Definition of Anchor Bolts in Wood Frame Construction

Design

• Anchor bolts, also called L-bolts or J-bolts, can be anywhere from six to 12 inches long, and are usually 1/2-in. diameter. They are made with a 90-degree bend at the end opposite the threads. This bend helps to hold them firmly in concrete when they are set into a foundation. An anchor bolt is threaded for several inches on one end to accept a nut which is torqued down on top of a sill plate.

Usage

• Anchor bolts are set into the surface of a new concrete foundation or slab when the concrete is still wet. The bolts are placed every few feet along the perimeter where the sill will be placed. Anchor bolts are sunk into the concrete for several inches, with the remainder of the bolt protruding vertically out of the concrete. After the concrete has dried, sill plates are drilled at measurements matching the locations of the anchor bolts and placed over the bolts. The sill plates are then secured by putting a large washer over the bolt on top of the plate followed by a nut which is tightened down.

Advantages

• Anchor bolts help to conform the sill plates to the face of the concrete foundation wall or slab. A sill plate that isn't secured to the concrete beneath it could warp, or could be bent when it is first put down. As long as the surface of the concrete is flat and level, the sill plate will be level as well, as long as it is firmly bolted to the concrete. This ensures a flat and level surface for the wall to be framed on. The wall frame is built directly on top of the sill plate.

Disadvantages

• There are few disadvantages to anchor bolts, which is why they are widely used. They can present a safety hazard before the wall is placed on top of them, because they protrude out of the concrete foundation in unexpected places and can cause unwary people to trip or scrape their shins. Anchor bolts that are left in wet conditions before use can become rusty, and need to be cleaned and sanded before being put into the concrete.