After an earthquake, government statistics show 155,000 housing units will become uninhabitable and 350,000 people will be displaced because they needed soft story retrofits. Most of them will be in Oakland and Berkeley, but soft story buildings can be found all over the Bay Area, including ordinary single family homes.
Most of the damage will be to large heavy multi-story buildings which prompted San Francisco to pass the Soft Story Mandate. When looking at the damage notice most of the buildings had wood siding and not stucco. The reason for that will remain unexplained until I have time to update this web page.
What is a Soft Story Home or Building?
These photos illustrates how garages with living area above the garage collapse. Imagine if there had been many thousands of pounds of living area above the blue garage instead of this very light roof. The same kind of damage occurs 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, to say this another way the lower story 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. Peter Yanev, a San Francisco Bay Area structural engineer made this excellent video that shows how soft story buildings can be protected.
Even Newer Homes need Soft Story Retrofits
This house was built before July 1, 1999 when it became illegal to brace garage door openings like this one, which attached the floor above the garage door opening to the foundation 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 tend to tip over. 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 is a soft story collapse to a newer home in San Fernando. 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 tippling over. This caused the garage to become separated from the main part of the house when the supports on either side of the garage door collapsed.
Moment Columns for homes require a large steel column be embedded in concrete and attached to the floor above. This is done by digging a trench near the garage door opening, placing steel known as rebar in the trench, attaching the bottom of the steel column to the rebar, filling the trench with concrete, and finally attaching the floor above the garage to the column.
When the floor above the garage door opening sways left to right this motion is transferred into the steel column and into the foundation.
Trench and Rebar Attached to Steel Column
Trench Filled with Concrete
Sometimes columns can be placed on the outside of a house and can be quite attactive
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.
The Unique Buildings in San Francisco
This gray house is a typical soft story home or small apartment building in San Francisco. The front of the house directly above the garage door opening has nothing underneath it to prevent it from swaying back and forth parallel to the street. If it sways far enough it will collapse because the existing small posts on either side of the garage door will simply tip over. In this home it is not only the garage that creates a soft story condition. Underneath the remaining front of the house you will not find a foundation or wall to support the floor except a few posts.
SOFT STORY RETROFIT METHODS
Below you will see three possible options for retrofitting one of these buildings. The illustration below is typical for soft story buildings in San Francisco such as the one pictured above. One option is to put a foundation under the front of the house and build high capacity shear walls on either side of this foundation. 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 two methods used an engineering principles called rotation or 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.
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
A similar soft story collapse. Again, a contributing factor was the wood siding
SOFT STORY RETROFIT METHODS
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. Notice how on the right the steel has been connected to the steel column
You can imagine how pleased you would be with yourself after building something so elaborate to save one more person’s life savings.
The Concrete is then Poured into the Ditch
Often we find sewer lines, drains, or water pipes when we did the trench. One must be extremely careful not to cut the pipes
A special type of pipe wrapping must be used before the concrete is poured to provide a barrier between the concrete and the pipe to prevent a deteriorating chemical reaction.
The top of the column is finally attached to the floor. And this is how the entire system works. When the earthquake forces pushes or pulls on the floor joists the column is attached to this force is stopped by the column which provides a counter force.
Such a large mass of concrete will shrink leaving an indentation in he floor. Reinforcing steel is then put into the indentation and filled with concrete.
The last bit of concrete fills in this indentation so that the garage floor can be driven over with no one realizing how much steel and how large a mass of concrete is under them.
The column is tucked behind the side of the garage door and invisible from the street and not an obstruction to parking.
This is another but lesser known method of protecting homes needing soft story retrofits.
This article could never have been written without the insights and guidance provided by research scientist and engineer Ed Keith. He has spent over 44 years of researching seismic design and the use of shear walls to protect homes as a leading scientist with the American Plywood Association, the largest shear wall testing laboratory in the world. He recently retired and I am forever in his debt for all he taught me.
This is an advanced retrofit and unorthodox retrofit procedure. It is advised that you not only read the material, but also spend some time studying the images and the linked-to web pages.
Using Shear Walls to address Garage Soft Story Conditions
If you install a shear wall on the back of the garage, and if the floor above the garage is more or less either a square or a rectangle , you can protect the building by putting shear walls on the back and two sides of the garage, without doing anything to the garage door opening itself. Using shear walls to protect a building like this is known as using the principle of rotation.
- The dark line on the perimeter represents the foundation, which is missing on wall line “D”.
- At the same time it shows where the outside edges of the floor above the garage are.
- The red lines represent earthquake forces, which will attack the garage from all sides.
- The red arrow in the upper left is pushing down against a side wall of the garage. This movement is resisted by Shear Wall A.
- Another arrow in the upper right is pushing down against the opposite side wall of the garage. This movement is resisted by Shear Wall B.
- A third arrow in the lower left is pushing against the back wall of the garage on wall line “C”. This movement is resisted by Shear Wall C.
- The fourth arrow to the left of the garage door opening is pushing on the garage door opening but there is no shear wall or foundation here. What happens?
- This is where rotation can be seen. The entire floor tries to twist or rotate as shown by the rotating circular black arrow, pushing down on the wall B and up on wall A as it rotates . This rotation transfers all of the force that would have caused the garage door opening to collapse into to the back wall as well as the side walls of the garage.
Thus saving the garage door opening from collapse
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.
Collapse of Northridge Meadows soft story apartment building left 16 people dead
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.
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
Another soft story apartment building being torn down after the 1989 Loma Prieta EarthquakeAs 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.
Large moment frames are an excellent way to protect apartment buildings. Courtest Nilgun Wolpe, C.E.
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.