Residential tall wall shear wall question

[TroyD]

I assist a few local custom home designers/drafters here in the Minnesota. Tall walls in entry rooms and great rooms, as you are probably aware, cannot be considered for resisting lateral loads using the braced wall prescriptive method in the IRC. That's when they call me for an engineered solution. Attached is an exterior elevation markup of a recent project. I evaluated the two story exterior walls as engineered shear walls, sheathed with 7/16" OSB. (The main floor shear wall may require a tighter edge fastener spacing). The half-height (cantilever) wall at the lower level can easily transfer the shear forces into the foundation via the ebmedded anchor rods. The h/b ratio of the tall wall segment between the windows exceeds 3.5, so I am neglecting it per AWC SPDWS 4.3.4. (I design the LSL studs and related connections for out-of-plane C&C wind loads). My question is in regards to resisting tension/uplift. This approach requires tension holdowns, due to the relatively short shear wall length considered at the main floor. It seems overly conservative to omit the tall wall portion as providing any shear or uplift resistance. The 4'x8' OSB panel edges are staggered so it should certainly engage some of the tall wall area.

I have some experience using the Force Transfer Around Openings (FTAO) method for shear wall design. It's h/b wall segment ratios are more accommodating for this situation. I think using FTAO method, I could distribute out the shear force proportionally around several windows and show that holdowns are not needed, because I would be able to consider more wall length. Perhaps I will transition to using this method in the future for these projects.

Can anyone with similar experience weigh-in or point out something I may have overlooked? Any input is appreciated.

 

[KootK]

1) Without making the modification shown below, I'd be reluctant to count on the tall wall in shear. Too far off reservation with respect to h/w ratio and, without all of the walls being forced to displace in tandem at the second floor level, I wouldn't have much confidence in any load distribution between walls that you came up with.

2) I like the modification that I've shown below mechanically but it may wall be that the blocking and strapping would be more expensive than the hold-downs. It's certainly weirder.

3) I agree with your assessment on FTAO. With the blocking in play, you could even take everything shown shaded below as your shear wall.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[phamENG]

Is your goal to find a more elegant engineering solution, or to save the project money? If the former, FTAO is probably a good option. I don't have a lot of experience with it yet, but I've played with it some and it's on my list of things to experiment with further. If the latter, stick with hold downs. The average hold down costs about $40 + labor and anchor. The additional analysis time will probably cost more. And besides, when considering the lateral stability of unique, architecturally designed homes a few hundred dollars for hold downs is the proverbial "cheap insurance."

For full disclosure, most of my shear wall designs are near the east coast. If we don't have discreet hold downs, most plan reviewers reject them immediately without considering calculated tension loads. That my color my opinion some.

 

[TroyD]

Thanks KootK and pham,

My goal was to avoid using holdowns - mainly because residential concrete laborers and framers are not very familiar with them. I worry that the embedded anchor rods will get located incorrectly, leaving me with a whole other issue to deal with when that happens. I agree the holdowns are inexpensive. The budget for these assignments is also small, so I don't do much detailing. I just mark-up the designers plan with the necessary hardware required and submit my calc package to the building official. Maybe I need to develop some typical framing details to include with the submittal, but every house is different.

I recently came across a residential tall wall shear wall example that used a number of Simpson LTP5 lateral tie plates and A35 framing angles at the window headers and king studs to transfer loads to the adjacent wall segments. It was a quasi-FTAO design of sorts. I think the next tall wall project I take I will try the FTAO method.

 

[Agent666]

I'd tend to do what kootk is noting, creating a virtual floor for want of a better term, capable of dragging the load around and turning it into two separate storeys. In this part of the world its looked at in terms of each wall is a storey high in our non specific residential codes, so this method sort of lends itself to breaking it up vertically.

 

[phamENG]

Your concerns about poor placement are well founded, but typical details (even though every house is different) can go a long way. Some of my biggest issues are at corners where two shear walls meet.

To share a horror story that may solidify your resolve: I got a really nice set of plans from an architect for a large 3 story duplex near the ocean (exposure D). I then detailed the lateral force resisting system on my own set of drawings and sent them off. Several months later, we got a call from the owner and contractor furious that they had failed their framing inspection. A little investigating revealed that the contractor had poured the foundations, framed, and sheathed the house without seeing the structural drawings. The moment frames hadn't been fabricated (their foundations also hadn't been poured), windows had been rearranged on the fly rendering walls that should have been shear walls useless - it was a mess. We didn't even know it was in construction until this happened.

 

[trackman417]

Maybe I'm not understanding or getting something here.

Why are you not detailing hold downs at the lower level shear wall to the cantilevered wall?

 

[boo1]

Troy done sweat the tiedown concrete anchor placement. If its off, drill and epoxy them in.