Earthquake Retrofitting,
Foundation Bolting & Cripple Wall Bracing
The primary purpose of earthquake retrofitting is to keep a home from being displaced from its concrete foundation. Retrofitting means making improvements to an existing building. The purpose is to make the building safer and less prone to major structural damage during an earthquake. 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. The terms house bolting, foundation bolting and cripple wall bracing are often used synonymously with earthquake retrofitting.
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This page provides an overview of earthquake retrofitting, for anyone who is new to its purpose and terminology. As in any overview, the descriptions here are oversimplified.
Foundation Bolting
Foundation bolting typically means that bolts are added to improve the connections between the wooden framing members of a building and its concrete foundation. 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. 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.
Careful planning, placement and installation of foundation bolts are critical for good bolting strength. It is important to use the proper type of bolt corresponding to the existing conditions of the home and its foundation. The expected type of bolt load or stress is another important consideration for bolt selection.
Two types of foundation bolts are typically used, referred to as expansion bolts and epoxy-set bolts.
Expansion Foundation Bolts
Expansion foundation bolts cost less to install but require good concrete strength to work well. They are typically used in houses with newer foundations and in situations where the earthquake movement is expected to shear or "slice off" the bolt.
Epoxy-set Foundation Bolts
Epoxy-set foundation bolts work better in older and weaker concrete foundations and in situations in which the movement is expected to pull the bolt upward and out of its concrete anchor. The strength of epoxy-set bolts can also be improved by making them longer and setting them more deeply into the concrete.
The type, size and shape of the bolting washer also makes a difference in bolting strength. Current retrofitting standards call for thicker and larger square washers as compared to the small, thin, round washers found in houses built prior to the mid 1990s. This allows the sill to be clamped more securely to the foundation. Some sources estimate that the use of the newer square washer increases the holding strength of the bolts to the mudsill by up to 60%.
Cripple Wall Bracing
Most houses have a short wood-framed wall in the sub-area crawl space. 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. In construction language, this is referred to as a cripple wall. Cripple wall collapse is a main source of earthquake related failure. The collapse of this wall will often result in the main floor dropping to the ground. The house is vaulted off to the side of the foundation as the cripple wall simply rolls out from under it.
Stiffening or bracing of the cripple wall to keep if from collapsing during seismic movement is a very important part of earthquake retrofitting. The bracing is accomplished by attaching structural grade plywood tightly to the wall framing. In engineering language, this strong or stiffened wall is referred to as a shear wall.
The stiffening effect is accomplished in the lengthwise direction of the plywood, which means that plywood run 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. Accordingly, it is important to brace all sides of a house for the best seismic protection.
It is usually not necessary and may even be undesirable to brace the entire lengths of the cripple walls. Over-bracing means unnecessary work and cost, and many engineers think that it is important to maintain some flexibility in the sub-areas of the house.
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. This will provide for good load transfer. 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. Good detailing of all of these connections is an important part of retrofitting technique.
Foundation Holdown Brackets
Some houses also require additional holdown brackets to anchor the shear walls. Holdowns are specially constructed right-angle brackets connecting from the cripple wall framing into the foundation. As the bracket name implies, they are designed to resist a shear wall lifting or rolling effect, which may also occur during seismic activity. Generally, the need for holdowns is a function of the height-to-width dimensions of a shear wall configuration.
Holdowns are usually installed at the ends of shear walls. They are not needed at all shear wall locations. Installation is done with an extra-long and more deeply set epoxy-anchored bolt, which provides for better resistance to the lifting stresses. If one is in doubt about the need for holdowns, there is no harm in installing them except of 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.
Angle Iron Struts and Foundation Bolting
Not all houses have cripple walls. In many newer houses, and some older ones, the floor framing is set directly on the mudsill. Compared to houses with cripple walls, houses built in this manner are considered to be less vulnerable to displacement from their foundations. There is still significant risk, however. The connection between the floor framing and the foundations is often too weak to withstand strong seismic movement. In this case, the foundation is literally at risk of being thrust out from under the house. The end result is the same – the house slips off its foundation. Even slippage of a few inches can do major damage to the house, allowing 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.
Current construction practice for new homes is to install steel straps in the concrete as the foundation is poured. The wet-set strapping extends upward from the foundation and is connected into either the floor or wall framing of the house, providing an anchoring method that extends well beyond simple mudsill bolting.
The equivalent method of improving house anchoring in retrofitting situations is accomplished by bolting angle iron struts into the side of the concrete foundation walls, working in the crawl space areas under the main floor of the house. The angle irons extend from the foundation upward into the floor framing where they are also bolted to the floor joists.
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. The struts reinforce any weakness in the original sill bolts and extend the anchor more completely into the house's main floor platform.
Many houses, especially hillside houses, have both cripple walls and areas with the floor directly on the foundation. In this case, a combination of retrofitted improvements may be called for, including mud sill bolting, holdowns, plywood shear walls and angle iron struts.
Soft Story Situations
Another condition found in many homes, especially those with attached garages, is referred to as a soft story. 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 or patio door or several windows. Such an opening has little or no stiffness or shear strength. Thus, comes the term soft story, meaning a story of a home built over a wall area that is "soft" in its resistance to lateral movement.
Current building practice in seismic areas is to install a rigid steel frame around the opening to stiffen it. Soft story conditions can sometimes be improved by retrofitting plywood where walls in areas of the house in the same plane, or at least parallel to its large opening(s). In cases where there is no place to construct parallel shear walls, engineer-designed steel frames may be needed to improve the support of the overhead rooms.
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