How to replace or change dia. of bars in slab?/ Steel replacement formula used on site.

 Let us consider an RCC slab of size 6m. x 2.5m. as shown below.

Now,  we need to replace the distribution bar of 10mm. dia. with 12mm. bar due to its non-availability.

Given data:

Distribution bar dia. = D1=10mm.

Spacing of 10mm bar = S1 =150 c/c.

To calculate:

Spacing of replaced 12mm. dia. bar.

Calculation:

The spacing of replaced bar is calculated by the formula

[(D1)² ➗(D2)²] = [S1➗S2]

Where,

D1 = dia. of specified bar = 10mm.

D2 = Dia. of replaced bar =12mm.

S1 = Spacing of 10mm. bar.

S2 = Spacing of 12mm. bar.

 [(10)² ➗ (12)² = [150 ➗ S2]

 [100 ➗ 144] = [150 ➗ S2]

 0.694 = [150 ➗ S2]

 S2 =  [150 ➗ 0.694]

 S2   = 216 mm.

The 12mm. rebar should be placed @ 216 c/c.

Note:

The c/c distance of the replaced bar should not be more than the maximum allowable spacing as per design criteria.

    Now the question is, 

When we replace it with higher dia. bars, does it cost us more?

The answer is NO.

 The weight of the total no. of bars in both cases is nearly equal.

Let us check:

For 10mm. bars

The no. of distribution bars

 = [Ly ÷ bar spacing ] +1

 = [ 2500 ÷ 150] +1

 = 16.67 +1

= 17.67 nos. 

By rounding off = 18 nos.

For 12mm. bars

The no. of distribution bars

  = [ 2500 ÷ 216] +1

 = 11.57 +1

= 12.57 nos. 

By rounding off = 13 nos.

The total wt. of 10mm. bar

=[ cutting length 🇽 no. of bars 🇽 wt./m.]

As the cutting length will be the same in both cases, let us cancel it.

= [18nos.🇽 0.62]

= 11.16 kg.

Total wt. of 12mm. bar

=  [13nos.🇽 0.88]

= 11.4 kg. ⋍ 11.16kg. ✔

To go through the articles on Estimation & calculation in civil engineering, click here.

Thank you for going through these calculation steps❤. Have a good day 😄.

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