Downhill Hillside Homes vs Uphill Hillside Homes

Before you can understand the methods of retrofit 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 are vastly different and it is the retrofit of downhill homes that are the concerns of this web page.

The above drawing shows the two basic types of downhill hillside homes. Parking is usually uphill from the house and, as with the house on the right, the living area is often on two or more levels.

The drawing on the left shows an uphill hillside home. Parking is downhill from the house and the living area is usually on one level though multiple levels may go up the hill in the back portion of the house. 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 web page do not apply to an uphill house.

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How Earthquakes Effect a Downhill Hillside Home
 

The following illustrations represent the main floor level for a typical downhill hillside home, the studs and walls that support the main level from below and the foundation that secures it 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 shows how earthquake forces going in a downslope direction effect the main floor and the area below it. While the entire house moves away from the hill, the greatest movement is located in the area between the two corners. The top of the studs of the lower level are displaced with the main floor and the bottom of the studs remain secure to the foundation. You can see that with extreme downslope force, the main floor would displace enough to tip the studs over and send the house down the hill.

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 that all people living in earthquake country are familiar with.

As with standard retrofit, the goal of retrofitting a hillside home is to keep it securely attached to its foundation. With hillside homes, however, the methods used to do this are much more involved than those used to retrofit houses on flat land; therefore it is more expensive to retrofit a hillside home.
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Factors that Contribute to Earthquake Damage in Hillside Homes    

Next we'll look at factors the Southern California Task Force found contributed to hillside homes becoming detached from their foundation. Preventing these situations 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.

Here 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 undermines 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.

This photograph shows an example of the above separation. The following photo shows a fuller view of the house this photo came from.

If the deformation had been greater so that the ledger support separated from the foundation completely, this entire house could have been destroyed.

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Methods to Retrofit Hillside Homes

The findings of the Los Angeles Hillside Task Force determined that the source of the greatest damage to hillside homes was the detachment of the main floor level from its uphill foundation. The goal of seismic retrofit of hillside homes focuses on strengthening the floor joist-to-foundation connection. The following drawings show typical details of retrofit techniques developed by the Los Angeles Hillside Task Force; they are not designed for a specific site. Generalized details such as these must be engineered by a professional retrofit specialist to accommodate the specific configuration and size of each house.

These drawings and photographs show examples of anchors that can be used to secure a hillside home to its uphill foundation. These anchors are of two basic types and, while we show a few samples here, there are many variations of each.

1. Primary Anchors - These are extremely strong anchors placed at the corners of hillside homes to prevent detachment of the main floor level from the foundation as caused by both downslope movement and rotation. They must be specifically designed based on the existing structural elements of each individual home and the components must be welded in place on site.
    
2. Secondary Anchors - These are placed under the main floor level along the uphill foundation, between the primary anchors, to prevent detachment caused mainly by downslope movement. Several secondary anchors are often installed and hardware is available with which to construct them.

Primary Anchors

One type of primary anchor.

In the following six photographs you can see the unique quality of these anchors and why they must be specifically designed for each individual home.

Here is a closer detail of the above primary anchor. The metal plate was bolted into the foundation then the cross bar was welded in place.

  As you can see, each house poses unique problems when it comes to placing the primary anchors in the necessary location. Its difficult to get a sense of size from these pictures but the angle iron sections used in primary anchors are 4 to 6 inch-wide steel that is 1/4 inch thick. This anchor is more than 48 inches wide across the top.

This plate is 5 inches wide at the top of the foundation and 24 inches long. It was cut to fit around the stud, then bolted 12 inches down into the foundation and the cross bar was welded in place.

Two more examples of primary anchors.

And interesting story about how primary anchors were developed...after the Northridge earthquake there was a hillside home that "should" have been destroyed but wasn't. When the Los Angeles Hillside Task Force investigated, they found that the architect, Art Levin, had installed anchors similar to these. That one addition to the construction of the house saved the home. Art Levin was a member of the Los Angeles Hillside Task Force and was very helpful to the Task Force as it developed its recommentations.

Secondary Anchors

One type of secondary anchor.

Here are two examples of the above type of secondary anchor. Please note, these floor joists are 2x6's so these anchors are not small pieces of metal. They are heavy duty steel, 4 inches wide on one side, 3 inches wide on the other side, 1/4 inch thick and 3 to 4 feet long. The bolts going into the joist are 1 inch in diameter. The bolts into the foundation are 3/4 inch in diameter; they penetrate 6 inches into the concrete and are secured with epoxy glue.

Another type of secondary anchor.

The above photographs show primary and secondary anchors that were constructed by Bay Area Retrofit. When designing primary or secondary anchors for hillside homes, Howard Cook of Bay Area Retrofit consults with local Bay Area structural engineers and often requests a review of design by Nels Roselund, S.E.
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