The U.S. Geological Survey (USGS) in Colorado estimates that there are about 500,000 detectable earthquakes in the world each year, of which 100,000 can be felt, and 100 of which cause damage. Let’s just be thankful that most earthquakes are minor in nature.
Yet, for as few of them as are serious, earthquakes cause billions of dollars in property damage all over the world. Although they are most prevalent in the Western United States, earthquakes can happen in almost any state.
In this tutorial, we will discuss what you can do to better prepare your home and protect your family in the event of an earthquake. You may be able to do some of this preparation work yourself, but some of the structural issues discussed in this tutorial may require the help of an architect or a licensed building contractor if you do not have the skills or tools needed. If you need to hire a contractor for any of this work, see the tutorial How to Check Out a Home Repair Contractor.
For earthquake safety actions you can take during and after an earthquake, take a look at this American Red Cross checklist. It is a good reference, so please print it out and keep it handy.
Home Structural Preparation for an Earthquake:
Preparing your home’s structure for an earthquake will involve improving your home’s ability to withstand the violent side-to-side motion of many earthquakes. This tutorial will focus on four main areas:
- Exterior wall-to-foundation connection (anchor bolts).
- Exterior wall-to-foundation connection (retrofit foundation plates).
- Cripple-wall shear reinforcement.
- Mobile home earthquake-resistant bracing system.
Let’s take a look at each of these sections and what it means to your home’s preparation.
Exterior Wall-to-Foundation Connection (Anchor Bolts)
Your home’s exterior walls need to be connected to your foundation walls. Logical as this seems, this practice was not common in home construction until the 1940s. Fortunately, it's now a code requirement that exterior walls are bolted to the foundation walls to resist lateral movement, like those found in an earthquake.
The horizontal 2x4 at the bottom of the exterior wall that rests on the foundation wall is called the sill plate (or mud sill in some parts of the country). The sill plate must be bolted to the foundation wall with galvanized anchor bolts so the wall does not move or slide. The spacing of anchor bolts in the sill plate can be no more than 6 feet on center and must be within 12 inches of any sill plate joints.
You can find the sill plate or mud sill in your crawlspace or basement at the top of the foundation wall. On top of the sill plate, you should see the anchor bolt head protruding through a metal washer and nut.
If you do not see anchor bolts at an appropriate spacing in your home, check with your building department to learn local requirements and to get help on deciding the proper action. There are different building code requirements on bolt diameter, length, and spacing for new construction versus retrofit construction. The length of the anchor bolt is also dependent upon the thickness of the sill plate, holding values required and the strength of the concrete foundation wall, but will generally range between 3.5” and 6.25” of embedment into the concrete. It is best to check your local building codes, building official or with an engineer or architect to determine the size of anchor needed.
If you have a crawlspace in your home, you may have a framed wall above the foundation and below the first floor, called a cripple wall. Retrofit anchoring of the exterior wall to the foundation is done one of two ways, either by anchor bolts (cripple walls) or retrofit foundation plates (no cripple walls).
If you elect to retrofit the anchor bolt installation on a cripple wall, you will need to drill through the sill plate and into the top of the poured concrete foundation wall. Foundation anchor bolts come in an expansion type for use in stronger, solid concrete and in an epoxy setting type for use in older, weaker concrete, cinder block or brick. You can improve the holding power of epoxy-setting-type anchors by using longer anchors installed deeper into the foundation. You can improve the holding power of either type of anchor up to almost 60% by using large 3" square washers.
Another method of anchoring the sill plate to the foundation is by using approved screw anchors such as the Titan HD made by Simpson. For a good overview of sill plate to foundation fastening, see Bay Area Retrofit's Bolting: Attachment Of The Mudsill To The Foundation.
If your cripple wall needs blocking, make sure to install that first, before installing the anchor bolts and washers (see Cripple Wall Shear Reinforcement later in this tutorial).
Exterior Wall to Foundation Connection (Retrofit Foundation Plates)
If your home has a basement or a non-cripple wall framed crawlspace, then the first-floor joists bear directly on the sill plate sitting on top of the foundation wall and do not sit on a cripple wall. In that case, it is difficult to gain access to the top of the foundation wall and drill into the sill plate, due to limited vertical clearance to drill holes for anchor bolts. This situation calls for retrofit foundation plates, which attach to the sides of the foundation wall and the side of the sill plate. The plates are anchored to the foundation wall with expansion or epoxy-set anchor bolts and with wood screws into the sill plate.
Cripple Wall Shear Reinforcement
Cripple walls are short exterior walls built on top of foundation walls to create a crawlspace. These walls have to carry the entire weight of the house, and if they are not braced for lateral (horizontal) movement, they can collapse during an earthquake. Once the cripple wall fails, the house will collapse or shift significantly, often off its foundation.
To reinforce these flimsy walls upon which your entire home sits, you need to create what is effectively a shear wall. To do this, you need to add plywood sheathing to create a rigid panel out of the studs, top plate and sill plate of the cripple wall. Once the cripple wall is converted into a rigid panel with the sheathing, it will now be able to resist lateral loads much better.
If the cripple wall sill plate is the same width as the studs, then blocking is not needed. However, if the cripple wall has a sill plate that is wider than the studs, you will have to add blocking at the bottom of the wall between the studs. This blocking provides a bottom nail surface for the sheathing and needs to be the same size dimensional lumber as the studs. For example, if you have 2x6 studs, use 2x6 blocking; likewise, for 2x4 studs, use 2x4 blocking. Each snug-fitting blocking section between the studs is usually installed with 10d galvanized nails, two per side (4 total).
Shear panel installation:
Cripple-wall sheathing needs to be installed on each wall but not for the entire length. Follow these guidelines for installation:
- Install sheathing to 50% of the cripple wall length for 1 story homes;
- Install sheathing to 70% of the cripple wall length for 1.5 to 2 story homes;
- Install sheathing so the length of run is no less than 2 times the cripple wall height.
- Install sheathing starting in each corner and working out.
- The centers of runs of sheathing can be no more than 25 feet center-to-center.
- Make sure the sheathing ends on the centerline of a stud. All ends of the sheathing MUST be fastened fully to the studs, top plate and sill plate (or mud sill).
- Fasten the sheathing along all edges with 8d common galvanized nails spaced about 4" on-center. In the center of the panels, nail into studs 12" on-center. If you are nailing into a double top plate, stagger the nails so one goes into the upper top plate, and then 4 inches apart, nail another nail into the bottom top plate, offset 4 inches laterally from the top nail. Repeating this staggered approach along the length of the top plate.
- Ventilate each space between the studs with a 3" hole centered between the studs. If the cripple wall is 18" high or less, you only need one hole. If taller than 18", then two holes (one at top and one at bottom) need to be provided. It is also recommended to locate the lower ventilation hole over the anchor bolt for inspection purposes. Lastly, placing a screen material over the holes is also recommended to keep small animals and insects out.
Mobile Home Earthquake Protection
If you have a mobile home, there are steps you can take to protect your home, as well. Take a look beneath your home. If the home is supported only with concrete blocks or metal jacks, you need better protection, have an "engineered tie-down system" or an earthquake-resistant bracing system (ERBS) installed. Installation time varies, depending on the system you used. Many of the systems can be used for either retrofit or new installations and can be relocated when you move. Many systems also have the ability to handle sloping sites.