By Erin Kelly, P.E.
In the previous blog post, we discussed that notable earthquakes often lead to changes in the upcoming edition in the building code, or at the least spur research to that effect. What about the buildings that were approved under prior building codes, but based on what we know now, may prove to be unsafe? Cities and states have the ability to pass acts and ordinances, which can mandate the retrofit of such structures.
We saw the first instance of this with the Field Act of 1933, in which all public-school buildings were required to be upgraded to be earthquake compliant. We also saw it in the Alfred E. Alquist Extension in 1994, which mandated that all hospitals must be earthquake-code compliant by 2030—work for which is still ongoing.
Currently, the most urgent and widespread ordinances are for buildings with soft stories. These ordinances are in place in several cities throughout California, but we’ll specifically discuss those of San Francisco and Los Angeles since those are the two largest municipalities.
First, what defines a building with a ‘soft story’? A soft story is defined to be a story in which the structural stiffness is calculated to be less than 70% of the stiffness of the story above or less than 80% of the average story stiffness of the three stories above. This configuration has been shown to suffer large ground floor displacements, leading to structural damage and even collapse during a seismic event.
The 1971 San Fernando Earthquake and the 1994 Northridge Earthquake both caused significant damage to buildings with soft stories in the Los Angeles area, and the 1989 Loma Prieta Earthquake caused similar damage in the San Francisco Bay Area.
In San Francisco, the seismic ordinance applies to wood-frame buildings with three or more stories, which were permitted for construction before January 1, 1978. The ordinance was rolled out in tiers, beginning with Tier 1, which included buildings used for education, assembly, or daycare. Tier 2 consisted of buildings with 15 units or more; Tier 4 consisted of buildings with ground-floor commercial use, or buildings located in a liquefaction zone; and Tier 3 consisted of buildings not falling into one of the other tiers. About 5,000 of the buildings in the city of San Francisco were subject to this ordinance. The tiers were used to stagger the mandatory completion dates with all seismic retrofits required to be completed by this year, 2020.
The mandatory ordinance in Los Angeles encompasses a much larger amount of buildings—nearly 13,500 (compared to San Francisco’s 5,000). The ordinance applies to wood-frame buildings permitted for construction before January 1, 1978, however, this ordinance also includes buildings with two stories. The Tiers are also slightly different, including that in the Los Angeles framework, they are called Priority levels. Priority 1 is for buildings with 16 units or more; Priority 2 is for buildings with three or more stories (less than 16 units); and Priority 3 is for buildings not included in Priority 1 or 2, which would mean two-story buildings with fewer than 16 units. The Ordinance states that from time of notice, the owner has two years to submit either proof of prior retrofit, or plans to retrofit and demolish; three and a half years to obtain a permit to start either construction or demolition; and seven years to complete construction. Following this framework, retrofit completion dates in order of Priority are 2022, 2023, and 2024.
While there is still a lot of work to do, we can have confidence in the fact that progress is underway and the cities in California will be much more resilient to earthquakes in the coming years.