufficient strength to eliminate the 0.92 adjustment, just needs to be approved by committee.

They were saying this is based on cyclic load tests. I'm curious - did this not show up in the old data? Or was it overlooked?

That I cannot say, all that testing was before my time, and I do not have direct access to that particular historic test data. I could ask some associates, but not sure what benefit such info would provide to us practicing today.

 

[phamENG]

(2024 will need to be the year I actually learn to read.)

You and me, both.

not sure what benefit such info would provide to us practicing today.

Sort of like the old wood designs using higher tension capacities, and then we realized that didn't really work. Good to know when the old way was wrong vs the new way just being more accurate/efficient or something like that. When I go into a building to renovate it, for instance, if I know that there is a lurking error in the way it was designed, it gives me an opportunity to consider it with better background and make a better decision on correcting it or letting it stand.

 

[Eng16080]

I'm not sure this meets the intent of Footnote 10 concerning the 8% reduction, but it seems like you could substitute 10d Galvanized Box Nails for 10d Commons and avoid the reduction. Maybe that's an oversight/loophole in the code.

 

[phamENG]

but it seems like you could substitute 10d Galvanized Box Nails for 10d Commons and avoid the reduction.

I don't think that's a loophole. I think if you put the hold down there, you have to take the reduction.

 

[Eng16080]

I meant because the footnote specifically mentions "10d common nails", you could avoid the reduction by using 10d galv. box nails instead. I'm guessing that's not the intent though.

 

[phamENG]

Oh, I know what you mean. I just think you're trying to read too well, if you know what I mean.

 

[YoungGunner]

However, it looks like all the drywall shear walls got destroyed. They maintained the same nominal values in 2021 as 2015, but now penalized more with the 2.8 ASD reduction factor. Ouch.

 

[phamENG]

I'm not too worried about that. I'm generally opposed to structural systems that can be rendered instantly useless by a roof leak.

 

[YoungGunner]

Also, the Errata removed the allowance to increase the 7/16 sheathing capacities to that of 15/32 if meeting either requirements of panel direction or stud spacing. Made me sad for that lost 20 plf.

 

[YoungGunner]

I'm generally opposed to structural systems that can be rendered useless by a roof leak.

Technically most structural systems could be rendered useless by a water leak over a long enough time, but I know what you meant

 

[phamENG]

I fixed it.

 

[Huevo]

Also, the Errata removed the allowance to increase the 7/16 sheathing capacities to that of 15/32 if meeting either requirements of panel direction or stud spacing. Made me sad for that lost 20 plf.

I shouldn't be surprised and yet here I am scratching my head at how this sort of change happened in an Errata. Thank you for pointing this out.