Soft story buildings and structures with unbraced cripple walls are among the most seismically vulnerable building types in the United States. Both conditions create situations where one level of a structure is significantly weaker than the levels above it, a mismatch that concentrates deformation and damage at that weak story during an earthquake.
Retrofit programs to address these conditions are mandatory in several California cities and strongly encouraged or required by code in many other high-seismic jurisdictions. Understanding what the structural engineering behind a retrofit involves helps property owners, contractors, and construction managers prepare for what the design process requires and what the completed retrofit changes about the structure.
What a Soft Story Building Is
A soft story is a story in a multi-story building that is substantially less stiff than the stories above it. The most common example is a multi-story apartment building with an open ground-floor garage or parking area, where the ground floor has fewer walls and more open space than the residential floors above. The upper floors are relatively rigid because they have interior and exterior walls distributed around the floor plan. The ground floor is soft because the open garage area eliminates most of those walls.
During an earthquake, the relatively flexible ground story absorbs most of the building’s lateral displacement. In severe shaking, this concentrated deformation can lead to partial or complete collapse of the ground story, while the upper stories remain intact. This failure mode was responsible for significant damage and casualties in the 1994 Northridge earthquake and has been extensively studied since.
What a Cripple Wall Is
A cripple wall is a short wood-framed wall that sits between the concrete or masonry foundation and the first floor of a wood-frame building. The wall creates crawl space access to the underside of the building. In older construction, particularly pre-1950s homes, cripple walls were not designed with adequate diagonal bracing to resist lateral seismic loads.
When subjected to earthquake shaking, an unbraced cripple wall can rack and collapse, causing the structure above to drop off its foundation. The collapse mechanism is sudden and can render a building irreparable. Cripple wall failures were widespread in wood-frame residential construction during the 1989 Loma Prieta and 1994 Northridge earthquakes.
Mandatory Retrofit Programs
Several California cities have adopted mandatory retrofit ordinances requiring soft story and cripple wall retrofits within defined compliance timelines. Los Angeles, San Francisco, San Jose, Berkeley, Fremont, and Oakland have all enacted mandatory programs of varying scope. Other jurisdictions, particularly in California, Nevada, Washington, and Oregon, have voluntary programs or building code provisions that strongly encourage retrofits of identified vulnerable buildings. Property owners in affected jurisdictions should confirm their compliance status and timeline with the local building department. Our engineering team works with property owners and contractors in high-seismic jurisdictions across the country to deliver permit-ready retrofit packages.
What the Structural Engineering Involves
Existing condition analysis
The retrofit design starts with an evaluation of the existing structure. For soft story buildings, this means documenting the wall layout and construction type at each level, identifying the open or weak areas at the soft story, and establishing the relative stiffness of each story. For cripple wall retrofits, this means documenting the cripple wall height, stud spacing, and the presence or absence of any existing bracing.
In most cases, an on-site assessment is required to confirm the as-built conditions. Original construction drawings are rarely available for buildings more than 40 to 50 years old, and field conditions often differ from what limited records exist. The engineer must verify conditions before the retrofit can be designed.
Seismic demand calculation
The engineer calculates the seismic demand on the building using either ASCE 7 procedures or, for soft story apartment buildings under mandatory retrofit programs, the Los Angeles Department of Building and Safety (LADBS) Ordinance procedures or FEMA P-807 methodology for flexible diaphragm buildings. The seismic demand is compared to the capacity of the existing structure at the soft story or cripple wall level to quantify the deficiency.
Retrofit design
The retrofit design adds lateral-force-resisting elements to the deficient story to bring it into compliance with the required performance objective. Common retrofit elements include:
Plywood shear panels: Structural plywood or OSB applied to the interior or exterior face of cripple wall studs or soft story framing to provide in-plane shear resistance. Shear panels are the most common cripple wall retrofit element.
Steel moment frames or braced frames: For soft story buildings where wall space is limited and openings must be maintained, steel moment frames or concentric braced frames are installed at the ground floor to provide lateral resistance without requiring new shear walls.
Hold-down anchors: Connections between the shear panels or frames and the foundation must transfer the overturning forces from lateral loading into the foundation. Hold-down hardware connects the vertical framing members to the foundation or to the floor framing below.
Mudsill anchors: For cripple wall retrofits, the connection between the cripple wall sill plate and the concrete foundation must be anchored to prevent sliding. Expansion anchors or epoxy anchors are installed through the mudsill into the concrete at required spacing.
Connection detailing
Connection design is where much of the engineering effort in a retrofit concentrates. The force path from the roof through each level to the foundation must be continuous and documented. Each connection in that force path must be sized to transfer the required load without failing. For older construction where the existing connections are inadequate, new hardware must be specified at each discontinuity.
What the Permit Package Includes
A soft story or cripple wall retrofit permit package typically includes: existing condition drawings documenting the current state of the structure at the retrofit story; structural calculations establishing the seismic demand and the required retrofit elements; retrofit plan drawings showing the location, dimensions, and specifications of every new structural element; connection details showing how each new element attaches to the existing structure; and a PE stamp from a licensed structural engineer. Our permit design services include retrofit engineering packages that meet the requirements of both standard IBC review and mandatory retrofit program compliance.
Right Angle Engineering and Seismic Retrofit Projects
Right Angle Engineering provides PE-stamped structural engineering for soft story and cripple wall retrofit projects in seismic zones across the country. Our licensed structural engineers deliver packages that satisfy both standard building permits and mandatory retrofit program compliance requirements. Start a project with our team to discuss your building’s conditions and retrofit requirements.
Frequently Asked Questions
What makes a building a soft story building?
A soft story building has one story that is substantially less stiff laterally than the stories above it. The most common example is a multi-family building with an open ground-floor parking or garage area, where the reduction in structural walls at that level creates a significantly weaker story compared to the residential floors above.
Is a soft story retrofit required by law?
In some jurisdictions, yes. Several California cities including Los Angeles, San Francisco, San Jose, and Berkeley have enacted mandatory soft story retrofit ordinances with compliance timelines. Property owners in these jurisdictions must retrofit or demonstrate compliance by the applicable deadline. Many other high-seismic jurisdictions have voluntary programs. Confirm the requirements for your specific jurisdiction with the local building department.
How long does a soft story or cripple wall retrofit take?
Engineering and permitting typically take four to ten weeks depending on the building complexity and the AHJ’s review timeline. Construction for a cripple wall retrofit on a single-family residence is often completed in one to three days. Soft story retrofits on multi-story apartment buildings may take one to four weeks for construction depending on the number of units and the retrofit approach.
Does a cripple wall retrofit require moving out of the home?
In most cases, no. Cripple wall retrofits are performed in the crawl space below the structure and do not require occupants to vacate. The work is typically completed in a day or two for a standard residential structure. Soft story apartment retrofits are more variable: work in garages and parking areas generally does not require unit vacancy, but structural work at the open first floor may require temporary relocation of vehicles and ground-floor occupants during construction.
What documentation does an engineer need to design a seismic retrofit?
The engineer needs the site address, building age and construction type, any available original drawings or permit records, and access to the building for an on-site condition assessment. For mandatory retrofit programs, the engineer also needs to confirm which ordinance applies and what performance objective the retrofit must meet.
References
Los Angeles Department of Building and Safety. Mandatory Retrofit Programs.