Tackling Seismic Retrofits
- Sep 29, 2014
By Homer Yim, Territorial Manager, Simpson Strong-Tie
Twenty-five years ago, the Loma Prieta earthquake struck the San Francisco Bay Area. Lasting only seconds, the quake measured 6.9 on the Richter scale and left thousands of residents homeless. The images were unforgettable: Neighborhoods on fire, failing bridges, and homes in the Marina District collapsing in heaps of shattered wood.
Almost immediately following the quake, the City of San Francisco initiated the Community Action Plan for Seismic Safety (CAPSS) working group to study impacts from the next major earthquake, which included a 30-year resiliency plan to prevent the collapse of buildings in San Francisco’s neighborhoods during the next, inevitable quake.
The City of San Francisco, under the City Administrator’s office of the Earthquake Safety Implementation Program (ESIP), recently launched the Mandatory Soft Story Retrofit Program under the recommendations of CAPSS.
San Francisco is a unique construction environment. The City is 98 percent built out with little or no new residential construction, while more than 60 percent of the population are renters. The Mandate aims to protect the unique look of San Francisco’s older buildings as well as the safety of its renters and business occupants. If a quake were to occur without these retrofits, many structures could collapse, and displace renters and businesses, while newer, more expensive properties would be built in place of the older, destroyed buildings. This could potentially change the character of both the architecture and demographics of the city.
Other cities and municipalities along the West Coast, from British Columbia to Southern California, are considering similar mandates to help reinforce soft-story buildings.
As a building owner, whatever your location, it makes sense to learn more about what a soft-story building is and how you can take early steps to reinforce your portfolio of properties, with or without a mandate.
What is a soft-story building?
Soft-story buildings are buildings built prior to recent building codes where the upper floors typically sit on a “soft story” such as a garage or open commercial space with windows. These buildings have more than 80 percent open area on one first story wall or more than 50 percent open area on two adjacent first story walls. These soft-story floors do not offer enough structural support for the floors above, making the building more prone to damage or collapse during an earthquake. In San Francisco, for example, these buildings typically contain five or more residential units that were permitted for construction prior to January 1, 1978. These structures, largely in San Francisco’s Marina, Mission and Panhandle districts, are vulnerable and at risk since they have been weakened by large openings such as garages or storefronts.
Elements of a soft-story building
A soft-story building usually:
- Is a wood-frame structure
- Has two or more stories over a “soft” or “weak” story, e.g. a garage or commercial space with many windows
- Has residential units or commercial businesses in upper floors
In San Francisco, thousands of building owners are required to retrofit their soft-story buildings during a specific timeframe. In other parts of the country, this hasn’t occurred yet, but owners can still take a proactive approach to ensure their residents are safe and their soft-story properties are secured. Though building owners in San Francisco have prescribed steps to follow for a successful soft-story building retrofit, here are some general steps to think about moving forward on a soft-story retrofit:
Reach out to your engineering team or partner with an engineer.
Your building needs to be structurally evaluated by a registered civil or structural engineer. Structural Engineers Association of California is a good resource for finding engineers in California. The engineering criteria, to meet the requirements of the San Francisco ordinance can be found at http://sfdbi.org/mandatory-soft-story-program. One of the options is FEMA P-807, which is also detailed in the city’s administrative bulletin AB-107 pursuant to 3406B.3 of the ordinance. This engineering option has a software tool that enables the engineer to evaluate the building and provide recommendations, if needed, on what structural elements are needed for a retrofit. This tool is unique in including existing material such as stucco, drywall, siding, or sheathing in the structural evaluation of the building. Simpson Strong-Tie introduced a functionality to this tool for engineers to specify the latest moment frame technology.
Choose an experienced building team.Look for a contractor with soft-story seismic retrofit experience. And before your contractor begins work, be sure to connect them with your structural engineer and/or architect, whose expertise will be needed to guide the builder. This team needs to be aligned to ensure your retrofit is done correctly. Choosing the right team will help avoid callbacks or costly delays, while minimizing project risks and costs.
Choose the right products. Work closely with your retrofit team when choosing retrofit products and technologies. A weak story in a soft-story building can be strengthened by using a system that includes prefabricated shearwalls built from braced panels, anchoring solutions for masonry, and special moment frame technology. Moment frames are typically made from beams and columns that have been welded and/or bolted together. They prevent collapse in a seismic event for their strong connections and ductile behavior. For instance, the Simpson Strong-Tie Strong Frame® special moment frame is bolted into the structure, helping reinforce large openings in soft-story buildings. This allows residents to stay in the building while the work is being completed. This particular product also doesn’t require any welding, which limits the risk of accidental fire.
These are simple steps to help mitigate your retrofit. Work with your engineer to determine if you own a soft-story building and if it needs a seismic retrofit. If you choose an experienced team and the right suite of products, the process should be smooth and the least disruptive to tenants. Although the costs associated with soft-story retrofits can vary, the end result will help you provide a safer building for your tenants and ultimately decrease your liability during an earthquake.
Homer Yim is a Simpson Strong-Tie Territorial Manager. Simpson Strong-Tie is a member of the Community Action Plan for Seismic Safety (CAPSS) working group and supports the industry efforts of the Earthquake Safety Implementation Program (ESIP), under the City of San Francisco’s Administrator’s office.