“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 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.
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.
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.
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.
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.
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.
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.
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.
For more information about seismic risk assessments and retrofitting, please see the Cascadia Risk Solutions website.