When wood shear walls don't line up 1

[Buleeek]

Hello everyone,
I am struggling with a two story residential house where the walls of the second floor almost entirely do not line up with the walls below. Even the outside walls end up on headers, second floor envelope is smaller (walls ends up in the middle of living room) or they are cantilevered (nothing below). I was wondering what other, more experienced, engineers think about the lateral design in such houses. How do you handle the design?

 

[jayrod12]

Transfer the loads in the second floor diaphragm. It's ugly, but sometimes necessary.

If I'm being completely honest, where I practice no one pays attention to lateral design of single family houses except on the extremely large custom homes. On regular homes it's generally ignored. I do know that in seismic areas (which are not my regular area) this sort of design is completed and the contractors are generally familiar with the blocking and strapping requirements of high load diaphragms.

 

[DaveAtkins]

Agree with jayrod12. It is a pain, but for the holddown force at each end of the shear wall, since you don't have a shear wall below, you have to transfer the uplift or downward force into a couple of joists. Or maybe put a built up beam directly below each shear wall.

DaveAtkins

 

[JoshPlumSE]

Transfer the loads in the second floor diaphragm. It's ugly, but sometimes necessary.

My thoughts are almost identical.

Though I've only used diaphragm transfer for structures where the majority of the lateral system lines up and I'm working out some messy detailing in the areas where it doesn't.

If it were like this EVERYWHERE, I might reconsider my lateral system on the lower floor. Like supplementing it with moment frames or a "strong wall" type of system at some locations that do line up. More costly for sure, but I just don't like the idea of designing a building where so little of the lateral system lines up.

 

[XR250]

In my area, lateral stability is on the plan reviewer's radar. Some reviewers make us actually draw out the braced wall lines etc.I usually don't make a big deal about it but if I did have an isolated sheerwall I would dump the shear into the floor diaphragm and the couple into some floor beams similar to what DA said.

 

[kissymoose]

I expect to get flak for this, but I use the IRC prescriptive method for lateral bracing for most of my house designs. I have no problem doing so as the loads in my area are not high, I have never seen lateral system issues in houses around here, and it's a legally accepted method of design. Offsets between stories are not a consideration in the prescriptive method of the IRC.

 

[DaveAtkins]

I expect to get flak for this

You won't get any flak from me! There is a reason for the IRC prescriptive method - it works. In a typical single family residence, there are exterior and interior walls all over the place which somehow work together to resist lateral loads.

DaveAtkins

 

[JoshPlumSE]

I expect to get flak for this,

I'd be okay with using IRC.... Just as long as there is an actual load path for both gravity and lateral loads. Meaning that you should NOT be using it for any case where there is an offset in the shear walls.

 

[Buleeek]

Thanks for all the replies.
1. If I were to use the prescriptive method, how would I connect/transfer the load below/onto the second floor diaphragm?
2. If I were to do the engineering method (segmented), and rest the shear on a beam, how will I make sure that beam will stay in place? Wood beams usually sit on built-up posts which of course it is all a pinned connection.
3. There are a lot of steel beams and columns. Should I try not to engage the steel (it is all pinned, no moment frames) or using the steel is beneficial in this case?


Ultimate wind 130 mph, exposure D, partially enclosed (I always go with partially enclosed for residential design- please convince me I am wrong). No seismic.

 

[DoubleStud]

Buleek, if you can, try to continue the upper floor plywood all the way to the main floor exterior wall. Then the lower roof will just overframe over.

 

[SE2607]

To the OP: Are you in a high seismic region? If so, not only do you have transfer diaphragms to deal with, but you also must consider the overstrength (Omega sub o) factors on all affected elements including the second floor shear walls. That will more than double the loads.

 

[DaveAtkins]

I always go with partially enclosed for residential design- please convince me I am wrong

Assume it is enclosed. Pre-engineered metal building designers do it all the time, even for buildings with overhead door openings.

And why Exposure D? Is this house on a large body of water?

Ultimately these are your decisions to make, but don't be too conservative on a residential design.

DaveAtkins

 

[Buleeek]

DoubleStud, I don't I following what you are proposing.
SE2607, No seismic region, only high winds
DaveAtkins, The house is near the ocean. I always assume partially enclosed in the ocean area, simply because what if the windows break due to high winds?

How do you transfer load from diaphragm to the first floor in this case ? Say from second floor to the first floor is about 10 feet.

Thanks,

 

[DoubleStud]

If the wall on upper floor is a few feet inward, that means you will have a lower roof covering the offset. I am just saying it is better to continue the floor diaphragm of the second floor all the way to the exterior wall of the main floor. This way you are not relying on the lower roof to transfer the lateral load.

 

[DoubleStud]

Like this.

 

[KootK]

I was wondering what other, more experienced, engineers think about the lateral design in such houses. How do you handle the design?

I do the diaphragm transfer thing often, despite not loving it. Rich people houses... I feel that it tends to worsen the unpredictability of the lateral response in these kinds of buildings for which predictability is pretty poor to begin with.

I would offer these comments as specific advice:

1) Be very careful about your detailing for transferring shear and chord forces through the system. It's surprisingly easy to miss something and a regular occurrence for me is to wind up requiring some kind of detailing that folks will view as unconventional.

2) Because of #1, wherever you need something unconventional, be very clear in your asking for those things in the construction documents. Localized transfer diaphragm nail spacing, strapping and blocking through plenum spaces below shear wall chords, lots of shear nailing on interior wall plates, uplift connections to transfer beams, top steel in interior footings if their uplift. All of this stuff tends to be non-IRC and, therefore, surprising to the folks putting the building together.

For single family dwellings, I actually try like the devil to not use interior shear walls at all if I don't have to.

 

[Buleeek]

I think I will install flat 2x_s to the bottom of TJIs and transfer the lateral load (diagonally) through the diaphragm on the first floor walls. What do you think about such an idea? The floor depth is 24", but my joists and beams are only 14" (so there is room for the flat 2x_s).
Thanks,