The primary purpose of earthquake retrofitting is to keep your home from being displaced from its concrete foundation — making the building safer and less prone to major structural damage during an earthquake.
This page provides an overview of earthquake retrofitting, for anyone who is new to its purpose and terminology. Foundation bolting typically means that bolts are added to improve the connections between the wooden framing members of a building and its concrete foundation. Careful planning, placement and installation of foundation bolts are critical for good bolting strength. Two types of foundation bolts are typically used — expansion bolts and epoxy-set bolts.
Expansion type foundation bolts, also known as mechanical foundation anchor bolts, are the basic type of bolt used in earthquake retrofitting to anchor the mudsill to the foundation — they cost less to install, but require good concrete strength to work well. The current ABAG Standard Plan A for earthquake retrofitting specifies that expansion bolts may be used as long as their installation does not cause cracking of the concrete. Epoxy-set foundation bolts work better than mechanical anchor bolts in older homes where the concrete may be weaker than in a newer home. The requirement for clamping washers installed with retrofit anchor bolts has been recently upgraded.
The use of this large clamping washer is important because it has been observed in recent earthquakes that the older type of round washer commonly found in houses built prior to the mid 1990s, sometimes pulled through the mudsill because they weren’t large enough to provide adequate holding strength. Sometimes there is not adequate vertical clearance under a home to properly anchor the mudsill to the foundation with conventional anchor bolts. Stiffening or bracing of the cripple wall to keep if from collapsing during seismic movement is a very important part of earthquake retrofitting.
The stiffening effect is accomplished in the lengthwise direction of the plywood, which means that plywood runs along the side walls of a house will brace it in the front-to-back direction while plywood run along the front and back walls will brace the house in the side-to-side direction. It is usually not necessary and may even be undesirable to brace the entire lengths of the cripple walls. Finally, following the principle that a chain is only as strong as its weakest length, it is also important to complete the connection from the foundations, through the walls, and into the overhead floor framing. There are several methods available for improving the connection of the framing to the foundation when a house doesn’t have cripple walls.
A custom connection upgrade is to use Angle Iron Struts, which connect the floor joists directly to the foundation in a similar fashion to the Simpson FJA anchors. The angle iron struts are installed in such a way as to provide both a vertical (lift resisting) and lateral (slide resisting) anchor for the house. Many houses, especially hillside houses, have both cripple walls and areas where the floor sits directly on the foundation. Another condition found in many homes, especially those with attached garages, is referred to as a soft story.
Soft story conditions can sometimes be improved by installing plywood bracing on a wall that is next to and inline with the opening.


Existing homes need to be retrofitted because our understanding of the effects of earthquakes as well as construction techniques have improved after the homes were built. Usually this means adding bolts through the piece of wood that lies flat on top of the foundation, referred to as the sill or mudsill, into the concrete. It is important to use the proper type of bolt corresponding to the existing conditions of the home and its foundation.
They are typically used in houses with newer foundations or when the concrete is in good repair. They also perform better when the earthquake force is expected to cause an uplift — a situation where the movement pulls the bolt upward and out of the concrete. This wall my be anywhere from a few inches to several feet in height, running upward from the top of the concrete foundation to the bottom of the main floor. The bracing is accomplished by attaching structural grade plywood tightly to the wall framing.
Accordingly, it is important to brace all sides of a house for the best seismic protection.
Over-bracing means unnecessary work and cost, and many engineers think that it is important to maintain some ductility in the sub-areas of the house. Holdowns are specially constructed right-angle brackets connecting from the cripple wall framing into the foundation.
In many newer houses, and some older ones, the floor framing (joist) rests directly on the mudsill. The choice of which connecting system is most practical for a given house comes down to a host of variables and ultimately includes a judgment on cost effectiveness.
Earthquake Safety devised the angle iron strut connection in the early 80s with input from several colleagues who were structural engineers. The struts reinforce any weakness in the original sill bolts and extend the anchor more completely into the house’s main floor platform. In this case, a combination of retrofitted improvements may be called for, including mudsill bolting, Simpson mudsill plates holdowns, plywood shear bracing and angle iron struts.
This is a situation in which a living space of the house, a bedroom for example, has been built over a large opening such as a garage door. If the configuration of the home doesn’t lend itself to this solution an engineered steel moment frame can be an option. BART carries about 360,000 travelers every day; and during the peak of the commute across the San Francisco Bay, BART carries as many travelers as the San Francisco-Oakland Bay Bridge. The terms house bolting, foundation bolting and cripple wall bracing are often used synonymously with earthquake retrofitting. There may be no existing bolts, or the existing bolts may be either weakened or too far apart to be strong enough for earthquake resistance. The expected type of bolt load or stress is another important consideration for bolt selection.


Load transfer is an engineering term which in everyday language means that the effects of the earthquake movements are transferred from one of the buildings components into the next – ground to foundation, foundation to sill, sill to wall, wall to floor. As the bracket name implies, they are designed to resist a shear wall lifting or rolling effect, which may also occur during seismic activity.
Installation is done with an extra-long and more deeply set epoxy-anchored bolt, which provides for better resistance to the lifting stresses. Compared to houses with cripple walls, houses built in this manner are considered to be slightly less vulnerable to displacement from their foundations.
A second method is to anchor the floor joist system directly to the foundation using Simpson FJA anchors. They offer a significant increase in connection strength, and for many projects they provide the most economical upgrade available for the existing conditions. This is particularly important if there is additional base framing (blocking) added between the cripple wall studs for the structural plywood anchorage of a shear wall (cripple wall bracing). Generally, the need for holdowns is a function of the height-to-width dimensions of a shear wall configuration.
If one is in doubt about the need for holdowns, there is no harm in installing them except for the added cost. However, a good understanding of shear wall mechanics is helpful in deciding whether to save the cost and work of holdown installation.
These are not over-the-counter connectors, but have to be custom made at a steel fabricator. Geological Survey statistical analysis has predicted a high probability of one or more major earthquakes hitting the Bay Area within the next 30 years. The house is vaulted off to the side of the foundation as the cripple wall simply rolls out from under it.
Even slippage of a few inches can do major damage to the house, allowing portions of it to sag and cause structural damage or to sever utility connections such as gas, water, sewer and electricity which extend from the ground into the house. The program will upgrade vulnerable portions of the original BART system to ensure safety for the public and BART employees.
Portions of the original system with the highest traffic will be upgraded not only for life safety but also to ensure that they can return to operation shortly after a major earthquake.
System extensions, built mostly during the 1990s, employed more stringent and up-to-date seismic criteria than the original system, and thus do not require upgrades. It also included reconfiguration of stairs, elevators and certain facilities housed in the building.



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