Tag: plywood

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: Weak Cripple Walls

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:

  1. 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.
  2. 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.