Family homes with a room built atop the garage are common across the U.S. In an earthquake, these buildings risk collapse, but they are not currently covered by most local seismic regulations.
By Alice Turner, Simpson Strong-Tie writing fellow (@SeismoAlice)
Citation: Turner, A., 2024, Single-family homes are falling through soft story ordinance gaps, Temblor, http://doi.org/10.32858/temblor.337
Early in the morning of Jan. 17, 1994, the magnitude 6.7 Northridge earthquake shook Los Angeles. At the Northridge Meadows Apartment complex, a three-story structure located just two miles from the earthquake’s epicenter, shaking caused the columns supporting the building’s open lower level to give way, bringing the two upper stories tumbling down. Sixteen people living in first-story apartments died when the building collapsed. In nearby Sherman Oaks, nine single-family hillside houses built on stilts collapsed. Two-thirds of the buildings that were destroyed or seriously damaged in the Northridge earthquake were “soft story” buildings — multistory buildings with an open ground floor level held up by columns or stilts, or single-family homes with a room built atop a garage.
At least 10 cities across California have since introduced ordinances to retrofit multi-family residences with soft stories to make them safe in an earthquake. But in other seismically active regions across the U.S., and for single-family homes in particular, there are few or no requirements to strengthen soft story buildings, leaving many families at risk.
What is a soft story building?
Soft story buildings have open space on the ground floor for parking, retail, or a garage. This means there can’t be any interior walls. But those interior walls provide resistance to the side-to-side (shearing) motion that occurs in an earthquake.
Without those interior walls, the building is “soft and weak,” says John van de Lindt, a professor in the Department of Civil and Environmental Engineering at Colorado State University. Although the remaining columns and walls in a soft story are strong enough to support the upper floors under typical circumstances, they cannot withstand the shearing motion in an earthquake. As a result, they collapse, bringing down all the floors above. “It’s like you are pulling the rug out from underneath the building,” van de Lindt says.
Some soft story buildings are easy to spot, such as large apartment complexes with tucked-under parking or retail on the lower floor. Other soft story buildings are more subtle to identify: Single-family homes with a room above the garage count as soft story buildings.
Not all homes and apartments built with an open ground floor are prone to collapse. Modern buildings constructed with updated seismic codes feature reinforcements, usually installed during construction. Buildings from the 1970s and 1980s, on the other hand, are likely to suffer soft story-related damage or collapse. Location matters, too; although older soft story buildings are prevalent across the U.S., their risk of collapse varies depending on whether the region experiences earthquakes. The risk is greatest in seismically active regions such as the West Coast.
In the Bay Area alone, there could be as many as 140,000 residential units in 18,000 soft story buildings. There are no available estimates of the number of at-risk, soft story, single-family homes in California or other seismically active regions. Residential structures with a living space above the garage are commonly found in dense suburbs.
Current soft story ordinances in the West Coast
Since 2009, many cities in Southern California have mandated retrofits for soft story buildings. In Los Angeles, the largest of these cities, around 65% of buildings requiring retrofitting had the work completed by 2022. But there are still thousands of potentially hazardous soft story buildings in Southern Californian cities that have not yet adopted soft story ordinances.
Other seismically active areas in the U.S. are lagging behind the cities of coastal California in this regard. In Washington, Oregon, and even inland California, many cities haven’t completed retrofits or demolitions of their old brick buildings (which are especially hazardous because the mortar holding the bricks together is generally weak), let alone passed ordinances to retrofit soft story buildings. In Anchorage, which has neighborhoods with higher seismic risks than Los Angeles, approximately 73% of the buildings in one region were constructed before 1990, including many with soft stories. In a report prepared in response to the 2018 magnitude 7.1 Anchorage earthquake, scientists from the Earthquake Engineering Research Institute suggested that city councils follow the example of Los Angeles and enforce seismic assessment and retrofitting of old soft story buildings.
Retrofitting soft story buildings
To conform to retrofitting ordinances, soft story building owners need to carry out remodeling. There are “ways to fix” soft and weak stories in buildings, van de Lindt says. Owners can provide extra stability to the soft story by strengthening existing walls, adding new ‘shear walls,’ or adding a steel frame in soft areas of the building that can provide extra strength and stiffness to the soft story. But, owners must be careful, especially with older buildings, he says. “In old buildings, the bottom story is soft and weak, and the upper stories are soft and weak, too. They’re just not as soft and weak.” If the bottom floor is over-strengthened, the problem simply moves up a level.
To keep the costs down and prevent people from having to move out of their homes, engineers make compromises. In practice, the soft stories in old buildings are retrofitted but do not meet modern seismic standards. “This compromise brings the buildings up to about 60% of the current seismic standards, which is much, much better than what they had,” van de Lindt says.
Development of new ordinances
The city of Burlingame in California’s Bay Area is one of the many U.S. cities that do not have ordinances for soft story buildings. The city council is working to change that.
“In Burlingame, around half of the units in the city are condos or apartments, which is a large proportion for an American city,” says Kevin Gardiner, community development director at Burlingame. “A lot of them are soft story.” Many of these buildings are also mid-century or older and are not up to current seismic codes, he says. “If there should there be a large earthquake, people are very vulnerable.”
Burlingame is developing regulations for retrofitting soft story buildings. In doing so, the city must consider the needs and wants of all community members, including renters, apartment complex owners and managers, and homeowners, says Gardiner, who is spearheading the process. The city will also learn from what has and hasn’t worked in other cities in California. Cities that have taken a stricter regulatory approach have had better success than those that merely incentivize retrofitting, Gardiner says.
Studies in New Zealand found that voluntary programs have lower compliance; building owners in New Zealand are not always motivated to strengthen their earthquake-prone buildings (Egbelakin et al., 2011). With limited budgets, most local governments prioritize retrofitting pre-1970s and multi-occupancy buildings, taking a phased approach in which the most hazardous buildings affecting the most people are targeted first. But by focusing on the largest buildings, cities have left single-family homes behind.
Single-family homes singled out
Whereas cities across the West Coast are improving protections for highly visible soft stories, such as parking garages and high-rise apartment buildings, single-family homes have largely gone unnoticed — and unprotected.
“Most regulations target apartment buildings. But the hazard is the same for these single homes,” van de Lindt says. “You don’t want [homeowners] to think that the hazard is only for apartments and not realize that the hazard is also in their own home.”
Single-family homes are often the lowest priority for soft story ordinances. Of the more than 10 cities across California that have introduced ordinances, eight cities use language that explicitly excludes single-family homes with soft story deficiencies. (Soft story ordinances in Santa Monica and West Hollywood implicitly include single-family homes.)
The Association of Bay Area Governments, a regional planning agency for many of the local governments in the region, expressed similar thoughts to Gardiner in its soft story retrofitting guidance document. The guidance states, “if a goal is to allow the majority of residents to be able to stay in their homes after a disaster, it is important to prioritize fragile housing types that house the largest numbers of residents.”
Still, following other cities in California, Gardiner thinks the regulations in Burlingame will focus on multi-occupancy buildings like apartment complexes. “You want to have the most impact,” Gardiner says. “Go for the buildings that are the most vulnerable and have the most units in them.”
Seismic retrofit ordinances require property owners to complete the retrofits. It can be a hard — and expensive — decision. To help homeowners finance retrofits and encourage them to undertake the project, California recently introduced a statewide incentive program which, with federal funding assistance, will provide an estimated 375 total grants of up to $13,000 each to owners of two-story homes built before 2000 that have a soft story. The program is a test case for the effectiveness of incentivizing, rather than mandating, soft story retrofitting.
Twenty-eight years ago, the Northridge earthquake was a wake-up call for Southern California: soft story buildings needed to change. In the hardest hit cities, lessons have been learned, and ordinances for seismic retrofitting are being implemented to ensure that multi-occupancy soft story buildings are safer. But even in these areas, single-family homes are the lowest priority for regulations, and it is still unknown how many are unsafe.
Alice Turner is Temblor’s Simpson Strong Tie Fellow. She is a distinguished postdoctoral fellow at the University of Texas Institute for Geophysics, where she uses seismic observations to understand the mechanics of earthquakes, both on the Earth and on other planets. Simpson Strong Tie is sponsoring their second Temblor science writing fellow to cover important earthquake news across the globe.
References
Egbelakin, T., Wilkinson, S., Potangaroa, R., & Ingham, J. (2011). Enhancing seismic risk mitigation decisions: A motivational approach. Construction Management and Economics, 29(10), 1003–1016. https://doi.org/10.1080/01446193.2011.629664
Copyright
Text © 2024 Temblor. CC BY-NC-ND 4.0
We publish our work — articles and maps made by Temblor — under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) license.
For more information, please see our Republishing Guidelines or reach out to news@temblor.net with any questions.
