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Key Takeaways
- Soft-story buildings have a weak first floor — typically due to tuck-under parking, large storefront windows, or open commercial spaces — that lacks the lateral strength to resist earthquake forces.
- The 1994 Northridge earthquake collapsed or severely damaged over 200 soft-story apartment buildings in Los Angeles, killing 16 residents and displacing thousands. It remains the defining case study for this building vulnerability.
- Multiple California cities now mandate soft-story retrofits, including Los Angeles (2015), San Francisco (2013), Berkeley, Oakland, Santa Monica, and others — collectively affecting tens of thousands of buildings.
- Common retrofit methods include steel moment frames, plywood shear walls, and steel braced frames, each suited to different building configurations and budgets.
- Costs typically range from $60,000 to $200,000+ for multi-unit apartment buildings, depending on size, method, and local requirements.
- Tenants may be temporarily relocated during construction, and many cities regulate how retrofit costs can be passed through to renters.
- Building owners should consult a licensed structural engineer to evaluate their specific building and determine the most appropriate retrofit approach.
What Is a Soft-Story Building?
A soft-story building is a multi-story structure where one floor — almost always the ground floor — is significantly weaker and more flexible than the floors above it. The term "soft story" refers to a story that has substantially less stiffness (resistance to lateral movement) than the story directly above.
In practical terms, the most common soft-story buildings in the United States are wood-frame apartment buildings built over ground-level parking. These "tuck-under" parking configurations became extremely popular in California and other western states from the 1950s through the 1970s, when land costs pushed developers to maximize lot usage by placing parking beneath residential units rather than in separate structures or surface lots.
The defining feature is the imbalance: the upper floors have shear walls (the plywood-sheathed walls of individual apartments), which provide considerable resistance to lateral forces. The ground floor, by contrast, has wide openings for garage doors, driveways, and sometimes commercial storefronts — leaving few if any continuous wall segments to resist horizontal shaking.
How to Identify a Soft-Story Building
You can often identify a potentially soft-story building by visual inspection. Common indicators include:
Tuck-under parking — The most recognizable configuration. Two or three stories of apartments sit directly above an open or semi-open parking area on the ground floor, with slender columns or posts supporting the structure above.
Large ground-floor openings — Storefronts, showrooms, or commercial spaces with extensive glass frontage and few interior walls at the ground level, beneath residential or office floors with conventional wall layouts.
Visible difference in wall coverage — When you look at a building from the street, the upper floors have regular window patterns surrounded by solid wall surfaces, while the ground floor is predominantly open space.
Construction era — Buildings constructed between roughly 1950 and 1980 are the most common candidates, though soft-story conditions can exist in buildings of any age.
Not every building with ground-floor parking is a soft-story building. The formal engineering determination depends on a stiffness comparison between floors, typically evaluated by a licensed structural engineer using criteria from ASCE 7 (Minimum Design Loads for Buildings and Other Structures). Under ASCE 7, a "soft story" exists when a story's lateral stiffness is less than 70% of the story above it, or less than 80% of the average stiffness of the three stories above. An "extremely soft story" meets an even lower threshold.
Why Soft-Story Buildings Collapse in Earthquakes
Earthquakes generate horizontal ground motion that transmits lateral forces through a building's foundation and up through its structure. Every floor must be able to resist these lateral loads and transfer them to the floor below and ultimately to the foundation.
In a soft-story building, the weak ground floor acts as the failure point. When the earthquake's lateral forces exceed the ground floor's capacity to resist them, the columns and supports in that story deform excessively — the ground floor essentially shifts sideways while the upper floors remain relatively intact. This phenomenon is called story drift.
When story drift exceeds the structural capacity of the ground floor, the columns and connections fail, and the upper floors drop onto the ground floor. Engineers call this pancake collapse — the upper stories fall as a unit, crushing the ground level and everything (and everyone) in it. Because the upper floors are relatively rigid, they often remain largely intact even as the building collapses, which means the destruction is concentrated at the ground level.
The Physics of the Problem
The issue is fundamentally about stiffness discontinuity. Earthquake engineering relies on a principle: buildings perform best when stiffness is distributed relatively evenly across all stories. When one floor is dramatically weaker than those above it, seismic energy concentrates at that weak point.
Think of it as a chain — the soft story is the weakest link, and all the seismic energy the building absorbs focuses there. The upper floors, being stiffer, move as a block, forcing the softer ground floor to absorb disproportionate displacement. The columns in the soft story weren't designed for this level of lateral movement, and they buckle or shear.
The problem is compounded by the P-delta effect: as the ground floor displaces sideways, the weight of the upper floors (the "P" or gravity load) acts on the displaced columns at an angle, creating additional overturning moment that further destabilizes the structure. Once lateral displacement passes a critical threshold, this effect becomes self-reinforcing and collapse is rapid.
The 1994 Northridge Earthquake: A Turning Point
The magnitude 6.7 Northridge earthquake struck the San Fernando Valley in Los Angeles on January 17, 1994 at 4:31 a.m. It killed 57 people, injured over 8,700, and caused an estimated $20 billion in property damage (approximately $44 billion in 2024 dollars), making it one of the costliest natural disasters in U.S. history at the time.
Soft-story apartment buildings were among the most devastating failures of the Northridge earthquake. According to damage assessments by the City of Los Angeles and FEMA, over 200 soft-story buildings suffered significant damage or collapse. The most well-known was the Northridge Meadows Apartments, a three-story, 163-unit complex where the entire structure collapsed onto its ground-floor parking level, killing 16 residents — the single deadliest building failure of the earthquake.
Full history of the 1994 Northridge earthquake
The pattern was consistent across the affected area: buildings with tuck-under parking experienced ground-floor collapse while adjacent buildings with conventional ground-floor wall configurations survived with far less damage. FEMA's post-earthquake technical reports (FEMA 352 and related documents) specifically identified soft-story wood-frame buildings as a critical vulnerability class.
Northridge was not the first earthquake to expose this weakness — the 1971 San Fernando earthquake and the 1989 Loma Prieta earthquake both damaged soft-story buildings — but the scale of loss in 1994 drove a fundamental policy shift. Over the next two decades, cities across California began adopting mandatory retrofit ordinances specifically targeting this building type.
Mandatory Soft-Story Retrofit Programs
Following the Northridge earthquake, seismic engineers, city planners, and policymakers began developing mandatory retrofit programs. San Francisco led the way with its mandatory screening and retrofit ordinance in 2013, and Los Angeles followed with its landmark program in 2015.
Cities with Mandatory Soft-Story Ordinances
| City | Ordinance Year | Estimated Buildings Affected | Compliance Deadline | Status |
|---|---|---|---|---|
| San Francisco, CA | 2013 | ~4,900 | Tiered: 2017–2020 | Most completed; enforcement ongoing |
| Los Angeles, CA | 2015 | ~13,500 | 2022 (varies by tier) | Majority retrofitted; extensions granted |
| Berkeley, CA | 2005 (screening); 2014 (mandatory) | ~300 | 2019 (extended) | Largely completed |
| Oakland, CA | 2019 | ~1,400 | Tiered: 2024–2027 | In progress |
| Santa Monica, CA | 2017 | ~1,800 | 2021–2024 (tiered) | Active enforcement |
| West Hollywood, CA | 2016 | ~800 | 2022–2024 (tiered) | Active enforcement |
| Pasadena, CA | 2020 | ~300 | 2025–2028 (tiered) | In progress |
| Beverly Hills, CA | 2017 | ~300+ | 2022–2024 (tiered) | Active enforcement |
Note: Building counts and deadlines are approximate and may have been updated since original ordinance adoption. Check with your city's building department for current requirements.
San Francisco's Mandatory Soft Story Program
San Francisco's Mandatory Soft Story Retrofit Program (Administrative Bulletin AB-094) was enacted in 2013 and applies to wood-frame buildings with five or more residential units built before January 1, 1978, that have soft or weak stories. The San Francisco Department of Building Inspection (DBI) identified approximately 4,900 buildings meeting the criteria.
The program was organized into four compliance tiers based on building size and occupancy:
- Tier 1: 15+ units — earliest deadline
- Tier 2: Educational, institutional use
- Tier 3: 5–14 units
- Tier 4: 3–4 units (later added)
Building owners were required to submit engineering plans, obtain permits, and complete construction within the deadlines. San Francisco provided technical guidelines (Standard Plans) that allowed for streamlined permitting of common retrofit configurations.
San Francisco Mandatory Soft Story Retrofit Program
Los Angeles Mandatory Retrofit Program
Los Angeles adopted Ordinance No. 183893 in 2015 as part of Mayor Eric Garcetti's Resilience by Design initiative. The ordinance requires retrofits of wood-frame soft-story buildings with two or more stories and five or more units, built under building codes predating January 1, 1978.
The program identified approximately 13,500 buildings — the largest mandatory soft-story retrofit effort in U.S. history. Compliance timelines were tiered based on building classification, with most buildings given a series of deadlines for engineering assessment, permit submittal, and construction completion.
The City of Los Angeles Department of Building and Safety (LADBS) administers the program and provides resources for building owners, including retrofit guidelines and approved standard plans for common building configurations.
LA Soft-Story Retrofit Program
Retrofit Methods for Soft-Story Buildings
There are several established engineering approaches to retrofitting a soft-story building. The appropriate method depends on the building's configuration, size, the degree of stiffness deficiency, architectural constraints, and budget. A licensed structural engineer must evaluate the building and design the retrofit.
Steel Moment Frames
Steel moment frames are the most common retrofit solution for soft-story apartment buildings with tuck-under parking. A moment frame is a rigid steel frame — typically consisting of columns and a beam connected with fully welded or bolted moment connections — that resists lateral forces through the bending strength of its members and connections.
How it works: The steel frame is installed within or adjacent to existing parking openings. The frame's rigidity adds lateral resistance to the ground floor without requiring the openings to be filled in, so parking access is generally preserved.
Advantages: Preserves parking functionality and open floor plans. Relatively straightforward engineering and construction. Well-understood performance.
Limitations: Frames can be architecturally visible. Requires foundation work (new footings) to resist the concentrated loads. Higher material costs than plywood solutions.
Typical application: Multi-unit apartment buildings with tuck-under parking where maintaining open parking areas is necessary.
Plywood Shear Walls
Plywood (or structural wood panel) shear walls are sheets of structural-grade plywood nailed to a wood frame to create a rigid wall panel that resists lateral forces. In a soft-story retrofit, new shear walls are added at the ground floor to increase its lateral stiffness.
How it works: New plywood-sheathed wall segments are constructed in the ground-floor area, typically at strategic locations determined by the structural engineer. The walls are anchored to new or reinforced foundations below and connected to the floor framing above.
Advantages: Lower material cost than steel. Well-suited to smaller buildings or situations where some ground-floor openings can be sacrificed. Familiar construction techniques for most contractors.
Limitations: Reduces usable open space on the ground floor. May eliminate some parking spaces. Can require significant foundation work depending on existing conditions.
Typical application: Smaller buildings (2–3 stories) where some parking or ground-floor space can be given up, or where openings are not required.
Steel Braced Frames
Steel braced frames use diagonal steel members (braces) within a rectangular steel frame to create a triangulated structure that resists lateral forces through axial tension and compression in the braces.
How it works: Diagonal steel braces — typically concentric braces in a chevron ("V") or inverted chevron configuration — are installed in ground-floor openings. The braces transfer lateral forces directly to the foundation.
Advantages: Efficient use of steel. Can be designed to fit within existing openings. Good for situations where moment frames are impractical.
Limitations: Diagonal braces visually obstruct openings more than moment frames. May interfere with vehicle circulation in parking areas.
Typical application: Buildings where moment frames are too expensive but parking access must be partially maintained.
Other Methods
Cantilever columns: In some cases, existing slender columns can be encased in concrete or steel jackets to increase their lateral capacity. This is less common for wood-frame buildings but may apply in specific conditions.
Base isolation: Rarely used for soft-story apartment buildings due to cost, but technically possible for high-value structures. Base isolation decouples the building from ground motion using flexible bearings at the foundation level.
Cost of Soft-Story Retrofits
Retrofit costs vary considerably based on building size, the chosen method, local labor costs, foundation conditions, and the extent of ancillary work required (utility relocation, temporary shoring, finish restoration).
Typical Cost Ranges
| Building Type | Retrofit Method | Approximate Cost Range | Cost Per Unit (Rough) |
|---|---|---|---|
| Small (2-story, 5–8 units) | Plywood shear walls | $60,000–$100,000 | $8,000–$15,000 |
| Medium (3-story, 10–20 units) | Steel moment frames | $100,000–$200,000 | $7,000–$15,000 |
| Large (3-story, 20–40 units) | Steel moment frames | $150,000–$350,000+ | $6,000–$12,000 |
| Mixed-use (commercial ground floor) | Steel moment frames or braced frames | $100,000–$300,000+ | Varies significantly |
These ranges reflect California construction costs as of 2023–2024. Costs vary by region and market conditions. All cost estimates should be confirmed with licensed contractors through competitive bidding.
What Drives Costs Up
Several factors can push a soft-story retrofit toward the upper end of cost ranges — or beyond them:
Foundation conditions. If the existing foundation cannot support the new lateral forces, significant foundation work (new spread footings or grade beams) is required. In buildings built before modern code requirements, foundation upgrades can represent 30–40% of total retrofit cost.
Utility relocation. Gas lines, water mains, electrical conduit, and sewer lines that run through the ground-floor area may need to be relocated to accommodate new structural elements. This is especially common in older buildings with haphazard utility routing.
Architectural finishes. In buildings with commercial ground-floor tenants, restoring finishes after structural work adds cost. For parking-only ground floors, finish restoration is minimal.
Permit and engineering fees. Structural engineering fees typically range from $10,000 to $30,000+ depending on building complexity. Permit fees vary by jurisdiction.
Complete guide to earthquake retrofit costs
Financial Assistance
While most soft-story retrofit programs place the cost burden on building owners, some financial assistance programs exist:
FEMA Hazard Mitigation Grant Program (HMGP): Provides grants to state and local governments after presidentially declared disasters to implement hazard mitigation measures, which can include seismic retrofits. Funding is not guaranteed and is typically distributed through state emergency management agencies.
FEMA Hazard Mitigation Assistance Grants
FEMA Building Resilient Infrastructure and Communities (BRIC): An annual competitive grant program that funds mitigation projects including seismic retrofits. Applications are submitted by state, local, tribal, and territorial governments.
CEA Brace+Bolt Program: The California Earthquake Authority's Brace+Bolt program provides grants of up to $3,000 for qualifying homeowners to retrofit older houses. However, this program targets single-family homes (primarily cripple wall and bolting retrofits) and generally does not apply to the larger multi-unit soft-story buildings. Owners of single-family homes with soft-story conditions should check eligibility.
SBA Disaster Loans: After a declared disaster, the Small Business Administration may offer low-interest loans for property repairs and mitigation, which can include seismic retrofits for eligible building owners.
Local incentive programs: Some cities with mandatory ordinances have created financing programs, fee deferrals, or property tax adjustments to offset retrofit costs. Check with your local building department.
California earthquake insurance guide
Tenant Implications
Soft-story retrofits affect building tenants in several important ways. Construction typically takes 2 to 6 months, and some portion of that work may require tenants to temporarily vacate their units.
Temporary Relocation
Most soft-story retrofits can be completed while upper-floor tenants remain in the building for much of the construction period. Ground-floor work — including demolition, foundation work, and steel erection — is the most disruptive phase. However, certain work may require temporary evacuation for safety reasons:
- Heavy foundation excavation or drilling
- Welding or cutting near occupied spaces
- Temporary removal of structural supports (shoring sequences)
- Utility shutoffs (gas, water, electricity)
Cities with mandatory ordinances often have tenant protection provisions that address relocation requirements, notice periods, and relocation assistance.
Rent Pass-Through Rules
A critical concern for tenants in rent-controlled jurisdictions is whether building owners can pass retrofit costs through to renters via rent increases. Different cities handle this differently:
Los Angeles: Under the Rent Stabilization Ordinance (RSO), building owners may apply for a rent increase to recover retrofit costs. The Los Angeles Housing Department (LAHD) administers the pass-through process. Increases are typically spread over a multi-year period, and the amount per unit is capped. As of the ordinance's provisions, owners could apply to pass through up to 50% of retrofit costs over 10 years, subject to approval and per-unit monthly caps.
San Francisco: The San Francisco Rent Board allows capital improvement passthroughs for seismic retrofit work. Costs are amortized over the useful life of the improvement (typically 20 years), and the monthly passthrough per unit is capped at a percentage of the tenant's base rent.
Berkeley, Santa Monica, West Hollywood, and other rent-controlled cities: Each jurisdiction has its own capital improvement passthrough rules. Tenants and owners should consult the local rent board or housing authority.
Tenant Rights and Protections
Tenants in buildings undergoing mandatory seismic retrofits generally have the following protections (though specifics vary by city):
- Advance notice of construction timelines and any required temporary relocations.
- Right to return to the same unit at the same rent after temporary relocation.
- Relocation assistance from the building owner for temporary moves, which may include payment of temporary housing costs or a flat relocation fee set by city ordinance.
- Protection from eviction — mandatory retrofit work is not grounds for permanent eviction in most jurisdictions with tenant protection ordinances.
Building owners should familiarize themselves with local tenant protection requirements before beginning retrofit work, as failure to comply can result in penalties.
Building Codes and Standards
Soft-story retrofit requirements draw on several engineering standards and building code provisions:
ASCE 7 (Minimum Design Loads for Buildings and Other Structures): Defines the engineering criteria for identifying soft and weak stories and establishes the seismic design parameters that retrofit designs must meet.
ASCE 41 (Seismic Evaluation and Retrofit of Existing Buildings): The primary national standard for evaluating and retrofitting existing buildings. It establishes performance objectives (such as Immediate Occupancy, Life Safety, and Collapse Prevention) and analysis methods for retrofit design. Most mandatory ordinances reference ASCE 41 or its predecessor, FEMA 356.
International Existing Building Code (IEBC): Provides code provisions for alterations, repairs, and additions to existing buildings, including seismic retrofit. The IEBC is adopted by reference in most California jurisdictions.
California Existing Building Code (CEBC): California's state-specific adaptation of the IEBC, with additional requirements for seismic hazard zones.
Local ordinances and standard plans: Many cities with mandatory programs have developed their own standard retrofit plans and engineering guidelines that supplement the above codes. These standard plans allow streamlined permitting for buildings that fit common configurations.
Overview of earthquake retrofit methods and requirements
How the Retrofit Process Works
For building owners facing a mandatory retrofit or voluntarily strengthening a soft-story building, the process typically follows these steps:
1. Screening and notification. In cities with mandatory programs, the building department notifies owners of affected buildings and establishes compliance deadlines. Owners receive a screening notice based on building records (year built, occupancy type, number of stories and units).
2. Engineering evaluation. A licensed structural engineer evaluates the building to confirm the soft-story condition and determine what level of retrofit is required. The engineer will assess existing structural conditions, foundation type, soil conditions, and the degree of stiffness deficiency.
3. Retrofit design. The structural engineer designs the retrofit, selecting the appropriate method (moment frames, shear walls, braced frames, or a combination) and specifying all structural elements, connections, and foundation modifications. The design must comply with the applicable codes and local ordinance requirements.
4. Plan check and permitting. The completed plans are submitted to the local building department for plan check. Cities with standard plan programs may offer expedited review for conforming designs. Plan check fees and permit fees are due at this stage.
5. Contractor selection and construction. The building owner selects a licensed general contractor (ideally one with seismic retrofit experience) through competitive bidding. Construction includes demolition, foundation work, structural steel or wood framing, connections, and finish restoration.
6. Inspection and sign-off. The building department inspects the work at required stages (foundation, framing, final) and issues a completion certificate or certificate of compliance.
Typical timeline: From engineering evaluation to construction completion, the process usually takes 12 to 24 months, though this varies widely based on building complexity, permit review times, and contractor availability.
Sources
- Federal Emergency Management Agency (FEMA). "Evaluation of Earthquake Damaged Concrete and Masonry Wall Buildings — Technical Resources." FEMA 306, FEMA 307, FEMA 308. Washington, D.C.
- Federal Emergency Management Agency (FEMA). "Prestandard and Commentary for the Seismic Rehabilitation of Buildings." FEMA 356. November 2000.
- FEMA Mitigation Assessment Team. "Northridge Earthquake: Observations, Recommendations, and Technical Guidance." FEMA 403. 2003.
- American Society of Civil Engineers. "Seismic Evaluation and Retrofit of Existing Buildings." ASCE/SEI 41-17. 2017.
- American Society of Civil Engineers. "Minimum Design Loads and Associated Criteria for Buildings and Other Structures." ASCE/SEI 7-22. 2022.
- City of Los Angeles Department of Building and Safety. "Ordinance No. 183893: Mandatory Earthquake Hazard Reduction in Existing Wood-Frame Buildings with Soft, Weak, or Open-Front Walls." 2015.
- San Francisco Department of Building Inspection. "Administrative Bulletin AB-094: Mandatory Soft Story Retrofit Program." 2013.
- California Earthquake Authority. "Brace + Bolt Program." https://www.earthquakeauthority.com/California-Earthquake-Risk/Strengthening-Your-House/Brace-Bolt
- Federal Emergency Management Agency. "Hazard Mitigation Assistance Grants." https://www.fema.gov/grants/mitigation
- City of Oakland Bureau of Building. "Mandatory Soft Story Retrofit Program." Ordinance No. 13516. 2019.
- Reitherman, Robert. "Earthquakes and Engineers: An International History." ASCE Press. 2012.
- International Code Council. "International Existing Building Code (IEBC)." 2021 Edition.
This article is for educational purposes only. Seismic retrofit design and construction must be performed by licensed professionals — a licensed structural engineer for design and a licensed general contractor for construction. Building codes, ordinance requirements, cost ranges, and financial assistance programs may change. Consult your local building department and qualified professionals for current requirements applicable to your specific building.