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Bars each way, the laps people forget, and how many sticks to order for a slab grid.
ConCal takes the slab, footings, steel and labor from one drawing and turns them into a bid your customer can sign.
Count the bars in each direction, multiply by how long they are, then add your laps. The laps are the part that gets skipped.
Bars one way = (width ÷ spacing) + 1
The plus one is the bar on the far edge. Do the same for the other direction, multiply each count by its bar length, and add them together.
Worked example. A 24 by 30 garage slab with #4 bar at 18 inches each way. Long bars: 24 ÷ 1.5 + 1 = 17 bars at 30 feet, 510 feet. Short bars: 30 ÷ 1.5 + 1 = 21 bars at 24 feet, 504 feet. That is 1,014 feet in the mat.
Now the laps. Every bar is longer than a 20 foot stick, so each one gets a splice. That is 38 splices at about 25 inches each, or 79 more feet. The real total is about 1,093 feet, which is 55 sticks. Order off the 1,014 number and you buy 51 and come up 4 short with the mat half tied.
A Class B tension lap under ACI 318 is 1.3 times the development length, with a 12 inch minimum. For #4 Grade 60 bar in 4,000 psi concrete that is around 25 inches. It changes with bar size, concrete strength, cover and coating. On structural work, use the lap on the drawings.
| Bar | Diameter | Pounds per foot |
|---|---|---|
| #3 | 3/8 in | 0.376 |
| #4 | 1/2 in | 0.668 |
| #5 | 5/8 in | 1.043 |
The full write-up, including when mesh is fine instead: How much rebar do I need for a concrete slab?