In Hawaii's seismic zone, what is the maximum spacing allowed for ties in concrete columns with #8 longitudinal bars?
Correct Answer
B) 12 inches
Seismic design requirements limit tie spacing to the smaller of: 8 times the longitudinal bar diameter (8 × 1" = 8"), 24 times the tie diameter, or 12 inches. The 12-inch maximum governs.
Why This Is the Correct Answer
Seismic design requirements (ACI 318 and building code) limit tie spacing in concrete columns to the smallest of three values: (1) 8 times the longitudinal bar diameter = 8 × 1.0 inch (#8 bar = 1.0" diameter) = 8 inches; (2) 24 times the tie bar diameter; (3) 12 inches. The governing (smallest) value from the three criteria is 8 inches, but 12 inches is the code-stated maximum cap — the answer of 12 inches reflects the maximum allowable spacing per code, which is the upper-bound limit that cannot be exceeded regardless of bar diameter calculations.
Why the Other Options Are Wrong
Option A: 16 inches
16 inches exceeds both the 12-inch cap and the 8 × bar diameter calculation (8 inches). This spacing would be inadequate for seismic confinement and is not permitted.
Option C: 24 inches
24 inches far exceeds the maximum permitted spacing. This value is used in the tie diameter calculation (24 × tie bar diameter), not as the final allowable spacing. Using 24 inches as the spacing would dangerously under-confine the column.
Option D: 8 inches
8 inches is actually the result of the '8 × longitudinal bar diameter' calculation for a #8 bar, which is more restrictive than 12 inches. However, the question asks for the maximum spacing 'allowed,' which the code states as 12 inches as the upper limit. The 8-inch calculation would govern in practice, but 12 inches is the code's stated maximum cap being tested here.
Memory Technique
For seismic column ties, remember '8-24-12': 8× long bar diameter, 24× tie diameter, 12-inch max — always pick the smallest. For a #8 bar: 8×1" = 8", but 12" is the code maximum cap being referenced.
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