Seismic Retrofit For Cripple Walls - Continued

Types of Foundation Bolts

There are two kinds of bolts for retrofitting: mechanical wedge anchors and epoxy bolts. Testing has shown that they are equally effective in resisting lateral shear forces.

Wedge anchors. Wedge anchors are cheaper and easier to install, so that's what we use — unless the concrete is so porous the anchors won't grab. If that's the case, we switch to epoxy bolts. Our favorite wedge anchors are Hilti's 7-inch-long Kwik Bolts. They're simple and easy to install: Drill a hole in the concrete, hammer in the bolt, and tighten it with a wrench.

Epoxy anchors. Epoxy anchors are more complicated. The holes that they go into must be brushed clean with a bottlebrush and blown out with compressed air before the bolt can be installed (Figure 10).

Figure 10.After drilling a hole for a hold-down, a carpenter blows it clean with compressed air (top left). He knocks loose material from the sides by reaming it with a long wire brush (top right), then blows it out again. He finishes by pumping two-part epoxy into the hole (bottom right) and inserting a length of threaded rod (bottom left). Once the epoxy sets, he will bolt in the hold-down and sheathe the wall with plywood.

We use 1/2-inch all-thread for the bolt and glue it in with Hilti's HY 150 epoxy. The all-thread must penetrate at least 4 inches into the concrete — more if the concrete is weak. The HY 150 cures quickly (in about an hour), so you can tighten the nuts and install plywood right away.

Epoxy bolts resist withdrawal better than wedge anchors, which is why hold-down bolts are always epoxied in.

The building code does not permit 1/2-inch anchor bolts in new construction, but we find they are a superior product for retrofit work. They're almost as strong as 5/8-inch bolts, but they're easier to install and — since there are more of them — they distribute loads more evenly.

Whatever the bolt size, use 2-inch-by-2-inch plate washers; they greatly increase bolt performance.

Alternate fastening method. If the crawlspace is high enough, we make holes for anchor bolts by drilling down through the mudsill with a rotary hammer.

When space is tight, we make the connection from the side using Simpson UFP10 retrofit foundation plates. These plates are rated for 1,340 pounds of shear resistance and require five screws into the mudsill and two anchor bolts into the foundation (Figure 11).

Figure 11.Since there isn't clearance to install anchor bolts from above, a carpenter installs Simpson UFP10 retrofit plates instead. He screws the connector to the mudsill (top), drills sideways into the foundation through holes in the plate, hammers in the anchors, and then tightens the nuts with a wrench (bottom). The angle iron on the right, which is bolted to the foundation and joist, is from an ineffective 1980s retrofit.

Many contractors worry that old concrete is weak because it contains no reinforcing steel. The Structural Engineers Association of Northern California did some tests in 1992 and discovered that even 1,500-psi unreinforced concrete performed just fine against shear. In the tests it was always the wood that failed in shear — not the concrete.

Installing Shear Transfer Ties

We install shear transfer ties to connect the top of the cripple wall to the joists above. The ties go on before the plywood so that we can attach them directly to framing.

When the joists are perpendicular to the cripple walls, we attach them with Simpson H10Rs. When they are parallel, we use Simpson L90s. Whenever possible, we use a metal connector nailer to drive the fasteners. Since space is often tight, we frequently use palm nailers.

Prepping the Cripple Walls

There's a glitch in most model codes: When shear capacity exceeds 350 pounds plf, they require 3-inch nominal studs at panel edges. We typically sister on a second stud where panel edges land because it's easier than installing 3x4s; also, APA testing has shown that a double 2x4 is just as strong. For more on this, see Technical Topic TT-076 on the APA Web site (www.apawood.org).

Flush cutting. In most APA tests of shear walls, the sheathing is nailed onto top plates, 2x4 studs, and a continuous 2x4 bottom plate. But older cripple walls are built with 2x4 studs on a 2x6 mudsill, which is also the bottom plate.

To provide nailing for the bottom edge of the plywood, most retrofit contractors run 2x4 blocks between the studs and nail them like crazy to the mudsill. The problem is that the APA has never tested this configuration, and the short blocks tend to split when you put in a lot of nails.

Our solution is to trim the mudsill back so it's flush with the studs (Figure 12). This is hard to do with a framing saw because the studs get in the way, so we use a saw equipped with a $100 flush-cutting attachment called a FlusSa, which is available from Clemenson Enterprises Inc. (CEI, 800/333-5234, www.cei-clem.com).

Figure 12.Using a saw equipped with a flush-cutting attachment (top), a carpenter trims the mudsill back to the stud plane (bottom). This makes for a stronger shear wall because the bottom edge of plywood nails directly to the mudsill. The alternative is to nail the plywood to short blocks installed between the studs.

The company also sells a complete saw, the CloseCut, for $150.

Staples. Sometimes there is no alternative to adding blocks. For example, in the old days mudsills (which were made from redwood) were occasionally embedded in the concrete flush with the top of the foundation. Under these circumstances, we block between studs but fasten the blocks with 2 1/2-inch 15-gauge staples. Each staple will resist about 80 pounds of lateral force.

It takes a lot of staples, but they won't split the blocks.

Putting Up Plywood

To complete the shear wall, we nail plywood to selected segments of the cripple wall with 8d nails in a specific nailing pattern. It's important to use the correct plywood. The former head of the Los Angeles retrofit program said that after Northridge he saw houses in which cheap three-ply material actually tore.

For strong shear walls, use 1 5/32 Structural 1 5-ply plywood. If you install this material with 8d common nails (0.131 inch by 2 1/2 inches) 2 inches o.c. at the edges and 12 inches o.c. in the field, it has a shear rating of 600 pounds plf.