Home Earthquake Vulnerabilities: Hillside Home Structural Problems

In the previous post, I explained that hillside homes are among the more dangerous building types in earthquakes, and that one of my concerns with these types of homes in the Pacific Northwest is landslide risk.

This post will discuss common structural problems with hillside homes.

Earthquake risk with hillside homes varies greatly from house to house. If you live in a hillside home, I strongly recommend an assessment by a structural and/or geotechnical engineer, but here are some common vulnerabilities to consider.

Structural Problems with Hillside Homes

Earthquake vulnerabilities related to the structure of hillside homes are numerous and varied. I can make a general statement that most hillside homes in Portland do not meet the current seismic code just based on their age. I also can say, based on what I’ve seen and homes I’ve helped strengthen, that even many newer homes are at risk (although generally less than the older ones). I’ll explain this further below, but here are some of the top seismic vulnerabilities.

Top-heaviness

Earthquake forces increase proportionally with weight, and their effects on structures increase with height. Many hillside homes are multiple stories tall; “stilts” or deep crawl spaces below the lowest level make some even taller. Often, the garage, which has a heavy concrete slab, is at the top-level. This means the seismic demand on these homes is generally higher than other homes, and since many of them were built with earthquakes minimally considered at best, the odds of them “accidentally” being strong enough to handle a large earthquake are much lower.

Stiffness and Torsional Irregularities

Because the down-slope side of a hillside home is often considerably taller than the up-slope side, the house can be much more flexible on one side than the other. During strong shaking parallel to the hill, the physics on these homes works something like this: First, the earthquake “tries to” push the tall part of the house over. As the house flexes from the seismic loads, it encounters a greater stiffness from other parts of the house, namely the floor, or “floor diaphragm”. As the forces encounter the relatively stiff floor diaphragm, a twisting tendency toward and away from the hill at each end of the house occurs.

The action of a hillside house breaking away from the top foundation was documented as a common cause of collapse during the 1994 Northridge earthquake. The forces at the top can be strong either from the twisting motion described above, or from shaking perpendicular to, and away from, the hill.

Remedies to this problem include adequately anchoring the house to the top foundation and/or designing a seismic force resisting system parallel to the hill on the tallest side that has adequate strength, stiffness, and ductility (hint: the effective lateral system is not stilts with some diagonal wood braces).

What About Stilts?

Stilts are generally pretty dangerous in earthquakes. At least two homeowners in the west hills have told me they heard that stilts actually may dampen earthquake forces (i.e. “stilt houses are good in earthquakes”). This is somewhat amusing to hear as a structural engineer, but I think I understand why this rumor may spread.

Although it’s theoretically possible this statement could be true in some instances, the practical reality is that old stilt-supported houses have high seismic risk. If little or no diagonal bracing exists, the twisting action described above could occur, or the house could start moving (with some damping) and then continue moving until it crashes to the ground.

The existence of diagonal bracing may be helpful, but most existing diagonal bracing measures in hillside homes lack the stiffness or strength they need to meet the seismic forces we now expect, or they lack ductility. Diagonal wood braces are a terrible seismic force resisting system in that the bolted connections don’t have much strength and tend to fail in a very brittle, sudden manner.

Stepped wood shear walls

Correctly built plywood-sheathed wood shear walls can be an excellent method of construction for resisting seismic forces. A significant exception is stepped shear walls. This is a common condition in hillside homes that have stepped foundations or concrete stem walls.

The main problem with stepped shear walls is that the shortest wall segments “suck” seismic load into them due to their much greater stiffness. Seismic forces work their way into the stiffest elements, whether the designer, builder, or engineer wanted them to do that or not. Sometimes the shortest shear wall segment will fail since it attempts to handle the entirety of the seismic load on that side of the house. After failure of the shortest wall segment, the load shifts into the second-shortest wall segment, and so on, until the entire line of seismic resistance is gone and the house collapses. A seismic retrofit contractor in the San Francisco area has done a good job of explaining this failure mechanism in detail here.

Unfortunately, this failure mechanism is not well known in the Pacific Northwest, even among engineers, and is not adequately addressed in our current building code.

A sloped foundation wall is even worse than the stepped scenario. Even worse are “skirt” walls that make their way down the exterior of the house and stop just short of the foundation. A retrofit is strongly recommended in all these scenarios.

Foundation and footing-soil interface

A house that has a continuous foundation around the entire perimeter is inherently stronger than an equivalent house with isolated pad footings. A continuous footing that is tied together prevents foundation sliding failures and also helps reduce overall settlement, earthquake related settlement or otherwise.

Partial collapse from slope instability is also less likely; if a small landslide occurs beneath the house, the continuous footing may span across it.

Deep foundations, such as driven piles, connected together, are the best. Unfortunately, it is common practice in hillside homes to build shallow foundations, even on soft soils such as those that are common in the west hills of Portland. Homes with foundations that were built with the recommendation of a geotechnical report are generally at lower risk than homes built without geotechnical input. However, if you’re concerned with seismic risk, it’s important to hire a geotechnical engineer that is in tune with seismic risk (as is true with hiring a structural engineer).

I wanted to demonstrate in this post some of the complexities to hillside homes that seismic retrofits must consider if they are to actually work. From an engineer’s perspective, sometimes structures such as these can be more difficult to work on than much larger or taller buildings. The next post will give some final thoughts and recommendations.

For more information about seismic risk assessments and retrofitting, please see the Cascadia Risk Solutions website.