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Estimating Reinforcements for a Concrete Footing

Independent footing, also known as isolated footing, is a type of foundation attached to a single column and is the simplest and most common type of foundation. According to the plan and design of the footing, reinforcement for this footing can either be plain or have bends and hooks.



HOW TO CALCULATE FOR QUANTITY OF STEEL BARS FOR A CONCRETE INDEPENDENT FOOTING:


  1. Determine the length, width and underground protective covering of your concrete footing:

  2. Determine the following information based on the design of the your concrete footing.


  3. Determine the cut length of one reinforcement bar:

  4. From the information gathered, get the cut length of footing reinforcement bar. If the dimensions of the footing's length and width are different, get both cut lengths.

    Cut LengthL = Footing Length - (Underground Protective Covering * 2)
    Cut LengthW = Footing Width - (Underground Protective Covering * 2)


  5. Determine the steel bar commercial length to be used and the number of cuts that can be made per steel bar:

  6. Steel bars are usually commercialy available in five(5) lengths: 6m, 7.5m, 9m, 10.5m and 12m. In order to calculate the quantity of steel bars to be ordered, we need to determine how many cuts can be made in a piece of steel bar.

    No. of Cuts Per Steel Bar = Steel Bar Commercial Length / Cut LengthL
    No. of Cuts Per Steel Bar = Steel Bar Commercial Length / Cut LengthW

    If the quotient is not a whole number, it means the commercial length selected is not divisible with the cut length. This will result in extra cuts. In order to avoid excess cuts, select a commercial length that is divisible by the cut length.

    For example, if your cut length is 2.5 meters, a 6 meter long steel bar will produce an excess cut of 1 meter.

    No. of Cuts Per Steel Bar = Steel Bar Commercial Length / Cut Length
    = 6m / 2.5
    = 2.4 - Commercial Length is not divisible by the Cut Length and would therefore produce an excess cut (1 meter excess).

    However, if you select a 7 meter long steel bar, no excess cuts will be produced.

    No. of Cuts Per Steel Bar = Steel Bar Commercial Length / Cut Length
    = 7 / 2.5
    = 3 - Commercial Length is divisible by the Cut Length would not produce an excess cut.



  7. Determine the specified bar spacing or, if already provided, the number of bar cuts of your footing:

  8. Unless specified in the drawings, the total number of bar cuts you will need for the footing will depend on the specified spacing of the footing.

    Total Number of CutsL = (Footing Length / Specified Bar Spacing) + 1
    Total Number of CutsW = (Footing Width / Specified Bar Spacing) + 1


  9. Get the number of steel bars to be ordered:

  10. Number of Steel BarsL = Total Number of CutsL / No. of Cuts Per Steel BarL
    Number of Steel BarsW = Total Number of CutsW / No. of Cuts Per Steel BarW
    Total of Steel Bars to be ordered = Number of Steel BarsL + Number of Steel BarsW




SAMPLE PROBLEM 1


Compute for the quantity of steel bars needed for an independent footing that is 1500mm long and 1000mm wide, with the following given specification:

Protective Cover = 75mm
Bar Spacing = 200mm O.C.

SOLUTION

  1. Determine the length, width and underground protective covering of your concrete footing:

  2. Footing Length = 1500mm
    Footing Width = 1000mm
    Protective Cover = 75mm

  3. Determine the cut length of one reinforcement bar:

  4. Cut LengthL = Footing Length - (Underground Protective Covering * 2)
    = 1500mm - (75mm * 2)
    = 1350mm

    Cut LengthW = Footing Width - (Underground Protective Covering * 2)
    = 1000mm - (75mm * 2)
    = 850mm


  5. Determine the steel bar commercial length to be used and the number of cuts that can be made per steel bar:

  6. For both cut lengths, the commercial length that would produce the least wastage is the 6m long steel bar.
    No. of Cuts Per Steel Bar = Steel Bar Commercial Length / Cut LengthL
    = 6000mm / 1350mm
    = 4.44 ~ 4 pcs. per 6m steel bar


    No. of Cuts Per Steel Bar = Steel Bar Commercial Length / Cut LengthW
    = 6000mm / 850mm
    = 7.05 ~ 7 pcs. per 6m steel bar

  7. Determine the specified bar spacing or, if already provided, the number of bar cuts of your footing:

  8. Total Number of CutsL = (Footing Length / Specified Bar Spacing) + 1
    = (1500mm / 200mm) + 1
    = 8.5 ~ 8 pcs. of 850mm long steel bars


    Total Number of CutsW = (Footing Width / Specified Bar Spacing) + 1
    = (1000mm / 200mm) + 1
    = 6 pcs. of 1350mm long steel bars


  9. Get the number of steel bars to be ordered:

  10. Number of Steel BarsL = Total Number of CutsL / No. of Cuts Per Steel BarL
    = 8pcs. / 4pcs.
    = 2 pcs. of 6m long steel bars are needed.


    Number of Steel BarsW = Total Number of CutsW / No. of Cuts Per Steel BarW
    = 6pcs. / 7pcs.
    = 0.85pc. ~ 1 pc. of 6m long steel bars are needed.


    Total of Steel Bars to be ordered = Number of Steel BarsL + Number of Steel BarsW
    = 2pcs. + 1pc.
    = 3 pcs. of 6m long steel bars.


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