Adding Window to a Residential Wood Framed Wall

[dreber]

I'm really surprised to not find this topic in the eng-tips archive, but perhaps my google foo is lacking.

This is a super basic question that I'm sure comes up all the time, but I would like to know what your general approach is.

I have an existing two story wood framed structure with an exterior plywood wall. The client would like to penetrate this wall with a new window.

I propose to assume this wall is a shear wall, calculate the required wind/seismic shear load capacity, and then design the wall, as if it were new, using the "force transfer around opening" method. providing blocking and strapping above and below the window as required.

This would presumably avoid the need to add holdowns at the king studs.

The problem I can forsee is assuming the shear capacity of the existing ply on the "piers" to the left and right of the proposed window. It seems that no matter what I did, the shear demand on these piers would be increased. Assuming I'm unable to determine the sheathing and nailing before hand, what should I use as an allowable shear value? Clearly anything under 100 plf [ASD] would be OK, but if it were 250plf [ASD], what would you do?

Attached is a typ detail I plan to use for inspiration.

 

[NorthCivil]

I've seen details like this before and still can't wrap my head around them. what exactly are those straps doing? nothing I would wager.

If the client wants to blow a big hole through the shearwall, he has to do it right. its not that much harder to strip the cladding/interior gypsum, and add hold-downs. much better end product than this wacky detail.

Or, analyze the shearwall and see if you can get away without hold-downs. if not, hold downs it is.

 

[racookpe1978]

But you can define the sheathing around the window: Instead of particle board/foam rubber/plastic used in new construction, just specify 5/8 plywood (or whatever thickness you need). Granted, during the time that the reconstruction is going on, you'll lose strength until the sheathing is back on and nailed, but you could avoid even that: Strip the old siding off (which you have to do before you cut the window out), then add the plywood sheathing, then cut the window holes out.

Upper floor or lower floor will be mod'ed?

 

[XR250]

Where are you located? Is it a high wind or seismic area?
It is rare that I would worry about one 4 ft. hole in the side of house where I practice (central NC).

 

[Ron]

dreber....I would be more concerned about how you will interface flashing for proper waterproofing the wall around the window than the minor structural issues. Depending on the cladding, that will be much more difficult than adding structure.

 

[BridgeSmith]

I don't remember much from my timber design class, but I do remember that plywood and OSB are both very strong shear diaphragms, to the point that the in-plane shear capacity is controlled by the shear capacity of the nails or screws attaching the panels to the framing at the joints. If you needed more shear capacity, you'd tighten up the spacing of the screws.

You're not the first person to put a window into the bottom floor of a 2-story house. I'll bet there's some standard detailing for your situation.

 

[dianium500]

Take a look at the APA guidelines for shear diaphragm designs. The documents they supply explain exactly how to design around a window opening. I have never seen straps used in the way you are using them in your detail. I work in a high wind area, and we install the straps vertically. There are numerous documents detailing the shear capacity of a frame wall with 1/2" plywood and all sorts nail spacing in the APA documents. They also indicate spacing for unblocked diaphragms, blocked diaphragms...plywood installed vertically, you name it. Also note that the minimum spacing for nails per APA is 6" o.c. If you take the capacity for the minimum spacing and lowest nail size, and the design works, just indicate on the drawings the required conditions to be satisfied.

 

[KootK]

- How large is your opening? What percentage of the affected solid wall do you lose? What percentage of the entire wall line do you lose? As others have mentioned, my first stop would be doing everything in my power to justify not messing with the wall laterally.

- The horizontal blocking and strapping makes sense to me if you're doing FTAO. Textbook. Trouble is, as I see it, that you're going to need to fasten the exterior sheathing to the new blocking. I'm not sure that's possible. Other shear wall analysis methods like segmented and perforated may be friendlier in this regard.

- Most roads here will lead to some degree of shear capacity loss. The only approach that might not is if you would improve the fastening or blocking condition of the remaining walls to compensate. But, then, you'd be stuck having to install exterior side sheathing fasteners again.

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.

 

[dreber]

Sorry for the slow reply.

NorthCivil, The horizontal straps are there to adequately resolve loads from the "shear transfer around opening", which is elaborated in this link:

http://www.awc.org/pdf/education/des/AWC-DES415-ForceTransferAroundOpenings-170817.pdf

Stripping the cladding and adding interior hold downs is not a bad approach. However, new hold downs into old concrete must be designed for overstrength, and due to the frequently narrow edge distance between the epoxy hole of the hold-down and the edge of the existing concrete, you can't get very much tension capacity out of the existing concrete unless you take really insane measures (digging under the existing footing and passing the hold down rod through the existing foundation and into new concrete beyond).

Also, if you epoxy into concrete in the Bay Area, the building department will demand special inspection, which is an extra pain for the client.

racookpe1978, I'm assuming the upper floor will be left alone for now.

XR250, I'm in the Bay Area of California and the wall will be seismically controlled.

HotRod10, yeah, I was surprised that I couldn't find any existing info on google or eng-tips about this situation, hence my post.

KootK, I'm assuming about a 3ft wide window in an 8ft long shear wall. Another way to look at it is that we're loosing 38% of shear wall along the entire line. would lead to a 60% increase in shear forces in the remaining piers

I've had a bit of a brainstorm since posting this. How about I remove the interior finishes, saw out the existing exterior ply, install blocking, sisters, and framing for the window as required. Then I add new plywood sheathing to the _interior_ of the wall to make up for any lost shear capacity at the piers. Then, nail the new interior ply, strap, and install the window. That way, I can verify the existing holdowns, add new ones if the calcs demand it. This way, I have full engineering control over the sheathing and nailing that I am relying on.

Thanks for your thoughts!

 

[BridgeSmith]

"Also, if you epoxy into concrete in the Bay Area, the building department will demand special inspection, which is an extra pain for the client."

Our experience with the typical poor installation practices of adhesive anchorage systems justifies their scrutiny. We had a bridge with uplift at the abutments where we specified cast-in hold-down anchor bolts but the contractor conveniently "forgot" to cast them in. They used an adhesive anchor system instead, but in typical fashion, didn't mix the components thoroughly and didn't clean the holes. When they set the center girder piece, the hold-downs popped out and the ends of the girders popped up and it made for one heck of a mess. Now we detail u-bolts to make it impossible to epoxy them in later.

 

[hokie66]

U-bolts are good, but contractors can "forget" them as well.

 

[BridgeSmith]

At least the change from U bolts to straight ones is more likely to generate a change order, or at least a call to us from our field personnel. So far, so good; we haven't had a contractor "forget" the U-bolts yet.

 

[JoshPlumSE]

Dreber -

I think you've got a pretty good grasp on what's needed. Certainly a better understanding of FTAO than some who've responded. I think your general approach is right on.

Assuming I'm unable to determine the sheathing and nailing before hand, what should I use as an allowable shear value?

This is the only thing you've said that troubles me. I don't think you can get avoid having do determine the sheathing and nailing. I would think some selective demo in the area where the opening will be installed will give you most of the information you need to know. All you really need to do is remove exterior stucco or cladding in that area to better see the sheathing and nailing. Remove / patch a small piece and you've got the sheathing and the nail size, and spacing at field and edge. You should also know if it's blocked or not.

 

[KootK]

However, new hold downs into old concrete must be designed for overstrength

I didn't know this. Can you point me to the requirement? This would kill off a lot of the systems that I encounter commonly if it applies where I do work. In particular, the ubiquitous wall hold down connections at the edges of precast decks.

I've had a bit of a brainstorm since posting this.

Yeah, that sounds like a great approach.

The APA guides were mentioned previously. They've got an excellent one on FTAO (attached) that came out in January. There's also a nifty analysis spreadsheet available fer free.



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.

 

[dreber]

KootK, ACI 318-14 § 17.2.4.3

Anchors and their attachments shall satisfy one of options (a) through (d):

....

(d) The anchor or group of anchors shall be designed for
the maximum tension obtained from design load combina-
tions that include E, with E increased by Ωo. The anchor
design tensile strength shall satisfy the tensile strength
requirements of 172.344.​

You can of course satisfy one of the other requirements, but the sure fire way to know your anchor is adequate is to design it for overstrength load.