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Seismic strengthening diagram showing foundation bolting and blocking for earthquake retrofitting
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– expansion bolts and epoxy-set bolts. Find all solutions on soft story retrofit Los Angeles.
Mechanical type anchor bolt with square plate washer
Expansion Foundation 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. They are typically used in houses with newer foundations or when the concrete is in good repair.
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. In these cases the mechanical type anchor bolt is very effective when the bolt is properly installed according to the manufacturer’s specifications.
Photo: Mechanical type anchor bolt with square plate washer
Epoxy-set Foundation Bolts
Epoxy-set foundation bolts work better than mechanical anchor bolts in older homes where the concrete may be weaker than in a newer home. 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. In addition, epoxy anchors can be longer and therefore inserted deeper into the concrete. 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).
The requirement for clamping washers installed with retrofit anchor bolts has been recently upgraded. It is now mandatory to use hot-dipped galvanized 3″ x 3″ x 1/4″ square plate washers under the anchor bolt nut. This allows the mudsill to be clamped more securely to the foundation.
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. These plates are installed at intervals similar to anchor bolts in retrofit applications.
Cripple Wall Bracing
Earthquake bracing diagram comparing unimproved cripple wall with retrofitted cripple wall showing cripple wall bracing, double studding and blocking
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 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. 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 ductility in the sub-areas of the house.
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. 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.
Foundation bolting typically means that bolts are added to improve the connections between the wooden framing members of a building and its concrete foundation. 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.
Epoxy-set foundation bolts work better than mechanical anchor bolts in older homes where the concrete may be weaker than in a newer home. Sometimes there is not adequate vertical clearance under a home to properly anchor the mudsill to the foundation with conventional anchor bolts.