Earthquake Retrofit Overview
Retrofit is the process of bolting together and strengthening a building’s understructure so that it is better able to resist earthquakes. Recently constructed houses, if built according to code, have the structural strength they need to withstand the side-to-side (shear) force of an earthquake. Buthomes built before 1980 (in the Seattle area) or 1985 (in the wider Puget Sound region) lack many of the basic components of structural strength that are needed to stay strong during an earthquake:
- A sill plate that’s bolted or attached with plates to the foundation
- Pony walls that are reinforced with shear paneling
- First-floor joists (part of the floor structure) that have been connected to the sill plate or the top of the pony wall
In 1965, the state of Washington reformed Uniform Building Codes to require that new structures be bolted to their foundations. Past earthquakes have shown time and again that houses that aren’t bolted to their foundations are vulnerable to dramatic structural damage– jolting completely off their foundations. This kind of structural damage can’t be repaired, meaning that houses that have been disconnected from their foundations have to be demolished.
In 1975, the building code in Washington state was reformed again to require pony walls (the short wall between the foundation and the first floor in certain houses) to be strengthened with shear paneling— plywood that is attached to the wall with a special nailing pattern. Without strong pony walls, a house can cave in on itself as walls give under a quake’s shear (side-to-side) pressure. In the years following 1975, building codes have become even more stringent.
Houses that were built before the building codes were enforced (about five years after the code was revised in Seattle and ten years after in other parts of the state) are likely to be missing these important elements of structural strength.
Your house could also be at risk if:
- Unpermitted work (often remodeling) has been done to it– this work has not been approved by inspectors and often doesn’t meet building codes
- Parts of the understructure have deteroriated due to leaking pipes, exposure to the elements, or insect infestations
- Structural elements in your home have been built to meet current building codes, but the rest of the building
doesn’t meet code. This can actually put your home at greater risk, because greater strength in just one section of the home will decrease your house’s structural “give” (its ability to accomodate seismic motion) without increasing its overall strength
Retrofit is the process of adding these elements of structural strength to houses to give them the best chance of surviving an earthquake. If your house is at risk, consider scheduling a home inspection to have an experienced professional assess the risks in your home.
House Understructure 101
The foundation of a building is its basic source of support. Most foundations are flats of concrete with raised perimeters that support the building.
Many older houses have nonstandard foundation types. Cinder block foundations are made of hollow cinderblocks that have been cemented together using mortar. Pier-and-post or car-decking foundations are made of connected wooden posts
that have been embedded in a concrete pier. Some foundations are flats of brick. These foundation types have less shear strength (resistance to side-to-side motion) than standard concrete foundations do, and will need specialized structural improvements to be retrofit.
The sill plate is the 2×4 or 2×6 wood flat that covers the concrete foundation and connects it to the pony wall, or, if the house doesn’t have a pony wall, to the floor framing. In homes that have been built up to code, the sill plate will be bolted to the foundation.
A pony wall, also called a cripple wall, is a plywood-enforced wall that rests on top of the sill plate, supporting the flooring and the rest of the house. Single-floor homes and homes without a basement don’t have pony walls- the sill plate connects directly to the floor framing.
A crawlspace is the narrow area between the foundation and the flooring in a house that doesn’t have a basement.
Floor Framing (Joists)
The floor framing is a series of wooden joists, or boards, that range in size from 2″x6″ to 2″x12″ and run parallel beneath the floor, providing it with support. In most homes, joists are connected on either end by a perpendicular board (therim joist or rim board) that increases the strength of the network of joists.
Elements of Retrofit
Bolting the Foundation
Steel bolts or special foundation plates are used to connect the sill plate to the foundation. Bolts are used to make this connection if the house has a pony wall (a basement wall between the foundation and the first floor). Foundation plates are used if the home has a full-height foundation or a crawlspace and the first floor of the home must be connected directly to the foundation.
If the foundation is cracked, cracks may be filled in with epoxy to restore the foundation’s structural strength.
If the house has an unconventional foundation, special measures have to be taken to secure it, since these types of foundations interact with the understructure of the home differently than concrete foundations do. For post-and-pier or car-decking foundations, each post is connected with metal plates to its concrete base (the pier) and the beam it supports. The parallel beams that the posts support are braced with a long perpendicular piece of lumber similar to a rim board.
Brick foundations, which are used in some turn-of-the-century homes, are especially difficult to retrofit because of their structural instability- the mortar between the bricks can crumble under strong seismic pressure, causing the foundation to crumble. Special engineering is required. During retrofit, brick foundations are encased in concrete, giving them the structural strength of a concrete foundation.
Connecting the Floor and the Understructure
Framing clips are used to connect the pony wall (or, if the home doesn’t have one, the sill plate) to the floor framing.
Increasing Shear Strength
Sheets of half-inch 5-ply CDX plywood are nailed along the pony wall with a special nailing pattern to increase shear (horizontal) strength. If the pony wall is higher than 48″, special engineering is required. Hold-down anchors are typically installed at each end of the wall to provide further reinforcement.
If the joists are lacking a rim joist (a long piece of 2x lumber perpendicular to the joist system), they are added so that the floor framing can be attached to the pony wall, if the house has one, or to the sill plate.
- If your house was built before 1980 and you live in an area as prone to earthquakes as Puget Sound, retrofit is an important part of ensuring the well-being of your home and family. Here are some reasons to go ahead and have your house retrofit:
- Retrofit brings a home up to current safety codes and significantly reduces the damage it will sustain during an earthquake.
- Houses that are not secured properly may collapse or be shaken off of their foundations during a major earthquake, making them inaccessible and uninhabitable in the aftermath of a disaster. If a house is severely damaged, a family may be prohibited from entering it, even to claim possessions.
- 95% of houses that have been shaken from their foundations during an earthquake have to be demolished.
- Potential damages to a house not up to code in an earthquake are many times more expensive than the cost of retrofit.
- Loans for home retrofit are relatively easy to get and are often offered at a reduced interest rate.
- Earthquake insurance is not adequate protection against a severe quake. Insurance does nothing to keep you safe during the earthquake itself and may not provide for interim housing while your home is being repaired after the quake.
- Deductibles for earthquake insurance policies can be as much as 10% of the house’s value–$30,000 for a $300,000 home.
- A homeowner whose house has been wrecked by an earthquake must continue to make mortgage payments whether the home is habitable or not.
- Accommodations for people who have lost their homes in an area severely impacted by an earthquake are usually limited to schools, community centers, churches and tents. Of the 300,000 people displaced in Turkey by the massive August 1999 quake, more than 30,000 were still living in tents after a year.
- The devastating 1999 earthquake in Turkey was so devastating because most houses had not been brought up to the national safety standard. The houses that were up to code sustained only minor damage.
- No one should risk losing their house and home to an earthquake.