ANCHOR BOLTS AND HOLD DOWNS
The need for a solid connection between frame and foundation would seem obvious, and in fact most residential construction since the mid-1940s has utilized concrete anchor bolts which tie the mudsill to the foundation. But this feature alone is not enough to ensure that a particular house is earthquake resistant.
The installation of holdowns, along with the anchor bolts, and or brackets, ensure the house is properly attached to the foundation. Hold-downs are heavy, steel brackets designed to provide maximum strength at the connection. The bottom of the hold-down connects directly to the foundation by way of the anchor bolt; the upper end of the bracket connects to the wall studs by way of machine bolts or nails. The result is that the supporting walls are connected to the foundation. Almost all retrofits are done from inside the crawl space or basement and are not visible on the exterior of the house.
Steel reinforced foundations, hold-downs, anchor bolts, and limited clearance connections, are a key step in retrofitting a wood frame home. Another major area of potential weakness in most wood-frame buildings: insufficient lateral bracing in perimeter load bearing walls. In addition to proper connections, a building must have the capacity to resist the lateral forces it will experience during an earthquake. Engineers provide for this by designing shear walls, which are usually constructed from structural plywood panels and fastened to the framing of the house. Shear walls can be included in original construction or retrofitted to an existing building. Again, most retrofits are done from inside the basement or crawl space area, and are not visible outside the house.
Statistically, the collapse of perimeter cripple walls due to insufficient shear strength is the most common form of quake-induced structural damage to wood frame houses. The lack of necessary shear strength can be a result of either poor workmanship or inadequate building codes at the time the home was built.