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How to Estimate Beam Reinforcement

An ordinary person can estimate reinforcement as lumsum. I see many people estimate reinforcement as a percentage of concrete volume. Such as 2% or 1.5% of concrete volume.

But as a construction professional you shouldn't estimate rebar quantity as percent of concrete volume. You should estimate that as shown in the structural drawing.

Estimating rebar quantity is easy. All you have to get the cutting length for each type of bar in the beam.

Let's start estimating. We'll use the following image as a structural drawing of beam.

estimating, beam reinforcement

Process of Estimating Beam Reinforcement

From the image above we have found -
  • Top longitudinal bar, T - 3D20
  • Bottom longitudinal bar, B - 3D20
  • Extra top bar on support1, Et1 - 2D20
  • Extra top bar on support2, Et2 - 2D20
  • Extra bottom Bar, Eb - 2D20
  • Stirrup, S1 - D10 @ 4" c/c

If you don't know the name of bar on different location in a beam then read the following post.

14 Terms About Beam Reinforcement a Supervisor Should Be Familiar With

You will also need the following data to estimate the beam reinforcement -
  • Clear cover for reinforcement
  • Lap length
  • Bend length
You'll get this data from the general notes sheet of structural drawing.

Read More:
Concrete Cover for Reinforcement Bar
17 Technical Terms about Reinforcement a Construction Supervisor Should Know
4 Terms to Understand to Truly Estimate Rebar Quantity

That's fine.

We have enough data to estimate the beam reinforcement. Let's estimate.

Top Longitudinal Bar (T)

Apply the following formula to get the cutting length of top longitudinal bar.

Cutting Length of top longitudinal bar is,
= Length of beam - 2 x clear cover + 2 x bend length - 2 x bend deduction length + lap length

Length of beam:  20" + (22'-8") + 20" + (8'-4") + 20" = 36' (from above image)
Clear cover: 1½"
Bend length: 12D = 12 x 20 = 240 mm, say 9½"
Bend deduction length: Bend deduction length for 90° bend is equal to two times of bar dia = 2 x 20 mm = 40mm = 1½"

So, the cutting length of the top longitudinal bar is,
= 36' - 2 x 1½" + 2 x 9½ - 2 x 1½" + 0
= 37'-1".

There are 3 numbers of top longitudinal bar in the above image. So the total length of top longitudinal bar is,
= 3 x (37'-1")
= 111'-3".

We know the full length of a reinforcing bar is about 40'. The length of a top longitudinal bar, we are estimating, doesn't exceed the length of a full bar. So you don't have to add lap length.

Bottom Longitudinal Bar (B)

The formula for calculating the cutting length of bottom longitudinal bar is same as the formula for top longitudinal bar. So the length is also same as T1 . That is 37'-1".

So the total length of B1 bar is,

= number of B1 bar x cutting length of a B1 bar

= 3 x (37'-1")

=111'-3".

Extra Top Bar on Support1 (Et1)

Calculating cutting length of extra top bar follow the following formula -
Cutting length of Et1 = extended length from support + width of support - clear cover + bend length - bend deduction length

Extended length from support: 7'-5"
Width of support: 20"
Clear cover: 1½"
Bend length: 9½"
Bend deduction length: 1½"

So the cutting length of Et1 is,
= (7'-5") + 20" - 1½" + 9½" - 1½"
= 9'-7½"

There are 2 number of Et1 bar shown in drawing (image above). So the total length of Et1 is,
=2 x (9'-7½")
= 19'-3"

Extra Top Bar on Support2 (Et2)

Extended length fron support: 7'-5"
Width of support: width of support for Et2  should be the width of support2 + distance between support2 and support3 + width of support3 = 20" + (8'-4") + 20" = 11'-8"
Clear cover: 1½"
Bend length: 9½"
Bend deduction length: 1½"

So, cutting length of Et2 = (7'-5") + (11'-8") - 1½" + 9½" - 1½"
= 19'-7½"

Total length of Et2 = 2 x (19'-7½")
= 39'-3"

Extra bottom bar (Eb)

Cutting length of extra bottom bar is,
=Distance between support - 2 x distance between extra bottom bar and nearest support
= (22'-8") - 2 x (2'-10")
= 17'

Total length of extra bottom bar is,
= 2 x 17' (there are 2 bar for Eb shown in the image above)
= 34'

Stirrup (S1)

For estimating  stirrup you have to calculate the required number of stirrup for the beam and cutting length of bar for a stirrup.

Number of stirrup:
Required number of stirrup for 1st span,
= (22'-8")/4" +1 = 69 + 1 = 70 nos.

Required number of stirrup for 2nd span,
= (8'-4")/4" + 1 = 25 + 1 = 26 nos.

Total number of stirrups = 70 + 26 = 96 nos.

Formula for calculating cutting length of a stirrup's bar is,
= 2 x (a+b) + 24D (for 135° hook)

Where,
a = length of stirrup's long side
b = length of stirrup's short side
D = dia of stirrup bar

So cutting length of the bar is,
= 2 x (21"+9") + 24 x 10mm
= 69½" (240 mm = 9½")
= 5'-9½"

Total length of bars for all stirrups,
= 96 x (5'-9½")
= 556'

Summary:
Total reinforcing bar we have estimated for the beam so far -

20 mm ø bar -
T = 111'-3"
B = 111'-3"
Et1 = 19'-3"
Et2 = 19'-7½"
Eb = 34'

So total 20 mm ø bar is,
 = 111'-3" + 111'-3" + 19'-3" + 19'-7½" + 34' = 296'.

10 mm ø bar,
S1 = 556'

But steel reinforcing bars are measured in kg in the market. So you have to convert the bar length to kilogram.

Unit weight of 10 mm ø bar is 0.188 kg/ft and 20 mm ø bar is 0.75 kg/ft. Read the following post to know how to calculate unit weight of reinforcing bar.

How to Calculate the Unit Weight of Steel Bars

So reinforcement required for our example beam,
  • 20 mm ø bar = 296' x 0.75 = 222 kg
  • 10 mm ø bar = 556 x 0.188 = 105 kg
That's it.

When you want to purchase reinforcing bar for the beam you should add 5% more with your estimated quantity as wastage.

You Turn:
Please share your experience about estimating in the comment below.

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