For wood frame construction in California Seismic Design Category D, what is the maximum spacing for anchor bolts along the sill plate?
Correct Answer
B) 6 feet on center
CBC Section 2308.3.1 specifies that anchor bolts must be placed at maximum 6-foot intervals in Seismic Design Category D. Additionally, bolts must be located within 12 inches of each end of each sill plate. This closer spacing compared to lower seismic zones ensures adequate transfer of lateral forces from the structure to the foundation.
Why This Is the Correct Answer
Option B (6 feet on center) is correct per CBC Section 2308.3.1. In SDC D, anchor bolts must be spaced no more than 6 feet apart along the sill plate to ensure adequate transfer of lateral seismic forces from the wood frame structure to the concrete foundation. The code also requires bolts within 12 inches of each plate end.
Why the Other Options Are Wrong
Option A: 8 feet on center
Option A (8 feet on center) is incorrect. An 8-foot spacing may apply in lower seismic zones or under other codes, but CBC SDC D requires tighter 6-foot spacing. Larger spacing reduces the number of connection points, which is insufficient for high-seismic force transfer.
Option C: 4 feet on center
Option C (4 feet on center) is incorrect as a maximum — this spacing is more conservative than required and would be structurally acceptable, but it does not represent the code maximum. The CBC allows up to 6-foot spacing; 4 feet is tighter than necessary.
Option D: 12 feet on center
Option D (12 feet on center) is far too loose. A 12-foot bolt spacing would leave long stretches of sill plate unanchored, creating a critical failure point during seismic events when lateral forces attempt to slide the wood frame off the foundation.
Memory Technique
Remember: '6 feet for SDC D sill anchors.' Visualize a 6-foot ruler lying along the sill plate — every time you reach the end of the ruler, you must have an anchor bolt. The '12-inch from the end' rule is a separate bonus requirement to anchor plate ends.
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