vulnerabilities – Cascadia Risk by BJ Cure

Home Earthquake Vulnerabilities: Lack of Foundation Anchorage

This house apparently slid about 1 foot off its foundation (from right to left in the photo) during the 1994 Northridge Earthquake.

Lack of foundation anchorage is probably the most well known earthquake vulnerability with houses. Moderate and strong earthquakes regularly cause damage to homes due to this weakness, as the wood-framed house slips off the concrete foundation.

This type of earthquake damage isn’t usually lethal- although it sometimes is- but it typically results in huge economic loss to the house often requiring a complete demo and rebuild. It also typically forces the occupants to leave the house and find somewhere else to live for a long time. This would be a bad scenario to be in after a large Cascadia Subduction Zone earthquake, where power in the whole region may be out for weeks or months.

Homeowners in the Pacific Northwest are regularly encouraged to “attach their house to its foundation”. I certainly agree, and I want to explain some of the details and considerations.

Adding foundation anchorage is a relatively low-cost improvement compared to the cost of repairing or rebuilding later, and considering the high odds of a large earthquake in our future.

Beginning January 1st, 2018, homeowners in Oregon are now required to disclose this vulnerability when they sell a house.

Houses built in the last 40 years or so are unlikely to have this vulnerability, as the new home sale disclosure implies:

What does foundation anchorage refer to?

First, anchoring a house to its foundation doesn’t necessarily solve its seismic problems, it just addresses one of the more common vulnerabilities. See my first post on the topic of home earthquake vulnerabilities for a list of things to watch out for.
The words, “anchoring”, “attaching”, or “bolting” of a house to its foundation typically refer to attachment of the mudsill (the bottom horizontal piece of lumber in a wood-framed house) to the foundation and the associated shear transfer ties required to complete the load path between the first floor and the top of the foundation. At least, these terms should refer to the entire load path, because anchoring the mudsill alone is almost always not sufficient; seismic forces must get from the first floor diaphragm through the thickness of the floor joists and into the foundation. This usually requires at least one other line of clips, nails, screws, or some other type of attachment to complete the load path into the foundation.
To make things a bit more confusing, “foundation anchorage”, or similar language, sometimes refers to all of the typical crawl space work done by seismic retrofit contractors. Strengthening of a weak cripple wall, for example, is sometimes lumped into the general concept of attaching a home to its foundation. If your house has a weak cripple wall and inadequate foundation anchorage, the seismic retrofit must include strengthening of the cripple wall to complete the load path. I consider a weak cripple wall to be a separate vulnerability, and will cover it in my next blog post.

A seismic retrofit involving retrofit plates (Simpson “URFP’s”) attaching the mudsill to a concrete stem wall and shear transfer ties (Simpson “L90’s”) attaching the mudsill to the rim board at the top of the photo.

Why didn’t home builders attach houses to their foundations?

I haven’t figured out the exact answer to this yet; it’s just how homes were built. A house (that has not been seismically upgraded) built before about 1940 almost certainly has no anchorage at all or very weak anchorage.

Since around 1940, there has been a general increase in anchor bolt installation in homes. For about the past twenty years the seismic code has been pretty consistent. It’s standard to install 1/2″ or larger diameter anchor bolts at 4′-0″ O.C. (unless engineered to a different spacing) and to include a 3″ x 3″ plate washer on every bolt.

Homes built in the ’60’s, ’70’s, and ’80’s often have an intermediate level of foundation anchorage. The importance of increasing the foundation anchorage for these homes varies. Generally, the addition of anchorage for these homes is important if you are looking for some assurance that this vulnerability is addressed.

This home, built in the ’60’s, had occasional anchor bolts. The risk of failure at the foundation interface during an earthquake was lower for this house than many older homes; however, the house had other seismic weaknesses, and it was cheap “insurance” to add some anchorage.

What is the right way to attach a house to its foundation?

The most standard way to add anchorage is to add new bolts from above, through the mudsill into the top of the concrete wall. In my experience, screw anchors (they look like giant screws without the pointed end) are usually the most appropriate. They are better than expansion anchors typically, for a few reasons I won’t get into here. Epoxy anchors may outperform screw anchors in an earthquake, but they are more expensive to install and generally not the standard simply because of the expense. Usually, it’s more cost effective to install additional screw anchors.

3″ x 3″ plate washers are important to install on the anchors to reduce the chance of the mudsill splitting due to cross-grain bending during an earthquake. However, houses that have significant anchorage but no plate washers are at much lower risk than houses with minimal anchorage.

If you have an old house, and you have room to install anchors in this manner, then you likely have a weak cripple wall also. Don’t even bother installing anchors if you don’t address the weak cripple wall- both weaknesses are essential to address if the seismic retrofit is to have any significant value at all when the big earthquake strikes.

When there isn’t enough room to install anchors through the mudsill into the top of the concrete, the go-to method of attachment is typically proprietary retrofit plates, which attach to the side of the mudsill with screws, and into the side of the concrete wall with screw anchors. Simpson’s URFP and FRFP are probably the most well-known, and the URFP is shown in one of the pictures above.

There are numerous other appropriate ways to attach depending on the conditions.

How risky is it to leave my house unbolted?

Foundation anchorage sometimes gets oversold, in my opinion. Probably, a better way to state that is mis-sold.

Do I think all houses, say in Portland, without foundation anchorage will slide off their foundations during a Cascadia Subduction Zone earthquake? No, I don’t. There are so many unknowns- such as variation in ground shaking due to varying soil types and topography, variation in ductility and structural response of each house, etc.

I’d expect most or all old neighborhoods to have some houses with this type of damage, with some neighborhoods worse than others.

In addition, as I suggested at the beginning of this post, sometimes foundation anchorage is not the primary earthquake vulnerability with a house. A house built in the ’70’s with intermediate foundation anchorage and a soft story condition above the garage is one example. In that case, I would communicate all vulnerabilities I was aware of to the homeowner and may attempt to lead them toward a soft story improvement as the highest priority.

Many homes damaged in California earthquakes due to lack of foundation anchorage experienced extreme ground shaking (i.e. they were close to the ruptured fault) or had another factor pushing them over the edge, literally- such as soft soil or ground failure. Photos like the one below are often on the home page of companies trying to sell you a retrofit, implying your house will certainly look like that if you don’t retrofit it. No, that won’t happen to every house.

This home slid two feet off its foundation due to the M6.5 San Simeon Earthquake in 2003. According to what I’ve read, slope instability and lateral spreading contributed to the damage of this house. I’ve seen other “apocalyptic” house pictures where ground failure was a significant factor in the damage- not just a simple lack of foundation anchorage or other structural vulnerability. Earthquake risk with any individual building is a combination of geologic and structural factors.

I don’t want to minimize the importance of anchoring a house to it’s foundation, I’m just trying to provide a more thorough explanation of earthquake risk instead of beating the “attach your house to its foundation” drum. To be clear, if you live in earthquake country and your house is not bolted to its foundation, I highly recommend doing so.

Ground shaking in the Portland area during a magnitude 9.0 Cascadia earthquake would be very strong- but not necessarily extreme depending on where you’re at. One problem with a subduction zone earthquake is the long duration of shaking- perhaps 3 to 5 minutes. A house could go through hundreds of cycles during this type of event. This could mean that most unanchored homes in Portland would shift off their foundations. We just don’t know for sure what will happen.

The one thing I can say for sure is that houses without foundation anchorage are at elevated risk of significant damage during a large earthquake. Bolting a house to its foundation can be inexpensive insurance.

Other Considerations

Although there are numerous homes where foundation anchorage is the only significant earthquake vulnerability, plenty of homes are not that way.

I regularly encounter homeowners who were told somewhere to “attach their house to their foundation”, so they set off to do that and opened a big can of worms. They were expecting a retrofit to cost maybe a few thousand dollars, but discovered they were off by tens of thousands.

The following are some items to consider before pursuing a “textbook” seismic retrofit of attaching your house to its foundation:

Does the house have geologic vulnerabilities such as soil prone to liquefaction or landslide? The HAZVU online tool by DOGAMI is helpful. You may need a geotechnical engineer, and/or a structural engineer who is in tune with these risks.
Try to get a sense of the quality of concrete. I’ll write an entire post about this. Bad concrete is a common problem in Portland for old houses, and there is minimal benefit attaching to a foundation that will crumble apart during hundreds of back-and-forth earthquake cycles.
Forget the textbook retrofit if you have a brick or stone foundation.
Consider removing your brick chimney. At least, be aware of the life safety risk. Brick chimneys won’t knock your whole house down in an earthquake, but falling chimneys are a more common cause of earthquake-related deaths than houses sliding off their foundations.
Use common sense and look for any complexities with the house that could affect the seismic retrofit. Check out my list of home earthquake vulnerabilities, as well as the considerations noted directly below, and if you have other vulnerabilities or apparent complexities, I recommend contacting a contractor or structural engineer who specializes in seismic retrofitting.

Should I hire an engineer?

The City of Portland has stated this in their brochure regarding seismic retrofitting:

“You will need to hire an engineer or architect when
you have special conditions like a stone or brick
foundation, poor quality concrete, cripple walls
more than four feet in height, or your home is built
without a continuous foundation or on a grade
steeper than three horizontal to one vertical.”

I think this is pretty well stated. I would add a few things to this statement. First, hire a structural engineer, not an architect. The exception to this if you are doing a significant remodel or addition and need an architect’s assistance with this aspect of the project. Second, I recommend an engineer who specializes, and has interest in, residential seismic retrofitting. Third, I would add to the list above:

Split level houses
Complex floor layouts that are nowhere near a basic square or rectangle
Houses more than 2 stories tall
Houses with additions
Houses that have been lifted in the past
Brick houses or houses with significant amounts of brick, stone, or stucco veneer
Other non-standard conditions

Can I do this work myself?

Because of the high percentage of older homes in urban areas on the west coast, and the number of these homes that haven’t been seismically retrofitted, many cities and jurisdictions have tried to help homeowners by issuing standard plans and retrofitting details. The City of Portland, for example, has retrofit measures it recommends. Many other cities, such as Seattle and San Francisco, have similar recommendations.

So, yes, if your house is relatively “textbook”, you can do the work yourself.

Time v.s. Money

I don’t recommend attempting seismic retrofit work yourself unless you have a construction, engineering, or general handyman (or handywoman)-type mindset and skill set. You also need to do some research to make sure the retrofit gets done right. This is important, because if one element of the seismic load path is missing (like a single weak link in a chain), you may still have a severe earthquake vulnerability after doing a seismic retrofit.

So, you basically have to dedicate a lot of time (if you do the work yourself) and some money or some time and possibly a lot of money (if you hire someone).

If you do want to do the retrofit yourself, I currently recommend the FEMA plan set over Portland’s recommendations. It addresses the common home earthquake vulnerabilities below the first floor level, focusing on weak cripple walls, but also addressing lack of foundation anchorage. There’s a good deal of information there, though, that could wear most people out.

If you really want to geek out with home seismic retrofit knowledge, I recommend buying the book, “Earthquake Strengthening For Vulnerable Homes” by Thor Matteson. Thor is a structural engineer in the San Francisco area who has been engineering residential seismic retrofits for over a decade, and I’ve gained quite a bit of insight from him.

There is a percentage of homeowners who have the time to research the appropriate way to do typical seismic retrofit work. They are handy with tools, willing to buy whatever hardware necessary, and are willing to do the dirty work under their house.

The rest of us should hire a seismic retrofit contractor and/or engineer who has a good reputation and experience with this type of work.

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.