Anchor Bolts by John Bouldin

This month I want to review anchor bolt and strap requirements and their purpose. One of the major forces that can operate against wood-framed structures is shear. Shear is a lateral/horizontal force that creates a tendency for the building to either rack out of plumb, or for the framing to slide off the foundation. It may be caused by high winds, tidal surge or flooding, and seismic activity. In fact, much of the damage done by Katrina involved shear forces.

Anchor bolts (or straps) are primarily intended to prevent the framing from sliding off the foundation. They offer no help in preventing racking, and are of limited value in preventing uplift. In residential housing, the IRC calls for the following minimums for anchor bolt placement:

1. One not more than 12” from each end of the plate section or less that 7 bolt diameters from each end of the plate section – that means one on each side of the corner.
   
2. Spaced no more than six feet apart.
   
3. At least two anchor bolts or straps per individual section of sill plate.

If the property is located in a seismically active or high-wind zone, further requirements must be met. Check with the local Building Code Official to find out if a property is located within such a designated zone.

During a normal visual inspection, many of these bolts may not be visible due to band joist insulation, storage or other obstructions, and we cannot report on what we cannot see. But there are some instances where we would be particularly on the lookout for them.

In cases where sill plates have been repaired or replaced due to decay or insect damage there’s a good chance that the replacement plate may be notched around the bolt, rendering it ineffective. Another important area to look for anchor bolts is at the small shear walls of garages. Many times these walls are less than 24” wide, but each sill plate still must have at least two bolts.

A more recent phenomenon in newer construction is to have a sill plate properly bolted to the foundation, but the wall resting on top of it has its own separate bottom plate, which is only nailed into the bolted pressure-treated plate, and is usually notched around the bolts. Depending on the amount and type of nails, this may create problems. If the pressure treated plate is a corrosive type of treated wood, only hot-dipped galvanized or stainless steel nails may be used for this purpose, and it might require the services of an engineer to determine whether the nail solution meets the minimum requirements to prevent lateral movement. Certainly if these nails are regular common nails, we would expect them to deteriorate within a few years, and not be able to prevent lateral motion.

Best of luck, and we’ll see you next month!

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