Improving earthquake resilience in the Pacific Northwest
Author: BJ Cure
My name is BJ Cure and I am a structural engineer focused on improving earthquake resilience in the Pacific Northwest.
I help home and building owners assess their earthquake risks, make informed decisions, and strengthen their home or building when appropriate.
I can be reached by email at bjcure@cascadiarisk.com.
Minamisanriku, Japan, after the M9.0 earthquake and tsunami in 2011 source: Richard Eisner
If you live in the Pacific Northwest, you should prepare for a large earthquake.
I say this not because I know we will get a large earthquake in the near future, although that’s a notable possibility. Even a probability.
My main reason for saying this is that preparing for an earthquake covers all sorts of disruptive events that are seeming more likely and more imminent.
Consider the likely aftermath of a Cascadia Megaquake, concluded in the 2013 Oregon Resilience Plan:
These are not minor implications. Imagine being 1 to 12 months without clean and available drinking water, and 1 to 3 months without reliable electricity. Something like this will happen. It’s just a matter of time.
But back to my original point. Other disastrous events seem more likely than they used to be. I’ll name a few.
Natural disasters such as wildfires.
Civil unrest. Our nation is becoming more divided than ever, at least in modern history. And the last few years have not helped.
Economic collapse of some sort.The United States and other nations have been on an unsustainable spending spree for decades, but especially the last 15 years or so. The last few years have not helped on this front, either. It’s not going to end well.
I could go on. Global tensions are higher. Russia and Ukraine are currently at war. We are not guaranteed peace in North America.
I do not believe in spreading irrational fear (and I’m not). I suspect you feel it, too. I want to help save lives and help us to prosper as we move into an uncertain future.
The February 2023 earthquake in Turkey and Syria reminded us of how fragile the world is. Source: EERI
Chris Goldfinger went on record with a calculation that there is about a 1 in 3 chance of a Magnitude 8 or higher earthquake off the coast of Oregon in the next 50 years (See “The Really Big One” New Yorker article).
Let us assume that estimate of probability is accurate. Consider what kind of odds we have of any significant disruptive event in the next 50 years- say, a regional, national, or global event, but one that affects us in our location. They’re high… very high.
Many other disruptive events could have similar effects on life and infrastructure in the Pacific Northwest as a large earthquake.
I’ll throw something else out here for consideration. I noticed after the December 2022 M6.4 Ferndale earthquake in northern California, Temblor (an earthquake risk assessment and catastrophe modeling company based in the Bay Area) had this to say: “The (Ferndale) quake produced severe shaking in a lightly populated area, (and) brought the southern tip of the Cascadia Subduction Zone slightly closer to failure…”
Tectonic action is often happening beneath us and off the coast that remind us that the Cascadia Subduction Zone is indeed active, and a large earthquake is looming. The Ferndale earthquake was a recent reminder.
Damage to Molalla High School after the 1993 M5.6 Scotts Mills Earthquake, about 20 miles south of Portland, Oregon
Priorities?
There are many good personal and family goals worthy to be pursued, so earthquake preparedness does not always seem like it needs to be an urgent priority. In addition, our exceedingly distracting world does not help us achieve goals, in general.
Please consider putting earthquake preparedness at or near the top of your list. Or, if you don’t live in earthquake country, preparedness for a particular event that aligns with your region. I would argue that some things we are often encouraged to prioritize should go below earthquake preparedness.
The point of this post is not to get into preparedness details, but think of food, water, sanitation, personal fitness, etc. It’s a time-consuming endeavor and probably needs to be thought of more as a lifestyle change than a checklist of things to get done. A perk about preparedness is that it can overlap with activities that we tend to value in the Pacific Northwest: camping, biking, gardening, hunting, etc.
For information about seismic assessments or retrofitting of a home or building, please see the Cascadia Risk Solutions website.
Whether or not the big earthquake happens in the near future, now is the time to prepare for it.
This photo shows a classic soft story failure of a wood-framed residential structure. The building on the left used to look similar to the building on the right.
“Soft stories” are a common cause of catastrophic earthquake damage in many types of structures, including houses. Identifying and addressing a soft-story vulnerability is important for both home and building owners. It is potentially very dangerous and represents a high economic risk as well.
The term “soft story” has a technical background that I won’t go into here. A “weak story” or “open front” building means essentially the same thing. Because a building needs shear walls, or some other type of seismic force resisting system, to bring seismic forces to the ground, a building lacking walls on one or multiple sides of it can be particularly vulnerable during the shaking that accompanies a strong earthquake, as it lacks strength and/or stiffness to adequately resist those forces.
If you’re from California, you may have heard the term, “soft story”. Many people from California are very familiar with the term, in fact. Soft story buildings were a significant source of earthquake damage in both the 1989 Loma Prieta and 1994 Northridge earthquakes (the last two “big” earthquakes in California). 16 people died in the Northridge Meadows apartments in 1994, a building with tuck-under parking at the lowest level representing a severe soft story condition.
Jurisdictions in California have identified and required seismic upgrades to these types of structures, because they’ve represented a significant percentage of lives lost in past earthquakes. Los Angeles, San Francisco, and most recently, Pasadena, have gone this route.
Meanwhile, here we are in the Pacific Northwest, and I don’t hear these building types talked about much. While Portland is focusing on requiring seismic upgrades to URM (unreinforced masonry) buildings, the Northwest also has plenty of other vulnerable building types, including many soft story buildings. Many people live and work in these homes and buildings, and don’t realize the risk that these buildings represent.
What is a soft story?
For wood-framed buildings, a soft story typically means a structure lacks walls on at least one exterior face of the building, at the bottom level. While a soft story can occur at an upper story, it is more common at the first floor and is far more dangerous at the first floor.
Soft stories are common at the first floor level in buildings due to garages, tuck-under parking, and open storefronts in retail spaces.
Soft stories are more dangerous at the lowest level, as this level has to resist all the seismic inertial forces working their way to the ground from the upper levels.
Soft stories come in numerous shapes and sizes. For houses, a soft story often occurs at a garage with a living space above.
A garage with a second floor above represents a common soft story condition with houses.
A living space over a garage, or another soft story condition, doesn’t necessarily mean a house is vulnerable during an earthquake. The following are important considerations:
The soft story condition may have been addressed in the design of the house. Current building codes require some type of seismic force resisting system to address this common condition. These include narrow wood shear walls with holdowns, a wood portal frame system, or less common engineered solutions like a steel moment frame or a “3 sided diaphragm” (essentially designing the 3 strong sides of the garage to resist the forces and the induced rotation).
Soft stories vary in their hazard. Some soft stories are “softer” than others. Even a home with an apparent severe soft story condition on one exterior face may have enough redundancy with interior walls that it isn’t at high risk of collapse in reality.
A soft story at the first floor level gets more dangerous the more stories there are above it.
A soft story is one of many seismic risk factors. The condition gets more dangerous when combined with other seismic vulnerabilities.
Soft stories and the age of a house
Newer homes are less likely to be vulnerable due to a soft story. This is for reasons related to building codes and modern construction, mentioned above. Building codes in general made significant changes in the 1990’s addressing seismic details in wood-framed construction. In the Pacific Northwest, the early 1990’s also represented a “seismic shift”, so to speak, as the Cascadia Subduction Zone and its projected design ground accelerations worked their way into our building code. This means houses newer than the mid-1990’s represent a much lower seismic risk in general, even homes with soft stories. However, this is a general statement, and I sometimes encounter exceptions.
Houses newer than the mid-1990’s should have been built to take a soft story condition into account.
Other soft story conditions
Soft stories exist in numerous other conditions with houses. The appeal of an “open floor plan” has always existed, for example. Many houses built in the 1960’s and 1970’s have an architectural style with an exterior wall line almost completely consisting of windows on one side. FEMA P-50 (a seismic risk assessment methodology for houses that I use) flags two-story houses as higher risk if an exterior wall line at the lowest level consists of less than 25% wall segments. For 3-story houses, that number increases to 40%.
Large old houses with multiple remodels
Another common condition is a large, old house that has been remodeled multiple times. Often because an open floor plan is desirable, many old homes have had numerous interior walls removed. These walls add redundancy and help resist seismic forces, even if they were not designed or intended to do so. Sometimes, exterior windows were added and exterior shear wall strength has been reduced.
Many of these homes have been beautifully remodeled and have seen a great increase in market value, but they’ve ironically created a soft story condition, or something similar, and have increased the home’s seismic risk.
A soft story is sometimes created with an addition to a house, as apparently shown in the photo below.
A soft story condition at the rear of a home in the Portland area. Notice the posts with no bracing or walls for seismic support. This was likely the result of an addition, perhaps built in the 1980’s before seismic risk was taken as seriously by building jurisdictions in the area.
Split Level Houses
Much could be written about split level houses, which I won’t do at this time. Split level houses often attract more seismic damage than the average home, due to the discontinuity of floor and/or roof levels. A split level home combined with a soft story can result in the two-story portion of the house pulling away from the rest of the house and collapsing.
An earthquake engineer can look at this house and see a soft story vulnerability on the left side (at the front of the garage) and a smashed cripple wall on the right side. The two-story portion is leaning and close to collapse. The right side of the house apparently had a weak cripple wall that failed during the earthquake. The cripple wall failure is evident by the roof line a few feet lower than it should be against the two-story portion, and front porch stairs that remain after the main part of the house dropped a few feet.
Semi-Soft Stories
For lack of a better phrase, some soft story conditions come with a moderate, or low, seismic risk, compared to other obviously dangerous soft story conditions. Many old homes fit this criteria: they have a decent amount of exterior wall segments (perhaps around 25% based on the FEMA P-50 guideline previously mentioned), but the old shiplap or 1x plank siding just isn’t as strong or ductile as modern, well-nailed plywood sheathing.
In these situations, I try to communicate to homeowners that the risk is lower, but not nonexistent. Whether to seismically upgrade in these situations is a personal decision based on risk tolerance and economics.
I typically would classify a home similar to that shown in this picture as having a “semi-soft” story. However, you can see that this structure is severely damaged and is close to collapse. My suspicion is that the ground accelerations that caused this damage were severe. I wasn’t there, though- this photo was taken after the South Carolina Earthquake of 1886.
Soft Stories combined with other vulnerabilities
A soft story condition combined with other seismic vulnerabilities is particularly dangerous. This combination can push a house past the brink of collapse. Other structural vulnerabilities like a deteriorating foundation, lack of foundation anchorage, or weak cripple walls could make a house more likely to have catastrophic damage when combined with a soft story. Geological hazards such as soft soil prone to liquefaction and/or lateral spreading, or slope instability, are also dangerous when combined with a soft story condition.
While this photo is a somewhat “textbook” example of a soft story failure at the front of a garage, slope instability contributed significantly to this collapse. If the ground shifts enough during an earthquake, it of course puts extra demand on an already vulnerable structure.
Seismic risk involves many variables
Besides addressing the risk of soft story vulnerabilities with houses, this post should also draw attention to the fact that seismic risk is a complex interaction of many risk factors.
For homeowners or potential buyers concerned about seismic risk, I recommend FEMA P-50 seismic risk assessments because they address the numerous known structural and geological vulnerabilities with any specific house. The methodology is simplified, but it quantifies risk at a relatively low cost and even helps identify how a home would perform after constructing a retrofit that mitigates specific earthquake vulnerabilities, such as a soft story, lack of foundation anchorage, or a weak cripple wall.
For more information about FEMA P-50 seismic assessments, click here.
Near collapse of a weak story structure in the Marina District of San Francisco after the 1989 Loma Prieta earthquake. The Marina District experienced strong, amplified ground accelerations due to soft soil.
How can a soft story be strengthened?
There are many possible ways to add adequate strength and stiffness to a soft story in an existing building. For houses, plywood shear walls are the least expensive solution and often the best. I also often recommend and design steel “moment” columns. Usually, a new reinforced concrete foundation is required to support these new systems. These are the two systems I most commonly work with and will focus on these two.
Other systems such as wood portal frames, steel moment frames, braced frames, and concrete and masonry shear walls could also be used if it made sense to do so.
While strengthening weak cripple walls and adding foundation bolts doesn’t necessarily require engineering, a soft story does.
New plywood shear walls
If there is room on an existing wall segment to add plywood and holdowns to create a shear wall, this is the least expensive approach. A new plywood shear wall with a new footing is often required, however, assuming the intent is to build the new wall to current seismic code standards.
A structural engineer can determine what the minimum or recommended wall length would be, and an appropriate location for the wall can be determined by the engineer and homeowner.
The following three photos show a soft story condition strengthened with a new plywood shear wall and concrete footing.
This Google image shows a house with a soft story condition at the front of the garage. The house was built in the ’50’s and the owner’s child’s bedroom is directly above the garage. The homeowner elected to install a new plywood shear wall and footing at the front of the garage on the left side. This was the least expensive option, even though he had to replace the double garage doors with a single door.
This is the exterior siding on a new plywood shear wall at the front of the garage for the house in the previous picture.
An interior view of the same plywood shear wall. A new reinforced concrete foundation was created by cutting a new trench in the garage floor at the front of the garage. A small concrete stem wall was placed above the foundation which was flush with the top of the existing garage concrete slab.
Steel “moment” columns
Sometimes, particularly in a soft story condition at the front of a garage, there is no room to place a new plywood shear wall, or it’s not desirable to modify the garage door or the space inside the garage. In this case, a steel “moment” column or “cantilevered” column with a new concrete footing is often the best approach.
Think of the new steel column like a vertically oriented, extremely rigid diving board. While columns typically are intended to take vertical loads, a moment column is designed to take seismic loads (and usually no vertical loads at all). A moment column needs a new concrete footing with a large enough mass to resist the overturning or rocking that the cyclical seismic forces place on it.
For buildings in general, steel moment frames are more conventional than moment columns. A moment frame consists of two steel columns and a steel beam. A moment frame can be used when retrofitting a house for a soft story condition, but it is often difficult to fit and a moment column is often simpler.
This house had a severe soft story condition at the front with the tuck-under parking and two stories above. There wasn’t space for a plywood shear wall, so a steel column and large concrete footing was placed on the left side with a new wood beam over the garage to act as a collector for the seismic forces to transfer to the steel column.
Recent developments
A structural engineer in the San Francisco area has developed an “Earthquake Resisting Column” (ERC) with a “structural fuse” at the top of the column. The “fuse” is essentially a carefully designed rocker that dampens seismic forces and allows for design of a much smaller steel column and footing. He designed it primarily for the stereotypical tall and skinny classic San Francisco style house, where sometimes only inches of room exist each side of the garage door for a new steel column.
I’ve designed my first seismic retrofit using this type of column on a house in northeast Portland which will be installed soon. A video of this type of column in testing is shown here.
This home in northeast Portland has minimal walls at the front with a living space above. Many soft story mitigation measures were discussed with the owner, but we landed on an ERC by the Soft Story Brace Company. The new steel column will replace the far right double post shown in this photo and will have a new reinforced concrete footing.
For more information about seismic risk assessments and retrofitting, please see the Cascadia Risk Solutions website.
This FEMA photo, taken after the 1994 Northridge Earthquake, shows what can happen to a house with a weak cripple wall.
The most well-known earthquake vulnerability with houses seems to be inadequate foundation anchorage, as discussed in the previous post.
The twin sibling of an “unbolted” house is a weak cripple wall, which we will discuss here. These two seismic weaknesses often occur together in older houses. They are also the two primary earthquake weaknesses that seismic retrofit contractors address, and the main reasons that these type of contractors exist.
Weak cripple walls can cause even greater seismic damage to a house than weak foundation anchorage, as the house can fall a few feet- whatever the height of the cripple wall was- and also shift laterally. This vulnerability is therefore typically more dangerous and more significant economically. If the seismic damage of a house sliding off its foundation doesn’t require a complete demo and rebuild, a weak cripple wall almost certainly will.
What is a cripple wall?
A cripple wall is the wood-framed wall between the foundation and first floor of a house. It typically follows the exterior foundation. Not all houses have cripple walls; some bear directly on a concrete stem wall or footing. Other houses bear directly on a slab-on-grade foundation system, although this is uncommon in the Pacific Northwest.
Houses with crawl spaces often have a cripple wall concealing the crawl space around the perimeter of the house and filling the space between the first floor and the foundation. If you have an old house with an elevated porch and no basement, you likely have a weak cripple wall.
Houses with basements often have a concrete (or brick) foundation wall with the first floor framing sitting directly on the wall. There is no cripple wall in this situation, although it is fairly common to have a portion of the house with a crawl space and cripple wall, and a portion of the house over a basement with no cripple wall. Some basement walls only extend part of the way up to the first floor and have a cripple above them.
The damage shown to the house above is the result of a strong earthquake (Northridge, 1994) combined with a weak cripple wall. It may appear that lack of stability at the porch caused this damage, but the porch collapsed because of the weak cripple wall. Notice the red placard, which indicates that the home is considered too unsafe to enter, even for the homeowners to retrieve their belongings.
Why are cripple walls so prone to seismic damage?
Seismic retrofit work in wood-framed houses should primarily focus on the base of the house between the foundation and the first floor. Severe seismic damage is much more likely here, in general, than above the first floor.
There are two main reasons for this:
Seismic forces (and damage) increase with weight. Weight is greatest at the base of the house. Buildings and their components react to earthquake accelerations roughly proportionally to their weight. Also, where the house is closest to the seismic accelerations occurring in the ground, it “feels” them the most.
A house almost always has virtually no seismic redundancy below the first floor level. “Redundancy” is an earthquake engineering concept that indicates a structure has more potential seismic load paths than it needs- from the upper levels of the structure to the ground. Earthquake forces in buildings are resisted by the stiffest elements- which happen to be the walls in typical wood-framed house construction. Above the first floor, houses often have multiple interior walls that inherently draw seismic forces into them during an earthquake due to their stiffness. This means the seismic demand of any individual wall above the first floor is usually considerably less than that of the cripple walls below the first floor. Even if a house has few interior walls, the drywall or plaster sheathing adds redundancy. Exterior siding does, also. Although these non-structural wall coverings are often brittle with minimal strength, they can, and do, have some capacity to resist seismic forces (potentially even after cracks form).
Cripple walls are forced to bear the entirety of the earthquake forces exerted on a house, without the help of any other stiff elements- such as interior walls.
This cross section of a typical house shows the importance of strengthening a weak cripple wall for seismic forces, as described above.
What not to do with a weak cripple wall
Some contractors- and even some engineers- have attempted to improve the seismic resistance of houses by installing post caps at the interior posts in a crawl space or basement. This does virtually nothing to seismically strengthen a house.
I believe this error may be more common in the Pacific Northwest than in California, where seismic retrofitting is more mainstream.
The strap and post cap shown above were done in the name of a “seismic retrofit” in Portland. The strap is incorrectly installed and serves no purpose anyway. The post cap shown at the bottom of the photo doesn’t hurt anything- but is typically insignificant in reducing earthquake risk.
If you own a house that has been “retrofitted” in this manner, it’s possible the contractor didn’t understand the science behind seismic retrofitting. Make sure the cripple walls have been strengthened, including every part of the load path. It wouldn’t hurt to have a seismic engineer or qualified retrofit contractor look at the “retrofit” that was done to make sure the “retrofit” doesn’t need to be retrofitted.
Seismic forces are drawn into the stiffest elements of a structure, as explained previously. In a house with cripple walls, the cripple walls are by far the stiffest elements between the first floor and the foundation. The cripple walls have stiffness many orders of magnitude greater than the interior posts. The first floor “diaphragm” (engineering term) also acts as a single structural element that has a stiffness many orders of magnitude greater than the interior posts.
Seismic forces that manage to get into the interior walls above the first floor will hit the stiffness of the first floor diaphragm and move to the exterior cripple walls, essentially bypassing the interior posts below the first floor.
If a house has weak cripple walls and rigid interior post caps, the posts would still fail in an earthquake when the cripple walls fail, as they are far weaker than they need to be to resist strong seismic forces.
If you are looking for redundancy with the cripple walls, or a “belt and suspenders”, so to speak, to resist seismic forces, then overkill the cripple walls by strengthening more than necessary with increased length and/or increased nailing, instead of focusing on wood posts. I typically take this approach. At least one of the seismic contractors I regularly work with takes this approach, also.
A seismic retrofit in a house with weak cripple walls should strengthen the cripple walls themselves, not the interior posts.
One last comment on this topic: post caps aren’t a bad thing. It’s generally a good idea for beam-to-post connections to have more than a nail or two connecting them together. There are also some instances where post caps can decrease seismic risk. But even in those instances, they wouldn’t be the top priority in a seismic retrofit.
A detail from the FEMA plan set indicating how to strengthen a weak cripple wall. The FEMA plan set is a prescriptive set of drawings intended to help contractors or homeowners retrofit a house without engineering.
How can a cripple wall be adequately strengthened?
Plywood sheathing is the go-to method for strengthening a weak cripple wall. Plywood shear walls have been rigorously tested by the American Plywood Association and are typically used as the primary seismic force resisting system in modern wood-framed construction. Modern wood-framed buildings do well in earthquakes in general.
Well-built plywood shear walls (i.e. plywood-sheathed cripple walls) are strong, ductile, and somewhat flexible, which makes them an excellent method to resist earthquake forces.
In contrast to properly installed plywood shear walls, old cripple walls are often constructed of 1x planks. Nailing may be minimal. These walls were simply intended to close the gap between the exterior of the house and the crawl space, while providing nominal stability to the house. They weren’t designed to resist strong earthquake forces. The wood may have lost strength due to rot, the nails may be rusting, and the connections can be brittle (“brittle” in the context of seismic resistance means sudden failure).
Some important components (shown in the FEMA plan set detail above) of a modern plywood shear wall are:
1/2″ or greater thickness “Structural I” grade plywood
Tight nailing pattern around all edges of each plywood sheet, or “panel”
Nails installed at least 3/8″ from the edges of the framing members
Double studs, if required, are adequately nailed together
Nailing at intermediate studs does not exceed 12″
Seismic load path is addressed above the plywood sheathing
Foundation anchorage is strengthened from the mudsill to the foundation
While old houses, say, pre- 1940’s, have 1x planks for cripple wall sheathing, houses built from the 50’s to 90’s sometimes have weak cripple walls for different reasons. Many are sheathed with lower grade plywood, don’t have all panel edges blocked, are stapled instead of nailed, and occasionally have very poor nailing where a contractor repeatedly missed studs with a nail gun.
Other sheathing types, such as T1-11 siding, are weaker than plywood but likely stronger than 1x planks.
Hillside home cripple walls can be seismically weak, even in newer houses. See my post on hillside home structural problems for more on this topic.
Plywood shear walls were added in the garage of this split-level house built in the ’60’s. The exterior sheathing was questionable, and it was “cheap insurance” for the homeowner to have the walls strengthened.
Often, but not always, plywood sheathing is added to a weak cripple wall from the crawl space. This is often the least expensive and easiest way to strengthen a cripple wall, since the existing siding and sheathing doesn’t have to be removed.
Correct installation of plywood on a weak cripple wall is important and can be tricky. The seismic load path must be carefully followed from the first floor to the foundation. If one component of the load path is missed or inadequately strengthened, the cripple wall has a weak link and may not perform as intended during an earthquake.
Conditions vary from house to house, but a common seismic load path from the first floor to the foundation is as follows:
Through the first floor sheathing into the rim joist via existing nails
From the rim joist into the existing double 2x top plate via shear transfer ties
From the double top plate to the new interior plywood sheathing via nails
From the plywood sheathing to the existing mudsill via nails
From the mudsill to the existing foundation via new screw anchors
This typical load path can be followed in the sketch below.
This section indicates a common way to strengthen a weak cripple wall from inside the crawl space.
Houses with a strengthened cripple wall may perform better than seismically retrofitted homes without a cripple wall
Another benefit of plywood cripple walls is they have a great structural response to earthquakes. “Structural response” is another earthquake engineering term that refers to how a building responds to seismic loads.
Think of a building like shocks on a vehicle. As an earthquake hits a building, it responds in one direction and then rebounds back. Flexibility in a building can be good for earthquakes, similarly to the flexibility of shocks on a car. If the shocks are too tight, the vehicle feels the impact of a bump more, as do the passengers. Some flexibility in the shocks dissipates energy, as does some flexibility in a building.
Plywood shear walls have a good deal of flexibility as well as strength. This makes them a great system for resisting seismic forces, if installed correctly. A house with a strengthened cripple wall, in addition to being strong enough to resist earthquake forces, may actually experience less non-structural damage in an earthquake than the neighbor’s house that is bolted to its foundation without a cripple wall.
I believe a strengthened cripple wall is similar to “base isolation”, in that the house has a type of seismic damping system.
This makes strengthening a weak cripple wall possibly the best value in seismic retrofitting, in that it brings a home from a potential total economic loss, with some life safety risk, to a home that would likely perform well in an earthquake.
This house had unreinforced brick “cripple walls” between the top of the concrete basement walls and the first floor framing. Since URM (brick) is notoriously bad for earthquakes, the homeowner chose an upgrade that replaced the brick with plywood-sheathed cripple walls. The wood framing is new and the plywood was installed from the exterior in this case. Note the shear transfer ties at the top of the double top plate and the screw anchors with 3″x3″ plate washers through the mudsill into the concrete.
For more information about seismic risk assessments and retrofitting, please see the Cascadia Risk Solutions website.
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.
Last week, a magnitude 4.0 earthquake occurred between Mollala and Silverton, about 35 miles south of Portland. I made a joke on LinkedIn that I was too busy designing a seismic upgrade to notice until I read this article from Temblor the following day:
We might be overdue for a moderate earthquake. Quakes with magnitudes in the 5’s have been occurring near the Portland area roughly every 20 or 30 years since we’ve been keeping track of them. The last one, the “Spring Break Quake”, happened in 1993.
Moderate earthquakes can be damaging; for example, the M5.6 earthquake in 1993 knocked down chimneys and caused part of a brick wall to collapse at Mollalla High School.
We have the potential in the Portland area for shallow earthquakes that could have magnitudes in the 6’s. These could be quite damaging and even deadly. If the Portland Hills Fault were to shift, for example, It’s possible ground accelerations near the fault in Portland could exceed the accelerations we would experience during a 9.0 subduction zone earthquake (although shaking wouldn’t last as long).
The likelihood of a damaging local earthquake in the next 50 years is likely lower than the odds of a subduction zone megathrust earthquake occurring (approx. 15-20 percent likelihood of a subduction zone earthquake significantly affecting Portland in the next 50 years), but it’s another scenario to consider.
“Episodic Tremor” has come alive again recently deep in the Subduction Zone, as discussed in the Temblor article noted above. A reminder for us that things are happening below us…
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.
Pictures are worth a thousand words: this photo by FEMA clearly communicates the potential danger of hillside homes. These two homes were obliterated in the 1994 Northridge earthquake.
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).
A hillside home with numerous apparent structural concerns: torsional irregularity (described above), stilts with wood bracing, shallow pad footings, and a deck added recently with slender steel columns. The new deck adds additional weight to the house without the addition of seismic bracing. The house is also located on a slope mapped as “high” landslide risk.
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.
This house has relatively short “stilts” with diagonal bracing consisting of two wood “X” configurations. This is a poor system to resist seismic forces. I helped the homeowner add a continuous reinforced concrete footing and stem wall, with a plywood-sheathed cripple wall to replace the inadequate “X” bracing.
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.
A stepped foundation with a stepped plywood cripple wall concealed by the siding.
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.
This house has a skirt wall. The steel column visible in the near corner may indicate an adequately designed steel frame or bracing system. Some hillside homes have skirt walls with inadequate (often wood) bracing, or no seismic bracing at all.
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).
Some hillside houses have slowly inched their way down the slope over the years. This is a picture of a wood post on a shallow square concrete footing that is tilting downward.
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.
The view from hillside homes can be amazing, but this usually comes with higher earthquake risk.
“Resilience” has become a hot topic in recent years, and rightly so. It’s defined as a region’s ability to rebound after a disaster. We look at cities such as New Orleans after Hurricane Katrina, and now Houston after Hurricane Harvey, and recognize cities that were not resilient to a known disaster coming at some point.
A Cascadia Megaquake is our unprecedented disaster, at least, the one that we are methodically ticking closer to on the geological clock.
Our city and region have a long way to go to become resilient. If you want to be more convinced of this, please read the Oregon Resilience Plan Executive Summary. It’s been estimated that perhaps 80 percent of our buildings in Oregon do not comply with the current seismic code requirements (this does not mean most of them would fall down, but some of them would)! For most of Portland’s history, buildings have gone up, and remained, with little regard to earthquake forces or effects.
When I think of dangerous buildings to be in during an earthquake, URM’s (unreinforced masonry or brick), hillside homes, soft-story buildings, and old “tilt-up” buildings come to mind.
Yes, hillside homes can be among the most dangerous places to be in an earthquake, and this post is about the seismic hazards unique to this category of buildings.
A hillside neighborhood in northwest Portland.
The basic seismic retrofit that involves strengthening measures implemented in a crawl space or a basement is becoming familiar. But Hillside homes are often not in the conversation, and they need to be.
Hillside homes are common in Portland and other west coast cities. Many of them went up in the 1960’s, when earthquake risk was considered low. They have great views and character. Unfortunately, they can have catastrophic damage in earthquakes.
Hillside homes are by far the most dangerous demographic of single-family residential structures, as measured in recent California earthquake fatalities.
If you live in a hillside home, you are not necessarily in danger during an earthquake. Your structure is just more likely than other homes to be dangerous. I encourage you to take in the information in this post and get a sense of what the risks of your particular home are, so you can take appropriate action.
Some hillside homes seem to compete with each other over which one can defy gravity the most. I’m concerned that gravity may defy some of these houses when the big earthquake shakes for 3 to 5 minutes.
FEMA’s P-50-1 document gives us the following statistics from the 1994 Northridge earthquake (magnitude 6.7) in the Los Angeles area:
114 hillside dwellings were significantly damaged.
15 hillside dwellings collapsed or were so severely damaged that they had to be immediately demolished.
Another 15 hillside dwellings were close to collapse.
At least four people died in these homes.
Other earthquakes, such as the 1989 Loma Prieta earthquake near San Francisco, have also resulted in hillside home collapses and fatalities.
The remnants of a hillside home after the 1994 Northridge earthquake.
Geology Concerns
We have unique geological risks in the Pacific Northwest with hillside homes. The soil in the hills around here often consists of a top layer of clayey or sandy silt, somewhere on the order of 30 feet deep, underlain with bedrock. Earthquakes can trigger landslides, landslides are more likely in saturated soils, and saturated soils are a common condition in the rain-soaked northwest. This soft layer of soil can slip away under the right conditions.
Remember the winter of 2017? The west hills of Portland had numerous landslides earlier this year. Landslides happen during earthquakes even in dry conditions; imagine what would happen if the big earthquake strikes at the end of a soggy winter?
Landslide risk is not only a concern at the exact site of a house or directly below it; an unstable slope above could be equally damaging. Even a landslide just down the street could destroy the road that accesses the home and cause severe injury or death of neighbors.
I’m not suggesting that most hillside homes will collapse and slide down the hill. But landslide risk is important to know about if you live in the hills, and some houses are in high-risk areas.
A landslide that occurred in an Alaska neighborhood during the Great Alaska Earthquake (M9.2) of 1964.
The Oregon Department of Geology is expecting tens of thousands of landslides to occur during a full rupture of the Cascadia Subduction Zone. The most at-risk areas have been mapped for the entire state of Oregon on a macro level in an online interactive map called “SLIDO“; they include areas where past landslides have been documented and steep slopes with soil characteristics prone to landslides. “A Homeowner’s Guide to Landslides” by the Washington Geological Survey is another helpful tool homeowners can use to qualitatively assess landslide risk.
I’m concerned that the seismic risk to hillside homes in our region may be worse than California, just from landslide risk alone.
A snapshot of Portland on the “SLIDO” landslide hazard map by DOGAMI. Brown and red areas indicate past landslides. Notice that entire neighborhoods have been built on some of these areas.
What this all boils down to is that an adequate seismic risk assessment or retrofit of a hillside home will often need the input of a geotechnical engineer as well as a structural engineer.
If the soil appears sound and landslide risk appears to be low, at the very least a structural engineer that is attentive to slope stability and geological risks is needed. Sometimes a conservative design with the foundation (such as a continuous footing with significant reinforcing) can make up for limited soil information. I’ll discuss this more in my next post.
I’ve become a proponent of FEMA’s “simplified” seismic assessments and perform them regularly on houses. I highly recommend this as a starting point for those concerned about the seismic risk of a hillside home. They are affordable and take into account both structural and geological seismic vulnerabilities. This methodology makes a relatively thorough, first-pass assessment and helps quantify the benefit of a retrofit and the likely costs involved.
For more information about seismic risk assessments and retrofitting, please see the Cascadia Risk Solutions website.
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.
On August 24, 2014, a magnitude 6.0 earthquake struck near the California city of Napa. It was subsequently named the South Napa Earthquake. One person died and 200 were injured as a result of the quake. Damage was in the range of $300 million to $1 billion- not an insignificant amount.
Much of the damage associated with structures occurred in brittle buildings like those constructed with URM (brick) or with stone-clad veneer. But there was a good deal of damage to homes and other wood-framed structures, also.
I read an article recently revisiting damage from this earthquake, and I couldn’t help but notice some basic statistics and compare them to our Cascadia threat looming off the coast.
Consider just two data points: Ground accelerations and duration of shaking.
The recorded peak ground accelerations during the South Napa earthquake were .61g (61% of gravity). The significant shaking lasted for less than 10 seconds.
Compare this to a Cascadia Subduction Zone earthquake:
Ground accelerations in the Portland area are expected to be around .75g. The shaking will be greater in areas with soft soil, which comprise a good portion of the metro area. Areas near the rivers- the Columbia, Willamette, Tualatin, etc are also prone to liquefaction, which will further increase damage. Ground accelerations will also generally increase as you move further west.
Duration of shaking will be measured in minutes, not seconds. If the full subduction zone ruptures, the shaking could last as long as five minutes.
What does this simple comparison tell us? It should be a sobering reminder of our need to strengthen our infrastructure in the Pacific Northwest. Consider these points also:
California has had multiple earthquakes to help weed out the weaker buildings, so to speak- through damage, repairing, and rebuilding over time. We haven’t even had a “South Napa” (i.e. magnitude 6.0) in the Portland area in recorded history. As a result, we have an excessive amount of weak structures still hanging around.
Liquefaction will likely be a huge source of damage during the Cascadia quake. Liquefaction damage was limited in the South Napa earthquake due to drought conditions, but it was a significant source of damage during the 1989 Loma Prieta (magnitude 7.0) earthquake and the 2001 Nisqually (magnitude 6.8) earthquake near Olympia, Washington.
The need for retrofitting of homes by strengthening cripple walls, providing foundation anchorage, and using blocking and framing connectors to create an adequate load path is very much needed in the Pacific Northwest. Every significant California earthquake produces this type of damage.
1800 URM (brick) buildings in Portland alone will all likely have significant damage unless they are strengthened. This has been known for at least 20 years, but only a small percentage… I believe it is less than 10%… have been adequately retrofitted.
