Guide to Earthquake Vulnerable Housing Types

Certain building types are particularly vulnerable to damage in earthquakes. This page provides a summary of some of these building types. Hover over the photos to see examples of damaged buildings.

Take a quiz to find out if your home is at risk for damage in an earthquake. shaking, ground failure (liquefaction, landslide, fault rupture) and type of construction all contribute to housing damage.
Find out how many housing units in the Bay Area may become uninhabitable in an earthquake.

Jump to: single family living space over garage | single-family cripple wall | soft-story wood frame multi-unit | non-ductile concrete multi-unit buildings | other construction types

Single Family House Over Garage

This single-family housing type is very common in the Richmond and Sunset districts of San Francisco, as well as some dense single-family neighborhoods of the East Bay. It is also common in suburbs as a ranch-style home with rooms over an attached garage. The garage lacks the interior walls of the living space above it which may be unable to support the living space above it during an earthquake.

Expected damage: Ground shaking could cause the garage to sway and collapse, damaging the living area above.

Retrofit Approach: Retrofitting typically comes in the form of strengthening the garage walls by adding bracing elements or stiffening, such as shear walls, steel frames, or cantilevered columns. This type of retrofit typically requires evaluation by a professional engineer.

Strengthening your single-family home
Wood frame single-family homes in earthquakes

Single-Family Homes | Cripple Walls

Older (usually pre-WWII) houses are often not bolted to their foundations and lack bracing on the wood framed exterior walls enclosing the crawl space (cripple wall). Presence of a cripple wall is usually indicated by the presence of a crawl space below the home and/or stairs leading to the front door.

Expected damage: Damage can include the home sliding off its foundation or the collapse of the cripple walls.

Retrofit approach: Structural retrofit of these homes typically includes a combination of adding anchor bolts to prevent sliding off the foundations, adding plywood sheathing along portions of the cripple walls to prevent collapse of those walls, and using clips to attach the cripple wall to the floor above. Cripple wall solutions can be designed by a contractor and generally do not need an engineer unless the cripple wall is over 4 feet in height.

Strengthening your single-family home
Wood frame single-family homes in earthquakes

Soft-Story Wood Frame Multi-Unit Buildings

A soft-story residential building is one that has large openings on the first floor, typically for parking or commercial space, with residential units on the upper floors. In some cases, the first floor may also contain residential units. Most were built prior to 1990. More information about soft-story buildings.

Expected damage: Ground shaking causes such structures to sway and may cause the ground story to collapse, damaging the floors above it as well.

Retrofit approach: Structural retrofitting for muti-unit buildings is more complex than for houses and should be designed by an engineer. Retrofitting buildings with large openings involves adding bracing elements (steel frames or shear walls) at the lowest story level and tying the bracing to the floor above. In the weakest buildings, some upper story walls may also need strengthening.

Soft-story buildings in the Bay Area and what local governments are doing to strengthen them

Non-Ductile Concrete Multi-Unit Buildings

Concrete structures need embedded steel reinforcing bars to add ductility, or the ability to bend without breaking. Many pre-1980 concrete structures may not contain adequate reinforcement and may be deadly in an earthquake.

Expected damage: Lateral movement from earthquake shaking can put too much strain on non-ductile concrete buildings, pushing them past their breaking point and causing catastrophic collapse.

Retrofit approach: Retrofitting non-ductile concrete buildings includes jacketing or wrapping concrete structural columns to improve strength and ductility of the columns or adding interior walls to increase the strength and ductility of the entire structure. Non-ductile concrete structures are more varied than wood frame structures and evaluation or retrofit design often requires sophisticated engineering.

Non-ductile concrete buildings in earthquakes
The Concrete Coalition is assessing older concrete buildings in California

Other Construction Types

Other vulnerable residential construction types include unreinforced masonry buildings, hillside houses, and mobile homes. Mobile homes installed prior to 1995 that are not well tied to their foundations are prone to shifting off their supports in earthquakes, mobile home retrofit guidance can help keep your house secure. Non-structural elements such as water heaters and brick chimneys may also cause damage in an earthquake.

Last updated: 09.05.2014