Submitted by BillB on no date available
Adding Shear Strength to a Wall
A small, dark bedroom was crying for a big window and I answered. The resulting hole left a need for some strengthening measures.
When the house was built, some degree of shear strength was achieved with a 1 x 8 board set into the studs at about a 50 degree angle. I cut this brace out, along with about a half dozen studs when creating the rough opening for the new windows, dramatically weakening the wall. Even a solid wall needs modern shear bracing, let alone one with a huge hole for a window cut in the middle of it.
I won't go into how to frame the new opening or how to add framing to close the old opening, that's pretty basic stuff.
Brackets, Blocking, Straps and Panels
The diagram shows the 4 measures I took to add shear strength to the wall
- Angle brackets on the framing around the window openings
- Three rows of blocking
- Plywood Shear Panel (Only 1 of four sections shown at lower left)
The angle brackets were Simpson A44s at the corners and an AC to hold the vertical center 4x4 post to the window header. Simpson is a company that makes a huge selection of wood construction connectors.
Rows of blocking transfer lateral stress from above and below the window to the sides of the wall. Therefore, two rows of blocking are needed, one in line with the window header and one at the sill height of the window. I used 2x8 blocks for this and held them in place using Simpson A35 angle brackets. I added a third row in the middle of the wall, 4x4's this time, to reduce twisting of the studs and provide a nailing surface for the shear panels, which I'll explain in the paragraph on the panels. In case you're wondering, yes, all this blocking takes up insulation space, but that's unavoidable.
The second layer of strength is provided by straps, Simpson WB coiled wall bracing, nailed across the full length of the wall along the top and bottom blocking runs. I used 8d nails for this. Before installing the strap, I routed a 1/8" deep 1 1/4" wide slot in all the framing members involved to accomodate the thickness of the strap and the nail heads holding it. This way I had a flat surface over which to install the paneling and avoided any buldge that otherwise might have been visible in the finished wall surface.
The last strength layer, and probably the easiest to apply and the most effective, is a layer of 1/2" plywood over the whole wall surface. Use plywood rated for structural use and use common nails with full size heads. The spacing of the nails depends on where you're nailing. Along the edges of the panels, spacing should be 6" and in what's called the "field" of the panel, spacing is 12". The nails along the panel edges should be kept 3/8" from the edge. This last requirement dictated the row of 4x4 blocking at mid-height on my wall, as this is where the panel edges met, (4x8 sheets meet midway up an 8 foot high wall), and I needed the nailing surface of the 4x4's to keep the nails 3/8" from the panel edge. Check with your local inspector for local requirements concerning both nail spacing and panel thickness and type. One town near where I live requires 3/4" structural plywood and 3" nail spacing along panel edges with pre-drilled holes for the nails. You may need to submit a plan certified by an engineer to your building department before they will issue a permit for this work. If so, ask your local inspector for a reference or use the phone book and get some estimates.
Any holes cut in the plywood panels for electrical boxes should have rounded corners and smooth, clean edges. Force is more apt to be transferred around a rounded, clean hole. A square hole, especially one with cuts extending too far, will be a weak point.
Where I live there are no codes for adding shear strength to a wall and I think my inspector thought I was a bit nuts to do all this, but I was interested in the topic. There's a book called, "Wood-Framed Shear Wall Construction", by Thor Matteson, that I'd recommend if you're interested. Google his name and you'll find many builder's book sources.
The completed job from the inside. BTW, notice the bamboo shades; by pure luck, 2 of them covered perfectly. I priced a single bamboo shade to cover both windows and it came to $700. The 2 shades I got, at Target, were about $50 each. I bought some duck cloth, cut it to match the shade size and tacked it to the back of the shade for more privacy. So, if you can plan ahead, consider the window treatments and trim before deciding on a window size and beginning work.
Notice the glaring omission of hold downs. A true shear wall needs to be bolted to the foundation. Very briefly : With a good rotary hammer and 1" bit, (or whatever diameter is required by your building codes), drill a hole a good 12" or 18", (again, check your codes), through the bottom plate and into the foundation. Epoxy threaded rod into the hole to which you attach your hold down, (Simpson PDH, Predeflected Holddown), which is in turn attached to a 4x4 stud. The 4x4 studs are spaced per your local codes, probably 4 feet. You don't need to shear an entire house or wall. They're usually strategically placed, say at the corners of the house or every 20 feet or so. Again, this is a job for an engineer to figure out. I've done this at a friend's house and drilling that deep into concrete is no small task. Each hole will take a good 15 minutes, maybe a half hour if you hit rebar, which you will. The drill will cost you $500.00 but you can rent them and the bit might cost you $75.00.