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

Over 7 Stories, Built After 1939

Building Damage Varies by Construction Type

Similar to the post-1940 buildings under seven stories, this generalized building type includes a wide variety of construction systems. In addition to the systems described under the category of non-wood-framed buildings built after 1939 but only four-to-seven stories in height (steel frame, reinforced masonry and poured-in-place reinforced concrete), the use of pre-cast reinforced concrete has occurred in apartment buildings above seven stories, particularly large elderly housing complexes.

Construction and Performance of Buildings of Precast Concrete

Precast concrete structures are constructed by either welding steel together at the connections or by pouring in place concrete closure strips at the connections of the precast elements. Various portions of the structure may be poured-in-place concrete, including topping slabs. Beams, columns and exterior panels are often precast.

The principal seismic concerns with precast elements are the joints and connections of the precast elements. While precast buildings may sustain actual forces several times greater than the design levels, the joints may not have the necessary strength and ductility to withstand the event without damage or separation. Precast concrete beams with cracking at the joint with the column represents a potential loss of vertical support for the beam and the structure as a whole. Also of concern is the potential hazard posed by the racking and separation between the concrete frame and the precast exterior panels.

Construction and Performance of Buildings with Concrete Frames

Pre-1975 reinforced concrete moment-resisting frame 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. These buildings are subject to collapse in the event of an earthquake.

Construction and Performance of Buildings with Steel Frames

Post-1940 steel frame apartment buildings over 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.

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.

Buildings in the San Francisco Bay Area

Units in buildings of this type represent 1.0% of the total Bay Area housing stock. Their presence is significant only in the County of San Francisco where they represent 5.3% of the total stock, and 8% of the multi-family 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.