Earthquake retrofitting is the process of strengthening a building to make it more resistant to earthquake shaking and damage. The most common types of retrofits involve a house being bolted and braced to the foundation on which it rests, but there are many other ways that houses can be made more resilient.
Millions of older homes are in need of seismic retrofitting, largely because they were built before more modern earthquake building codes came into effect. A wood-framed house that isn’t properly anchored to its foundation is more likely to shift or even slide off the foundation during an earthquake, causing serious structural damage and potentially putting the occupants at risk of injury or death.
An essential component of any retrofit is the insertion of “seismic anchor bolts” into the concrete foundation, often referred to as foundation bolting or foundation drilling. This is a relatively inexpensive and highly effective way to make a house more resistant to earthquake movement. During an earthquake, these bolts transfer movement from the house to the foundation, where it will be dissipated. The weakest link in this chain is likely to be a failing connection between the house and the foundation, which is why careful planning of the location, type, and number of bolts is crucial.
Another important aspect of earthquake retrofitting for buildings is the use of dampers and braces to absorb seismic energy. These are typically made of special materials such as shape memory alloys that are designed to return to their original shape after being deformed, thereby reducing the force transmitted by an earthquake and protecting the structure from damage.
There are a number of signs that indicate the need for a seismic retrofit, including visible sagging or leaning in the house, cracks that are tightly clustered, horizontal or diagonal, and a visibly uneven or crumbling foundation. In addition, any house that is located in an area known for frequent or severe seismic activity is at a higher risk of needing an earthquake retrofit.
In some areas, governments offer grants to help offset the cost of earthquake retrofits for older homes. The state of California offers the Earthquake Brace and Bolt Program to provide up to $3,000 in grants to homeowners with wood-framed houses that are at a higher risk for earthquake damage.
A seismic retrofit will also make a house more resistant to the shifting and damage that can be caused by an earthquake, especially on raised foundations. Typically this is accomplished by adding plywood to crawl space walls and by using bolts to secure the foundation. In some cases, a raised foundation may need to be replaced with a new continuous foundation.
Most residential seismic improvements can be planned by an experienced contractor without the need for an engineer. In some cases, a prescriptive plan set may be available that can expedite the application process for an Earthquake Home Retrofit Permit. However, in the most complex situations it is advisable to consult with an engineer or to hire a design professional to prepare engineered plans.