In earthquake prone areas like San Francisco, homes and other buildings need to be built to withstand the forces of a seismic event. Unfortunately, there are many buildings that went up in times that predate our modern understanding of the way that these forces affect structures. For these buildings, some level of seismic retrofitting is in order.
With most homes, the areas that need the most attention will be in the understructure. Things like anchoring and cripple wall bracing can effectively protect most structures. Another common issue is buildings that have a soft story condition. This is where some portion of living space is built over a story that has insufficient strength to resist the lateral forces of an earthquake.
The soft story condition is generally the result of second or third story being built over an open space. In homes, it is commonly found in places where there is a room built over a garage or if there is a large picture window somewhere on the ground floor. When a home has a soft story, the lack of shear strength in the first floor will cause a collapse, with the upper floors coming down on to this open area.
If a home has a soft story condition, a seismic retrofit is recommended. With this type of retrofitting, the idea is to add the necessary strength to resist the forces of an earthquake, while also maintaining the functionality of the space. To achieve this, an engineer will need to consider a number of factors.
In the process of retrofitting a soft story, there are various methods that a contractor could use. One common method is to brace the walls with structural plywood. In effect, this adds rigidity to the space, and turns the walls into shear walls. Anchoring the walls to the foundation may also be a part of the seismic retrofit for a soft story.
In some cases, it might be necessary to add a steel frame to the structure. The steel frame is not added to increase the load-bearing capacity, the intention is to prevent the swaying that can occur during an earthquake and improve the stability of the ground floor. This frame can add the rigidity that is needed, but it tends to be a more expensive and labor intensive process. The addition of a steel frame is generally reserved for situations where adding plywood to the walls is not an option.
Montclair Construction is a company that performs seismic retrofitting in San Francisco and Oakland. We can analyze the structure of your home and identify the ways in which it is exposed to earthquake damage, and we can perform the seismic upgrades that will provide the protection that your home needs. Contact us today to schedule a consultation and inspection.
Here you can see a foundation being correctly built to withstand earthquakes:
