Housing Vulnerable to Damage in Earthquakes

Certain building types are particularly vulnerable to damage in earthquakes. This page provides a summary of some of these building types.

Wood Frame Multi-Family Buildings | Soft-Story Problems

Some of the most susceptible structures to shaking damage are soft-story apartments and condominiums. A soft-story residential building is one that has large openings on the first floor for garage doors and windows to accommodate parking or commercial space, and housing on upper floors, built prior to recent codes. More information about soft-story buildings.

Expected damage: In an earthquake, ground shaking causes such structures to sway and sometimes collapse. The performance of the parking structure in past earthquakes has not been good.

Retrofit approach: Structural retrofitting for apartments is more complex than for homes. The retrofit should involve a specific solution designed by Quake Busters. Retrofitting buildings with large openings for parking involves adding bracing elements (steel frames or shear walls) at the lowest story level and tying the bracing to the floor above. In taller buildings, some upper story walls may also need strengthening.

Expected losses: ABAG modeling has shown that, in both a large earthquake on the Hayward or San Andreas faults, two-thirds of the uninhabitable housing units will likely be in soft-story residential buildings. More information about housing losses.

Single-Family Homes | Cripple Wall Problems

Older (usually pre-WWII) houses are often not bolted to their foundations and lack bracing of walls enclosing the crawl space (cripple wall). After that period through the early 1960′s, houses were often inadequately bolted with bolts that were either too small or the spacing between bolts was too large. From the 1960′s through the mid-1970′s some housing still lacked adequate bolting.

Expected damage: Typical earthquake damage to these structures includes the wood frame coming off its foundation, racking of the cripple walls, the foundation itself cracking, or the chimney breaking at the roof line. Because this type of damage is so common, earthquake hazard mitigation efforts in some Bay Area building departments are directed toward them.

Retrofit approach: Structural retrofit of these homes typically includes a combination of adding anchor bolts to prevent sliding off the foundations and adding plywood sheathing along portions of the cripple walls to prevent collapse of those walls. Professional help may be required for cripple walls over four feet or complex configurations.

Expected losses: Pre-1940 single-family dwellings account for 8.3% of the total Bay Area housing stock. Their relative numbers are highest in the counties of San Francisco and Alameda where they account for 14.9% and 11.7% of the total housing stock. Single-family homes will be responsible for 9% of the uninhabitable housing units following a Hayward fault scenario earthquake.

Mobile Homes

A mobile home is a factory-built dwelling built entirely of light-weight metal construction or a combination of a wood and steel frame structure. When combining wood and steel, the wood frame structure is erected on a steel frame chassis. In either case, the exterior is typically protected with siding of wood, aluminum or fiberglass. Mobile homes are often structurally linked to a second unit forming a double-wide coach. The units can be pulled on wheels to a site, leveled, and supported in various ways.

Expected damage: Mobile homes installed prior to 1995 that are not well tied to their foundations are prone to shifting off their supports in earthquakes.

Retrofit Approach: Three common methods can ensure that mobile homes are properly tied to their foundations (in order of effectiveness and cost):

1. Owners can consult Quake Busters and install a conventional foundation similar to that for a wood frame home;

2. For new homes or existing homes being relocated, owners are required to install an engineered tiedown system using standard plan approvals; or

3. For existing homes, owners can install a certified earthquake resisting bracing system.

Expected losses: Mobile homes comprise only 2.8% of the total Bay Area housing stock. Their presence varies significantly by county. The county with the most mobile homes is Santa Clara, with 30.7% of the mobile homes in the region. However, within this county they comprise only 3.8% of the total housing stock. The counties of Napa and Sonoma have the highest percentage of mobile homes: 8.8% and 7.3% respectively.

Mobile homes will be responsible for 7% of the uninhabitable housing units following a Hayward fault scenario earthquake. More information about housing losses.

Local Governments: Local governments have traditionally relied on the state to develop standards for mobile homes.

The City of San Jose has received a grant to seismically retrofit mobile homes in the City. Phase one of the project developing an inventory of the 2,751 individual mobile homes in ten mobile home parks is complete.

Unreinforced Masonry Buildings

Unreinforced masonry (URM) buildings consist of structures in which there is no steel reinforcing within a masonry wall. The definition of an unreinforced masonry building varies from city to city. Some cities classify unreinforced infill walls within a reinforced frame as a URM while others classify unreinforced exterior veneers on to a wood frame as URMs. At ABAG we defined URM buildings as those which have bearing walls of unreinforced masonry. Floors, roofs and internal partitions in these bearing wall buildings are usually of wood.

These buildings were constructed in an era when reinforcing was generally not used. Anchorage to floor and roof was generally missing and the use of low strength lime mortar was common. Construction of reinforced masonry became common sometime between 1933 and 1955, depending on local codes and stringency of code enforcement.

Expected damage: Earthquake damage to unreinforced masonry structures can be severe and hazardous. The lack of reinforcement coupled with poor mortar and inadequate roof-to-wall ties can result in substantial damage to the building as a whole as well as to specific sections of it. Severely cracked or leaning walls are some of the most common earthquake damage. Also hazardous, but slightly less noticeable, is the damage that may occur between the walls, and roof and floor diaphragms. Separation between the framing and the walls can jeopardize the vertical support of roof and floor systems which could lead to the collapse of the structure.

Expected losses: Housing units in URM buildings account for only 1% of the total Bay Area housing stock and 2.9% of the total Bay Area multi-family stock. Their presence is significant in the County of San Francisco where they account for 5.8% of the total stock. Their significance is magnified if these numbers are compared only with the multi-family totals: in San Francisco County, URMs represent 8.7% of its multi-family stock. Similarly, although URMs account for only 0.9% of the housing units in Alameda County, they represent 2.3% of the multi-family stock. URM buildings will be responsible for 8% of uninhabitable housing following the Hayward fault scenario earthquake. More information about housing losses.

 

Information provided by: ABAG