Why People Love Hillside Homes

The photograph above shows why people love hillside homes; the views are fantastic. But notice there is police tape across the photo and a cement slab that goes nowhere. The next picture is of this same location from an aerial view.
 

There appear to be two, possibly three destroyed homes in this picture.

The following photo shows a closer view of the rubble that lies down the hill...beyond that concrete slab that once was a driveway.

As verification of location, notice that the brick pillar in this picture matches the style of the brick pillar in the first picture.

Why is it that these homes were completely destroyed in the Northridge earthquake while the homes around them withstood the earthquake forces? The answer to that question was the focus of an extensive engineering study.

What Engineers Discovered After the Northridge Earthquake

Over 300 hillside homes collapsed or were severely damaged in the Northridge earthquake and people were killed in those homes. The Los Angeles Department of Building and Safety and the Structural Engineers Association of Southern California Task Force on Hillside Homes investigated and evaluated approximately 60 of the destroyed homes. Their findings were unexpected.

Not only were older hillside homes damaged in the Northridge earthquake, but even brand new hillside homes that were braced with plywood shear panels collapsed and were destroyed.  That was not what engineers expected! They expected the newer homes to be protected by the plywood shear walls that totally covered and supported their tall cripple walls. A team of engineer researchers studied the damage to these homes and their findings were incorporated into a document entitled "Emergency Regulations for New Hillside Buildings."

The cause of damage to the newer homes was, to quote the report, "a deflection incompatibility with a rigid but sloping foundation. The damage to these structures was due largely to the irregular shape of these walls resulting in varying stiffnesses and load distributions to the shear walls that were not accounted for in the design."

In plain English....Hillside homes have sloping or step foundations which means the cripple walls have an "irregular shape", i.e. short on the uphill side and tall on the downhill side. Because they are of varying heights they have "varying stiffnesses." Stiffness is an engineering term that refers to an object's resistance to deformation. Just as a short squat piece of cardboard is stiffer than a tall thin piece of cardboard, so are short cripple walls stiffer than tall ones. This is what caused much of the severe damage seen in hillside homes.

It is an engineering principle that stiffer elements absorb more of the earthquake forces than flexible elements. 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.

This was a completely unexpected finding. Because of this discovery, while the city of Los Angeles still allows plywood shear walls to brace hillside homes above the main level, it no longer allows plywood as the earthquake bracing material that connects the house to the foundation. The L.A. Building Department has approved alternate construction methods that can be adapted for retrofit but they can be quite expensive. Cities in the San Francisco Bay Area still allow hillside homes to use plywood as the bracing material on stepped and sloped foundations despite the above findings.
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Diagrams and Photographs of Shear wall Failure

The following diagrams document the findings of the Southern California Task Force and represent the kinds of damage that were observed in the destroyed hillside homes. They are provided by Nels Roselund, a structural engineer and member of the Southern California Task Force. This information was first published by the Pacific Earthquake Engineering Research Center in Publication 2000/03, "Framing Earthquake Retrofitting Decisions: The Case of Hillside Homes in Los Angeles" by D. von Winterfeldt, N. Roselund and A Kitsuse.

Horizontal foundation:

Illustration 1

These two illustrations represent a standard rectangular shear wall on a horizontal, one level foundation. You can envision the vertical lines of the drawing on the left side as being the 2x4 studs of the cripple wall that have a sheet of plywood nailed to them. The drawing on the right shows how the 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.

The following diagrams represent untested shear wall configurations. Nels Roselund, S.E., knows of no tests or analytical methods for determining the strength of shear walls on stepped or sloped foundations.

Stepped foundation:

Illustration 2

This diagram represents the first of two ways in which a plywood shear wall on a stepped foundation can respond to earthquake forces. Combinations of both of these failure modes were observed in the Northridge earthquake. In the above situation, the mudsill split at the anchor bolts or the anchor bolts bent, either of which resulted in the mudsill sliding along the top of the foundation. In this configuration, the shear wall may remain intact, moving with the mudsill without tearing, or as in the photograph below, where sliding of the mudsill is evident, the shear wall may be reduced to rubble.

Illustration 3

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.

Sloped foundation:

Illustration 4

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.

The following four photographs show two sloped foundations following the Northridge earthquake and what lay down the hill from them.

The house below was destroyed when the hill below it gave way in the Northridge earthquake. Seismic retrofit cannot prevent this problem.

To go to Hillside Homes - Page Two, click here.
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