Foundation Repair, Foundation Contractor, Earthquake Retrofitting, Earthquake Contractor Seismic retrofitting is a structural strengthening process that begins at the foundation and related to earthquake preparation and disaster mitigation. Cal-Quake Construction is a retrofitting contractor who specializes in foundations and structural strengthening. Cal-Quake provides quality retrofits and foundation replacement or repairs to the entire state of California.
How can you prepare your house to better withstand the next damaging earthquake?
You can reduce earthquake damage to your house’s structure by taking steps to strengthen your home, such as:
- Bolting your house to the foundation and bracing your cripple walls.
- Strapping your water heater to the house’s frame.
- Securing fragile items on your shelves with earthquake putty and straps.
What’s the earthquake risk in California?
California is earthquake country, with more than 2,000 known faults crisscrossing the state.
According to the Uniform California Earthquake Rupture Forecast (UCERF3), within the next 30 years, there is a:
- 99.7% chance a 6.7M or greater earthquake will strike California.
- 75% chance a 7.0M or greater earthquake will strike Southern California.
- 76% chance a 7.0M or greater earthquake will strike Northern California.
What can you do to get yourself and your family prepared?
- Create a disaster-preparedness plan.
- Put together disaster supplies for your home, work and car.
- Purchase earthquake insurance to help you repair, replace, rebuild and recover.
What is seismic retrofitting?
Houses built before 1979—before residential seismic building codes were consistently enforced—may be more vulnerable to earthquake damage. Even a moderate earthquake can severely damage a pre-1979 house that has not been retrofitted to current building standards.
A seismic retrofit strengthens a house to make it more resistant to earthquake damage.
One way to seismically retrofit an older house is to bolt the house to its foundation and brace the walls around the crawl space with plywood.
If your house has?
- Has a wood frame
- Was built before 1979
- Is on a raised foundation (you would have a crawl space or basement under the house)
How a brace and bolt retrofit works. Many pre-1979 houses have a short (less than 4-foot), wood-framed wall surrounding the crawl space under the house. This short wall is known as a “cripple wall.” To help prevent this type of house from sliding off of its foundation during an earthquake, the cripple wall needs to be braced with plywood and the house bolted to the foundation.
In order to understand the seismic retrofitting process, here are some basic concepts. The majority of structures were built to withstand one type of force or load as the engineers call it — the force of gravity. That’s an up-and-down force. Unfortunately, the most damaging component of earthquake force is from side to side, producing what engineers call a lateral load. Therefore, properties that were adequately built to resist up and down forces are potential collapses under the lateral stresses of earthquakes. When dealing with conventional wood framed structure, there are three basic types of lateral failure, and as a consequence there are three different needs in seismic reinforcement. Bracing against one type of lateral failure does not protect you from the other two. That’s why a retrofitting design should be planned by a knowledgeable professional — a scatter shot approach can leave your property more vulnerable.
1. Cripple wall failure – This occurs in wood-frame structures on taller crawl space walls, known as cripple walls, they are built with vertical 2X4 studs. They can be found between the foundation and the floor joists.
Even in recent minor quakes, such properties have been destroyed when these walls collapsed. Un-reinforced cripple walls are a weak link for getting the earthquake loads from the lower floor to the foundation. Plywood reinforcement on the inside faces of the cripple-wall studs can save the structure from destruction in a future quake. Shear failure happens when the bottom of a building moves under the force of a quake but the top doesn’t.
2. Sliding failure – Sliding failure occurs when a property is not securely bolted to the foundation. Thus an earthquake can cause the entire building to literally slide off its foundation (while often remaining otherwise intact). Sliding failures are usually prevented by ordinary foundation bolts and framing anchors. Properties with deficient anchor bolts or none at all, are a worry!
Is the property bolted? Maybe and maybe not. Many — but not all — properties built have either deficient or nonexistent foundation bolts. It is best to have the adequacy of existing foundation bolts checked by an expert.
Under the influence of earthquake forces, the foundation begins oscillating before the roof even starts to move. The result is that the top and bottom edges of the structure’s walls shear, or move horizontally past each other. Shear failure produce characteristic diagonal or X-shaped cracks in plaster, stucco and concrete. The heavier the building, the greater the shear forces produced — and the greater the potential damage.
3. Soft Story – Many properties, residential, commercial and apartment buildings have what is called a “soft story” condition. This term is used to describe any building that has a habitable room or rooms above a garage, carport or porch area that was not specifically designed to transmit shear or lateral forces to the story above.
Failures of these types of building or structure with soft story conditions can lead to loss of lives in an earthquake. Many counties in California are currently drafting ordinances to require retrofitting of all soft story buildings.
In California, there is an earthquake hazard disclosure law requiring the seller to disclose to the buyer at the time of sale the existence of certain known earthquake hazards, such as lack of bolting, existence of cripple walls with no shear paneling, hot water heaters that are not properly strapped, etc.. One of the conditions required to be disclosed as a hazard is the existence of a “soft story” condition. Visualize two dominoes standing on their small ends. Load four or five bricks on the dominoes with no difficulty because the dominoes are strong enough to carry the load, or downward force. But, if you go to the side of the dominoes and apply a tiny shear force: a shake or with even a breath; the whole thing will come crashing down…
We make certain to consult our structural engineers to address the specifics of your particular structure.
photos courtesy of: www.sfgate.com
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