Have you ever wondered what a soft story building is and why it poses an extra risk during an earthquake? In this guide, we will explore the concept of soft story buildings, the dangers they present, and the retrofitting measures that can be taken to enhance their structural integrity. With over 15 years of experience in the field, we will provide you with expert insights and trustworthy information to help you understand and address this critical issue.
What is a Soft Story?
Soft story buildings are characterized by an open ground level with significantly fewer walls than the floors above. This design feature makes the first story of the building less rigid and capable of withstanding the forces exerted during an earthquake. Technically, a building is considered a soft story if the first story has 70% or less stiffness than the second story or 80% less stiffness than the average stiffness of the three stories above it. One common example of a soft story building is an apartment complex with parking on the ground level.
During an earthquake, the lack of stiffness in the first story makes it unable to adequately support the levels above it. This can lead to the phenomenon known as “pancaking,” where the first story collapses or has the potential to collapse. Soft story buildings have proven to be particularly vulnerable in past earthquakes, contributing to significant damage and loss of life.
The Impact of Soft Story Buildings
Soft story buildings have been responsible for substantial damage and casualties in earthquakes. In the 1989 Loma Prieta earthquake and the 1994 Northridge earthquake, soft story buildings caused the displacement of many residents and the tragic loss of lives. Approximately half of the displaced residents and 30 fatalities were directly attributed to soft story buildings in these two seismic events.
Soft Story Ordinances
Recognizing the inherent risks associated with soft story buildings, cities throughout California have implemented soft story ordinances to ensure the safety of their communities. These ordinances require the retrofitting of pre-1990s soft story buildings to reduce the risk of collapses in future earthquakes.
Cities like Los Angeles, San Francisco, and Oakland have been at the forefront of this initiative. Los Angeles, in particular, took the lead by identifying over 13,500 soft story buildings through a comprehensive survey. San Francisco and Oakland followed suit, conducting their surveys and identifying thousands of soft story buildings in their respective jurisdictions.
Each city has specific criteria for determining which buildings are subject to the soft story ordinance. For example, in Los Angeles, the ordinance applies to buildings that are 2+ stories and 4+ units, while in San Francisco and Oakland, it applies to 3+ stories and 5+ units.
Do I Live in a Soft Story Building?
If you are unsure whether you live in a soft story building, there are publicly available surveys conducted by cities like Oakland, San Francisco, and Los Angeles that can provide you with information about the status of your building. These surveys can help you determine if your building is subject to the soft story ordinance and in need of retrofitting.
Retrofitting Solutions for Soft Story Buildings
Retrofitting a soft story building involves strengthening and stiffening the first story to prevent collapse during an earthquake. Several retrofitting techniques can be employed, depending on the specific structural characteristics of the building:
Steel Moment Frame
A steel moment frame consists of two columns connected by a beam. This frame is typically installed around large first-level openings, such as parking areas or storefronts. By connecting to the existing supporting members, the steel frame enhances the stiffness and strength of the first story.
A shear wall, usually made of concrete or wood, connects the second floor of the building to a new foundation. Shear walls are designed to counteract earthquake forces and typically require walls in two perpendicular directions. However, this retrofitting solution may impact the utilization of the building due to the space occupied by the walls.
Diagonal bracing involves adding additional stiffness to the building. Braces are installed between existing columns, creating rigid triangles that enhance the structural integrity. Like shear walls, diagonal bracing may restrict the building’s use by interfering with existing openings.
A fusing system is a specialized engineering solution that attaches a piece of steel, known as the “fuse,” to the building’s floor joists. The fuse has its independent foundation and acts as sacrificial element. During an earthquake, it absorbs the energy and protects the rest of the structure, much like the crumple zone of a car protects passengers.
These are just a few examples of retrofitting solutions that can be employed to strengthen the first story of a soft story building. Consulting with structural engineers and architects will help determine the most suitable retrofitting strategy based on the specific characteristics of your building.
Soft story buildings pose significant risks during earthquakes. To ensure the safety of residents and reduce the potential for catastrophic collapses, soft story ordinances have been implemented in cities across California. Retrofitting these buildings is of utmost importance and can be achieved through various techniques like steel moment frames, shear walls, diagonal bracing, and fusing systems.
If you suspect that you live in a soft story building, it is crucial to take action. Consult the publicly available surveys conducted by your city to assess the status of your building. Engaging the expertise of professionals such as structural engineers, architects, and contractors will guide you through the retrofitting process and pave the way for a safer future.
Remember, when it comes to soft story buildings, prioritizing the well-being of residents and the structural integrity of buildings should always be the top priority.