Soft Story Retrofits in the San Francisco Bay Area

Soft Story Retrofits-A Matter of Life and Death

Why worry?

After a the  earthquake, government statistics show 155,000 housing units will become uninhabitable and 350,000 people will be displaced, most of them will be in Oakland, Berkeley, and San Francisco because of un-retrofitted soft story buildings. People will be killed and apartment buildings will be torn down exactly as occurred  when the Northridge Meadows in Los Angeles collapsed in 1994, leaving 16 people dead.  As a result Oakland, Berkeley, and on a very large scale San Francisco now mandates soft story retrofits and Los Angeles.

What is a Soft Story Home or Building?

GARAGES ARE A COMMON CAUSE OF SOFT STORY CONDITIONS BUT WINDOWS AND DOORS CAN DO THE SAME

GARAGES ARE A COMMON CAUSE OF SOFT STORY CONDITIONS BUT WINDOWS AND DOORS CAN DO THE SAME

 

THIS DOORWAY IS LEANING BECAUSE THE FLOOR ABOVE THE OPENING DID NOT HAVE ANY FOUNDATION TO BE ATTACHED TO

THIS DOORWAY IS LEANING BECAUSE THE FLOOR ABOVE THE OPENING DID NOT HAVE ANY FOUNDATION TO BE ATTACHED TO

THIS IS THE KIND OF DAMAGE SEEN IN RESIDENTIAL NEIGHBORHOODS AFTER AN EARTHQUAKE

THIS IS THE KIND OF DAMAGE SEEN IN RESIDENTIAL NEIGHBORHOODS AFTER AN EARTHQUAKE

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. 

THIS HOME WAS BUILT BEFORE THE 1994 NORTHRIDGE EARTHQUAKE CHANGED THE BUILDING CODE REGARDING SOFT STORY RETROFITS IN JULY OF 1999

THIS HOME WAS BUILT BEFORE THE 1994 NORTHRIDGE EARTHQUAKE CHANGED THE BUILDING CODE REGARDING SOFT STORY RETROFITS IN JULY OF 1999

 

THIS SOFT STORY HOME IN SAN FERNANDO WAS DAMAGED BECAUSE THE LIVING AREA ABOVE THE GARAGE WAS NOT ATTACHED TO FOUNDATION AT THE FRONT

THIS SOFT STORY HOME IN SAN FERNANDO WAS DAMAGED BECAUSE THE LIVING AREA ABOVE THE GARAGE WAS NOT ATTACHED TO FOUNDATION AT THE FRONT

THE SAME DAMAGED HOME SHOWING THE DESTRUCTION CAUSED IN ONLY A FEW SECONDS

THE SAME DAMAGED HOME SHOWING THE DESTRUCTION CAUSED IN ONLY A FEW SECONDS

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.

 

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.

SAN FRANCISCO HAS THE HIGHEST NUMBER OF SOFT STORY HOMES IN SAN FRANCISCO

TYPICAL SOFT STORY HOME IN SAN FRANCISCO

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.

San Francisco building soft story retrofits use one of two methods.

Two possible options for a San Francisco soft story retrofit

USING ROTATION

This is another but lesser known method of protecting homes needing soft story retrofits.  This method uses shear walls and the principle of rotation. 

Soft story retrofits can also be done using shear walls alone

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A soft story failure in Santa Cruz

This house could have been saved if it had a soft story rotation retrofit.

This soft story home in Santa Cruz could have been saved with shear walls and rotation

This was a common failure in the 1989 Loma Prieta Earthquake

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

This is another soft story home that collapsed in the 1989 Loma Prieta Earthquake

A similar soft story collapse

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.

Cutting Concrete

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.

 

 

 

 

 

 

 

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.

A soft story retrofits using a steel column, also known as a cantilevered column.

Steel column connected to concrete and floor above

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.

Moment Frames are used in San Francisco because the buildings are so heavy

Moment frames are also used in soft story retrofits for larger houses

Collapse of Northridge Meadows soft story apartment building left 16 people dead

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.

The Northridge Meadows is a good example of why soft story buildings need retrofits.

 

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.

Northridge Meadows damage to soft story created by parking area

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.

This soft story damage occurred in the San Fernando earthquake

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.

 

This soft story building was many miles away from the Northridge Earthquake

 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.

Damaged soft story buildings also impact businesses

 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.

It appears this soft story building also caught fire.

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.

This heavy soft story apartment building in San Francisco needs large steel moment frames.

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