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Concept
Unreinforced masonry building
Summary
An unreinforced masonry building (or UMB, URM building) is a type of building where load bearing walls, non-load bearing walls or other structures, such as chimneys, are made of brick, cinderblock, tiles, adobe or other masonry material that is not braced by reinforcing material, such as rebar in a concrete or cinderblock. The term is used in earthquake engineering as a classification of certain structures for earthquake safety purposes, and is subject to minor variation from place to place.
URM structures are vulnerable to collapse in an earthquake. One problem is that most mortar used to hold bricks together is not strong enough. Additionally, masonry elements may "peel" from the building, and fall onto occupants or passersby outside.
In California, the 1933 Long Beach earthquake resulted in a near-immediate statewide ban on construction of new unreinforced masonry school buildings. A State law enacted in 1986 required seismic retrofitting of existing structures. Retrofits are relatively expensive, and may include the building being tied to its foundation, tying building elements (such as roof and walls) to each other so that the building moves as a single unit rather than creating internal shears during an earthquake, attaching walls more securely to underlying supports so that they do not buckle and collapse, and bracing or removing parapets and other unsecured decorative elements. Retrofits are generally intended to prevent injury and death to people, but not to protect the building itself. According to the 2006-04 California seismic safety commission report, there are still 7800 URM buildings with no retrofitting in the state, including 1100 in the city of Los Angeles.
The California law left implementation and standards up to local jurisdictions. Compliance took many years, and as of 2008, most (but not all) of the unreinforced masonry buildings in San Francisco have undergone retrofitting.
There is particular cause for concern in regions which can generate strong earthquakes, but only rarely.
Official source
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