In California, earthquakes are a fact of life. The next one may strike any day, and it may just be that big one.
Is your home ready for it?
If you haven’t had a professional assessment of your home’s seismic worthiness, you don’t have time to waste. Learn the failure mechanisms that destroy homes during earthquakes, check if yours is prone to any of them, and get help.
The guide below lists common seismic weaknesses, and the best means of resolving them. It’s a good starting point for you to get acquainted with earthquake damage and things you can do to prevent it.
1. Anchor Your Home to Its Foundation
If your house is not anchored to its concrete foundation, earthquake loads can cause it to slide off. The implications of this can be severe, as the movement may rupture gas lines and start fires, and the structural damage may be grave enough to warrant the home’s demolition.
If your home has a raised floor foundation, normally, the joists of your lowest floor will either be supported by a cripple wall, or rest directly on the sill plate. To determine if your home’s frame is secured to the foundation, head down to the crawl space and look for the heads of anchor bolts installed along the sill plate. If the house is built on a slab-on-grade foundation, you will not have a crawl space, but an unfinished garage should give you visual access to the anchor bolts.
If you find no evidence of anchors, holes will have to be drilled through the sill plate and concrete to install them. If there is not enough space for drilling, steel plates or retrofit foundation plates have to be used to fasten the sill down to the concrete.
If you are planning a seismic retrofit for your home, you may be eligible for an incentive program to help you offset the costs of the project. For instance, the Earthquake Brace + Bolt Program offers homeowners in certain parts of the state an incentive of up to $3,000 for eligible retrofits. Be sure also to ask your building department about incentive programs specific to your jurisdiction.
2. Brace The Cripple Wall
If you have a crawl space, there is a chance it contains a cripple wall. These walls support the weight of the entire house and require strong lateral reinforcement to resist seismic loads. Unbraced cripple walls may buckle during earthquakes, causing the structure above to shift or collapse. Failures of this nature are a common cause of earthquake damage.
Adequately braced cripple walls have plywood or diagonal sheathing nailed to their studs, which makes them shear walls by definition. If your cripple walls are not braced in this manner, rectify the situation as soon as possible. A structural engineer will consider all variables to design a strong, yet economical bracing system for your cripple walls.
Our engineers are standing by to help you with your seismic retrofitting needs. Call us at (877) 704-5687 for a FREE consultation.
3. Retrofit The Soft Story
A soft story condition exists in a building where a room is situated above the garage. The garage door opening and the weight of the space above weaken the wall, leaving it particularly vulnerable to seismic loads.
Depending on its configuration, you may strengthen the wall with plywood panels, steel straps and hold down anchors, or reduce the size of the opening by adding a narrow shear wall. In some cases, a steel moment frame may be the only viable option.
If such a condition exists in your home, you should consult an engineer to find a suitable bracing solution.
4. Hillside Home? Call an Engineer
Down-slope hillside houses suffer massive damage during earthquakes. Those supported by columns and concrete pads, known as stilt homes, are notably vulnerable. During seismic activity, their diagonal tension bracing stretches and weakens, allowing the earthquake’s forces to detach the home from its foundation.
Homes with stepped foundations have their own seismic flaws. The shear panels that brace their cripple walls are stepped along with the footing, with the shortest panel at the top of the slope. Stepped shear panels cannot resist lateral loads in unison; instead, the shortest segment absorbs the entire load until it fails. The next shortest panel follows suit, and so on, until the house collapses.
Regardless of the foundation type, if your hillside home hasn’t had a seismic retrofit, you need a professional engineer to assess your situation and plan one. Call us at (877) 704-5687 to discuss your hillside home’s seismic retrofitting needs.
5. Replace Unreinforced Masonry Foundations
Some older homes may have foundations built from brick or stone. Often, such foundations are not reinforced with rebar the way a typical concrete foundation would be by today’s standards. When resisting seismic loads, unreinforced masonry foundations may break apart or loosen around the anchor bolts, causing your home to slide off.
To mitigate earthquake damage, you should replace your unreinforced masonry foundations with concrete and rebar. If your foundation is made of stone or brick, call a licensed engineer to evaluate it.
6. Reinforce Masonry Walls and Chimneys
Unreinforced masonry walls and chimneys are common earthquake victims. Without bracing from steel rods, bricks, stones, and concrete blocks are like Lego blocks without their studs and tubes. Mortar is just not strong enough, and when an earthquake strikes, they crumble.
To determine if your home has this weakness, it’s best to find your house’s as-built drawings and turning them over to a licensed engineer. You may have luck locating them at the building department; if not, your engineer should be able to establish whether or not your walls are reinforced during a site visit.
Retrofitting measures may include tying the walls to the building’s diaphragms, or bolting them to a steel frame. Your engineer will advise you on a solution that meets the needs of your home.
7. Install Shear Walls
Your home’s studs offer excellent compression strength to resist the vertical loads they transfer down. Without bracing, however, they cannot withstand an earthquake’s horizontal loads. To brace the studs, a sheathing board is attached, and the resulting assembly becomes a shear wall. Hold down devices may be required to secure the shear wall to the farming and eventually the foundation, below. Be sure to consult your structural engineer to determine whether you need these.
Older homes built under less stringent seismic code (or none at all) may lack appropriate shear bracing in their design. This seismic weakness exposes homes to racking during earthquakes. Their walls buckle, leading to substantial damage or collapse.
Unlike many seismic retrofitting measures, adding shear walls should not incur major costs. The upgrade can be done by exposing the frame and nailing plywood boards to the studs where your engineer deems appropriate.
8. Make Sure Your Design Is Earthquake-Resistant
Factors such as large openings, overhangs, and irregular wall shapes may contribute to your home’s seismic weakness. Large segments of floor-to-ceiling windows may create a soft story condition and reduce precious shear wall area. During an earthquake, their glass may shatter and complicate your evacuation efforts. Overhangs lacking adequate reinforcement may detach from the house and collapse.
If you‘re not sure whether or not your house is designed to withstand seismic activity, call a licensed engineer for a consultation. Small modifications with long-lasting benefits may be possible, and our engineers will be happy to plan them for you. Call us at (877) 704-5687 for a FREE consultation.
9. Brace Your Water Heater
Unbraced water heaters can inflict serious damage to your home during an earthquake. Their collapse can rupture gas lines and water pipes, causing fires and floods as you try to evacuate your home. With proper bracing, these calamities are avoidable.
First, determine whether your water heater is already secured. If the device is freestanding, it should be strapped to the frame of your house, and the straps should be bolted to the studs. If you do not see evidence of such bracing, your water heater is at risk of collapse during seismic activity.
To provide sufficient bracing, you may use an architect-certified restraining kit available in most hardware stores, or call a plumber to secure your water heater to the studs. This simple upgrade won’t cost you much, and will have a far-reaching impact on protecting your life and property.
10. Plan to Prevent Fires
When the ground shakes, gas pipes may burst and cause leaks, fires and explosions. While you can’t prevent earthquakes, you can do your part to stop them from causing fires. Make sure that you use flexible piping to connect natural gas to your appliances and know when to shut off the gas supply to your residence.
We Are Here To Help
Over the past 13 years, we’ve designed and retrofitted 1000s of projects right here in earthquake country. If your home needs a seismic retrofit, we will do our best to find an economical solution that protects your family and home from the next seismic event. Call us at (877) 704-5687 for a FREE consultation and quote.
- United States of America, Seismic Safety Commission, California. (n.d.). Homeowner's Guide to Earthquake Safety. Retrieved May 16, 2019, from https://ssc.ca.gov/forms_pubs/hog.html
*Note: The content published above was made in collaboration with members of Design Everest.
You must have an account to comment on the post. Register with us if you are an AEC professional!