Soft-Story Collapse: Is Your Building at Risk?
Earthquakes strike without warning, and not all buildings are designed to withstand their force. If you own or manage a commercial or industrial property, it’s crucial to understand whether your building is at risk. Soft-story buildings—characterized by open ground levels and limited structural support—are among the most vulnerable to soft-story collapse during seismic events.
Recognizing these risks is the first step toward safeguarding lives, protecting your property, and maintaining business operations when the next earthquake hits.
What Makes Soft-Story Buildings Prone to Collapse?
Soft-story buildings are particularly vulnerable to earthquake damage due to their open ground levels. These buildings are often used for parking or commercial spaces and lack the lateral support needed to withstand seismic forces.
Large openings in the walls, especially on ground floors, create what is known as a “soft-story condition.” This is most commonly seen in buildings with storefronts where the front of the structure is largely open, leaving the walls unable to resist lateral forces during an earthquake.
During an earthquake, ground shaking causes the foundation to move side-to-side while the upper floors shift even more. This imbalance increases the risk of a soft-story collapse, especially in buildings constructed before modern seismic codes.
Examples of soft-story buildings include:
- Apartment Complexes with Tuck-Under Parking: Multi-unit residential buildings with parking spaces beneath raised living areas.
- Commercial Buildings with Open Storefronts: Retail spaces with large glass windows and minimal structural reinforcement at the ground level.
- Mixed-Use Buildings: Structures combining residential units above and retail or office spaces below.
- Garages and Carports Under Residential Units: Residential buildings with ground-level carports supporting multiple floors above.
- Hotels and Motels: Lodging facilities with an open parking level below or large open common area walls beneath guest rooms.
- Industrial Buildings with Open Floor Plans: Warehouses or manufacturing facilities with large openings on a wall or series of walls at the ground floor.
- Community and Public Buildings: Libraries, schools, or community centers with open parking or gathering spaces designs.
Soft-Story Collapse Risks: Why Action Can’t Wait
The potential danger of a soft-story collapse in a densely populated area is severe, as it could lead to extensive injuries or loss of life. In urban areas, where these buildings are prevalent, thousands of residents, employees, and visitors are potentially at risk.
According to the California Legislature’s joint informational hearing report, Seismic Resilience in California’s Building Stock and Multi-Hazard Mitigation:
- The Bay Area faces a 72% chance of a magnitude 6.7 earthquake by 2043.
- Southern California faces a 93% chance of a magnitude 6.7 earthquake by 2043.
These probabilities highlight the likelihood of major seismic events and the urgency of addressing structural vulnerabilities.
Combining these probabilities and structural weaknesses underscores the critical need for seismic retrofitting to reduce the potential for catastrophic outcomes.
Recognizing these risks, many California cities have enacted local legislation requiring retrofits to stabilize these vulnerable structures and safeguard occupants.
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In an earthquake, a soft-story building can tilt and collapse within seconds,
posing serious risks to those inside.
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Historical Examples of Soft-Story Collapse
Soft-story collapses have been documented in several significant earthquakes, emphasizing these buildings’ dangers.
Two notable examples underscore the dangers of soft-story structures in earthquake zones.
1989 Loma Prieta Earthquake (San Francisco Bay Area)
The 6.9 magnitude Loma Prieta earthquake hit Northern California, inflicting severe damage in San Francisco’s Marina District, a neighborhood densely populated with soft-story buildings.
Numerous commercial and multi-story residential buildings with ground-floor parking or storefronts suffered significant structural failures.
This earthquake highlighted the inherent instability of soft-story designs in bustling urban centers, serving as a wake-up call for San Francisco to initiate proactive retrofitting initiatives to strengthen these vulnerable structures.
1994 Northridge Earthquake (Los Angeles)
The 6.7 magnitude Northridge earthquake wreaked havoc on soft-story buildings across Los Angeles, with some of the most severe impacts observed in multi-story apartment complexes and commercial spaces with unsupported lower levels.
The collapse of the Northridge Meadows apartment complex, which resulted in the tragic loss of 16 lives, brought nationwide attention to the deadly consequences of structural weakness in soft-story designs.
In response, Los Angeles prioritized new retrofitting regulations, recognizing the urgent need to fortify high-risk buildings against future earthquakes.
A soft-story building collapse crushed cars during the 1994 Northridge earthquake. (Source)
Why Retrofitting Is Essential for Preventing Collapse
Seismic retrofitting directly addresses the structural weaknesses of soft-story buildings, reducing collapse risk and enhancing safety for occupants. Key benefits of retrofitting for collapse prevention include:
- Improved structural stability: Retrofitting strengthens the structural integrity of soft-story buildings, making them better equipped to withstand seismic forces.
- Regulatory compliance for safety: Many jurisdictions now mandate retrofitting for soft-story buildings. Adhering to these requirements protects lives and ensures legal compliance, which is crucial for building owners.
A soft-story seismic retrofit project performed by Saunders Seismic.
Assess and Protect Your Soft-Story Building Against Collapse
If your building has open ground levels (like tuck-under parking) and was constructed before 1996, it is highly susceptible to soft-story collapse. Assessing your building’s seismic safety is the first step in determining your risk.
Don’t wait until the next earthquake strikes! By acting now, you can prevent the devastating consequences of soft-story collapse and ensure the safety of those who live and work in your building.
Contact Saunders Seismic today for an assessment and learn how seismic retrofitting can make your building safer.