Calculate the required development length for a #6 rebar in 4,000 psi concrete with normal weight aggregate in a seismic application per CBC:
Correct Answer
C) 30 inches
For #6 rebar in 4,000 psi concrete in seismic applications, the development length is calculated as 30 inches to ensure proper load transfer.
Why This Is the Correct Answer
Per the California Building Code (CBC) and ACI 318, the development length for a #6 bar (0.75-inch diameter) in 4,000 psi normal-weight concrete in a seismic application is 30 inches. Seismic applications apply an additional 1.25 multiplier over the standard development length, increasing the bar's required embedment to ensure ductile load transfer during seismic events. The 30-inch figure is the CBC-recognized result for these specific conditions.
Why the Other Options Are Wrong
Option A: 24 inches
24 inches corresponds to the non-seismic or less-conservative calculation for a #6 bar. In standard (non-seismic) conditions, 24 inches may be sufficient, but the seismic multiplier (1.25×) pushes the required length beyond this value to ensure the bar does not pull out during a seismic event.
Option B: 36 inches
36 inches is excessive for a #6 bar in 4,000 psi concrete and would be more appropriate for a larger bar size (such as #8) or lower-strength concrete under seismic loading. Using 36 inches would waste material and is not the CBC-specified value for these conditions.
Option D: 18 inches
18 inches is dangerously insufficient for seismic applications. This length might apply to very small bar sizes in high-strength concrete under standard conditions but is far below the seismic requirement for a #6 bar in 4,000 psi concrete. Using 18 inches would risk catastrophic pull-out failure.
Memory Technique
For #6 rebar in 4,000 psi concrete: standard ≈ 24 inches, seismic = 24 × 1.25 = 30 inches. Remember: seismic adds 25% to development length. '#6 at 4K psi seismic = 30 inches' as a benchmark value.
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