Impacts of California Earthquakes on Buildings from Shaken Awake!

In this 1996 ABAG study, existing residential buildings buildings were broadly classified according to the three sets of categories described below: the material of the structure of the building, the height or number of stories of the building, and the age of the building. These three categories enable classification of the Bay Area’s housing stock into the following thirteen building types.

You can select one of these building types for data on habitability following past California earthquakes, or scroll down for more information on the classification system used.

Mobile Homes
Unreinforced Masonry Buildings
Non-Wood-Framed Buildings, Four to Seven Stories, Built Before 1940
Non-Wood-Framed Buildings, Four to Seven Stories, Built After 1939
Non-Wood-Framed Buildings, Over Seven Stories, Built Before 1940
Non-Wood-Framed Buildings, Over Seven Stories, Built After 1939
Wood-Framed Buildings, Four to Seven Stories, Built Before 1940
Wood-Framed Buildings, Four to Seven Stories, Built After 1939
Wood-Framed Buildings, One to Three Stories, Multifamily, Built Before 1940
Wood-Framed Buildings, One to Three Stories, Multifamily, Built After 1939
Wood-Framed Buildings, One to Three Stories, Single-Family, Built Before 1940
Wood-Framed Buildings, One to Three Stories, Single-Family, Built After 1939
Other (Non-Classified) Construction

Classification Description

For the purposes of seismic analysis of the extensive Bay Area housing stock, buildings have been broadly classified according to three basic characteristics: 1. the nature of the building’s structural system; 2. the configuration or shape of the building; and3. the age of the building.

1. Structural Type

In very simple terms, the complete structural system of a building consists of vertical elements (such as columns and walls) and horizontal elements (such as floors, roofs, beams and girders). The material of these components, as well as the method by which they are connected, are fundamental in determining the essential characteristics of the structural system.

Because of the large variation in housing stock in the nine Bay Area counties, a detailed classification scheme which would take into account both horizontal and vertical elements of the structure, as well as the various methods by which they are connected, has not been used in this report. Rather, this project has focused on identifying the material and nature of the principal vertical structural elements. Thus, this report categorizes building structures according to the material of its principal vertical elements. This “material/structure” classification follows established groupings from the building industry. In the building industry, the nature of a structural system is generally classified according to the material of its components:

Light material; includes wood frame, mobile homes, and light metal;
Masonry; includes brick and concrete block (reinforced or unreinforced);
Steel frame; includes braced frames and moment resisting frames, with and without concrete or masonry infill walls; and
Concrete; includes cast-in-place and precast or a combination of the two.

Based on these definitions, this report classifies the various structural types according to the principal material of which the structure is built. Thus, a wood frame is classified as a “Wood” structure; a concrete structure, whether precast or poured-in-place, is classified as “Reinforced Concrete”; masonry buildings are classified as either “Unreinforced Masonry” or “Reinforced Masonry” structures; and a steel frame building is classified as a “Steel Frame” structure.

It should be noted that the common multi-family type building which consists of a concrete podium with a wood frame of several stories above has been classified within the wood-frame category: “Wood.”Two classifications used in the report which do not follow the above rules are the categories of “Mobile Homes” and “Other.” Since Mobile Homes are accounted for in the U.S. Census and are a clearly distinguishable building type with particular seismic concerns, they have been classified into a category of their own. The category defined as “Other” in the report does so according to the definition of the U.S. Census; “Other” is used in the U.S. Census when a householder lives in a structure other than in the alternatives defined by the Census Bureau. This option includes shelters such as boats, tents, caves, or railcars.

Based on these issues, this report classifies the structure type of a building into the following categories:

Mobile Homes (MH)
Wood (Wd)
Unreinforced Masonry (URM)
Reinforced Masonry (RM)
Reinforced Concrete (RC)
Steel Frame (SF)

Since wood-frame buildings are by far the largest component of the housing stock in the Bay Area (they account for up to 93% of the area’s housing stock), the structural types have been further broken down to wood and non-wood classifications. The combined three structure types of Reinforced Concrete, Reinforced Masonry and Steel Frame represent only 2% of the Bay Area’s total housing stock and they have therefore been consolidated into a single structural type: non-wood.

The five categories used in this report to classify the type of structure of the Bay Area’s housing stock are:

Mobile Homes, which represent 3% of the total;
Unreinforced Masonry, which represent 1%;
Wood, which represent 93%;
Non-wood, which represent 2%; and
Other, which represent 1%.

2. Configuration

While the structure of a building plays an important role in the mitigation of seismic forces, its configuration will likewise influence its earthquake resistance. Configuration deals with issues such as building height, building shape and general width-to-height proportions. It might also include the location and arrangement of the major structural elements within the building. Due to the size of the Bay Area’s housing stock, and due to the fact that height is perhaps the most important element in a building’s configuration, this report categorizes a building’s configuration only by its height. Accordingly, for the purposes of this report, the height of a building determines whether it is classified as:

Low (L); three stories and under
Medium (M); between four and seven stories
High (H); eight stories and over

When classifying a building according to its height, parking levels have consistently been included. Whether the parking level is below grade, half a level below grade, or above grade, it has been included in the story count of a building. Thus, for example, a building with three stories of living space and a first story of parking is considered as a four story building, or a “Medium” (M) height building.

3. Age

The age of construction plays an important role in the seismic performance of buildings. Over time, building technologies have improved, and code requirements have increased. The age of a building can give clues as to how buildings were detailed and put together. The age of a building can also suggest information about the quality of the materials used and the quality of the craftsmanship during construction. Since significant changes in construction practices were typically triggered by earthquakes, the age of a building determines whether construction occurred before or after a code change or before or after the use of a particular building technology. Age therefore can serve as an indicator of how well a building will resist earthquakes in the future.

It is generally accepted that changes in construction practices and codes occurred after World War II (W.W.II). Some of the first construction legislation predated W.W.II and was triggered by earthquakes: the Long Beach earthquake of 1933 triggered the Field Act and the Riley Act; the Uniform Building Code was first published and organized in 1927. However, almost three-quarters of the total housing stock in the United States has been built since 1940 and changes in construction practices, particularly in housing construction, did not take place until after W.W.II with the massive suburbanization of cities, and the increased demand for single family dwellings (Hayden, 1986). In the San Francisco Bay Area this number is slightly higher; almost 80% of the region’s housing stock was built after 1940.

The extensive use of plywood and other modern building materials, together with the wider existence and implementation of building codes, occurred with the large scale developments following the war. Although certain building regulations were in place before the beginning of W.W.II, and certain construction technologies were already in existence by the time of the war, it was not until the suburbanization of cities and the increased demand for housing that there were significant changes in construction practices (Gideon, 1948; Kostof, 1985; Hayden, 1986).

The significance of W.W.II as the cut point for this report in the classification of building age is corroborated by preliminary statistics from the Northridge earthquake. Insurance company statistics on percentage losses to single family homes show that there was no significant difference in performance by year of construction other than pre- and post-W.W.II buildings. These companies speculate that, to some degree, improvements in the codes and construction practices have been offset by the extensive use of complex configurations, large amounts of glass and cathedral ceilings in newer construction (personal communication, Robert Dowen).

One of the most noticeable differences between pre- and post-1940 buildings is the degree of parking within both multi-family and single family buildings. It is common for most multi-family buildings built after W.W.II to have either the entire ground floor or most of it dedicated to parking.

At the same time, owner-occupied units have been getting larger and larger in each decade since W.W.II; in 1976, 84% consisted of five or more rooms (Hayden, 1986).

Although 1940 has been identified as the key cut-off point for the report, the actual decade of construction has been catalogued for each entry. In addition, when the information was available and a precise record of construction existed, it was catalogued. Typically, the decade of construction was identified through field visits or from data obtained from building departments. Hence, a building built in 1936 has been identified as a 1930s building; a building built in 1967 as a 60s building, a building built in 1940 as a 40s building, and so on.

While the age of buildings has been identified by decade or by year, the report classifies building age according to if they are pre- or post-W.W.II (1940s) vintage. Based on the developments in the building codes and construction practices described above, World War II or, in terms of decades, 1940, has been identified as a turning point in the development of construction practices.

This report classifies the age of buildings according to two periods:

1940 to present


ABAG, the Association of Bay Area Governments, is the regional planning and services agency for the nine-county San Francisco Bay Area.

This page was last updated 9/29/03 by jbp.