ABAG Shaken Awake! Report -
Non-Wood-Framed Buildings

4-7 Stories, Built After 1939

Building Damage Varies by Construction Type

This building type includes a wide variety of construction systems and materials. Even though combinations of materials are common, perhaps the most typical structural systems within this category can be classified as either being reinforced concrete (bearing wall or frame structures), reinforced masonry, or steel frame structures. Precast concrete apartment buildings under seven stories are generally less common.

Most of the buildings in this building type date from the 1960s. The 1960s was an era in which there were some innovations in construction and when the construction of multifamily buildings was extensive. Multifamily buildings under seven stories with the primary structural material being something other than wood or reinforced concrete are generally from this period.

Construction and Performance of Buildings of Reinforced Masonry

A construction system generally identified with the 1960s is that of reinforced masonry or concrete block bearing walls. This construction system involves the use of concrete blocks, or concrete masonry units (CMU) to form walls which are reinforced with steel bars and then filled with concrete. These walls are usually bearing, but can also serve as shear walls. Floor and roof diaphragms may be constructed in a variety of ways. Among the more common are steel decking with or without concrete fill, cast-in-place concrete and glulam and plywood floors. Usually these buildings are easily identified by their form: typically they are rectangular in plan, have flat roofs, and have exterior circulation balconies. Also, most often the CMUs are left exposed.

Reinforced masonry buildings are subject to far less damage than unreinforced masonry structures. Diagonal cracking of walls, particularly of shear walls and columns, indicate a loss of strength. Corner cracking in CMU walls at door and window openings can jeopardize the vertical support of floor and roof framing. In addition, separation between the horizontal diaphragms and the shear and bearing walls connections may occur. In general, performance problems are less in reinforced masonry buildings constructed after about 1978.

Construction and Performance of Buildings with Concrete or Steel Frames

Reinforced concrete or steel frame apartment buildings under seven stories are far less common than CMU construction during this period.

Of greater concern are the pre-1975 reinforced concrete moment-resisting frame buildings. These buildings were constructed prior to the lessons of the 1971 San Fernando earthquake being incorporated into new ductility requirements in the codes. Concrete moment-resisting frames gained popularity during the 1950s when designers treated concrete as a steel frame. As mentioned above, these buildings are subject to collapse in the event of an earthquake.

Steel frame apartment buildings between four and seven stories exist, but are rare. They usually will date from the 1960's, when different construction systems were tried. The lateral forces can be resisted by a braced frame, a moment-frame, or masonry or concrete-infill shear walls. In the case of braced-frame structures, diagonal bracing typically buckles, stretches, or pulls apart at the connections. After the braces buckle, deformations of the structure could become large with the consequence of greater damage.

Moment-resisting steel frames might experience considerable side sway under a severe earthquake. Severely damaged moment-frame structures may have a residual story drift causing the entire building to remain out of plumb. Steel frames with concrete or masonry shear walls have had the best experience of any type of large structure. Wall stiffness limits motion and consequent drift damage. While walls might crack, repair costs are low.

Although buildings seven stories and under, with a concrete parking podium which supports a wood frame above, are widespread during the post WW II period, for the purposes of this study, these buildings are classified as wood structures. In the 1980s, the combined use of materials, namely concrete, wood and steel, is more common in multi-family housing buildings above three stories. Wood structures that include a one or two story concrete platform, with a wood frame that has steel elements within it, is common in apartment buildings between five and seven stories. As the building increases in height so do the steel elements within it.

Buildings in the San Francisco Bay Area

As with non-wood-framed buildings built prior to 1940, units in these buildings account for only 0.2% of the total Bay Area housing stock. Their presence is significant only in the denser parts of the region. Even in San Francisco, they represent less than 1% of the total stock and only 1.1% of the multifamily stock.

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

This page is based on a 1996 ABAG report. It was last updated 9/29/03 by jbp.