CSLB General Building (B)Framing and CarpentryMedium
An inspector checks anchor bolts securing a sill plate to the foundation in a high seismic design category. Per code requirements for these conditions, what size plate washer must be installed under each anchor bolt nut?
- A3 inch x 3 inch x 0.229 inch
- BNo plate washer is required if lag screws are used instead
- C2 inch x 2 inch x 0.229 inch
- D1 inch diameter round washer only
Show answer & explanationAnswer & explanation
Correct answer: A. 3 inch x 3 inch x 0.229 inch
In Seismic Design Categories D, E, and F, code requires a minimum 3-inch by 3-inch by 0.229-inch plate washer under the nut on each foundation sill plate anchor bolt. This larger washer distributes clamping force over a wider area of the sill plate, reducing the risk of the wood splitting or the nut pulling through under seismic loading.
Why the other options are wrong
- B. Anchor bolts (not lag screws) are required at sill plates, and plate washers are still required.
- C. Undersized; does not meet the minimum plate washer dimension for these seismic categories.
- D. A standard round washer does not provide adequate bearing area.
Sill Plate Anchor Bolt Plate Washer
A square steel washer required under sill plate anchor bolt nuts in high seismic areas to distribute clamping force and prevent the plate from splitting or pulling free.
- Minimum size: 3 in x 3 in x 0.229 in
- Required in Seismic Design Categories D, E, F
- Nut must be tightened snug, not overtightened to crush wood fibers
Memory trick: 'Three by three keeps the sill plate free (from splitting).'