Shear Wall Hold Down Slip 1

[phamENG]

I'm taking a close look at some of my shear wall calculations and deflection compatibility between co-linear shear walls. The hardest thing for me to be happy with is the deflection of the panel caused by hold down elongation/slip.

Simpson publishes (both in their catalog and also in the third party reports) vertical displacement of the anchorage due to elongation and slip at maximum allowable load. I realize this value is non-linear, but is there anything out there that could be used to approximate this value with some accuracy at load levels below max allowable? I don't generally design my shear walls to hit allowables exactly, and like any self respecting, OCD engineer, I can't stand that I'm using a max value in a less than max situation.

I can see some value in assuming that it just maxes out on that piece of the deflection puzzle (and, indeed, this is how I typically handle it), but it's just not...precise.

How are others treating this term in the deflection equation? Is this a valid concern, or am I trying to round to the thousandths with inputs truncated at multiples of a hundred?

 

[Agent666]

Have you asked Simpson. They probably have a lot of test data and could probably provide something?

 

[phamENG]

Not yet. It was late and I figured I'd ping the group here. Doesn't sound like many people have interest, though, so it's probably not something to worry about. Next time I talk with the local Simpson rep I'll ask the question. Thanks.

 

[XR250]

I have never worried about it as I know the guy down the road certainly doesn't. Maybe in a multi-story application it would be prudent.

 

[KootK]

I'm sure that there's interest, just not readily available answers. As approximate as I feel that wood shear wall drift estimates are, I suspect that any increased resolution at hold down slip would get lost in the noise of that.

 

[RWW0002]

I would think a linear approximation would be reasonable (i.e. holdown slip proportional to load demand). I have seen this approach used by others and have used it myself.

Keep in mind that 2015 SDPWS allows the shear distribution in a line to be proportional to the shear wall strength provided several requirements are met. This simplification basically assumes equal deflection of panels with only a slight aspect ratio modification. Approximating holdown slip deflection (my maxing it out or by assuming linear response) and distributing based on the resulting deflection seems reasonable (an could be put into the splitting hairs category compared to the other approved simplification).

 

[phamENG]

Thanks, everyone. I'll stick with using max for now - if I ever need to sharpen my pencil a bit more I might use the linear approximation. I took a look at the FTAO guides that APA publishes and it looks like they use the max in their example calcs, too. Like KootK said - it'll probably get lost in the noise to do much else.

 

[KootK]

I'd still be curious to know what Simpson would have to say on this. In my head, I've always envisioned the slip happening mostly early on as slack gets taken up in places and the elongation being linear throughout.

 

[Craig_H]

Is it not a footnote in the Simpson catalogue that you can linearly reduce the holddown elongation/slip based on the actual serviceability load demand? I often find myself messing with the hold downs to get some wall segments' deflection within limits, so reducing that elongation where possible seems worthwhile to me (especially in wind-governed design).

 

[RWW0002]

Craig, it sure is good catch.

 

[Craig_H]

Thanks for that screen grab! Now that I have my own catalog in front of me, the limit states catalogue says the exact same thing (just with "Allowable Tension Load" replaced with "Factored Resistance"). Since deflection in limit states design is dictated by serviceability (unfactored) load combinations, the hold down deflection/slip can often be significantly reduced from the catalogue value. For wind governed design, you get a reduction approximately equal to your wind load factor, even if you are at the full strength demand under ULS load combinations.

 

[RWW0002]

How often is shear wall deflection actually governing your wind controlled design? Except for potentially multi-story designs where take-up devices are generally used, I am not sure that many engineers are checking shear wall deflection as a serviceability limit state for (generally fairly stiff) wood shear wall buildings.

 

[Craig_H]

It's my basic practice to check for all but the simplest projects. If I'm getting involved, there's generally something weird going on. Custom homes, tall walls, lots of glass cutting the wall segments up, you get the idea.

 

[Huevo]

How often is shear wall deflection actually governing your wind controlled design? Except for potentially multi-story designs where take-up devices are generally used, I am not sure that many engineers are checking shear wall deflection as a serviceability limit state for (generally fairly stiff) wood shear wall buildings.

It's not always the deflection limit that is important but using the different stiffnesses of the shear wall segments so that the load sharing can be appropriately determined for each segment.

 

[RWW0002]

Huevo - I am familiar with shear distribution based on segment deflection (see posts above). I have just never seen it checked as a serviceability limit state for wind controlled design, especially to the extent of resizing holdowns to control deflection as Craig_H references above..

Craig - Makes sense I guess. It seems to me that as long as unit shear and holdowns are reasonable, panel deflection is generally not going to control, but maybe it does more than I realize.

 

[Huevo]

Reading fail on my part...

 

[phamENG]

Craig and RWW0002 - thanks for that! I must have skimmed over it somehow. I found the concrete offset explanation in the third party reports, but they didn't get into linear reductions.

Essentially everything I do is wind driven, and I do all of my wood design in ASD. So I at least glance at the deflection as I run the calculations. As long as I'm in the H/200 neighborhood at 0.6W, I don't usually worry too much about it. If it's deflecting more than that, I'll drill down into serviceability checks at lower MRIs to make sure everything checks out.

 

[Craig_H]

Perhaps individual situations and approaches differ here. I'm often trying to make a relatively skinny piece of wall carry load, and in those situations, hold down elongation & slip is a major driver for deflection. There may occasionally be instances where I could get fancy and start doing FTAO, but if sizing up the hold down a size or two solves the problem, I will often do that and move along.