Soft Story Retrofits-A Matter of Life and Death
After a large earthquake, government statistics maintain 155,000 housing units will become uninhabitable and 350,000 people displaced, most of them because of un-retrofitted soft story buildings. People will be killed and apartment buildings will be torn down, as occurred when the Northridge Meadows in Los Angeles collapsed in 1994, leaving 16 people dead. As a result, San Francisco now mandates soft story retrofits and Los Angeles has started a mandatory soft story retrofit program.
What is a Soft Story Building?
This photo of a garage illustrates how a soft story collapses. Imagine if there had been many thousands of pounds of living area above it. The same thing happens whether it is a 40-unit apartment building or a simple house. Walls with large windows and garage door openings create a soft story building because they are much more flexible and easily shaken apart than the rest of the building (softer than the living area above). Imagine your house or apartment building swaying back and forth on top of glass windows or garage door openings. They cannot resist this movement and the building can collapse unless is has a seismic retrofit. Peter Yanev, a San Francisco Bay Area structural engineer made this excellent video that shows how soft story buildings can be protected.
NEW HOUSE SOFT STORY
This house was built before July 1, 1999, when it became illegal to brace garage door openings with such narrow walls on either side of the garage door opening. The 1994 Northrdige Earthquake proved that walls this narrow act like posts and tend to collapse. All soft story houses built after July, 1999, must have shear walls on either side of the garage at least four feet wide or use narrow specially-made steel shear walls.
Here you can see how the connection between the living area above the garage and the main house was not sufficient to keep the supports on either side of the garage from collapsing. This garage separated from the main part of the house when the supports on either side of the garage door collapsed.
The Unique Buildings in San Francisco
The gray house on the left is a typical soft story home or small apartment building in San Francisco. The floor directly above the garage door has no foundation or walls under it to prevent swaying from left to right. The existing small posts on either side of the garage door will simply tip over. It is not only the garage that creates a soft story condition. Underneath the front door, only posts, which can easily tip over, support the floor.
The porch floor will also be separated from the main floor. In essence a house like this is only supported by posts.
SAN FRANCISCO RETROFIT METHODS
Here you see the two possible options for retrofitting one of these buildings. The first one is to put a foundation under the front of the house and build high capacity shear walls on either side. High capacity shear walls enable one to use a minimal length of foundation and get twice as much strength than if one had plywood and bolts on only one side. These shear walls are then attached to the floor above to keep it from swaying. The other method is to use a large steel column in the garage door opening such that when the floor moves left to right it pushes and pulls on the steel column.
This is another but lesser known method of protecting homes needing soft story retrofits. This method uses shear walls and the principle of rotation.
A soft story failure in Santa Cruz
This house could have been saved if it had a soft story rotation retrofit.
Another Santa Cruz soft story failure
This is another example of where the a retrofit would have saved the house. Notice how the glass is not even broken. This is because of the ability for sheet rock to resist earthquakes
Moment Column Installation
The technician here is cutting the garage slab so that he can remove part of it and dig a trench. On either side of the trench below can be seen the concrete slab of the garage floor. This is a very dusty undertaking if proper precautions are not used to keep dust to a minimum. This is done either by using a special saw with a vacuum attachment or by flooding the cut with water.
Steel goes into the concrete
Once the trench is dug out it is leveled so that there is no more than a 10% grade as required by the building code. The size of the trench, the amount of steel used, and the amount of force it will need to resist are all determined by a structural engineer. To the right you see a technician working on a moment column foundation for a large apartment building. These foundations are the same as those used in houses except they are much bigger.
A FULLY INSTALLED MOMENT COLUMN
Moment Columns for homes require a large steel column be embedded in concrete and attached to the floor above. When the floor above the garage door opening sways left to right this motion is transferred into the steel column and into the foundation. This method has been tested in actual earthquakes and has proven effective. Notice how the concrete has been troweled smooth so that cars can still drive over it.
MOMENT FRAMES FOR HOUSES
Sometimes large houses are heavy enough to need a moment frame. Here you see a completed San Francico soft story retrofit that will protect this home from catastrophic damage. This kind of retrofit is needed for soft story homes in the San Francisco Bay Area to survive. This is especially true for homes built before the 2001 International Building Code came into effect including in it many of the lessons learned in the 1994 Northridge Earthquake.
Multi-Unit Soft Story Retrofits
The collapse of the Northridge Meadows apartment building in the 1994 Northridge earthquake caused many of the inhabitants of the living areas adjacent to the parking to be crushed to death. Altogether 16 people died. Here you see both a front view and a rear view of this building. The exact same thing can happen in the San Francisco Bay Area if steps are not taken to protect the public.
San Francisco’s Mandatory Retrofit Program
The mandatory soft story retrofit program in San Francisco will go a long way in preventing this from happening. If you own a soft story building, think how you would feel if your building collapsed with the same fatal consequences? It is a stroke of luck that no one was killed in the buildings that fell in the 1989 Bay Area Loma Prieta Earthquake.
San Francisco Soft Story Failure
This soft story failure occurred in the Northridge Earthquake due to parking under the main floor. This is often called “tuck under” parking.
San Francisco Soft Story Failure
This damage occurred in the 1989 Loma Prieta earthquake, which devastated part of San Francisco. Look carefully at the garage door openings on either side of the entrance way and notice they are leaning to the left. This is because they could not remain upright when the three stories they supported started swaying left and right. The tenants of this building were forced to re-locate and temporary shoring was immediately installed so an aftershock would not cause a collapse.
Soft Story Damage far away
This is a photo of damage to a soft story building that was 22 miles from the epicenter of the Northridge Earthquake. If the shaking had been more intense, or the building had been closer to the fault, it would have collapsed. If it had been a much heavier three story building, such as those found in San Francisco, it would certainly have collapsed.
Why Soft Story Damage?
The cause of all this damage is always the same: too much weight is being supported by a limited amount of wall bracing. The building code does not recognize walls less than 27 inches wide as having any earthquake resisting value. In most soft story buildings, such as the ones shown here, the fronts were supported by nothing but posts. Buildings like this can be found all over San Francisco, a terrible disaster waiting to happen.
Possible Soft Story Devastation
As stated earlier, it is expected that over 360,000 people will be displaced after the earthquake and thousands of people will be killed. There will also be untold injuries.
Most of this will be due to the upper stories collapsing onto the lower stories where many buildings in San Francisco have apartment units. This is what happened to the Northridge Meadows apartment building mentioned above.
It will take years to re-build the Bay Area after such a catastrophe and impossible to rebuild these historic structures that give San Francisco so much of its character.
Moment Frames for Large Buildings
These are the kinds of moment frames used in very large, heavy buildings. A series of these frames were installed in a row of garage door openings of a large apartment building.