Tag: URM

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:

  1. 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).
  2. 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.
  3. A soft story at the first floor level gets more dangerous the more stories there are above it.
  4. 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: Hillside Homes and Geological Concerns

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.

The next post will discuss common structural earthquake vulnerabilities with hillside homes.

South Napa v.s. Cascadia- and our need for seismic upgrades in the Northwest

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.


Collapsed chimney from the South Napa Earthquake.  (See more pictures here)

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.

Rogue One, structural engineering, and earthquakes

Most structural engineers have experienced a glazed-over look in someone else’s eyes when describing what they do for a living, followed by a response like, “so, you’re an architect?”

There remains ignorance in the public about what structural engineering (and engineering in general) is for.  Other engineering disciplines may be even more confusing; I doubt most people could define the term, “geotechnical”.  Personally, I’m still not 100 percent sure what an industrial engineer does.

I think the fault of this ignorance lies primarily with engineers.  We have an incredibly cool profession and if people understood better what we do, we would probably make more money, quite frankly. Especially if we were passionate about taking our skills and orienting them toward serving the community, region, and world to make it a better place (which is the reason all professions should exist).

Structural engineering seems to be making more inroads into the public sphere in recent years.  It’s always amusing when Hollywood addresses your career. This happened in the latest Star Wars movie, Rogue One (slight spoiler alert if you haven’t seen it).  The movie specified a large building, which reminded me of a dark version of a Dubai hotel, as a site dedicated to structural engineering (among other things) for the Empire.  And there was also the scene of a group of engineers (in lab coats?) being assassinated for apparent faulty design of the Death Star.  Considering the fact that this Death Star weakness led to the downfall of the Empire in subsequent episodes, this was understandable using Imperial logic, I suppose.

The Structural Engineers Association of Oregon has this clear statement defining the profession of structural engineering (good job, whoever wrote it):

Structural Engineering is the practice of analyzing and designing buildings, bridges and other structures to resist forces induced by gravity, wind, and earthquakes and to safely transfer these forces to the ground.

See here for more: http://www.seao.org/resources/aboutstructengr/

Regarding engineering in general, there are a number of good definitions online, but here is my very simple one:

Engineers apply science and mathematics to the real world to solve real world problems.

Engineers and the engineering profession should act as a bridge between the theoretical realm of science/ mathematics and real life.

Consider the large problem of an impending Cascadia megaquake. The science indicating that these earthquakes have happened and that the subduction zone is locked and building up energy has been settled for about 20 years.

Emergency management at the state and federal level has been aware of the threat for a long time also.  The Oregon Resilience Plan, which was a state funded plan addressing the effects of a Cascadia megaquake and its consequences, was published in 2013.

Journalists helped disperse the information about this topic into the homes and hearts of Pacific Northwest residents (thank you Sandi Doughton and Kathryn Schulz, to name two).  For the last couple of years, there has been more mainstream awareness of this issue than ever.  And my experience has been that people are still baffled by the topic and wondering what to do about it.

Here is my exhortation to engineers, particularly those involved in the disciplines of infrastructure (civil, structural, and geotechnical at the forefront). The Cascadia earthquake threat is large enough to involve all of our individual efforts for years. Please consider what part you can play in increasing your personal, community, and regional resilience. We all know more than the average person about earthquakes and what they do to the ground and to structures. Don’t hide in your cubicle or office. Do what you can with your career to help and you will be saving lives when the earthquake happens.

In Portland, engineers know that there are about 1800 URM (brick) buildings which may partially or completely collapse in a large earthquake.  We know many older homes have weak cripple walls, dangerous unreinforced chimneys, and “soft story” weaknesses which will result in damage, injury, and in some cases, loss of life. We know there will likely be long term loss of power and drinking water. We know industrial areas are set to contaminate our rivers with millions of gallons of liquid fuel. Wow, that’s just the tip of the iceberg. Let’s get to work.

Engineers are a key to helping bridge our gap between what we now know (the science) and resilience. But not just engineers… every one of us can help and I would argue that we have a duty to make steps toward preparedness.

Science =>  =>  =>  => => (our gap)  =>  =>  =>  =>  => Resilience

Journalists

Engineers

Emergency Planners/ Responders

City and State Leaders

Heavy Industry

Everyone

You and me