PROTECTING YOUR HOME FROM EARTHQUAKES
BOLT THE FOUNDATION
Bolting the wood frame of an older house to the concrete foundation can significantly reduce earthquake damage. Mobile homes, portable classrooms, and modular buildings can slide off their foundations during earthquakes. Their supports need to be braced to resist horizontal forces. If portable classrooms are used at your local school, you should ask school officials whether they are properly braced.
Bolting a structure to the foundation is a fairly easy 4-step task. For this project you'll need these materials: 1/2" or 5/8" diameter foundation bolts that are at least 7" long with nuts and square plate washers; rotary hammer drill with an appropriately-sized carbide tip drill bit; (right-angle drill if possible); short-handled sledge hammer for setting bolts; adjustable wrench; measuring tape and chalk line; dust mask and eye/ear protection and a torque wrench.
FOUR EASY STEPS
First, mark the places for each bolt on the mud sill. Make the first mark between nine and twelve inches from the corner, and then measure another four to the next bolt, and so on. Continue this pattern along all of the foundation walls. Place an extra bolt within nine to twelve inches from any joint or step in the mud sill.
Then follow the ABC's.
A. Drill the holesB. Clean the holesC. Install the bolts
C. Install the bolts
- A. Drill the holes
Using the rotary hammer drill equipped with an appropriately-sized carbide bit, drill down through the mud sill at least five inches (12.7 cm) into the concrete.
B. Clean the holes
Use flexible tubing to gently blow the concrete dust out of the hole. (This is especially important if you are using chemical anchors.)
Expansion bolts are designed to be hammered into place. This can be done without damaging the bolt's threads by turning the washer and nut past the end of the bolt and tapping on the end of the bolt shaft to hammer the assembly into place. Once
the bolt is in place, tighten the nut down firmly using an adjustable wrench.
If your house is built on a hill or even a slight grade, chances are you have some step-like off-sets in your foundation to compensate. Every step must be bolted down even if it is adjacent to another bolted step.
- FOUNDATION ANCHOR PLATE
If you don't have working room above the mud sill to drill straight down and can't find a right angle drill, you can secure the mud sill to the foundation with an anchor plate. This is a metal plate that is nailed or screwed to the top of the mud sill and bolted to the side of the foundation.
When you tighten the nut on an installed expansion bolt, the bolt's
other end expands to grip the concrete. When the bolt is inserted
properly, you will actually feel it "grab" the foundation as you
tighten the nut. Test at least one out of every four new bolts
for tightness with a torque wrench applying 40 foot-pounds of
CHEMICAL ANCHORS (epoxy bolts)
If you have an older foundation and worry about cracking it with
the pressure of expansion bolts, consider using chemical anchors
(also called epoxy bolts). Always follow the manufacturer's installation
instructions. Measure, drill and clean the holes per the manufacturerÕs
instructions. Be careful not to drill deeper than the bolt's length.
Before you place the bolt in the hole, inject the epoxy mixture
into the hole. Press the bolt into place and wait for the epoxy
to harden (usually 24 hours). Once the epoxy has hardened, tighten
the nut with an adjustable wrench until the washer just begins
to indent the wood mud sill. Chemical anchors can be a bit more
time-consuming to install. However, they are very effective, and
are the preferred method.
© Text and Graphics, 2001 Stan and Holly Deyo, except where otherwise credited