Q: I have a den above a garage in a soft-story house. I am considering a floor-to-ceiling bookcase and a baby grand piano. The piano will weigh 800 to 1,000 pounds. As for a wall of books, I have no idea how much that would weigh. Do you see the need for some kind of reinforcement so it won’t sag the floor, etc.?
A: Putting that much extra weight in the den will stress the framing below, especially on a soft-story structure, which has ground-floor space located where a wall might otherwise be. This makes the building’s frame prone to twisting and buckling in an earthquake.
Beefing up the floor joists is a must. You should also install shear panels on the garage walls to provide additional lateral support in the event of an earthquake.
The piano and the books will make the garage section even more top-heavy. Installing shear walls on these walls will give it the best chance to survive a tremor of moderate to severe magnitude.
Your first step is to head down to the local building authority with a plan and secure a permit. Ask the inspectors about joist size and don’t be surprised if they require some engineering. It is basically a seismic retrofit, and buying an engineer’s report is money well spent.
Identify the joists’ thickness, the spacing between them and the length they run without intermediate support in the garage. Check a joist span table to determine the width of joists you might need given the span and the distance between joists. Given the added load of books and piano, we would build beefier — either with a shorter span or a thicker joist. Span charts can be found in the Uniform Building Code, in any good construction manual or in many places on the Internet.
An alternative is to double up the size of the joists by "sintering" new wood onto the old joists, essentially making 2-by-4s into 4-by-4s. From a retrofit standpoint, it is the best alternative because the new material will fit onto the top plate of the garage, forming a 90-degree angle, allowing the studs to be attached to the joists as part of the shear wall.
Installing shear panels is more than just nailing some plywood to the studs. The goal is to make the foundation, the mudsill, the wall and the floor joists into a monolithic unit by tying these components together. It is accomplished with specialized metal fasteners and a skin of plywood or oriented strand board. …CONTINUED
The mudsill must be bolted to the raised concrete foundation, studs must be nailed to the mudsill and floor joists must be secured to the top plate of the stud wall. The whole provides significant structural defense against earthquakes. Specialized fasteners are readily and economically available.
Simpson Strong-Tie makes these connectors and also provides an excellent Seismic Retrofit Guide. It’s well worth the look.
If the mudsill is not bolted to the foundation, that’s where to begin. Foundation bolts are required to be 5/8 inches in diameter and located 12 inches from each corner of the house or break in the mudsill and no more than 6 feet apart. If the foundation is bolted but you’re short on bolts, Simpson makes a Universal Foundation Plate to connect the stem wall of the raised concrete foundation to the mudsill.
You must rent a hammer drill equipped with a carbide drill bit to drill the holes in the concrete foundation to receive the anchor bolts. Hopefully, the mudsill is the same width as the studs of the cripple wall. If it’s not, it’s just a bit more work. If a 2-by-4 wall sits on a 2-by-6 mudsill, nail 2-by-4-inch blocking on top of the mudsill between the cripple studs. This allows the shear panel to be nailed on all four edges. Use four ten-penny (10d) nails to secure the blocking to the mudsill.
The next step is to secure the floor joists to the top plate of the cripple wall. A 90-degree metal strap is available, again through Simpson, for this purpose. The brackets are nailed to the rim joist and then to the perpendicular joists.
It can be pretty close quarters getting these brackets nailed in place, so we’d suggest you rent a compressor and a palm nailer to make short work of the job.
With the mudsill secured to the foundation and the stud wall secured to the rim joists, install the plywood sheathing. Use 1/2-inch plywood or oriented strand board. Install the panels vertically. The edges of the shear panels should be nailed to the top plate of the stud wall, the mudsill or block and to the studs with 8d nails 4 inches on center. In addition, nail the panels to the studs in the field every 12 inches. Make sure the edge of each panel begins on a stud and ends on a stud so the panels are fully nailed.
Once the panels are nailed off, use a hole saw attached to a drill motor to drill a 3-inch-diameter hole top and bottom through the sheathing at each stud bay for ventilation.
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