Tag: assessment

Home Earthquake Vulnerabilities: Hillside Home Recommendations

Home and building owners, as well as renters, in hillside neighborhoods need accurate information about their earthquake risk so they can make informed decisions.

Hillside homes can have high, even catastrophic, earthquake risk. The previous two posts discussed common geological and structural problems with hillside homes.

But if you live in or own a hillside home, what should you do? Move away? Just live with the risk, and hope “The Really Big One” doesn’t happen in your lifetime? Get the house seismically upgraded? Get more information?

Yes, those are the four options that come to my mind:

1. Move Away?

I’ll address this first, because many reading this are concerned about earthquake risk. Of those of you who own and/or live in a hillside home, I’m guessing a high percentage of you didn’t know how dangerous this type of home can be in an earthquake. It’s likely no one told you anything about earthquakes when you bought or moved into the house, in fact, it may not have even entered your mind at the time. But here you are, and now you are thinking of moving, perhaps.

Moving away makes sense if:

  • You are confident the house is dangerous, and
  • You’re confident that it would be too expensive for your budget to fix, and/or
  • You aren’t too attached to the home, or maybe you don’t even like it.

Moving out of a hillside home that you perceive to be dangerous makes more sense, in my opinion, than staying and living with the risk. But you may want to consider my last point (#4 below) before moving.

2. Keep the house and live with the risk?

I certainly don’t recommend this. But there are some situations that are less risky than others.

If you own multiple homes and are rarely occupying the house, risk (at least life-safety risk) is obviously lessened simply due to the fact that you aren’t around much. If you don’t have kids and travel often, that’s a similar situation.

If you have a family, especially with a spouse or kids staying home during the day, you genuinely could be putting their lives at risk by not addressing potential seismic vulnerabilities in the place where most of their time is spent.

If your house is looming over your neighbor’s house down the hill, they could be at risk also due to your home’s earthquake vulnerabilities. You may both have earthquake insurance policies (unlikely), but that can’t make up for loss of life. Just something else to think about before you decide to do nothing.

Interestingly, there may be an economic argument against doing nothing also. Suppose your hillside home is worth a million dollars. Let’s say the chance of a severe earthquake affecting this house in the next 50 years is 20 percent (this is in the ballpark of what seismologists have estimated). Suppose the odds of collapse of the home during the earthquake are 50 percent (arbitrary number), with severe damage likely even if it does not collapse. A retrofit costing in the tens of thousands of dollars, or even $100,000, isn’t necessarily unreasonable in this circumstance, for those with the available capital and desire to keep the home.

There are situations where a retrofit could be more costly, but this would be difficult to know without more information (which is why I like Option #4 below).

It’s also possible that the house has low risk of seismic damage. In that case, it may be reasonable to live with the risk. But how would you know this? You probably need a specific assessment to be sure.

A collapsed California house after the Northridge Earthquake. This will likely happen to some hillside homes in Oregon and Washington when we get our “Big One”. Please take a look at this picture and ponder if you are okay living with this risk before choosing to do so.

3. Have your home seismically upgraded?

Of course, I recommend a seismic upgrade for many hillside homes. I’m concerned about the risk of a Cascadia Megaquake, and what it will do to hillside neighborhoods. This is why I’m writing this and specializing in this type of work.

But the choice to upgrade the home has to work economically. Hillside home seismic upgrades can be expensive, and not only does the money need to be there to pay for the upgrade, but the benefit should be worth the cost to the homeowner. I recommend spending the time necessary up front so you have a good ballpark figure of the cost.

Don’t Mess Around With Cascadia

I strongly believe in conservatism with seismic upgrades. Our Cascadia Subduction Zone could produce a magnitude 9.0+ earthquake that could last 3 to 5 minutes. This earthquake will last much longer than earthquakes that have caused the collapse of hillside homes in California in recent decades. Homes that have poor seismic force resisting systems (such as stilts with wood bracing) could degrade with each cycle of ground shaking, and there could be hundreds of cycles in this type of earthquake.

My point is this: if you are going to do an upgrade, do something that will actually work. Don’t just pay someone a few hundred bucks to take a quick look at your house and give you a few cheap recommendations. Count the cost ahead of time and be willing to pay for the thorough upgrade that you really want, that will really do what it needs to do when the ground shakes longer than an average pop song.

Hillside home seismic upgrades are complex, and involve much more than just “attaching the home to the foundation”.

You will need a structural engineer who specializes in hillside building seismic upgrades. I’m trying to be that guy because there’s a need there, but if you find someone else who qualifies, that’s great! More engineers need to be doing this, in fact, we really need an “army” of specialty engineers and contractors retrofitting homes and buildings ahead of the earthquake who are passionate about this kind of life-saving work.

You will, in many cases, need a geotechnical engineer also. Geological risks can’t be ignored and can sometimes drive the cost of seismic rehabilitation through the roof. If landslide risk is high, mitigation may be expensive or even virtually impossible. Make sure you figure this out with a geotechnical investigation, and make sure their recommendations are followed in the seismic upgrade. Or, if landslide risk is apparently low, at least have a structural engineer consider slope stability in the seismic upgrade including a conservative design with a new foundation if needed.

A stepped foundation collector I designed for a hillside home built in 2001. The intent of this design is to prevent the stepped shear wall failure (described in the previous blog post) by directing seismic loads into the high part of the foundation. I communicated to the homeowner that this type of failure was unlikely for her house, but I couldn’t rule it out. She wanted to strengthen the house. I believe it was a reasonable decision that gave her house a “belt and suspenders”- i.e. some redundancy, to help her and her family sleep better at night.

Hillside Retrofit Economics

Some hillside homes are almost beyond hope of an adequate seismic retrofit due to high landslide risk or a combination of structural problems. It is possible that effective strengthening measures could cost in the hundreds of thousands of dollars if the owner wants to really mitigate their slope stability or significant structural weaknesses. There is little benefit to retrofitting a home structurally if the ground it sits on is unstable.

The earthquake risk of hillside homes varies significantly from house to house and from site to site, and the cost of a necessary seismic retrofit can vary from $0 (no retrofit necessary) to extremely expensive. The decision to upgrade, move away, or live with the risk is a personal decision based on life-safety concerns, risk tolerance, and personal economics.

Due to the variability of cost and the many factors affecting the seismic risk of hillside homes, there is a need for good, up-front information for hillside homeowners.

4. Get More Information.

I hope the information in these blog posts about hillside homes is helpful for making decisions. Since the information is not house-specific, however, many need to go a step further.

Consulting a structural and/or geotechnical engineer is appropriate, and I am happy to do this. My preferred approach is to use a developed seismic risk assessment methodology. I currently use FEMA P-50, P-58, and ASCE 41, depending on the situation. You can learn more about these assessments here.

I believe seismic risk assessments have great value, and are a good first step in the decision process. If you hire me for an assessment, you will get a structural engineer’s opinion (a qualitative assessment) as well as an analytical (quantitative) assessment. This first-pass information can be done quickly at a relatively low cost, to help develop the “big picture” of what a potential retrofit would look like and what the potential benefits are.

“FEMA P-50” is a good seismic methodology that applies to most hillside homes. It will grade the house (with a letter grade from A to D-) based on how well it will perform in our largest expected earthquake. When I assess a home this way, I develop retrofit concepts and then grade the house pre-retrofit and post-retrofit. Sometimes I will provide a “lean” retrofit option, in addition to a more thorough retrofit option, if that makes sense for the particular structure.

Even if your hillside home was engineered relatively recently, it doesn’t hurt to have it double-checked. Engineers make mistakes sometimes, and hillside home retrofits can be difficult to design correctly.

If you’ve read all three of my blog posts on hillside homes, you can hopefully tell that I’m trying to sound the alarm regarding earthquake risk with these types of homes. However, not all hillside homes are in danger.

My main point is that there are many variables to seismic risk with these unique structures. To make an informed decision about what to do, hillside homeowners need accurate information that takes all these variables into account. This information may lead you in many different directions depending on your specific house and personal situation.

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

Home Earthquake Vulnerabilities: An Overview

One of the top priorities in preparing for an earthquake is making sure your home is safe.

Many homeowners in the Pacific Northwest are concerned about how their home will perform in a large earthquake, but they are confused. Some think earthquake insurance is the next step, but haven’t thought much beyond that. Others (wisely) have considered earthquake retrofitting.  But that opens the door to all sorts of questions, like:

  • How do I verify that the retrofit will actually be effective?
  • Does my house go from bad to awesome in terms of earthquake performance, or bad to okay, after the retrofit?
  • Is my house okay without any seismic strengthening?
  • What else should I do besides the retrofit? What do I need to do myself?
House with failed cripple wall- South Napa earthquake, 2014

My goal is to provide as much useful, free information as possible, and shed some light on a confusing topic.

Earthquake Vulnerabilities are no Mystery

Although earthquake awareness has increased much in the Pacific Northwest, many people are so overwhelmed by the thought of it that some make statements like this:

“There’s no way we can know what will happen to our house after a 9.0 earthquake”- Typical pessimist’s home earthquake risk assessment

While it’s true that we can’t know for sure what will happen, we can make good estimates based on past earthquake data and engineering principles. Plenty of helpful information is out there, and it’s available to those of us who have searched for it and used it in our work. I’d like it to be more available to the general public, which is why I’m writing this.

I’ve been amazed at the wealth of information available at sources such as FEMA or various earthquake engineers I’ve spoken to in California who have designed earthquake strengthening measures for buildings and then seen them tested with actual earthquakes.

Methodologies to assess earthquake risk have been in development for decades, and are based on actual earthquake damage to various building types.

FEMA’s P-50 (for houses) and P-58 (for various building types) methodologies are very helpful resources for assessing earthquake risk, and in my opinion, their usage needs to be marketed more to home and property owners.  I use both methodologies as well as structural engineering principles.

The vulnerabilities that cause damage to homes in earthquakes are well documented, but not easily accessible to the typical homeowner in the Pacific Northwest.  So… what are they?

One simple way to categorize the different variables affecting any individual building’s earthquake risk are below-ground and above-ground variables.

Below-Ground Variables

The below-ground variables are the geological site characteristics, such as the distance from the earthquake source and the soil type. Ground shaking will generally increase the closer you are to the earthquake source. This is common sense.

What many don’t know is the effect that soil type can have on ground accelerations.  In the 1989 earthquake in San Francisco, for example, ground shaking was five times stronger at the Fisherman’s Wharf area (with soft, saturated soil) compared to the Chinatown area, which is on bedrock and only a half mile away.

In some cases, a site that a house (or any structure) is built on can be so poor that a seismic upgrade is not even worth considering, at least, from an economic perspective.  The only reasonable choice for a homeowner in this scenario may be to either move away or simply live with the risk.

Near collapse of a “weak story” building on soft soil after the 1989 Loma Prieta earthquake in San Francisco. There are many buildings in Portland and Seattle that have both of these vulnerabilities.

Other below-ground hazards include liquefaction and lateral spreading, which tend to occur in sandy, saturated soils in low-lying areas, and landslides in the hills.  I also include tsunami risk in this category; although it’s technically not below the ground, it’s a feature unique to the site where a building is located.

With our abundance of water in the Northwest, and the potential for an earthquake shaking 3 to 5 minutes, geological hazards pose a great risk in many areas.

There are helpful free online resources to allow home or building owners to quickly assess their geological hazards.  For example, the Oregon Department of Geology and Mineral Industries has an interactive map where all of these different site hazards can be viewed for any location in Oregon (the mapping is on a macro level and does not eliminate the need for a site geotechnical investigation, but is still helpful information).  OPB’s “Aftershock” tool combines ground shaking, distance from the Cascadia Subduction Zone and soil type to give you a qualitative explanation of what to expect at your specific address.

These tools, however, are not building-specific, and for this reason, they do not accurately quantify the earthquake risk of your home. They are helpful tools- and I recommend using them- but there will be a huge variability in earthquake damage from one home to another, even in the same neighborhood, because of the differing construction of each home.

Above-Ground Variables

Above the ground, every structure will respond differently in an earthquake. Every home has its unique geometry and construction, which will affect the way it reacts to the forces.

There are plenty of exceptions, but in general, newer homes perform better than older homes.

A building will shake roughly proportional to its weight and height, which means that a smaller one-story house will typically do better than a larger two or three-story house.

Wood-framed houses tend to perform well in earthquakes, if they don’t have any significant vulnerabilities. Wood-framed construction is flexible, which dampens earthquake forces.  This is true even with older wood-framed homes, although damage is typically greater.  This is one reason why a brick house would likely perform worse than a wood-framed house in the same neighborhood.

The following common above-ground vulnerabilities tend to generate earthquake damage:

  • Brick Chimneys. Chimneys are heavy, tall, skinny, and brittle.  This is a dangerous recipe. Even in moderate earthquakes, chimney damage is common and can result in injury or death.
  • Weak Cripple Wall. A “cripple wall” is the wood-framed wall between the home’s foundation and its first floor.  A house with an elevated porch often has a cripple wall, particularly if there is no basement. This is a common weakness in older homes, and failure to strengthen a cripple wall can result in the house suddenly dropping and shifting laterally a few feet during an earthquake.  This usually results in a complete economic loss of the home.
  • Inadequate Foundation Anchorage. In hindsight, it’s amazing that builders didn’t think it was necessary to attach wood-framed houses to the concrete basement walls or foundations way back when, but that’s how they commonly built homes.  It’s also amazing that many relatively new homes sometimes have inadequate anchorage, even homes built after the building codes required it. The code began catching up to our knowledge of a potential large subduction earthquake about 20 years ago, but I sometimes see homes built as late as the early 2000’s with missing nuts and plate washers on many of the anchor bolts. Inadequate anchorage is a common failure mechanism in earthquakes which results typically in total economic loss as the house slides off the foundation during strong shaking.
  • Deteriorating concrete or brick basement walls and foundations. This is a common structural problem with homes around 100 years old in Portland. It’s a hazard that should be addressed regardless of earthquake risk. It’s also important to not attempt a textbook retrofit that attaches to poor concrete or brick without an expert’s input.
  • Soft or Weak Story homes. A practical definition of a “soft story” is an exterior wall line that has very few wall segments (i.e. it is mostly composed of windows or openings). A common example of this is a garage door with a living space above it and very little wall width each side of the garage door.  The narrower the walls each side of the garage door, the greater the likelihood of severe damage.  Another similar issue with older homes is that after a century of different owners, the current floor plan is open with more windows and less walls than it originally had. If enough wall segments are removed, very little lateral strength remains. A weak story combined with liquefaction-prone soil is particularly dangerous in earthquakes.
  • Hillside Homes. By far the most dangerous demographic, these homes can suffer severe damage during an earthquake.  Not only is the structure often weak and top-heavy, as in the case of homes on “stilts”, but they can have catastrophic landslide risk. They also often have other structural problems such as torsional weakness and lack of ductility with bracing or shear walls.
  • Split Level Homes, Complex Floor Plans and Roof Lines. Complexities to homes add character, but sometimes they are problematic for an earthquake load path. The more discontinuities in roof, floor, or wall lines, the more likely separations will occur.
  • Elevated Porches and Decks. These types of “add-ons” to a house sometimes detach from the house during an earthquake and collapse without adequate bracing.
The remnants of two hillside homes after the 1994 Northridge earthquake in the Los Angeles area.

In the past decade or two, a number of contractors have established a niche for residential earthquake retrofitting. Typically, an earthquake retrofit contractor will provide services primarily relating to weak cripple walls and inadequate foundation anchorage.  Rightly so, because these vulnerabilities are common and relatively inexpensive to fix compared to say, a home on stilts or with a severe soft story problem. But as I’ve established, there are many variables of earthquake risk both with the site of a home and the structure itself, and these risks aren’t always communicated or addressed.

Bracing For Cascadia

Many people have latched onto phrases like, “everything west of I-5 is toast” (a quote made somewhat infamous after the 2015 New Yorker article, “The Really Big One”), and they envision a post-earthquake Northwest where all or most buildings are destroyed. Some suppose the tsunami will enter the Willamette Valley and Portland. Neither of these ideas are true whatsoever (and that’s not what the quote meant). I expect most buildings to remain standing after our big earthquake. I expect most homes to do even better than other buildings overall, as they have done in past earthquakes.

That’s not to say the earthquake won’t be a major disaster. It will certainly be. Power outages for 1 to 3 months in the Portland area, which is what the state expects, is a disaster.

As far as home preparedness goes, we need a realistic view of our earthquake risks. We need to make sure we don’t have a home that is prone to damage. We want to ride through the earthquake uninjured if possible, so we can help others. And as most of us know, there are numerous other tasks we need to do to prepare for the earthquake, so let’s make sure our homes are safe to the best of our ability.

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