FAQ soft-story and retrofit solutions

Q. What does the term “soft story” mean in the context of design and construction?


Soft-Story structures (also known as weak-story) are identified as buildings with inadequate stiffness 

(soft) and/or strength (weak) at the first floor to prevent significant damage or collapse in an earthquake. These structures have been designed for open, common space at the first floor to accommodate car parking or retail space.  The first floor framing along open areas is typically post and beam construction that are designed to support vertical loads from the structure above but provide very little lateral support.  Levels above the first floor are typically apartments above the first level that have more walls dividing the living area which provides more lateral strength and stiffness than is available in the soft/weak first story.

  

Q What types of buildings are typically involved in soft-story retrofit projects? 

The soft-story retrofit ordinances concern multi-family wood-framed residential structures such as apartment buildings, condominiums and residential/retail mixed use.  Typically the first floor space in these buildings have one or more exterior faces of the building with very narrow wall lengths, if there are any walls at all making these buildings very susceptible to excessive damage or collapse in the event of a significant earthquake. The multiple unit buildings were common construction prior to the adoption of the 1978 building code provisions and are a concern because they may house many people including families.

   

Q. When did new codes or regulations begin to appear in connection with soft-story retrofits? Where are these codes or rules in effect currently? 

The increased risk associated with Soft-Story residential buildings was emphasized in the 1989 Loma Prieta earthquake and again in the 1994 Northridge earthquake. In the wake of the Northridge earthquake the City of Los Angeles adopted procedures and criteria to voluntarily strengthen soft-story wood framed buildings.  Through the Association of Bay Area Governments (ABAG) and San Francisco Bay Area Planning and Urban Research Association (SPUR) cities in the Bay Area also identified the need to address these types of structures. Their efforts led to San Francisco adopting their retrofit ordinance in 2013.    

Q. Are such codes and regulations in the process of being adopted or discussed elsewhere?  

In 2013 the City of San Francisco adopted a soft-story ordinance.  Two years later  the City of Los Angeles adopted a similar mandate.  Other jurisdictions, including the cities of Oakland, Berkeley, Fremont, Alameda, and Santa Monica have adopted policies to identify, evaluate and/or strengthen these vulnerable buildings. 

 

Q Can you discuss some of the solutions, perhaps using examples of structure types? Also, some ideas of costs? 

Because soft-story structures are designed to provide parking for tenants the walls supporting the structure are very narrow.  To strengthen the first story a lateral load resisting system needs to be designed and installed.  To preserve the parking spaces that system needs to provide lateral bracing in very narrow wall lengths and it needs a foundation capable of providing the necessary anchorage and bearing support.  The Hardy Frame Shear Wall System includes light-gage steel “Panels” and structural steel Special Moment Frames.  Both Panels and SMF’s provide very high lateral load capacities and both should be considered as potential solutions.  Panel solutions do tend to be a little wider and they do generate higher forces on the foundation making our Special Moment Frame a better solution at very narrow wall conditions.  Panels range in cost from $300 to $450 and SMF’s can range from $4,000 to $7,000 depending on the jobs specific requirements. 

Q What kind of professional service providers are recommended as resources for such projects (e.g. contractors, architects, other?) 

Building owners who need or want to strengthen a soft-story structure should begin by contacting a structural engineer with knowledge and experience in this specific type of structural design.  The engineering firm will assess the building, create a structural design and building documents then submit the project to the building department for permit.  During the course of construction the engineer or a representative will provide some on-site inspections of critical work and will sign off on the project when complete.   

Owners will also need to select and hire a reputable contractor capable of performing the retrofit work. If possible select a contractor that has experience or even one that specializes in retrofit work.  For retrofit construction it is common that one crew of a few people will perform all structural work including concrete, installation of the shear wall systems and all required connections to the structure.    



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