Hold down for a bearing wall question with contiguous shearwall?

[sponton]

I am designing a wood building. Due to some construction problems we had to remove the sheathing in one of two contiguous bearing/shear walls because contractors were struggling to actually nail the sheathing. Anyways, we were having a discussion at work because somebody picked up one of my details and told me the bearing walls needed hold downs otherwise the nailing would just pull out. Detail is attached. Now my train of thought, as that of my boss, is that the contiguous wall is taking the shear, and the bearing wall isn't exactly connected to the diaphragm as to actually transfer or take the shear forces to the top plates or in that matter to anything connected to that wall. However, I do believe that in flexible diaphragms this is only half true, I mean had this been concrete in the sense that stiffest elements take the most load, I wouldn't doubt the uselessness of a tie down, since proximity and stiffness would pretty much let the sheathed wall handle the load transfer but in the case of wood and flexible diaphragms, I can only be half sure of whether or not the bearing wall takes some of the shear forces, or at least suffers from some deformation that can indeed pull out some of the nailing. and would indeed need tie-downs/hold downs to transfer everything to the foundation.

Any technical expertise on this topic out there?

 

[KootK]

While I see where your colleague is coming from, I mostly agree with you and your boss. As I see it, these two things are true-ish:

1) Every stacking bearing wall sheathed in any material is going to effectively be an unintended shear wall right up until it fails in some way which may well be vertical separation from the walls below (overcoming of gravity load and nail pullout).

2) If the tied down, intended shear walls in the building are doing their job, the vertical separation in uninteded shear walls should be pretty small, more or less matching that of the tied down walls.

Conclusion:

Yeah, you may well pull the nails out a bit. Not enough to cause any problems though. No hold downs required. And this phenomenon is not unique to bearing walls that just happen to be former shear walls located right next to current shear walls. In fact, it would be more true for stacked walls located further from the intended shear walls because diaphragm deflections would exacerbate wall rotations.

I'm amazed that you were able to get continuous sheathing across the party wall joint. Every time that I've pushed for that, I've been denied. You must be a better architect tamer.

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.

 

[woodman88]

If the shearwall is designed to prevent uplift of the shearwall, how is the bearing wall going to lift up? I would see it, maybe, if the space between the wall were two feet to four feet. But as the amount of uplift at the nails, in a code event, would not affect the structure only allow some noise and cracking at the ceiling/floor to the wall. IMHO.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[KootK]

As long as we're on a related subject, does a stud wall sheathed on only one side require blocking to be considered fully braced (weak axis and torsional)?

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.

 

[sponton]

I normally take the weak axis as fully braced, first because you have the obligatory blocking as per code and then you have the sheathing + continuous nailing which in addition to all the stuff covering the wall does not really allow studs much lateral movement.

 

[DaveAtkins]

If the overturning moment of the upper shear wall exceeds its resisting moment, then a holddown is required at each end, to connect to the lower shear wall. Or am I missing something?

DaveAtkins

 

[sponton]

@DaveAtkins, that's the thing, it isn't connected to the diaphragm to act as a shear wall per se, so technically speaking it shouldn't take any shear/

 

[KootK]

@Sponton: with regard to the one wall not being connected to the diaphragm, are you intending for that unsheathed ladder truss to remain entirely unsheathed and be flexible with regard to shear? The diaphragm is connected to the ladder truss and the ladder truss is connected to the wall. So, unless the ladder truss is flexible wrt shear, the non-shear wall is in fact connected to the floor diaphragm.

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.

 

[sponton]

@KootK we would ask the truss manufacturers to design it for a value of 150 plf for shear but the connections wouldn't be strong enough for them to take the shear transfer, that's what I mean, it's kind of tricky with wood, I'd assume that the stronger connection in the sheathed wall would take the hit first but I still don't feel like neglecting any influence of the shear on the unsheathed wall.

 

[KootK]

I simply wouldn't worry about the unsheathed wall at all. Detail it as you normally would. As long as the designated shear wall is designed to do its thing, the other wall will be fine.

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.

 

[sponton]

@KootK

pfft, that's not even half of my concern now, I was doing the wind analysis by hand and I don't know if my numbers are very off, but even the exterior walls on the third floor do not provide enough shear resistance (most sections are less than 2') and I was thinking on how to approach it, I don't know if it's permitted to have the single interior shearwall take the whole hit or do i extra-reinforce the ext. walls even if they don't really pass by either strength or service, this a regular 3 story and 4 story building [I attached the plan so you can have a look].

The architect restricted us to 2x4's, the ext. walls are already overbearing, the first floor is (3) 2x4's at 12", One of my coworkers said I should run the thing on Woodworks and have it design it by it, but still I'm hesitant on how it could differ so much from my work. I was using TEDDS to do the shear wall analysis/design (either perforated or segmented) but i don't really trust that thing.

 

[sponton]

 

[KootK]

Yeah, for this, I would take it all down the middle. That's how it'll tend to behave in my opinion anyhow. I know that there are other members here who will likely disagree with that approach however.

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.

 

[jayrod12]

As much as I don't like it that's likely how I would do it as well. Hope that there is enough capacity on the central wall.

Although can you when you're obligated to consider it a flexible diaphragm?

 

[KootK]

The method only works if the diaphragm is assumed/proven to be rigid. I view it as another permutation of the three sided building really.

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.

 

[sponton]

The funny thing is that the Corridors (where the beams C run across) are not to be sheathed (you know architects), although in my office they want to model it as a single building I must say it would act like two different blocks and an allowance for the kick (or ram, not sure how you call that the collision of bdgs due to displacement) must be taken into account unless bdg is extremely rigid. This thing is not even complicated at first glance but it's giving me a headache already.

And then they are not to use sheathing in ext walls, they just expect the gypsum to do all the work. I wouldn't live in this bdg even if i was paid to.