What is a shear wall and what is it for?

Question

I have a wall in the middle of my house which I have been told is a shear wall. The image below is a diagram I put together real quick to show what it looks like. The grey lines on the right are perpendicular exterior walls. The orange-ish section on the right is actually exterior since the foundation jogs a bit there.

It doesn't seem like the 1x6 that crosses in the middle does much. I intend to put French Doors in this wall but need to know what I'm getting myself into.

The wall runs parallel to all the joists and trusses in the house and sits directly under one truss. The far left side of the wall actually has a vent running up through it directly through the truss above. When they built the house, they cut a 12" gap in the truss, which makes me not so sure if the wall is load bearing or not.

Oh, and I live in Seattle.

Answer 1

The brace is providing lateral support. If you build a wall with the top and bottom plate plus some studs, it's easy to shift the wall out of square and turn it into a parallelogram. Under load, a house would take an 8' shift to one side and flatten to the ground. With the brace, the wall remains square and holds perpendicular walls plumb.

In typical construction and low risk areas, the exterior wood sheathing and interior drywall provides this lateral support and avoids the need for any lateral bracing. However, in older homes or in higher risk areas, particularly where there are hurricanes or earthquakes, extra support may be added. In my own home, this was done with a metal T that was installed into a grove that was cut in the studs, but older homes will more likely notch the studs and install a piece of lumber.

Before removing this, I'd want to consult with a structural engineer. They may require that you relocate the brace to one or both sides of the doorway, and that will require opening up your walls even more. Removing this structure will likely show little signs of danger until there is extreme weather or a seismic event, so if you were to remove it without proper adjustments to the rest of your structure, don't assume that everything will be ok.

Note that with a truss resting directly on top of this wall, you should assume there is some load passing through, especially if there's any structure above that truss in your house. The fact that it's been cut for ducting may mean that the wall is the only thing keeping that truss from collapsing. When you create a doorway, it's best to install some temporary bracing and make the header a load bearing structure that appropriate for the span.

Answer 2

Shear walls resist lateral loads like from an earthquake or hurricane. They are important parts of the building structure, even if they do not support any of the weight of the building in the traditional "load-bearing" sense (although they might).

If that really is a shear wall, you must not modify it without the approval of an engineer or other qualified professional. You will be setting yourself up for serious problems if you dismantle the structure of your house (not to mention difficulty selling it, doing other work that require permits, getting insurance coverage). I would say that adding double-doors to that wall would constitute a serious change in the structure. There are definitely ways to do it, but taking a sawzall to the wall is not the right one.

Earthquake and hurricane design can be counter-intuitive, and actually a lot of the country is susceptible to earthquakes even though most people don't realize it. You don't need to live on the San Andreas Fault.

Often in wood structures the shear walls are made of specially spec'd and nailed plywood, so if that really is a shear wall I would expect it to be plywood. I'm not sure that single 1x6 provides much lateral support—it may just be leftover from construction or something.

Who told you it was a shear wall, and what made them think that?

Answer 3

I have worked with engineers on shear walls in florida. I have seen many examples of the amount of shear protection. I have never seen a diagonal 1x6 used in an interior wall. For example just some cross bracing would provide much better protection. A layer of plywood under drywall would be better. My point is that unless we are missing something in the picture (I guess the 1x6 could span higher and lower - but it is still a 1x6) then it shouldn't be that hard to compensate for losing it - I am in no way suggesting that you don't need it but I am suggesting that they didn't engineer that much shear strength for that wall. I would ask an engineer - "what do I need to do to provide the same shear strength as I currently have?"

I think the bigger question for the engineer is what support you need to carry your structural load across the french doors.

But before you talk to an engineer I would talk to your city building department. You need to figure out how they would approach this, what they require, and if they have any specific code for shear walls. Florida is specific for hurricane rated housing - I am not sure what code Seattle has or if it differs from hurricane rating. My first thought is Florida would have higher restriction because they lose roofs and hurricanes have a lot of lateral force... this is another question though.

Answer 4

I'm a carpenter contractor in the chicago land area they added the 1x6 in the wall because the small piece of plywood on the exterior is not wide enough to really hold the wall plum over time take out the sway and add a k brace at both side of the new opening k brace are not that strong but adding two will do the job. Mark