As a general category of structures, unreinforced masonry (URM) buildings are typically considered to have the worst performance. The masonry walls are typically rigid and are unable to deform without cracking. In addition, the roof and floor diaphragms are often not anchored to the walls, which results in separation of the walls from the building. Several jurisdictions such as Los Angeles and San Francisco now have mandatory requirements for the seismic upgrade of this category of structure. Typical damage to unreinforced masonry structures is as follows:
- Parapets (collapse of parapets can occur even in moderate shaking)
- Floor or Roof to Wall Separation (inadequate anchorage to roof and floor diaphragms otherwise known as out-of-plane failure)
- Collapse and Falling Damage (loose bricks or portions of walls)
- Wall and Pier Cracking (diagonal cracking in narrow piers)
- Diaphragms (failure of wood framed roof and floor systems.
