Beams are extremely important parts of a building. They are one of the major load-bearing members of the structure, transferring loads from slabs to columns. When you construct a beam, you have to put steel reinforcements in it in order to combat the tensile forces generated at the bottom side. Today, we will see how should we go about calculating steel quantity in beams.
They say learning on the work is the best way to learn anything. This is true about construction as well. So, we will learn about calculating steel quantity in beams using a live example.
Let us say, we have a simply supported beam. Let the cross section of the beam be 300 mm by 500 mm (width x height). Also, let’s suppose this beam is 4 meters long.
Moving onto the available reinforcements. We have 20 mm bars to be used the main reinforcement, and the support bar dia is 16 mm. Furthermore, we will use 12 mm stirrups at 100 mm gaps. The midsection will have an increased stirrup spacing of 150 mm.
Now, given this design, we will have to calculate the steel quantity in this beam.
It is quite simple to find out the steel quantity in beams if you take a step-by-step approach. You will need only the following three steps to find out the solution:
Now we will compute the amounts in each step one by one.
Actual length = length of beam + development length – bend
Where,
Knowing that development length is the grip between steel and concrete.
Therefore,
Length:
Main = 4000 + (50d×2) – (2d×2)
= 4000 + (50×20×2) – (2×20×2)
= 4000 + 2000 – 80
= 5920 mm
= 5.92 meter.
For 2 bars it will be total:
= 5.92×2
= 11.84 meters.
Weight:
The weight of steel = d2/162 × length
=20 × 20/162 × 11.84
= 29.2 kg
Calculating for support bars (16 mm)
Length = length of beam + Ld – bend
Where,
• for 90 deg bend = 2d
Therefore,
Support = 4000 + (50d×2) – (2d×2)
= 4000 + (50×16×2) – (2×16×2)
= 4000 +1600 – 64
= 5536 mm
= 5.536 meter.
For 2 support bars, total length
= 5.536 × 2
=11.072 meter.
Weight = d2/162 × length
= 16 × 16/162 × 11.072
= 16.47 kg
Length is divided into 2 way 1st is at support and 2nd is at mid support.
For end supports: (spacing 100 mm)
Length / 3:
= 4000/3
= 1333.3 m
No of stirrups
= Length/spacing + 1
= 1333.3/100+1
= 14.33
So, we have to use 15 pieces, in real life work.
Total no of stirrups for end supports
= 15×2
= 30 pieces
For mid supports: (spacing 150 mm)
Length = 1333.3 m
No of stirrups
= length /spacing + 1
= 133.3/150+1
= 7.88
Therefore, we have to use 8 pieces in real life.
Cross section of stirrups is
• Width = 300 mm
• Height = 500 mm
A is horizontal length of stirrup
= 300 – (2 x clear cover) – (2 x half of bar dia)
= 300 – (2 x 30) – (2 x 6)
= 228 mm
B is vertical length of stirrup
= 500 – (2 x clear cover) – (2 x half of bar dia )
= 500 – (2 x 30) – (2 x 6)
= 428 mm
Cutting length of one stirrup is
Cutting length:
= (2 x A)+(2 x B)+(2 x 10D)-(3d x 2)-(2d x 3)
Where, 10d is hook and 3d is bend 135 for deg and 2d is bend for 90 deg, and in one stirrups 5 bends are required.
Therefore, cutting Length:
= (2 x 228)+(2 x 428)+(2 x 10 x 12)-(3 x 12 x 2)-(2 x 12 x 3)
= 456 + 856 + 240 – 72 – 72
= 1408 mm or 1.408 m
For total 38 stirrups
= 1.408 x 38
= 53.50 m
Weight of stirrups is
= d2/162 x length
= 122/162 x 53.50
= 47.55 kg
Therefore, total weight of steel
= Main bars + Supports bars + Stirrups
= 29.2 + 16.47 + 47.55
= 93.22 kg
That’s it, we got the answer! You will need a total of 94 kilo steel for reinforcing the given beam. Make sure you add some error or environmental adjustment margin.
