weak story – Cascadia Risk by BJ Cure

Home Earthquake Vulnerabilities: Soft Stories

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.

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.