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Posted by Alan Burr, SE, MIStructE on Tuesday, March 20, 2012

Case Study: Soft-Story Building Retrofit in San Francisco's Marina District

Soft-Story Building Retrofit in Marina District

The 2010 Edition of the San Francisco Building Code includes a provision for the Voluntary Seismic Upgrade of Soft-Story wood framed buildings. The provision, which is documented in Administrative Bulletin AB-094, provides a definition of soft-story buildings, and design and acceptance criteria for seismic upgrades to these buildings. AB-094 also defines what is required for building owners to qualify for incentives for voluntary seismic upgrade work permit processing.
This article presents a case study of the seismic evaluation and retrofit for a typical soft-story building in the Marina District of San Francisco.

“Soft-Story” Defined

Buildings defined as soft-story buildings in AB-094 include

  • Type V (wood framed) buildings constructed before May 21, 1973, and with an open ground floor.

  • At the ground floor, the building must have at least 50% of the floor area as • Occupancy Classification A (assembly), • B (business), • M (mercantile), • S (storage, open, parking) or • U (private garages).

Many buildings in San Francisco and the Bay Area qualify under these categories, including multi-unit residential buildings with parking or retail businesses at the ground floor level, such as our case-study building.

Codified Incentives to Retrofit

AB-094 includes provisions for streamlining the permit process for soft-story retrofits. This makes it easier for owners to obtain permits by expediting the plan review and reduces the cost of the permit by exempting owners from Planning and Plan Review fees. In addition, AB-094 contains a provision that owners who take advantage of the voluntary program would be exempt for 15 years from compliance with any subsequent legislation that imposed mandatory seismic retrofit upgrades for soft-story wood-framed buildings.

Soft-story Retrofits may become Mandatory

The San Francisco Department of Building Inspection is considering making AB-094 mandatory. The intent of this is to • improve the seismic performance of multi-family residential properties, • reduce the number of residences with severe damage and which will likely be uninhabitable, and • thereby reduce the number of people requiring temporary housing after a major earthquake.

Other Good Reasons to do the Work

We know that a major earthquake in the Bay Area is inevitable and that it will cause significant harm and economic loss. We believe that there is a moral obligation to address this known seismic hazard as soon as possible. We also believe that there are economic as well as life-safety benefits to doing this work now, such as reducing building damage and repair costs and the benefit of being able to plan for the work in advance. There may also be insurance cost benefits for retrofitted buildings. If AB-094 does become mandatory, then the cost of retrofitting soft-story buildings will inevitably rise as contractors take advantage of the captive market.

Case Study - Building Description

The case-study building is a four story wood-framed, multi-family residential building with 21 units in three stories over a garage at street level. Figures SK-1 and SK-2 show plans of the building at street level and at a typical residential level. The construction is wood frame, with wood beams supporting the second floor framing and above. Partitions at the upper levels are a mixture of plywood and gypsum board. Most of the partitions at the garage level have very limited shear capacity. From the plans there is clearly a significant number of partitions in the transverse direction at the upper residential levels of the building compared to at the garage level. At the residential levels most of the partitions stack over the three upper floors and do not align with the limited number of partitions at the garage level. Also, many of the walls at the garage level are non-structural partition walls and many of these walls are located in the storage area to the rear of the building, contributing to a lack of structural stiffness at the garage level. In the longitudinal direction there is no soft-story condition, due to the side walls which run full length on both sides of the building.

In the Marina and other areas of San Francisco there are many corner buildings which have open storefronts on both street façades. These buildings may have soft-stories in both directions, requiring additional seismic retrofit work.

Applicable Code Standard

AB-094 specifies that the stiffness and strength of the partitions and walls at each level be determined and compared using the methodology in ASCE 31-03, standard for the Seismic Evaluation of Existing Buildings. From the analysis and per the definition in the code, the building was found to have a soft-story condition in the transverse direction.

Proposed Seismic Retrofit

Based on the analysis a structural design was performed and structural plans prepared for a seismic retrofit of the building to correct the soft-story condition. The basic components of the retrofit are as shown in SK-3. The proposed seismic retrofit consists of providing a new lateral-load-resisting structure at the front of the building and shear walls at the interior of the building. The new structural system at the front of the building consists of a structural steel moment-resisting frame around one of the two garage doors. The new interior plywood shear walls will utilize existing stud walls where these are in good condition. All new elements will require new foundations. The new steel frame and shear walls provides additional structural stiffness and strength to the building it its transverse direction at the ground floor level and correct the soft-story condition for the building.

Because this building is located in the Marina District, the soil conditions below the building were also considered. The building was found to be on loose sandy soil with a high potential for liquefaction. The geotechnical engineer determined by testing that the potential for liquefaction induced settlement is about 8” and that there is also the possibility of a loss of bearing capacity at the site during a large earthquake. Foundation options for mitigating this hazard include: 1) constructing an 18” thick mat foundation slab below the building across the entire site, in order to improve the soil below the mat, 2) constructing a mat foundation without soil improvement, 3) constructing a grid foundation system of grade beams supported on piles and 4) constructing a grid foundation system of grade beams without piles. The anticipated performance of these four options varies from 1 to 4, with 1 having the best performance and 4 having the worst. The cost of the four options also varies from 1 to 4, with 1 having the highest cost and 4 the lowest. Ultimately, the selection among these alternative foundation systems will be based on the owner’s budget and desired performance for the building.

As a case-study, this building is considered typical of the soft-story conditions that may be encountered at similar buildings throughout the City and Bay Area. The soil conditions and recommendations for foundations however are not typical, but are specific for buildings on poor soil conditions that are subject to liquefaction. The foundation requirements for buildings not on liquefiable soil would be more conventional, consisting of spread footings and grade beams.

References
  • City of San Francisco Administrative Bulletin, AB-094 dated January 1, 2012
  • Here Today - Here Tomorrow: Earthquake Safety for Soft-Story Buildings prepared for the Department of Building Inspection (DBI) City of San Francisco by Applied Technology Council (ATC) February 19, 2009

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