Homes commonly found in the Oakland, Berkeley, parts of El Cerrito and much of San Francisco are built on steep hillsides. The structural deficiencies inherent in hillside homes caused numerous casualties in addition to catastrophic losses of property. These photos show what happened to hillside homes in the Northridge earthquake and clearly point out why hillside home retrofits are so necessary.
In response to this tragedy, the City of Los Angeles developed the Los Angles Hillside Code for hillside home retrofits. This code does not rely on plywood shear walls to protect hillside buildings for the reasons outlined below. Unfortunately, the California Building Code still relies on retrofit standards that failed to prevent this catastrophic damage, and the principles used in hillside home retrofits are often not even being used for new houses in California. Meanwhile, the clock is ticking on the Hayward Fault.
Here is a photograph that shows why people love hillside homes; the views can be spectacular. Notice the police tape going across the photo. As you can tell by the shadows, this picture was taken late in the day. This is what lies down the hill, past the police tape and beyond the cement slab that once was a driveway.
Why do Hillside Homes Kill People?
Why is it that this home was completely destroyed in the Northridge earthquake while other homes around it withstood the earthquake forces? The answer to that question was the focus of an extensive engineering study conducted by the Los Angeles Department of Building and Safety and the Structural Engineers Association of Southern California Task Force on Hillside Homes. Their findings were unexpected.
What the researchers found was that in hillside homes, the plywood on the shortest, stiffest shear panel (Panel A in our illustration) absorbed the earthquake forces until it failed. Then the plywood on the next shortest and stiffest panel (Panel B) absorbed the forces until it failed. Then the plywood on the next shortest shear wall (Panel C) absorbed the forces until it failed, and so on, until the house collapsed. The research concluded that in steep hillside homes, shear panels do not work together as one unit but rather work sequentially, one after another, failing one after another until the house collapses.
Before you can understand the retrofit methods used to protect hillside homes, you must be able to distinguish a downhill hillside home from an uphill hillside home. The retrofit of the two types of construction is vastly different and it is the retrofit of downhill homes that is the concern of this presentation. This drawing shows two types of downhill hillside homes.
An uphill hillside home is no more susceptible to earthquake damage than a home built on flat land. The hillside home retrofit methods discussed on this presentation do not apply to an uphill hillside home.
These two illustrations represent a standard rectangular shear wall on a standard horizontal foundation. You can envision the vertical lines of the drawing on the left side as being the 2×4 studs of a wall that have a sheet of plywood nailed to them. The drawing on the right shows how the 2 x 4 studs and shear wall deform when earthquake forces hit it. The little triangle thing represents “delta”, an engineering term that indicates the amount of deformation that takes place. NOTE: This standard rectangular shear wall configuration has been tested by engineers and the load limits of it are known and understood. The building code strength values for shear walls are based on this rectangular configuration.
In the above diagram, the mudsill stayed attached to the foundation but the top of the shear wall moved, resulting in tearing of the plywood panel at the foundation attachment. This is an idealized depiction of tearing, but it shows the kind of “stretching” that the shear wall is subjected to during an earthquake.
This diagram represents the effects an earthquake has on the shear wall of a sloped foundation. Notice that the short upper end of the shear wall displaced, but the tall lower end remained in place. This kind of damage might be interpreted as uplift related to overturning, however, the top edge has not lifted, instead the upper edge of the shear panel has moved horizontally away from the foundation slope.
Photo of a sloped foundation.
The photographer, after taking this picture, turned to his right and took his next photo.
These are the remains of the house that disconnected from the uphill ledger and fell down the hill. Notice the line of trees just beyond the rubble and the debris past the trees.
This is the debris beyond the trees….the next door neighbor’s house. Notice that the floor diaphragm and roof are intact. All supporting structures are totally destroyed.
A typical hillside home with hillside hazards. Picture in your mind where the main floor level of this house is located and the downstairs walls that support it.
This illustration represents the main floor level for a typical downhill hillside home and the downstairs walls that support it. The vertical lines represent the studs of the downstairs walls sitting on the foundation that secure the house to the hillside. The studs and walls may be those of a living area below the main floor or they may be the studs and shear walls of the cripple wall that supports the main floor.
This illustration represents the main floor level for a typical downhill hillside home and the downstairs walls that support it. The vertical lines represent the studs of the downstairs walls sitting on the foundation that secures the house to the hillside. The studs and walls may be those of a living area below the main floor or they may be the studs and shear walls of the cripple wall that supports the main floor.
This drawing demonstrates the effect of cross-slope forces on a hillside home. The main floor remains secure at an uphill corner then rotates around that point, moving the house away from the hill. In reality, earthquake forces are generated in all directions; the combined effect is a circular motion familiar to people living in earthquake country.
Next we’ll look at factors the Southern California Task Force found contributed to hillside homes becoming detached from their foundation. Preventing these factors is the goal for retrofitting hillside homes. In this illustration, earthquake forces pull the floor joists off the mudsill. The joists move on the mudsill, pull out the nails holding them in place, slide off the mudsill, fall off the foundation and tumble down the hill.
Debris built up at the base of the house raises the soil level above the foundation level, resulting in moisture-related structural damage. Corrosion of nails and bolts, and rot damage to wood undermine the mudsill and the joist-to-mudsill connection. When earthquake forces pull on the joist, the connection is too weak to resist destruction.
In this situation the floor joist is nailed into a ledger support that is bolted to the foundation wall. If, due to deformation of the shear walls, displacement of the house is great enough, the joists pull free of their nailed connection and the main floor level falls from the ledger support.
Photo of ledger detachment.
Imagine if the ledger had detached completely; the house would not have survived the downhill plunge.
This photo shows where a concrete slab ripped away from its uphill attachment….possibly the house in the picture shown at the beginning of this presentation.
An angle iron secondary anchor. It is difficult to get a scale of the actual size of the anchors we will be showing. In this photograph, the metal is 4 inches wide by ¼ inch thick. This anchor is almost three feet tall.
Similar secondary anchor and foundation plates.
A photo of a primary anchor bolted built from the previous detail.
Another photo of a primary anchor. All of these components were welded in place on site.
Photo of the same primary anchor. You can see why each primary anchor must be engineered for each individual house.
This house failed when the hill below it gave way in the Northridge earthquake. Primary and secondary anchors do not help a house when the soil itself gives way under the house.