Calculate quantities of reinforcing steel (rebar) based on square feet and center by gathering information about the physical size of the project under study, an instrument to take measurements and a calculator to help with arithmetic. Notice whether the project has the shape of a rectangle, square or some other shape. Although similar calculations determine the rebar needed for almost all two dimensional shapes, this discussion only addresses calculating the rebar for a rectangular concrete slab.
Identify, Measure and Calculate
Before figuring rebar quantities needed to reinforce a rectangular concrete slab, label the longer side of the rectangle “longitudinal” and the shorter side “latitudinal.” Then determine the size of the spaces between the rebars in both the longitudinal and latitudinal directions. Note that the rebars form a grid after placement, with the longer rebars running in one direction, while the shorter rebars run at right angles to the longer ones.
Measure the longitudinal length of the rectangle. Divide the measured distance by the size of the space between the latitudinal rebars, then add one rebar to the resulting number. This determines the number of rebars needed in the latitudinal direction.
Example: When a longitudinal length measures 20 feet and the size of the latitudinal rebar spaces equals 16 inch centers, this calculation follows: 1. Change the longitudinal feet to inches: 20 feet times 12 inches equals 240 inches. 2. Divide the number of inches by the size of the spaces between the latitudinal rebars: 240 inches divided by 16 inch spaces equals 15 rebars. 3. Add one rebar to the resulting number: 15 rebars plus 1 rebar equals 16 lengths of rebar.
Next, measure the latitudinal length of the rectangle. Divide the measured distance by the space between the longitudinal rebars and add one rebar to the resulting number. This determines the number of rebars needed in the longitudinal direction.
Example: When a latitudinal length measures 16 feet and the size of the longitudinal rebar spaces equals 16 inch centers, this calculation follows: 1. Change the latitudinal feet to inches: 16 feet times 12 inches equals 192 inches. 2. Divide the number of inches by the size of the spaces between the longitudinal rebars: 192 inches divided by 16 inch spaces equals 12 rebars. 3. Add one rebar to the resulting number: 12 rebars plus 1 rebar equals 13 lengths of rebar.
Figure the number of longitudinal rebar times the length. Then figure the number of latitudinal rebar times the length. Add the calculations together to determine the total linear footage of rebar needed to do the work. 1. Multiply the length of the longitudinal rebars by the number required: 20 feet times 13 rebars equals 260 rebar feet. 2. Multiply the length of the latitudinal rebars by the number required: 16 feet times 16 rebars equals 256 rebar feet. 3. Combine the results: 260 rebar feet plus 256 rebar feet equals 516 total rebar feet.
Tips: Rebar steel might cost less if standard lengths are ordered then cut to fit on the job site.
When estimating rebar, subtract from the longitudinal and latitudinal lengths the appropriate clearance from the formwork. Quantities ordered cut to precise dimensions might cost more than rebar purchased in standard lengths.
Purchasing a calculator named ConcreteCalc Pro helps simplify the entire rebar calculating process. The device is available from Construction Book Express: ConcreteCalc Pro and www.Contractor-Books.com. (http://www.constructionbook.com/2005-concretecalc-pro/concrete-calculator/) (http://www.contractor-books.com/CI/ConcreteCalc.htm)
Further consideration should be given to contingencies like splices. Information found on the Reed Construction website sums it up like this: “When estimating the amount of reinforcing steel, either bar or mesh, if no lap specifications are given, add 10% to your quantities for lapping, splices and waste.” (http://www.reedconstructiondata.com/rsmeans/estimating-tips/concrete/)
The calculations in Steps 1-3 of the example result in a web of rebar steel that fills the entire rectangular form work. An adjustment to the rebar length must be made because building departments do not allow rebar to come too close to form work. In many cases, building officials mandate a clearance minimum of 2 inches. In the rectangular slab example, the simplest solution dictates adjusting the length of each rebar by cutting 4 inches off one end after the material reaches the job site. This allows clearance from the form work of 2 inches on each end when the rebars are set in place.
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