As a site engineer,
you need to calculate the cutting length of bars according to the slab
dimensions and give instructions to the bar benders.
For small area of
construction, you can hand over the reinforcement detailing to the bar benders.
They will take care of cutting length. But beware, that must not be accurate.
Because they do not give importance to the bends and cranks. They may give some
extra inches to the bars for the bends which are totally wrong. So it is always
recommended that as a site engineer calculate the cutting length yourself. In
this article, we will discuss how to the calculate length for reinforcement
bars of slab. Let’s start with an example.
Where,
Diameter of the bar
= 12 mm
Clear Cover = 25 mm
Clear Span (L) =
8000
Slab Thickness =
200 mm
Development
Length(Ld) = 40d
CALCULATION:
Cutting Length =
Clear Span of Slab + (2 x Development Length) + (2 x inclined length) – (45°
bend x 4) – (90° bend x 2)
Inclined length =
D/(sin 45°) – dD/ (tan 45°) = (D/0.7071) – (D/1)= (1D – 0.7071D)/0.7071= 0.42 D
As you can see
there are four 45°bends at the inner side (1,2,3 & 4) and two 90° bends (
a,b ).
45° = 1d ; 90° = 2d
Cutting Length =
Clear Span of Slab + (2 X Ld) +(2 x 0.42D) – (1d x 4) – (2d x 2) [BBS Shape
Codes]
Where,
d = Diameter of the
bar.
Ld = Development
length of bar.
D = Height of the
bend bar.
In the above
formula, all values are known except ‘D’.
So we need to find
out the value of “D”.
D = Slab Thickness
– (2 x clear cover) – (diameter of bar)
= 200 – (2 × 25) – 12
= 138 mm
Now, putting all
values in the formula
Cutting Length =
Clear Span of Slab + (2 x Ld) +(2 x 0.42D) – (1d x 4) – (2d x 2)
= 8000 + (2 x 40 x
12) +(2 x 0.42 x 138) – (1 x 12 x 4) – (2 x 12 x 2)
∴ Cutting
Length = 8980 mm or 8.98 m.
So for the above
dimension, you need to cut the main bars 8.98 m in length.
