Earthquake Retrofit

A soft-story building is a structure which has a weaker first floor and is unable to carry the weight of the stories above during an earthquake. The first floor generally would have large openings in the perimeter walls such as garages, tuck under parking or even large windows.

The property owner must comply with the ordinance within the following time limits:

From the receipt of the Order to Comply:

• 2 years: Submit proof of previous retrofit, or plans to retrofit or demolish
• 3.5 years: Obtain permit to start construction or demolition
• 7 years: Complete construction

From past earthquakes, multi-story buildings with weak and/or open front wall lines creating a “soft-story” (i.e. buildings with tuck-under-parking) performed poorly and collapsed. The goal of the mandatory retrofit program, under Ordinance 183893 and Ordinance 184081, is to reduce structural deficiencies by the most economical and feasible method. Without proper strengthening, these vulnerable buildings may be subjected to structural failure during and/or after an earthquake.

Buildings that are most vulnerable have been identified with the following criteria:

• Consist of 2 or more stories wood frame construction
• Built under building code standards enacted before January 1, 1978
• Contains ground floor parking or other similar open floor space

The program does not apply to residential buildings with 3 or less units.

Soft story retrofit refers to a structural engineering technique used to strengthen buildings, particularly those with weak or flexible ground floors, commonly known as “soft stories.” Soft story buildings typically have a ground floor that lacks sufficient shear strength or stiffness, making them vulnerable to collapse or significant damage during earthquakes or other lateral forces.

The retrofit process involves reinforcing the soft story with additional structural elements such as steel braces, moment frames, or shear walls. These elements help redistribute the building’s weight and forces more evenly, improving its resistance to seismic events.

Soft story retrofitting has become increasingly important in areas prone to earthquakes, such as California, where many older buildings were constructed before modern seismic building codes were established. Retrofitting helps enhance the safety and resilience of existing structures, reducing the risk of injury, loss of life, and property damage during earthquakes.

Building owners, municipalities, and engineering firms often collaborate to assess the seismic vulnerability of soft story buildings and implement retrofitting measures as needed to meet current safety standards and regulations. Additionally, financial incentives and assistance programs may be available to encourage building owners to retrofit their properties.

Living Space Over Garage House Retrofit

This type of house has one floor of living space over an attached garage, also known as a “soft story.” It is vulnerable to damage because the garage is not strong enough to withstand earthquake shaking, because it lacks structural soundness, as it:

• typically has a large open space (the garage area), with walls not braced to resist earthquake motion, and
• has one or more large doors that are not retrofitted to withstand earthquake shaking.

Garage-door openings can be retrofitted by installing plywood sheathing or steel panels on each side of the garage door. Some garage doors require a steel frame to be properly retrofitted for earthquakes. The back wall may require a new foundation with reinforcing bars, and the side walls likely will need to be strengthened with new anchor bolts, hold downs, and plywood.

Raised Foundation House Retrofit

A house with a raised foundation has an area underneath the first floor, which is referred to as a crawl space. The first floor in a raised foundation house may be supported by:

• A continuous foundation where the wood floor is supported directly on concrete stem walls,
• A continuous foundation where the wood floor is supported on short, wood frame walls, called cripple walls, that extend from the concrete foundation to the bottom of the first floor, or
• A continuous foundation with both stem walls and cripple walls.

Note: Industry professionals use the term “cripple wall” to define “crawl space walls.” These types of walls are short OR less-than-full-height wood stud walls that surround the crawlspace under your first floor.

BOLTING AND BRACING RETROFIT

Older houses—those built before 1980—may not be properly anchored to the foundation, making them more vulnerable to shaking and more likely to slide off their foundation during a quake.

For houses with short (less than full-story height) wood-framed walls between the foundation and the wood-framed first floor—in the crawl space under the house—a retrofit is needed to strengthen this area and make it less vulnerable to shake damage.

Both stem wall and crawl space wall (cripple wall) houses may require bolting. Foundation anchor-bolts are installed to these walls.

If a house has crawl space walls, in addition to bolting, plywood bracing may also be needed to further protect against earthquake damage. The walls are braced by nailing plywood sheathing to the inside of the walls that surround the crawl space.

Post & Pier House Retrofit

A post and pier house (or post and beam house) is a type of raised foundation home, in that there is an area created underneath the dwelling floor.

The wood frame floor is supported by upright wood posts that rest on individual concrete blocks or pier pads around the perimeter and interior of the crawl space.

The wooden posts or concrete piers are set into the ground to bear the weight of the house. But because a post and pier house has no continuous concrete-perimeter foundation, it is particularly vulnerable to shifting, and potentially collapsing, in an earthquake.

A post and pier retrofit involves adding new continuous concrete footing all around the crawl space, and adding new plywood bracing on new crawl space wall studs all around the crawl space.

IS A POST AND PIER HOUSE RETROFIT WORTH IT?

Post and pier houses in California’s hazard areas are vulnerable to damage from earthquake shaking. In the event of a large earthquake, this could mean major structural damage or destruction to vulnerable houses, and devastation in communities across the quake’s shake zone.

If your home is damaged, you may not be able to live inside it while repairs are made. And remember, while you may be able to take out loans to repair or rebuild, you’ll have to pay to live elsewhere all while still paying your mortgage.

Hillside House Retrofit

Houses built on hillsides or steep slopes are often set on tall, narrow posts or columns, with or without diagonal bracing, and may have walls that “step down” the hill.

This means the foundation must be able to withstand the weight of the house bearing downhill, and support the house’s entire structure, for it to not collapse or slide away.

Houses built on hillsides or steep slopes (steep hillsides) without special seismic detailing may be in danger of swaying and could even collapse during an earthquake. Two major foundation issues include:

• houses supported on one or more sides by tall poles or columns and,
• houses with a crawlspace under the floor with a very tall wall at one end and stepped walls on one or more sides.

Certain structural elements inherent in hillside houses create vulnerabilities, and many may not be properly braced to withstand earthquakes.

For example, drainage systems could weaken or shift the soil, and tree roots or soil erosion can further challenge the house’s stability and ability to resist shake damage.

These factors of a hillside house can lead to significant damage and even collapse during an earthquake.

HILLSIDE HOUSE RETROFIT DETAILS

Because they are uniquely built to the hillside or slope that they rest on or along, the retrofit for a house on a hill cannot be developed from a standard plan set.

Since hillside houses require an engineered retrofit, it is advised to hire a licensed structural or civil engineer to assess the structure, and then develop a particular plan for your specific house’s retrofit needs.

A house partially or fully supported on poles or columns without engineered cross bracing may require a retrofit with new bracing elements or new foundations and shear walls along the perimeter of the house.

A house with a tall crawlspace under the floor may require the following:

1. Foundation: the hillside house will need a continuous perimeter foundation. If it already has one, the foundation must be free of deterioration and soil underneath must be intact.
2. Anchorage of framed crawlspace walls to the foundation: The crawlspace walls need to be anchored and bolted to the foundation. If anchor bolts and/or retrofit anchors already exist, they will be assessed to make sure they are in accordance with modern building codes.
3. Braced wall sheathing: Crawlspace walls need to be braced with materials that can better withstand shaking. The wall sheathing will be assessed to see if it meets modern standards. If it does not, the walls can be replaced or updated to be compliant.