The Pacific coast of the United States is riddled with
earthquake faults; sooner or later one of them will rupture
with an intensity far greater than was experienced in the 1994
Northridge earthquake. That event caused more than $40 billion
of property damage, half of which was due to failures of
The San Francisco Bay area, where I live, is threatened by a
number of faults, including the Hayward Fault. The Association
of Bay Area Governments predicts that a rupture of the Hayward
Fault — with an expected magnitude of 7.2 or greater
— will render 150,000 housing units uninhabitable. Other
population centers on the West Coast face similar
Earthquake retrofitting is the science of attaching a house to
its foundation so that when it is violently shaken it remains
standing. I started my seismic retrofit business in 1993 after
a couple of stints doing residential earthquake damage
assessments as a contract worker for FEMA.
The first was after the 1989 Loma Prieta quake, which caused 67
deaths and some $7 billion worth of damage in the San Francisco
Bay area. The second was in 1992; I did assessments following
the Ferndale earthquake on the far north coast of the
The Greatest Risk
The most interesting thing I learned doing inspections was how
little shaking it takes to destroy or seriously damage a house
that is poorly attached to the foundation, especially when it
has cripple walls. In many cases, homes fell off their
foundations and were damaged so badly they were uninhabitable.
Houses that stayed on their foundations typically suffered
little more than plaster and drywall cracks.
Cripple walls. Until about 1950, it was the style to build the
first floor of West Coast homes a few feet off the foundation
by supporting them with short studwalls called cripple walls.
Nearly all the houses in the town where I live were built this
way. It's easy to tell: If there are more than two or three
steps up to the entry door, the house probably has cripple
walls. As originally built — with no plywood sheathing
— older cripple walls have very little lateral strength,
so a good shake can knock them down (see Figure 1).
Figure 1.Any house that isn't properly attached to
its foundation can slide off in an earthquake. The problem is
even worse when the house is built on cripple walls; it doesn't
take much shaking to make cripple walls collapse.
If you look at photos of houses where the cripple walls
collapsed, you'll see that frequently the house itself remains
more or less intact. You might think it would take no great
effort to lift a relatively intact house back onto the
foundation. But when a house falls, it tears out the plumbing
and gas connections, damages the wiring, racks walls out of
square, and causes drywall and plaster to loosen or fall off.
I've been in many homes where the fall caused cast-iron drain
lines to punch up through the building, lifting toilets a
couple of feet off the floor.
It's very expensive to repair a fallen house. Frequently it
costs nearly as much as building new (Figure 2).
The building at top slid off its
foundation during a magnitude-6.5 earthquake in 2004. The house
itself is in pretty good shape and would have likely survived
unscathed if the cripple walls and foundation attachments had
been reinforced. The cripple walls under the house above
collapsed during the Northridge earthquake, causing serious
structural damage. With damage this severe, it's usually
cheaper to tear the house down than to repair it.
Fixing the Weakest Link
In many houses, cripple walls and poor foundation connections
are the weakest structural links; fix those deficiencies and
the house has an excellent chance of surviving even a major
earthquake. This doesn't mean the house won't benefit from
strengthening the walls above a cripple wall — but the
expense is high relative to the benefit.
A good seismic retrofit gives customers a lot of bang for the
buck because the work is concentrated on the weakest part of
the building — where the foundation is attached to the
The average retrofit costs $6,000, an amount many homeowners
are willing to spend to protect their No. 1 investment. We have
been retrofitting two homes a week in the Bay area for the past
11 years and always have a backlog of a few months' work.
Building code. The work we do
requires a building permit, but surprisingly, the building code
offers little guidance on the proper way to retrofit a
The topic is addressed by a single long sentence in Section
3403.2 of the Uniform Building Code. A building official once
described the intent of the sentence as the "do no harm"
policy, but admitted that in practice it sometimes becomes a
"do no good" policy instead.
Currently, stamped engineered drawings are required if you
apply for a permit and call the job a seismic retrofit. This
nearly doubles the cost of a simple retrofit and discourages
many homeowners from having the work done.
Many contractors get around this by not calling the work a
seismic retrofit on the permit application. On these jobs, the
homeowners often mistakenly think that the building department
is checking to make sure the work will resist earthquakes. But
in fact, the inspectors are checking only to see that the
plywood and framing hardware are installed to code, not that
they are installed in the quantity, size, and location required
to resist earthquake forces.
Consequently, many of these homes have only partial — or
no — protection from seismic forces.
To address problems in the code and make it more affordable to
do the work, the city of Berkeley is currently developing the
country's first comprehensive seismic retrofit building code.
It will be prescriptive, so if the project is simple, there
will be no need to hire an engineer.
The Goal of Retrofitting
A single-story 1,000-square-foot house weighs about 50,000
pounds. When that much weight rocks back and forth on top of
cripple walls, the cripple walls readily collapse.
Our goal in retrofitting a house is to connect it firmly to the
foundation and stiffen the cripple walls by turning all or part
of them into shear walls. That way, when an earthquake occurs,
the lateral forces are transferred through the shear walls into
Shear walls. In a wood-framed house,
a shear wall is a studwall that's connected to the foundation
with anchor bolts, sheathed with plywood fastened in a tight
nailing pattern, and tied to the floor above with shear
transfer ties. All three components — foundation bolts,
plywood sheathing, and shear transfer ties — must be
there for the retrofit to work. If overturning forces are
expected — or if the shear wall is tall relative to its
width — hold-downs are added to this list.
The mudsill and bottom plate can be bolted to the foundation,
but if the cripple walls aren't sheathed with plywood, the
walls can fail. The bolts and plywood can both be in place, but
if the floor isn't properly connected to the wall with shear
transfer ties, it may slide sideways in a large earthquake
Reinforcing Cripple Walls
Figure 3.A house with cripple walls must be
reinforced at three distinct weak spots below the floor: The
floor must be tied to the cripple walls, the cripple walls must
be stiffened with plywood and tied to the mudsill, and the
mudsill must be bolted to the foundation.
How Strong Is Strong Enough?
The big questions are always "How many bolts, how much plywood,
and how many shear transfer ties?" To determine the answers,
it's necessary to use a simple formula called the base shear
formula, which is V = 0.185 x W.
"V" represents the shear force (measured in pounds) at the base
of a building. The value "0.185" is the anticipated force of
ground acceleration from a major earthquake in a particular
geographic area. It's based on proximity to known earthquake
faults. You can find out the value for a particular location by
calling a structural engineer.
"W" represents the weight of the building. Single-story homes
weigh about 50 pounds per square foot. Two-story homes weigh
about 80 pounds per square foot of first-floor area.
Designing a Solution
Here's an example of how to design a retrofit for a house with
cripple walls. Let's say you're dealing with a one-story house
that's 35 feet by 40 feet, or 1,400 square feet. If we multiply
the area by 50 pounds per square foot, we find the building
weighs 70,000 pounds. Plugging that number into the base shear
formula tells us how much shear force is expected to strike the
building at its base:
V = base shear force (horizontal to the base of the
V = 0.185 x weight of house
V = 0.185 x 70,000 pounds
V = 12,950 pounds
To simplify, let's round the number up to 13,000 pounds. A
properly designed retrofit for this home must resist at least
13,000 pounds of lateral force at each of the following places:
where it sits on the foundation, against cripple walls, and
where the floor connects to the cripple walls (Figure 4).
Shear Force at The Base of a Building
Figure 4.The heavier the house, the greater the
lateral forces that must be transferred to the foundation in an
earthquake. Here, forces are shown from only two directions,
but they can come from any direction, so the cripple walls and
foundation must be equally reinforced on all
Overturning. In addition to resisting
lateral shear forces, reinforced cripple walls must resist
uplift forces at their ends. Engineers refer to this as
overturning, the tendency of tall objects to tip over when you
push against them. In an earthquake (or high wind), the house
is pushed back and forth, which pulls up on the ends of shear
walls. This is why hold-downs are typically installed at each
end of a shear wall — to keep it from "overturning"
Figure 5.Now that hold-downs are installed, this
wall will be sheathed with plywood and turned into a shear
wall. The hardware is necessary where a wall is tall enough
that overturning forces could result in uplift. It isn't needed
on shorter cripple walls.
The APA/Engineered Wood Association has tested various
shear-wall assemblies and rated them for the amount of lateral
force they can resist per lineal foot (plf) of length. These
tests form the basis for shear-wall design in U.S. and Canadian
building codes. A nominal 1/2-inch plywood shear wall
edge-nailed 3 inches on-center with 8d nails has a shear rating
of approximately 500 pounds plf.
To calculate the amount of uplift, you multiply the height of
the wall (in feet) by the shear rating of the assembly (in
pounds plf). The uplift on the end of a wall 8 feet high and
rated for 500 pounds of shear will be 4,000 pounds (8 feet x
500 pounds plf).
Calculating uplift gets more complicated, however, because
hold-downs must be sized to resist the "net uplift force,"
which is uplift less the weight bearing down on the shear wall
from above. Fortunately, overturning is rarely a major issue
for cripple walls, because they are usually short and tend to
slide rather than overturn.
Sliding of the cripple walls, as discussed earlier, can be
prevented by installing anchor bolts.
What to Install
Based on our calculations, the house in our example will be
attacked by 13,000 pounds of earthquake force in each
direction. Retrofitting it will require the following
Anchor bolts. To strengthen the
connection between mudsill and foundation, we install anchor
bolts (Figure 6). If a bolt is rated to resist 1,000 pounds of
shear (its shear value), then we know it will take 13 bolts to
protect the house in the north-south direction and 13 to
protect it east-west. The bolts should be divided between
opposing sides. Since you can't install half a bolt, we would
round up to 14 and install seven bolts on each side of the
Figure 6.The author's crew typically adds anchor
bolts to prevent mudsills from sliding horizontally in an
earthquake. Bolts perform better when they are installed with
2-inch-by-2-inch washer plates.
Shear transfer ties. The same method
is used to calculate the number of shear transfer ties needed
to attach joists to the top plate of the cripple wall. We
typically use Simpson H10R ties for this connection (Figure 7).
Their shear value is shown in the column labeled 1.33/1.60 in
the Simpson Strong-Tie catalog
(www.strongtie.com). This particular column
refers to short-duration lateral loads caused by wind and
earthquakes. The H10R ties resist about 500 pounds of lateral
force, so we would need 26 in each direction, or 13 per
Figure 7.When this house was built, the joists
were simply toenailed to the top plate. As part of the
retrofit, the crew fastened the joists with shear transfer ties
— H10Rs — and the locking between them with
Plywood bracing. To brace the cripple walls, we typically
sheath them with plywood from inside the crawlspace (Figure 8).
If the shear-wall configuration (plywood and nailing pattern)
is rated to resist 500 pounds plf, then we need 26 lineal feet
of plywood on the east-west and north-south walls. Since this
bracing is divided between opposing walls, each side gets 13
feet of plywood.
Figure 8.With the anchor bolts and framing
hardware installed, this carpenter finishes the job by
sheathing the cripple wall with plywood. For best results, use
at least 15/32 Structural 1-grade 5-ply plywood.
If the calculations showed net uplift from the overturning
force, we would install a hold-down at each end of all shear
walls. The strength of the hold-down is rarely an issue. The
weak link is usually the old unreinforced concrete foundation;
if the uplift force is large enough (more than 2,000 pounds) it
may pull out the retrofit hold-down bolt, rendering it useless.
One solution — an expensive one — is to shore up
the building and replace that section of foundation, but it's
often simpler to pour a new footing beside the old one and
install the hold-downs in it (Figure 9).
Figure 9.The foundation under this shear wall can
support the vertical load but is too weak and shallow to accept
hold-downs. Instead of replacing it, the author poured a new
reinforced footing alongside the existing one. To tie into the
new footing, the hold-downs and anchor bolts are on the
interior face of the wall and attached through a "reversed"
mudsill. The crew nailed the plywood to the back of the mudsill
before standing it up and nailing it to the studs.
Anchor-bolt spacing. It's a common mistake to space new anchor
bolts evenly around the building. That approach is fine when
joists land on the mudsill, but when you turn a cripple wall
into a shear wall, those bolts need to be installed where the
plywood panels are. We install them every 2 feet at shear-wall
locations. The shear transfer ties can be anywhere along the
wall; we install them near the plywood because we're already