Earthquake Brace & Bolt Division
Seismic Retrofitting Basics San Francisco Bay Area
The horizontal (or lateral) motion in earthquakes create the forces that can damage wood framed homes. Homes tend to fail in one of three ways – all of which are between the foundation and the first floor joists.
- The sill plate can slide off of the foundation.
- The cripple wall (or pony wall) can collapse (or rack).
- The floor joists can slide off of the mudsill or cripple wall.
There are a few variations on these themes, but most of our work involves installing structural elements proven to prevent these three types of failure. These standards and methods, developed in 1999 by Seattle Project Impact, are consistent with other building department permit requirements. Insurance providers and structural engineers generally ask that the same methods be used.
- Bracing the cripple walls with plywood.
- Bolting the braced cripple walls to the foundation.
- Attaching the floor of the house to the braced cripple walls.
Earthquake Retrofit FAQ's
CEA offers earthquake insurance premium discounts for older houses that have been retrofitted to better withstand earthquakes. Learn More
Some information about the earthquake resistance of your house can be obtained simply from knowing when it was constructed. Almost any house built before the 1980s can be improved in such a way as to reduce earthquake risk. Even some houses built in the 1980s can be improved. Houses built 1990 or after usually have the earthquake resistive features installed during the original construction.
Seismic failures occur when a house either is displaced from (slides off) its concrete foundation, or when walls below main floor areas of the house collapse. In the first case, improvement consists of strengthening the connections between the house and the foundation. Improvements to prevent the second type of failure consist of both supplementing any existing connections to the foundation and strengthening the sub-area supporting walls.
The existing strength of your home, as well as its earthquake vulnerabilities, can be determined through a visual inspection by a trained and experienced earthquake specialist. Typically, this person will make recommendations for strengthening which, if implemented, will reduce the risk of both damage to the property and personal injury to its occupants.
There is no such thing as a standard house or standard earthquake retrofit. Accordingly, this is not a simple question to answer in the abstract. Larger and more complex houses, such as houses with split floor levels, cost more to earthquake retrofit than smaller and simpler ones. Ease of access will also affect your retrofit cost.
The typical cost to completely earthquake retrofit a medium size home with one floor level, a crawl space area relatively free of obstructions and in good condition, and with wooden framed walls in the crawl space (referred to as cripple walls) is about $10,000 to $12,000. A house of similar size and good access, but with no cripple wall (where the floor platform sits directly on the concrete foundation), can have connection improvements installed for $7,000 to $10,000. Larger homes, homes with many floor levels, steep hillside homes, and homes with finished basement area rooms, or homes with rooms over garages will cost more to retrofit. In these situations, a complete earthquake retrofit can cost $15,000 or more.
While it is fortunate that you did not have previous damage, and this certainly gives comfort, it would be a mistake to rely on such a past experience to predict the effects of any future earthquakes.
There are a number of factors that affect the severity of earthquake damage. These include soil conditions, how a house was built, length and strength of the shaking. Most critical, however, is the distance between your home and the source of the earthquake; its epicenter.
The USGS (U.S. Geological Survey) shows in its 1990 public information booklet, The Next Big Earthquake, a very dramatic graphic comparing shaking strength with distance from an epicenter. This graphic makes several comparisons with the 1989 Loma Prieta Earthquake, measuring 7.1 on the Richter Scale, and with the epicenter on the San Andreas fault in the Santa Cruz mountain area. Interpreting this graphic takes a bit of study. Its implications can be understood by taking just one example. If the epicenter of this same 7.1 earthquake were on the Hayward fault in the Oakland hills, instead of in the Santa Cruz area, shaking in the Oakland hills would be 12 times as strong as it was in that locality in 1989. By moving the same size earthquake some 60 miles closer, the intensity of the shaking is increased many fold. How much will the ground shake?
This is the bottom line. An earthquake of about the same magnitude as the 1989 Loma Prieta Earthquake is being predicted for the Bay Area. If this happens, the strength of shaking in Bay Area cities will be much greater than in 1989, as much as 12 times as strong. Relying on the 1989 experience is risky. Preparations need to include expectations for an intensity of shaking never before experienced by most homes in the Bay Area and surrounding communities.
Yes, certainly. Decisions about earthquake-related improvements are very much like decisions about safety features to include when buying a new car. More features cost more, but they will also provide greater reduction in the risk of personal injury and property damage.
The expression “earthquake proof” is used in common language, but there is no such thing as an earthquake “proof” house. Also, there is no way to determine the exact improvement needs of an individual home. This is because the actual seismic stresses a home may experience are unpredictable.
Engineers can make calculations based on certain assumptions. Improvements can be made based on assumption of the worst possible earthquake situation. However, there may be situations where it is not possible for the homeowner to make all the recommended improvements because of budget or other considerations.
Also, given a choice, many of us would rather invest our home improvement budget in something other than seismic improvements. We have heard many variations in this homeowner dilemma: “… I would rather spend my limited home improvement money on things I can see and enjoy, like remodeling my kitchen. Yet, I don’t want my remodeled kitchen to be lost if my house is ruined.”
There is a way to make decisions about seismic improvements in spite of this dilemma. In order to both reduce earthquake risk and still have funds left to remodel the kitchen, we suggest some of the following as criteria for determining earthquake improvement priorities:
- Stage the improvements. Do some of the recommended improvements every year or so until all the desired improvements have been made. Identify the weakest and most vulnerable conditions in your home. Improve these first.
- Some improvements may be more important to improve safety rather than to protect the property. Do the safety-related improvements first.
- Do the improvements that may be more cost-effective, or possibly the simplest and most straightforward to install. Defer the more difficult or more disruptive improvements for later.
- If any remodeling is planned, make improvements in other areas first; defer the improvements in the planned remodel areas and make the seismic improvements during the remodeling work.
Even though improvements can be staged and even deferred, it is important to remember that full seismic protection will not be achieved until all recommended work is completed.
An engineer’s consultation is certainly an option. But, it is not always required, since seismic improvements are voluntary. An engineer will charge a fee for a consultation and plan.
It is common and accepted practice for seismic improvements to be planned by a contractor, especially if the contractor is an experienced seismic specialist. In addition, many cities now have “prescriptive plans” which an experienced contractor can use in planning the retrofit.
There are situations where an engineer’s consultation is advisable, or where engineered plans are necessary. These situations would be large, complex, multilevel houses; houses such as stilt houses on steep hillsides; houses with very large openings in lower level walls; or houses with severe foundation and/or drainage problems.
Following the recent major earthquakes in California, various governmental agencies, private engineers, and university research groups have made extensive examinations of failed buildings, identifying failure points and other weaknesses in construction. Their observations along with their recommendations for construction improvements are found in research publications. Many of the recommendations have been adopted into revisions of the Uniform Building Code, especially since 1989, and have been incorporated into new buildings constructed since that time. Some, or even most, of the recommended corrective measures can also be added to existing structures, a practice commonly referred to as “seismic retrofitting”, “seismic strengthening” or, more commonly, as bolting and bracing.
Additionally, the California Seismic Safety Commission, the Association of Bay Area Governments, and the International Conference of Building Officials have jointly sponsored training programs for contractors engaged in the business of offering seismic improvements for homeowners.
At this time there is no formal accrediting agency or certification process for seismic retrofitting, even for engineers. Experience, formal and informal training of employees, and a company’s track record of retrofitted homes are the foundation for its expertise. As a final note, it should also be pointed out that the training of employees who install the seismic improvements is also very important. The best-planned retrofit will not protect your home if it is not properly installed.
Most recommended strengthening practices stem from a report issued by the University of California, Engineering Department, in 1982. Other recommendations for seismic improvements come from field observations and reports issued by governmental agencies, private engineers, and other sources following major California earthquakes, particularly the 1989 Loma Prieta quake and the 1994 Northridge earthquake. Construction standards determined by either the Uniform Building Code, or by common practice, were changed considerably in the 1990s , following the recommendations of these reports.
Accordingly, if your home was built before these seismic standards were adopted, particularly before the 1980s, to the extent that your home can be upgraded to meet or be closer to these standards, you will be able to reduce its seismic risk. If your home was well built originally, seismic improvements may be of even greater value, since you will have both the quality of the original construction and the added improvements to reduce seismic risk. If you cannot count your house as being originally a well-built home, seismic improvement may be absolutely critical.
There is no current code for retrofitting for several reasons. Houses are very different in construction style, accessibility, and seismic vulnerability; it is difficult if not impossible to apply one standard to such a variety of situations and conditions. Attempts to develop a consensus among experts regarding retrofitting standards have not been very successful. Most importantly, though, is that the seismic retrofitting is voluntary; any homeowner can do as much or as little as he or she wishes.
Many local building departments have developed a “prescriptive plan” for retrofitting. Again, though, because houses differ so much in style and construction and, mainly, because homeowners differ in their needs and their expectations, we have found that a customized plan will be necessary to accommodate any given retrofitting situation.
Building departments require permits for seismic improvements, and charge a fee based on the valuation of the proposed work. The building department does do what is commonly referred to as a “plan check”, meaning a review of the proposed work by a staff engineer. During the retrofit installation the city inspector will review the work, providing you an independent verification of proper installation. This means, of course, greater assurance of a good performance when the improvements are put to test during an earthquake. Specifically, inspectors will check the bolts and other hardware to make sure that attachments to the concrete and wood framing members make the strongest possible connections; that plywood used for shear improvements is properly fit and nailed; and any other code-specified details.
Inspectors also make sure that other peripheral code items are taken care of during the installation. If a plywood shear wall is to be built inside a garage, for example, code requires reinstallation of sheetrock over the new plywood to provide a fire barrier between the garage and living area of the house. There are also some other code-required installations that must be done whenever any building permit is involved. Smoke detectors in your home, for example, may need to be upgraded.
Contrary to the fears which homeowners sometimes express to us, we find that inspection by the building department is done to protect homeowners rather than to be annoying or harassing. Building inspectors are friendly and competent, and will not cause problems for people who know and follow the rules. The rules are there to assure health and safety, not just to be troublesome.
The retrofitting contractor will obtain the building permit and deliver it to your home. All permit documents are owned by you, and should be retained with your property records for future reference. They may be useful, particularly, for insurance purposes or for selling your property.
Of course you can retrofit your own home. However, many people who have attempted to do this give up and call a licensed contractor to complete the work. There are several reasons. Many of the important details regarding the handling of problematic situations under a house are not covered in the very generalized do-it-yourself manuals. Making certain these details are handled properly often makes a critical difference in the quality and strength of the retrofit. Prior to the installation of a retrofit, a good plan is needed. Knowing what to do, where to do it, and how to do it are extremely important. Retrofitting is very hard work. Very undesirable and uncomfortable working conditions are found under most houses. Good tools and equipment are needed to help the work, to make sure it is well done, and to protect personal safety during the work.
Many homeowners find they can save money by doing some of the peripheral work themselves. Sometimes cabinets need to be removed and replaced. Plaster and other wall coverings may need to be removed and the debris handled. Many homeowners, especially if handy at all, choose to complete some of the preparation and follow-up work themselves to save some money, and leave the critical structural improvement work for the experts. We encourage this type of participation whenever it is practical.
It probably does. The simple answer to this question is that it is not enough just to see bolts. Weakened or insufficient bolt connections may lead to failure. Additionally, bolting and bracing are both needed. If the walls supporting a house are weak they may simply collapse from under it.
Code requirements for bolting as well as standard construction practice have changed in response to increased earthquake concerns, especially since 1990. Many houses have half-inch diameter sill bolts placed at intervals of six feet. The practice now is to install larger diameter bolts (5/8″ or 3/4″ depending on the scale of the home) at closer intervals. Additionally, since bolts primarily prevent lateral or sliding movements, other types of hardware called hold downs are installed to resist effects of the house lifting off the foundation during seismic movement.
The type of washer used also makes a considerable difference in seismic resistance. Following failures observed after the 1994 Northridge earthquake, the building code now requires the use of a larger, thicker, square plate washer in place of the small, thin, round washers that were used for many years. This has the effect of anchoring the sill from the top (clamping it down) rather than relying on the bolt extending through the wooden sill for sliding resistance. Northridge observers found situations of improper drilling, in which the hole drilled for the bolt was too large in diameter. This allowed for sill movement around the bolt, which split the wood. The clamping effect of the square washer compensates for this.
Most importantly, however, is that bolts do not last forever. Steel decays and loses its strength, particularly when a home has not had good foundation drainage. Rusted bolts cannot be detected by visual inspection alone, or in any other way short of costly testing. The decay does not reveal itself above the sill, but is inside the concrete where it cannot be seen. Bolt weakening is usually discovered only at failure. One engineer who has advised us thinks that new bolts should be added about every 30-40 years because of steel failure.
Many houses are built with the floor platform set directly on the foundation sill. In this case, even if the bolts are sound, the connection does not extend far enough into the floors and walls of the home to resist displacement. Read the answer to question 15 for more description of this situation
General practice is to use either a mechanically anchored bolt, commonly referred to as an expansion bolt or wedge anchor; or a bolt that is bonded to the concrete with epoxy adhesive, commonly referred to as an epoxy bolt or epoxy-set bolt. Both material cost and installation time are greater for epoxy bolts; thus, they cost more to install.
Expansion bolts have a cone shaped base with a stainless steel sleeve wrapped around the bolt near this slightly widening base. A hole is drilled into the concrete a little deeper than the bolt length. The bolt is inserted into the hole. Then, as the nut of the bolt is tightened it draws the coned end into the sleeve, causing it to expand , and wedging the bolt into the hole at its base. The bolt is anchored to the concrete at its base, and puts a tension load on the concrete as it is wedged in place.
In older houses where the concrete may be weaker, or when a bolt is going to be asked to resist the effect of being pulled out of the concrete, the epoxy-set bolt should be used. Epoxy-set bolts are installed by drilling a hole in the concrete slightly larger in diameter than the bolt, to give room for the epoxy material. The epoxy material is inserted into the hole in a liquid form, and the bolt is driven into the hole. After some set up time, the epoxy adheres to the concrete in a bonded attachment usually stronger than the concrete itself. Epoxy bolts are threaded the full length of the bolt. The epoxy material does not adhere well to the steel, but sets within the openings of the threaded steel, anchoring the bolt much like a screw is anchored within a hole in a piece of wood.
There are several advantages of epoxy-set bolts. They simply “rest” inside the hole in an unloaded, static state rather than placing a wedging stress on the concrete, as the expansion bolt does. The strength of the connection can be increased by installing a longer bolt, more deeply set into the concrete and with more length of attachment. The epoxy anchoring material is also impervious to water. Rusting and other decay of the bolt is reduced; bolt longevity is increased. Epoxy materials have been tested. Bolt manufacturers tell us that there is no reason to expect epoxy failure over time.
Thus, bolt choice is influenced by several factors: cost, condition and quality of the concrete, and the manner in which the bolt will be loaded. Most engineers recommend epoxy bolts for all mud sill bolting. If there is any doubt about the better choice, use the epoxy-set bolt.
The risk of damaging the concrete while installing the bolts is very minimal. Special drilling equipment is used in order to reduce this risk.
Hold downs are steel brackets attached to the studs of a cripple wall and anchored into the foundation with epoxy-set foundation bolts. The purpose of a hold down is to resist the overturning and uplift forces acting on a shear wall. They are typically used where the wall is tall in relation to its length, or if the wall is expected to carry extremely high seismic loads. An engineer or experienced retrofit contractor will know when their use is appropriate.
Many houses have a short wood framed wall, referred to as a “cripple wall“, running from the foundation to the main floor. This wall needs to be strengthened to avoid seismic collapse. With this collapse, the house appears to have been vaulted to one side on its sub-area walls. Improvement in the stiffness of these walls is needed in order to transfer the earthquake movements from the ground through the cripple walls to the house. This forces the foundation and the house to move together.
Differential movement is reduced by turning some of the sub-area walls into “shear walls”, which consists of attaching structural plywood to the walls to create the desired strengthening or stiffness.
Many houses, especially those built in the 1960s and 1970s or later, are constructed with the floor framing set directly on the foundation sill. If this is the case, even if the bolting is sound, the connections may not extend far enough into the floors and walls to keep the house from sliding off its foundation. Homes without Cripple Walls
Failures in past earthquake situations show the floor framing sliding off the bolted sill; a failure in the typically toe-nailed connection between bottom edge of the floor joist and the flat top surface of the bolted sill. Recently, construction technique has improved to compensate for this. Steel straps are set in the wet concrete as the foundation is poured. The straps then extend upward, across the flat sill, to connect into either the floor or wall framing of the house.
This connection between the foundation and the floor framing can be improved by retrofitting steel struts and/or foundation plates and framing clips. These add both lateral or sliding resistance and vertical or lifting resistance to the main body of the house. They improve the connection of the house framing to the foundation better than that provided by mudsill bolts alone.
You can determine whether your house has a cripple wall simply by looking under it. If the sub-area walls are solid concrete extending to the floor, the addition of the steel struts is to be considered. If there are short walls with vertical wooden “studs”, the sill will be bolted and the wall braced with plywood. Some houses will have a combination of both of these.
Yes, as long as it is recognized that any brief summary requires oversimplification. In engineering language we say that retrofitting increases the ability of the substructure of a house to transfer the “loads” or stresses of an earthquake, especially lateral loads, from the house to the ground so that everything moves together. In order to make this transfer, all connections must be in place and must be strong enough to complete the transfer of movement. Like the failure of the weakest link in a chain, if any transfer point fails, the house itself may fall, slide, or otherwise be moved off its concrete foundation. If this happens, extensive structural damage is likely to be the result.
Retrofitting a house consists of adding connectors, where they are found to be missing, improving existing weak connections between the major structural elements of a house, and bracing of walls that are weak and subject to failure under earthquake stress.
It would be nice if all houses had new, steel reinforced foundations, but they do not. It would also be nice if we all had the budget to replace foundations when they are in marginal condition, but we do not. Quite a dilemma — to own an older home, with a weak foundation, live in earthquake country, and have no money. New owners of older homes, particularly, may experience this dilemma, given the recent rapid increases in the cost of buying a home in the Bay Area.
The most important question is whether the concrete is strong enough to provide good anchorage for foundation bolts. In some cases, it is obvious that foundation replacement is the only solution; one can simply see how weak the concrete is. In less clear situations, there are several options.
The strength of concrete can be determined with test instrumentation. This process is fairly costly and not usually done unless the foundation condition is highly suspect. Very few homeowners choose this option.
Foundations can be evaluated visually and by sound. It is easy to see the crumbling of the concrete, signs of years of poor drainage, excessive subsidence, severe cracking and other conditions which suggest that the concrete will fail if heavily loaded. A light tap on the concrete with a steel hammer also reveals differential sounds, which say a lot. A sharp “ping” sound indicates decent core strength; a dull “thud” sound indicates poor strength. Minor cracks in the foundation are usually not a reason to forgo a seismic retrofit.
Under marginal conditions and with foundation replacement costs very tight, anchoring to the existing concrete may still a good decision. Weaknesses in the concrete can be compensated for in a number of ways. With poorer concrete, epoxy-anchored bolts should be used. In addition, both the strength of the connection and better distribution of loads can be accomplished by using more bolts, placing them closer together, and by using longer, more deeply set bolts. If only parts of the foundation are weak, it may be possible to concentrate the improvements in the areas of the house with the better concrete, and still achieve worthwhile seismic risk reduction.
No. You will need to have the brick foundation (or any portions that are brick) replaced before the remainder of the retrofitting can be done.
This question is asked frequently. Essentially, though, it is unanswerable. The movement during an earthquake is too complex, and there are too many unknowns to be able to answer this question directly.
In past earthquake situations, two houses built at about the same time, right next to each other, and with many other similarities have performed very differently. Size and shape of the house, condition of the concrete foundation, how well it has been maintained, modifications and changes in the original construction which may have created structural weakness, and many other variable factors will affect earthquake damage.
Given this, it is important to think of earthquake damage prevention as a risk reducing practice rather than have a more absolute expectation as to what may be accomplished. The comparison may be something like maintaining a good diet and exercising to reduce the risk of heart disease. Both are good health practices; they reduce the risk of a heart attack, but there is no guarantee of such.
The term risk reduction also means at least two things. The probability of any damage at all is reduced. Also, if there is damage, the extent of the damage is reduced (along with what may be required to repair the damage).
But, the news is good. Observations, made repeatedly, from past earthquake situations show that amount of damage, time required for repair and recovery, loss of property, and cost of repairs are significantly and considerably less in buildings where improvements have been made, even during the larger earthquakes. This is why engineers, building officials, insurance companies, public safety agencies and others who have observed past earthquake situations recommend, almost without exception, that homeowners have their houses improved for better earthquake resistance.
What is the probability of another large quake?
Why Use Rooted Retrofitting for your Earthquake Retrofit?
- Licensed, bonded and insured.
- Every member of our team is trained and certified.
- Experienced with all types of retrofit.
- References available upon request.
- Works in conjunction with a licensed structural engineer.
- Always secures a building permit for retrofit work.
- Familiar with recent changes to the retrofitting code.
- Familiar with Project Impact standards.
- Qualified to address all your structural needs.
- “One stop shopping” for all retrofit related tasks, including sheetrock & siding.
- Maximizes dust control, minimizes time disruptions, cleans up properly.
- In and out in less time than any other contractor.
To schedule an appointment, please give us a call today at (925) 378-2204
Hugh and Todd showed us the current code for new construction using very large bracing designed exactly for preventing a house from slipping off of its foundation. Mom received a very professional detailed quote the same day and after doing some quick math (retrofit work versus possible earthquake destruction costs) mom decided this was defiantly the best form of insurance for her. ‘Rooted Retrofitting’ proved to be fair, timely, professional and above all provided a detailed history, then current code education in a manner that Mom seemed to easily understand, making her confident with her decisions.