the aftermath of California's recent Loma Prieta and Northridge
earthquakes, there is a heightened awareness of the need to strengthen
existing wood- framed residences. Insurance companies are enforcing
stricter guidelines to qualify for earthquake insurance, as well
as requiring seismic retrofits before issuing renewal policies.
Many cities and counties in California have also adopted new ordinances
in their building codes to address seismic safety.
the 1989 Lorna Prieta quake, the average cost to repair houses
bucked off their foundations was $25,000 to $35,000. Many homes
were so badly damaged that they were written off completely
had to be demolished. By contrast, the average cost to provide
sill bolts and cripple wall bracing to an undamaged house ranges
$1,500 to $2,500. The cost-effectiveness of correcting these deficiencies
city of Santa Barbara, Calif., where my company does most of its
work, has a voluntary residential seismic retrofit program that
simplifies the permit process for homeowners. The city offers standard
pre-approved plans and details that help the home- owner save on
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 un-reinforced 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.)
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 hardware.
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 any good.
homes in our area have cripple walls-short stud-framed walls
sit on the stem wall foundation and support the floor framing above.
Strong cripple walls are critical for earthquake resistance.
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
floor framing above. I inspect the cripple walls for rot, and make
sure they are framed properly to receive shear wall panels.
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 hardware.
1 .Where there is good access, the least expensive retro- fit
involves threaded rod anchors epoxied in place, plywood shear
paneling, and framing anchors between the top of the cripple
wall and the floor framing.
I have defined the problem, the next step is to choose the proper
metal connectors to tie the foundation to the mudsill or cripple
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/s-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 (Figure
1). This is the least expensive solution.
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/s-inch-diameter holes. The dispenser costs over
$100, but can be rented for around $5 a day.
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 blowout the residue with compressed air.
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.
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.
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 l/z-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 nailer.
next step is to drill 3-inch holes into the ply-wood 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 cloth.
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
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.
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.
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, an L-shaped 12-gauge steel connector that
rests on top of the plate and laps over the side of the stem wall
(Figure 2). 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
FA anchors aren't cheap; they cost around $6 to $8 each, so we try
to limit the number we use. Plus, they require us to drill two holes,
whereas dropping in threaded rod requires only one hole. But in
tight quarters, there's no better choice.
a New Stem Wall
3. In cases where the old foundation is too weak to hold anchors,
it's necessary to pour a new wall. In the case shown here, a
new stepped stem wall was poured tight to the existing one,
with new cripple walls built on top. Simpson A3SF clips secure
the cripple wall to solid blocking between the floor joists
above (see photo}.
a New Foundation
sometimes the foundation isn't stable enough to hold bolts, so we
have to pour a new concrete stem wall. A recent retrofit job illustrates
the point. When I first visited this residence, I was pleased to
find two rare conditions for a retrofit -a very comfortable clearance
and a lighted crawlspace. The owner wanted recommendations for strengthening
the sub floor framing and a labor-and-materials bid for bolting
his cripple walls to the existing concrete foundation.
particular house sat on a sloped lot, but instead of stepping
foundation, the builder simply sloped the stem wall to follow the
grade-making it much more vulnerable to slipping in a quake.
cripple walls were inadequately bolted and un-braced. Even worse,
the foundation concrete had been mixed with unwashed aggregate
beach sand and was in very poor condition. It was obvious the existing
concrete would not hold an anchor bolt, let alone hold up under
alternative was to raise the house on large steel I-beams, remove
the existing foundation, then form and pour a new one - an expensive
proposition. But once we determined that the existing floor was
level, we decided instead to pour a new stepped foundation tight
to the inside of the existing one (Figure 3).
first excavated for a new footing, slightly "undermining"
the existing footing. Then we drilled and placed #4 rebar dowels
in the old stem wall on 4 foot centers, In the new footing trench,
we installed continuous #4 rebar around the perimeter, and attached
it to the dowels with short vertical pieces of rebar.
pouring the new foundation, we plated and framed new cripple walls
around the perimeter. These were bolted to the new foundation with
standard S/s-inch j-bolts. To connect the floor to the new cripple
wall, we blocked between the existing floor joists directly over
the cripple wall and used Simpson A35F metal brackets to connect
the blocking to the double top plates. We covered the cripple wall
with l/z-inch structural-l plywood edge-nailed 6 inches on-center,
with 12-inch centers in the field.
4. New post-and-pier supports are built in place using precast
pier blocks set in a 2-foot-square concrete footing. Note how
the original post, at right in the photo, has no mechanical
of the houses we work on have a center girder supported by a series
of posts and piers. The posts are typically resting on the piers
with no mechanical anchor and are attached to the girders above
with nails only. In that case, we install new 2-foot-square by I-foot-deep
pier footings between existing posts. We also replace any posts
located under girder splices. To form the new pier footings, we
lay a grid of four
#4 rebar pieces beneath a precast block that comes with embedded
straps for attaching to the post (Figure 4). After pouring the new
piers, we install new posts, strapped at the bottom and connected
at the top with a plywood gusset. Finally, we use Simpson H- series
clips to tie the floor joists to the girders.
Scoggins is a field representative for Allen Associates.