Engineering and Construction

The foundation sill plate, usually made of preservative-treated wood for added resistance to insects and moisture, is bolted to the foundation as a base for the wood framing (Figure 5.18,  A, B, and C). A single sill, as shown in the details here, is all that is required by code, but in better-quality  work, the sill may be doubled or made from thicker stock for greater stiffness. A thicker sill plate may also  be required in areas of high seismic risk, to provide a stronger connection between the wood framing and  the foundation anchor bolts. Because the top of a foundation wall is usually somewhat uneven, the sill may  also be shimmed up at low spots with wood shingle wedges or plastic shims to provide a more level, even base  for subsequent framing. A  sill sealer, made of compressible or resilient  material, should be inserted between  the sill and the foundation to reduce air infiltration through the gap. Some  sill sealer materials also discourage  moisture wicking up from the foundation into the wood framing (Figure 5.22). Conventional anchor bolts  are sufficient to hold most buildings on their foundations, but tall frames or those subject to high winds  or strong earthquakes may require more elaborate attachments (Figures 5.39-5.41).

Figure 5.22 Carpenters apply a preservative-treated wood sill to a sitecast concrete foundation. A strip of compressible glass fiber sill sealer has been placed on the top of the concrete wall, and the sill has been drilled to fit over the projecting anchor bolts. Before each section of sill is bolted tightly, it is leveled as necessary with wood shingle or plastic shims between the concrete and the wood. As the anchor bolts are tightened, the sill sealer squeezes down to a negligible thickness. A length of bolted-down sill is visible at the upper right. The basement windows were clamped into reusable steel form inserts and placed in the formwork before the concrete was cast. After the concrete was cast and the formwork was stripped, the steel inserts were removed, leaving a neatly formed concrete frame around each window. A pocket in the top of the wall for a steel beam can be seen at the upper left.

Figure 5.39 Strap tie hold-downs are made of galvanized steel straps with hooked or deformed ends cast into the concrete foundation wall (top). After the wall is framed, the exposed length of strap may be nailed directly to framing or, as seen here, nailed through the sheathing into the studs or posts behind (bottom). The number, size, and spacing of nails used to fasten the strap depend on the magnitude of the loads that must be resisted and the capacity of the wood member to hold nails without splitting. In the top image, anchor bolts cast into the top of the foun- dation wall that will be used to secure the sill plate in place are also visible.

Figure 5.40 Hold-downs made from threaded rod and steel plate anchors can resist much greater forces than strap tie hold-downs. They may be used at each fl oor level to securely tie the full height of the building frame to its foundation. For the type shown here, the nuts at the ends of the threaded rods may require retightening after the fi rst heating season to compensate for wood shrinkage, which can mean that access holes must be provided through the interior wall surfaces. Other models rely on spring-loaded tapered shims or other mechanical compensation so as to be self-adjusting.

Figure 5.41 A heavy-duty seismic hold-down similar to that illustrated in Figure 5.40. The anchor rod with one end cast into the concrete foundation wall protrudes through the preservative-treated wood sill where its threaded end is bolted to the anchor. The anchor in turn is bolted to a 4 4 (89 89 mm) post with fi ve bolts of substantial diameter. A conventional foundation anchor bolt with an oversized square washer is also partially visible to the right of the hold-down. Also note the thicker-than-normal, 3-inch nominal (64-mm) sill plate, a common feature in wood light framing designed for high seismic forces.