House Bolting, Foundation Bolting & Cripple Wall Bracing 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.
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.
Two types of foundation bolts are typically used — expansion bolts and epoxy-set bolts.
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.
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 ¼" square plate washers under the anchor bolt nut. This allows the mudsill to be clamped more securely to the foundation.
Foundation Plates. Sometimes there is not adequate vertical clearance under a home to properly anchor the mudsill to the foundation with conventional anchor bolts. Simpson Strong-Tie has several anchors which can be used in situations like these. The most commonly used one is called the Universal Foundation Plate or UFP10. These plates are installed at intervals similar to anchor bolts in retrofit applications.
Cripple Wall Bracing
Most houses have a short wood-framed wall in the sub-area crawl space. This wall may 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.
Homes without Cripple Walls
In many newer houses, and some older ones, the floor framing (joist) rests directly on the mudsill. Compared to houses with cripple walls, houses built in this manner are considered to be slightly less vulnerable to displacement from their foundations. There is still significant risk, however. The connection between the floor framing and the foundation consists of a series of “toenails” which are often too weak to withstand strong seismic movement and the house can slide off its foundation.
Foundation Hold-down Brackets
Some houses also require additional hold-down brackets to anchor the shear walls. Hold-downs 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 Hold-downs is a function of the height-to-width dimensions of a shear wall configuration.