After the magnitude 6.0 South Napa earthquake at 3 am on August 24th, two large historical buildings in downtown Napa were red-tagged and deemed unsafe because of falling debris onto the sidewalk. The falling bricks posed a life-safety risk, and we can only imagine the potential danger to pedestrians if the quake had occurred at 3 pm instead of 3 am. It is important to note that these two older buildings (“unreinforced masonry” buildings, or URM buildings) had previously been seismically retrofitted.
We can see how URM buildings performed in the South Napa earthquake, to understand what may happen with similar URM buildings in the event of an earthquake whose epicenter is closer to Santa Clara and Alameda Counties.
What is “URM”?
URM stands for “unreinforced masonry,” and brick or other masonry that is unreinforced is highly brittle and very susceptible to damage due to earthquake forces.
Here are some additional characteristics of URM buildings:
- Of late 1800s-early 1900s vintage and constructed of wood-frame floors and roofs with unreinforced, (typically brick) exterior walls.
- Most common earthquake damage is brick walls pulling out of plane from the building and collapsing in a pile of rubble. Such buildings are also vulnerable to complete collapse.
- As of the mid-to-late-1940s, they were usually privately-owned buildings because public officials recognized the danger to the public and facilitated their retrofit as early as the late-1930s onward.
In the 1990s after the 1989 Loma Prieta Earthquake, most local government jurisdictions adopted mandatory retrofit ordinances requiring owners to retrofit such structures to a minimal standard that primarily mandates improved out of plane anchorage of the URM walls to the building framing.
Napa URM Building Damage
A list provided by the California Seismic Safety Commission showed 40 URM buildings in the city of Napa, as recorded because of a 2006 Napa City Ordinance. About 20 of the 40 buildings were retrofitted as of 2006.
Here are what initial screenings of damage to URM buildings found:
- Equal numbers (seven of each) of red-tagged retrofitted and un-retrofitted buildings, marking 14 of the 40 URM buildings unsafe.
- Four of the buildings were red-tagged because of adjacent unsafe buildings, and not necessarily because of issues within the structures itself.
- Two of the red-tagged buildings were downgraded to yellow-tags because issues with dangerous parapets were immediately addressed.
- In the week after the earthquake, eight of the 14 initially red-tagged were still deemed unsafe, four of them being retrofitted buildings.
As is usually the case with URM buildings, earthquake damage was observed with URM wall out-of-plane failures, typically where walls lack ties to roof and/or walls. However this also occurred in some instances in retrofitted URM buildings where ties were present, as pictured to the right.
Lessons Learned for URM Building Owners
Those URM buildings that had been seismically retrofitted prior to the earthquake in general performed better, in terms of life-safety and collapse prevention, because they showed less severe and more localized damage. For example, an entire block of URM buildings in Napa that had been comprehensively retrofitted performed fairly well. Some exceptions included URM buildings with parapets and corners left vulnerable and susceptible to damage.
Despite the fact that retrofitted URM buildings in general performed better, the earthquake confirmed the difficulty of preventing damage in URM buildings even with comprehensive retrofits. Overall, stone masonry performed below average, and there were multiple examples where straight epoxy dowels pulled out of the URM building walls or the shallow embedment of the dowels contributed to the disintegration of walls.
Other URMs in the Bay Area
Though it has now been almost 30 years since California adopted a law encouraging the retrofit of URM buildings, there were thousands of Bay Area URM buildings still in need of retrofit as of 2006.
Here are some numbers of URM buildings, as of the 2006 survey by the California Seismic Commission, in select counties of the Bay Area (source: San Jose Mercury News article)
- There were 1,520 URM buildings in Alameda County still need of retrofit; in Contra Costa County, 329; and in Santa Clara County, 102, respectively.
- However, some cities such as San Francisco, Oakland, and San Jose have made significant strides in retrofitting these buildings.
- At the time of the survey in 2006, Oakland had 1,200 URM buildings in the city, but now there are about 80 to 90 masonry buildings that have not been retrofitted. These tend to be smaller buildings where owners may have been able to demonstrate that their buildings are exempt per certain sections of the retrofit ordinance.
The August 24th earthquake was certainly a wake-up call for California. More than 200 people were injured, and while there were no immediate fatalities, one person died about two weeks after the earthquake because of injury due to falling debris inside her home. The earthquake occurred at 3 am at a time when many people were asleep, but if it had occurred during business hours we may have had a much different result, especially because of the falling stones from URM buildings, some of which had already been retrofitted. This is an opportunity to assess the seismic retrofit performance of URM buildings. It is also a wake-up call for other other states in the U.S. that lack seismic retrofit programs.
*Images courtesy of CA EQ Clearinghouse/Private Contributor
The Earthquake Engineering Research Institute (EERI) and the Pacific Earthquake Engineering Research Center (PEER) presented their initial observations on the performance of URM buildings at a reconnaissance briefing in September 2014. Here is a reference slide deck and an initial report on the South Napa earthquake, and information is also available at a virtual earthquake clearinghouse.