When I inspect a house for a seismic upgrade, I start with
the foundation. The foundations I see range from poorly
mortared sandstone and brick to unreinforced block or poured
concrete. In some cases, there is no foundation at all —
just a series of wood posts resting on masonry pads set on the
ground. (In such cases, it’s just a matter of time before
the whole house is sitting on the ground.)
When there is a foundation, I check the strength of the
concrete, block, brick, or stone by drilling a test hole or
chipping at the surface with a steel bar. I have to find out if
the masonry is hard enough to hold a bolt or other
Working up, I next explore the condition and placement of
existing hold-down bolts attaching the mudsill to the
foundation. I also look for rot and termite damage to wood
members. In many cases, there are no sill anchors at all; in
other cases, the mudsill is too rotten for the anchors to do
Many homes in our area have cripple walls — short
stud-framed walls that sit on the stemwall foundation and
support the floor framing above. Strong cripple walls are
critical for earthquake resistance. Not only must they be
anchored to the foundation and secured to the framing above,
but they must also receive shear panel sheathing to handle the
severe lateral forces that a quake causes. At the top, the
cripple wall has to be secured with hardware clips to the floor
framing above. I inspect the cripple walls for rot, and make
sure they are framed properly to receive shearwall panels.
Probably the most common condition I run into, however, is
an unsecured mudsill sitting on a foundation with the floor
joists resting directly on it. Such low-clearance situations
make retrofit work difficult and typically require special
The Basic Retrofit
Once I have defined the problem, the next step is to choose the
proper metal connectors to tie the foundation to the mudsill or
cripple wall plate.
Every retrofit job is different. Let’s look at a basic
job first — a firm masonry foundation with a sound
cripple wall, having insufficient anchors but plenty of room to
work. For areas that can be easily reached by an impact drill,
we typically drill 5/8-inch holes through the plates and about
41/2 inches into the foundation. We then insert lengths of
1/2-inch threaded steel rod, epoxied into place (see Figure 1).
This is the least expensive solution.
1. Where there is good access, the least expensive
retrofit involves threaded rod anchors epoxied in place,
plywood shear paneling, and framing anchors between the top of
the cripple wall and the floor framing.
Advantages of epoxy.
Because most of the foundations we see are made of old
concrete, we prefer the epoxy anchoring system to expansion
bolts, which might crack the stem wall as they expand. We use
Simpson’s Epoxy-Tie Adhesive Anchor system (Simpson
Strong-Tie, 4637 Chabot Dr., Suite 200, Pleasanton, CA 94588;
800/999-5099). The epoxy costs $22 for fifteen
5/8-inch-diameter holes. The dispenser costs over $100, but can
be rented for around $5 a day.
Before epoxying either threaded-rod or the Simpson anchors,
it is important to clean out the predrilled holes. We scrub the
hole with a nylon brush and then blow out the residue with
compressed air. The hole must be perfectly clean: Any dust will
reduce the epoxy’s bond. It’s also important to
monitor the mixing of the two-part epoxy. A consistent gray
color indicates a proper mix, which means the epoxy will cure
to full strength.
After filling the hole half full with epoxy, we insert the
anchor and slowly turn it until it contacts the bottom of the
hole. We wait a full day until the epoxy cures before
disturbing the anchor.
Once the epoxied anchors are in place and the sill is bolted
down, we turn our attention to the cripple walls. Here, we
follow the guidelines provided by the City of Santa Barbara,
which recommend that 4-foot-wide 1/2-inch structural sheathing
be installed at corners and every 25 feet along the
house’s length. Shear panels are also installed at the
sides of any access doors or vent openings. The city also
recommends that the length of each shear panel should be at
least twice its height. The required fastening schedule is 8d
nails 6 inches on-center on the edges and 12 inches on-center
in the field. This work goes quickly with a pneumatic
The next step is to drill 3-inch holes into the plywood at
the top and bottom of each stud bay to allow for ventilation.
To keep rodents out, we cover the holes with 1/4-inch hardware
Finally, at the top of the cripple wall, we usually install
Simpson H1s or H5s to every other joist to secure the floor
framing to the cripple wall plate.
Limited access — when an unbolted sill sits right on top
of the stem wall in a tight crawlspace, for instance —
usually dictates the use of more expensive connectors.
Occasionally, we get lucky and find that the floor is framed
with 2x12 joists. This gives us enough room to use our Hilti
TE15 right-angle hammer drill in the restricted area between
floor and plate. Then we can drop in an epoxied threaded-rod
anchor or a Simpson RFB retrofit bolt. More often, however, the
joists are smaller, so we have to use a Simpson FA6 or FA8,
L-shaped 12-gauge steel connector that rests on top of the
plate and laps over the side of the stem wall (Figure 2).
2. The Simpson FA series anchors are useful in tight
quarters for anchoring the mudsill to the foundation wall. The
side plate bolts to the foundation wall with epoxied threaded
rod; the top can be nailed with a palm nailer.
This configuration allows us to drill into the concrete
horizontally, instead of having to position the drill upright.
The top of the FA anchor can be nailed to the plate with a
pneumatic palm nailer.