A 30 ft by 24 ft slab has one layer of No. 4 bars at 12 in. on center each way, with bars placed at both edges. What total bar length is required before laps and waste?
Correct Answer
A) 1,494 lf
Bars running 30 ft are spaced across 24 ft: 24/1 + 1 = 25 bars, so 25 x 30 = 750 lf. Bars running 24 ft are spaced across 30 ft: 30/1 + 1 = 31 bars, so 31 x 24 = 744 lf. Total is 1,494 lf. Other choices: 1,440 lf misses the edge-bar count, 1,248 lf omits one direction count, and 1,548 lf adds a lap allowance not stated.
Why This Is the Correct Answer
Bars running 30 ft are spaced across 24 ft: 24/1 + 1 = 25 bars, so 25 x 30 = 750 lf. Bars running 24 ft are spaced across 30 ft: 30/1 + 1 = 31 bars, so 31 x 24 = 744 lf. Total is 1,494 lf.
Why the Other Options Are Wrong
Option B: 1,440 lf
reinforcing quantity, choice b: 30 x 4 x 12 gives 1,440 lf. That calculation follows multiplies three stated factors.
Option C: 1,248 lf
reinforcing quantity, choice c: 1,248 lf backs into 1,248 when divided by 1; the accepted setup gives 1,494 on that same base.
Option D: 1,548 lf
reinforcing quantity, choice d: 1,548 lf backs into 1,548 when divided by 1; the accepted setup gives 1,494 on that same base.
Memory Technique
Remember reinforcing quantity as "base, adjustment, final unit"; the last unit should read 1,494 lf.
Reference Hint
Open Placing Reinforcing Bars, CRSI, 10th Ed., 2019; use the index entry or section for "Reinforcing bar quantity takeoff - grid spacing" to confirm that Bars running 30 ft are spaced across 24 ft: 24/1 + 1 = 25 bars, so 25 x 30 = 750 lf.
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