ABAG Shaken Awake! Report -
Unreinforced Masonry Buildings

Typical Unreinforced Masonry Building 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.

Typical Damage to Unreinforced Masonry Building

Source -- Courtesy of Degenkolb Associates

Unreinforced Masonry Construction

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. For this report, URM buildings are 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.

Unreinforced Masonry Buildings in the San Francisco Bay Area

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.

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 9/26/03 by jbp.