Related Information Homeowners Guide to Earthquake Retrofit

While shear walls can strengthen a house, they are only one of several steps that can be taken to lessen the likelihood of earthquake damage. Good seismic design starts with the foundation. Three kinds of foundations are covered in this section: post and pier; poured concrete walls; and slab-on-grade.

In areas designated seismic zone 3 and 4, the Uniform Building Code requires foundation plates or sill plates to be bolted to the foundation with 5/8-inch-diameter bolts spaced no more than six feet apart imbedded a minimum of 7 inches. Everywhere else requirements are less, so this will be ok everywhere. One bolt must be placed within 12 inches of each end of each section of the plate. If an existing house has no connections, or if the connections don't meet code requirements, retrofitting should be considered. Anchor bolts are the most common way to attach a house to its foundation, but a thorough inspection of the foundation should precede bolting. The concrete must be strong enough to hold the anchor bolts. If the concrete is weak or deteriorating and drilling holes for bolts is likely to cause cracks or crumbling, the foundation should be replaced. For building 3 stories and above, foundation bolting may need to be custom designed, consult an engineer.

Anchor Bolts Anchor bolts are manufactured in a variety of types and sizes. Mechanical wedge anchor bolts are the type most commonly used in seismic retrofit applications (Figure E). However, epoxy anchors offer considerable advantages, and are gaining in popularity. On a mechanical anchor a metal collar near the tip of the conical end is what makes mechanical wedge anchor bolts work. Once the bolt is in place, a few turns on the nut will lift the bolt and expand the collar, locking it in place. Anchor bolts must be installed before panels are nailed to the wall framing. Once installed, the panels will block access to the sill plate. In new construction, L bolts or J bolts, inserted when he foundation is poured, are the most common type of anchor bolt.

Post-and-Pier Foundations The most common problem with post-and-pier foundations during an earthquake is the tendency of posts to shift off piers and beams to shift off posts. Failure at either of these points can cause the substructure, and sometimes the entire house, to collapse. But steps can be taken to strengthen post and pier foundations. Bolting posts to piers and beams is the most common method. Lateral support can be achieved by installing 2x4s as diagonal bracing from one post to the next. Predrilled steel T-straps are recommended for attaching beams to posts. For attaching posts to piers, predrilled, shop-fabricated, heavy gauge, bent sheet metal connectors are recommended (Figure F). Poured Concrete Foundation Walls In new construction, poured concrete walls are the most common type of foundation for supporting raised wood floors. In houses built prior to modern building codes the foundation and sill plate are not always attached, but retrofitting with anchor bolts can substantially improve a home's ability to withstand an earthquake. There are two ways of bolting sills to concrete foundations: vertical and horizontal.

Vertical bolting can be accomplished only if there is enough workspace between the top of the sill plate and the bottom of the wall top plate. Horizontal bolting is useful where space between foundation and floor is minimal. Vertical bolting - Vertical bolting is the easiest way to attach a house to its foundation. Mechanical wedge anchor bolts are the only hardware needed. Using a rotating-hammering drill or rotohammer, simply drill the holes for the anchor bolts through the sill plate and into the foundation, then install the bolts. Horizontal bolting - In horizontal bolting, a structural steel plate is needed to tie the sill plate to the foundation. The steel plate is attached to the sill plate with lag screws and to the foundation with anchor bolts.

Slab-on-grade Foundations Bolting sills to slab-on-grade foundations requires removal of exterior siding or interior plaster or wallboard. The code requirements for bolting slab-on-grade foundations are the same as those for poured concrete foundation walls and the job is done in the same way. Anchor bolts are the only hardware needed. Shear Wall Hold-down Anchors Once the sill plate is bolted to the foundation, shear wall hold-down anchors should be installed at each corner of the house. While anchor bolts help keep a building seated on its foundation, shear wall hold-down anchors help prevent walls from overturning. A steel bracket, like the one pictured in Figure H, is recommended. Anchor bolts are suggested for attaching the brackets to the foundation. Lag screws or machine bolts can be used to fasten the brackets to wall framing (Figure H).

 Homeowners guide to earthquake retrofit