Holddowns at multi-level wood shearwall design 1

[bricep]

Hi all, this is my first post here! I couldn't find a topic specifically pertaining to this, so I decided to make a new thread for it. Basically, I am wondering how those of you who do a lot of wood shearwall design handle overturning forces, primarily when it comes to tension. When I have a single story structure, or even a two story structure, I just use segmented shearwall design. If there happens to be a tension force I need to design for, each shearwall end post gets its own holddown at the foundation level. This means that if a tension force from a shearwall above does not land directly over the shearwall end post below, then both shearwall end posts receive their respective holddowns at the foundation level (rather than relying on the strap above to transfer the tension load out to the end posts of the shearwall below).

Once a third story comes into play, however, and especially when the wall segments have end posts that do not align with the segment end posts below, I have a huge smattering of holddowns all over my foundation stem wall! At this point, do you guys and gals switch to a perforated shearwall design to keep the tension strapping to the ends of the entire wall elevation? Or do you stick with segmented design and determine that the shearwall panel below transfers the tensile overturning above out to the end posts of the wall segment below?

I'd love to hear your thoughts on this, there is a HUGE amount of book keeping that needs to happen once that third story comes into play, and I would really like to simplify my approach as much as possible. BTW I do primarily residential design here in Seattle.

-Brice

 

[jayrod12]

Many times I will see if I can get the shearwall hldown locations to line up top to bottom. Even if that means the actual holddown (and post) is not at the end of the wall segment. On most of my projects the floor layout (and door layout) is similar floor to floor and therefore can normally find segments of walls from top to bottom that don't have a door in them.

 

[KootK]

Jayrod's solution is my default if it's not too much of a penalty at the bottom. You'll likely get mixed opinions on this but another possible solution is to pass the tie-down forces from stud group to stud group laterally as you move down the building. It would be a pain to detail it in a way that won't thoroughly confuse your contractor though. KISS in reverse. If your walls are pre-fab, that might make it a bit easier. This is a bit like relying on concentrated loads to hold down shear walls which bothers some folks, me included.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bricep]

Thanks you guys, I have done the approach where there will be a stud pack with panel edge nailing in the middle of one of the walls above to align the strap and holddown locations. Usually I use a hatch pattern to indicate an area to receive a particular nailing pattern if above the standard 8d commons @ 6" oc. The issue I run into with that a lot of the time is that due to the fact that the overlapping wall segments are generally significantly shorter than the overall usuable wall segments above would be otherwise, my overturning is far in excess of what any reasonable holddown scenario could provide.

I may just need to do a free body of a mid-wall strap. The strapped studs of course will receive panel edge nailing in addition to the strap nailing. It would really be nice to be able to use the full length of a wall segment without having to put so much hardware in at the lower level. It seems that perforated shearwalls are not a popular approach?

 

[jayrod12]

Too much calculating and strap detailing to me to do perforated. Rather just deal with the holdowns required for segmented

 

[bricep]

Yes I was looking through perforated calculations myself, it seems that there are quite a few requirements that could easily generate an error in the calculations. I was trying an automated calculator for a project for perforated (Tekla Tedds) and I cannot find many scenarios where the calculation would be straight-forward enough to employ on many projects.

I drew a free-body diagram, and it seems that if you place a strap in the field of a wall, and provide panel nailing at the strapped stud, you will increase the shear in the wall by T/(2*H) where T is the tension force, and H is the height of the shearwall. I may just start beefing up my shear panels to meet the extra capacity required and not put the additional holddown below.

My main issue, as stated above, is confusing the residential contractors I work with by trying to force things to align. Obviously, they will know where the end of the wall is above because they HAVE to know where the windows go, and those dimensions are readily provided in architectural documents.

If someone has tried the method of increasing required shearwall capacity by T/(2*H) please let me know, or if I'm way off please also let me know why, I would really like to hear your thoughts and hopefully simplify the residential design approach among those of us who do this work!

Thank you

 

[CCox]

Perforated shearwall design for me. I use these as much as possible to minimize holdowns. I even use these on one story buildings. Risa 3D has a wall panel component that is intuitive to design these. You can draw an elevation of a multi story wall with openings wherever you want. You can choose what shearwall type you want.

I try to get holdowns to line up as much as possible, but sometimes that doesn’t work. In those cases, I have a typical wall elevation detail showing holdowns that don’t align. Developing typical details like this for perforated, segmented and force transfer shearwall can save you some time.