How to bolt you house?
California Government Code Section 8897 says, "... there
are approximately 1,200,000 homes in the State of
California which may not be bolted or anchored to their
foundations or do not have adequate cripple wall
bracing."
"Homes built prior to 1940 in Los Angeles were built
without being bolted to their foundations. Without
this anchorage, a large earthquake can move the ground
and the foundation right out from under the house.
Older homes were damaged or destroyed in this manner by
every major earthquake in California's history." (LA
Dept. of Building and Safety)
"Some wood frame homes built as recently as the 1970's
were shaken off their foundations in recent earthquakes.
Even though these modern homes were bolted down, they
failed because of weak bracing materials on the cripple
walls. The cripple wall must be braced with
materials like plywood. If not, the next
earthquake may damage the cripple wall and move your
home off its foundation, even if you've bolted it down."
(LA Dept. of Building and Safety)
It is best to have a licensed contractor or inspector
check your foundation and cripple walls, make sure they
are secured and up to the current building code.
FOUNDATIONS AND FOUNDATION-TO-WALL CONNECTIONS:
While shear walls can strengthen a house,
they are only one of several steps that can be taken to
lessen the likelihood of earthquake damage. Good
seismic design starts with the foundation. Three kinds
of foundations are covered in this brochure: post and
pier; poured concrete walls; and slab-on-grade.
The Uniform Building Code requires foundation plates or
sill plates to be bolted to the foundation with 1/2-inch
diameter bolts spaced no more than six feet apart. One
bolt must be placed within 12 inches of each end of each
section of the plate. If an existing house has no
connections, or if the connections do not meet code
requirements, retrofitting should be considered.
Anchor bolts are the most common way to attach a house
to its foundation, but a thorough inspection of the
foundation should precede bolting. The concrete must be
strong enough to hold the anchor bolts. If the concrete
is weak or deteriorating and drilling holes for bolts is
likely to cause cracks or crumbling, the foundation
should be replaced.
A WORD ABOUT ANCHOR BOLTS:
Anchor bolts are manufactured in a variety of
types and sizes. Mechanical wedge anchor bolts are the
type most commonly used in seismic retrofit
applications. A metal collar near the tip of the conical
end is what makes mechanical wedge anchor bolts work.
Once the bolt is in place, a few turns on the nut will
lift the bolt and expand the collar, locking it in
place. Anchor bolts must be installed before panels are
nailed to the wall framing. Once installed, the panels
will block access to the sill plate. in new
construction, "L" bolts or "J" bolts, inserted when the
foundation is poured, are the most common type of anchor
bolt.
The most common problem with post and pier foundations
during an earthquake is the tendency of posts to shift
off piers and beams to shift off posts. Failure at
either of these points can cause the substructure, and
sometimes the entire house, to collapse. But steps can
be taken to strengthen post and pier foundations.
Bolting posts to piers and beams is the most common
method. Lateral support can be achieved by installing
2x4s as diagonal bracing from one post to the next.
Pre-drilled steel T-straps are recommended for attaching
beams to posts. For attaching posts to piers,
pre-drilled, shop-fabricated, heavy gauge, bent sheet
metal connectors are recommended
POURED CONCRETE FOUNDATION WALLS:
In new construction, poured concrete walls
are the most common type of foundation for supporting
raised wood floors.
In houses built prior to modern building codes the
foundation and sill plate are not always attached, but
retrofitting with anchor bolts can substantially improve
a homes ability to withstand an earthquake. There are
two ways of bolting sills to concrete foundations:
vertical and horizontal.
Vertical bolting can be accomplished only if there is
enough workspace between the foundation and floor.
Horizontal bolting is useful where space between
foundation and floor is minimal.
VERTICAL BOLTING: Vertical bolting is the easiest
way to attach a house to its foundation. Mechanical
wedge anchor bolts are the only hardware needed. Simply
drill the holes for the anchor bolts through the sill
plate and into the foundation, then install the bolts.
HORIZONTAL BOLTING: Horizontal bolting, a
structural steel plate is needed to tie the sill plate
to the foundation. The steel plate is attached to the
sill plate with lag screws and to the foundation with
anchor bolts.
SLAB-ON-GRADE FOUNDATIONS:
Bolting sills to slab-on-grade foundations
requires removal of exterior siding or interior plaster
or wallboard. The code requirements for bolting
slab-on-grade foundations are the same as those for
poured concrete foundation walls and the job is done in
the same way. Mechanical wedge anchor bolts are the only
hardware needed.
SHEAR WALL HOLD-DOWN ANCHORS:
Once the sill plate is bolted to the
foundation, shear wall hold-down anchors should be
installed at each corner of the house. While anchor
bolts help keep a building seated on its foundation,
shear wall hold-down anchors help prevent walls from
overturning. A steel bracket is recommended. Anchor
bolts are suggested for attaching the brackets to the
foundation. Lag screws or machine bolts can be used to
fasten the brackets to wall framing.
Bolting your house step-by-step is shown below:
