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
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
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
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
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,
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
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
It takes a lot of staples, but they won't split the
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
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.Howard Cookowns Bay Area Retrofit in Berkeley,