Unreinforced Masonry Buildings
An unreinforced masonry (URM) building is a building made of brick, hollow concrete blocks, stone, or other masonry with no steel reinforcing bars embedded in it, making it susceptible to severe damage and collapse during an earthquake
Figure Sources: FEMA P774
The most common type of unreinforced masonry building in the United States is constructed of brick walls with wood-frame floors and roof. The brick walls bear the weight of the floors, roof, and roof beams.
An earthquake, even a minor one, can cause severe damage or collapse to an unreinforced masonry building. In many cases, the damage may require closure until repairs are made, a long and costly process. Unreinforced masonry buildings are dangerous to occupants, adjacent buildings, and to pedestrians during an earthquake. Even if the entire building does not collapse, falling masonry can be lethal. A square foot of wall weighs 120 pounds or more. During the 2001 Magnitude 6.8 Nisqually Earthquake in Washington, the most common form of damage included the following hazards:
- Brick shedding from parapets and chimneys
- Cracking in URM walls and piers,
- Damage to masonry arches
- Damage resulting from fallen bricks
For more reading: FEMA P774
The 2003 Magnitude 6.5 San Simeon Earthquake of Central California highlighted the importance and effectiveness of retrofitting. None of the nine unreinforced masonry buildings that were retrofitted in Paso Robles, the nearest affected area, experienced major damage. Meanwhile, many of the URM buildings that were not retrofitted were damaged so severely that they had to be demolished. Cities throughout the U.S. have different unreinforced masonry building retrofit requirements. In some cities in California, retrofit ordinances have been passed and their deadlines have already passed. Various cities that still have vulnerable URM buildings are currently working to pass ordinances.
Summary reports of the status of URM retrofit laws in Washington and California can be found in the following links:
Slide Show Images Source: Link