How much overlap is required for a #4 reinforcing bar with a 30DIA minimum lap at the splice?

The correct amount of overlap required for a #4 reinforcing bar with a minimum lap splice is determined based on the diameter of the bar, which in this case is specified as 30 times the diameter (30DIA). For a #4 bar, the diameter is 0.5 inches, so the calculation for the minimum lap splice would be:

30 times 0.5 inches equals 15 inches.

This calculation is aligned with industry standards and building codes that dictate the minimum splice length required for adequate strength and integrity of the reinforced concrete structures. Therefore, the answer of 15 inches accurately reflects the engineering requirements for splicing a #4 reinforcing bar.

The other options, which suggest overlap lengths of 10 inches, 12 inches, and 20 inches, do not meet the requirement based on the standard lap splice calculation, thereby reinforcing that 15 inches is the correct answer.