Lateral concerns for making an opening in a house

[milkshakelake]

Client wants to make an opening in an existing rear wood wall. This wall is about 100 years old and definitely wasn't designed as a shear wall, but functionally, it's a shear wall. When it comes to ASCE 7 code, alterations/removals to shear walls will require shear walls added somewhere else.

I was thinking of replacing part of the existing wood stud wall with a new shear wall, or at least add sheathing on a second side. I'm concerned about:
1. Overstrength: does this make it a soft story?
2. Do I need to do anything with the shear wall above? Like between second floor and roof.
3. The proposed shear wall would be on top of a brick foundation wall. I'm concerned about how to connect a holddown to something like that. It's a bit silly because holddowns didn't exist when this building was made and lateral systems weren't really a thing for this type of building, and ASCE 7 would come out like 70 years later...but I still need to follow modern codes when doing this alteration.

 

[Celt83]

What does the floor plan look like, anyway to justify a 3-sided diaphragm?

Anything new I'd probably add in board of the brick and carry down to a new foundation, unlikely you'll find any products for anchoring to the top of an existing brick wall.

 

[milkshakelake]

Floor plan is kind of crazy and has different walls on each floor, so it's not a 3 sided situation. But thanks for the idea, I might use that next time.

Yeah, I'm thinking of making a concrete pier down to foundation level to receive the holddowns.

 

[SWComposites]

The opening doesn't look that much larger than the total existing opening.
And you are presumably going to need a large header over that new opening, can you just extend the header across the adjacent wall segments and make a frame out of it?

 

[milkshakelake]

@SWComposites The header is the easy part. I don't want to keep the existing one, will put a new one in. The issue is the lateral system and the code-related issues. For example, does this create a soft story or a vertical irregularity necessitating overstrength design?

Even if it's not that much bigger than the existing opening, the authorities in my jurisdiction go kind of crazy over things like this, so I need to make sure it's buttoned up nicely. Good thing this forum is anonymous...those same people would be pissed off if I mentioned that.

 

[ChorasDen]

What's your seismic design category? What's the spectral response? Chapter 12 of ASCE gives you all the info you need for determining if overstrength checks or soft story considerations are required.

I'm with Celt, I would probably start by considering this a 3-sided diaphragm and see if I could get comfortable with that approach. 3-sided diaphragms will induce extreme torsional irregularity, which needs to be considered in the analysis including amplification of accidental torsion, and limits your allowable seismic categories.

Next option might be to use a Strong Wall? Would need to have a foundation that accommodate the strongwall requirements, but sounds like you are already considering going that route. I'd have to reread ASCE, but I think the strong wall option may prevent your soft story concerns.

 

[milkshakelake]

Seismic design category is B so it's not too bad. But going through the typical overstrength checks will be a nightmare for such a small job. I usually do that for bigger jobs where the design fee is justified. I think I'll go with the Strong-Wall route as you mentioned and make my life easy and beautiful. I guess the justification is that because it's so stiff, it doesn't create a soft story.

 

[ChorasDen]

Soft story is defined as increasing displacement of the wall-line compared to the story or stories above right? So I would expect that if you get any strong wall in there, it will be sufficient to justify not running soft story checks. I can't imagine how you might rationalize the stiffness of 100 year old existing walls, but I think you could easily argue the the strong wall is equivalent to an engineered wall line of xxx ft, and then back calc the justification that way.

 

[milkshakelake]

Yeah, that's what a soft story is (though I think it's with diaphragm, not wall line). Anyway, I agree that a strong wall should be stiff enough to justify not checking it. Thanks!

 

[Flotsam7018]

I assume you've already verified the 10% DCR is exceeded per IEBC 807.5. Of course, for old structures it's always fun to try and justify the final statement in that exception accounting for all additions/alterations since original construction .

I'd don't think I'd be anchoring a strong-wall to a brick wall given the high hold down forces.