Bar Bending Schedule.
In this Article we will Only Focus on How and Where to start and what are the basics.
Bar is any type of Rebar which is used as a reinforcement in RCC.
The Bar May a Mild Steel Bar or HYSD Bar or TMT Bar .
Bar Bending schedule termed as "Calculation of steel required for the construction of
a building.we used steel to make concrete reinforced and to provide tension in structure.
But how much steel required for constructing 15 floor building? How much steel we have to ordered? all these questions answered in BBS.
In
Bar bending schedule, the bars are organized for each
structural units (Beams or columns or slabs or footings etc) and detailed list
is prepared which specifies the Bar location (Bar in footings, slabs,
beams or columns), Bar Marking (to identify the bar in accordance with the
drawing), Bar Size (length of the bar used), Quantity (No. of Bars used),
Cutting length, Type of Bend and Shape of the bar in reinforcement
drawings.
How BBS Has Changed From 1950-2020
How BBS Has Changed From 1950-2020
From 1950 to 2020 lots of modifications
and enhancements happening in our world. In 1950, three-storeyed buildings are
high rise buildings now we are constructing a building with 200 floors+. There
is a massive growth in the construction industry. Due to the vast increase in
world population demands increased facilities, more need for space and more
construction.Father
of Estimation B.N. Datta has given certain recommendations for the usage of
steel in different components of buildings. But, he didn’t mention any values
if we use more bars in a single structural member.
Bar Bending Member
|
Percentage
|
Slab
|
1% of total volume of concrete
|
Beam
|
2% of of total volume of concrete
|
Column
|
2.5% of total volume of concrete
|
Footings
|
0.8% of total volume of concrete
|
At
that time we’ve used only four bars in columns;
now we are using 12+ bars in columns based on load
analysis. So, the percentage of steel is increased in a column which
reveals that the above-cited values are outdated. ( They are outdated “not
wrong”) He wrote that book in 1950. Now we are in 2019. He gave recommendations
according to the potentiality of construction at that time. Now we are
constructing 200 + floors in the small area.
Bar Bending Schedule(BBS):-
Before dealing with BBS, It is very important to learn the Basic of BBS.The Below table is kick start guide for BBS.
S.No.
|
Particulars
|
Result
|
1.
|
Standard Length of the Steel Bar
(Bars are sold at standard Length) |
12m or 40'
|
2.
|
Weight of Bar
for length = 1m |
D2/162
(were D = Dia of Bar) |
Ex:
|
If length of bar is 12m with 10mm
Dia then ,
Weight of bar = D2/162 Therefore for length 1m = 1m x D2/162 = 1 x 102/162 = 0.61 Kgs For length 12m = 12 x 102/162 = 7.40Kgs |
7.40Kgs
|
3.
|
Density of Steel
|
7850kg/cum
|
Below We will discuss the
different concepts of Bar Bending schedule which are very useful while working
with BBS. All these concepts are used in BBS design calculation of any
structural member. So be familiar with the below concepts.
1. Hook Length or Cutting length of
Stirrups:-
The hook length is commonly provided for
stirrups in beams and ties in columns. In general, Hooks are added at the two
ends of the rebar in stirrups or ties.
Hook Length = 9d (d is dia of the bar)
From above fig, length of hook =
[(Curved Portion) + 4d] = [(4d+d)+4d] = 9d
2. Bend Length:-
The Bend length calculation is different
for Cranked bars (bent up bars) and bends at corners.The bars are usually cranked in Slabs
and bars are bent at corners in Stirrups or ties.
(i) Bend Length calculation in Cranked
Bars:-
As Shear stress is maximum at
supports in Slab. To resist these stresses we usually crank the bars at the
ends of supports in the slab. The below figure depicts the bent up bar in Slab.
To calculate the bend length the below procedure is followed.
111
From the above figure as the bar is bent
at an angle θ0 the additional length (la)
is introduced.
Where, la =
l1 – l2–(i)
Tanθ = D/l2 ;
Sinθ = D/l1
Hence l1
= D/Sinθ and l2 = D/tanθ
Therefore from (i) :- la = D/Sinθ – D/tanθ
Giving different θ values as 300 ,
450, 600 results different additional length la values
as below.
θ0
|
D/Sinθ
|
D/tanθ
|
la =D/Sinθ – D/tanθ
|
300
|
D/0.500
|
D/0.573
|
0.27D
|
450
|
D/0.707
|
D/1.000
|
0.42D
|
600
|
D/0.866
|
D/1.732
|
0.58D
|
900
|
D/1
|
0
|
1D
|
1350
|
D/0.707
|
D/-1
|
2.42D
|
The additional length is added to
the total length of the bar if the bars are cranked at a certain angle.
Example Calculation
considering Bent up bar in Slab (Cranked bar):-
To keep the crank bar in position, an
extra bar of length (L/4) is provided below the crank bar as shown in the below
figure.
Therefore, the total length of bar =
L+0.42D+0.42D+(L/4)+(L/4) = 1.5L+0.84D
Remember
D = Depth of Slab-Top Cover-Bottom cover.
(ii) Bend Length calculation when bars
are bent at corners:-
The important standards used while
calculating the bend length at corners
1. 45° Bend length = 1d
2. 90° Bend length = 2d
3. 135° Bend length = 3d
Here, ‘d’ = Diameter of bar
Example Calculation
considering stirrup with the bends at corners:
From above fig, There are 3 bends which
are bent at an angle of 900 and two bends are bent at an angle of 1350
Total bend length = 3 x 900 Bend length + 2 x 1350 Bend length = 3 x 2d + 2 x 3d = 12d = 12 x 8 = 96mm
Bar Shapes
|
Total Length of Hooks
|
Total Bend Length
|
Total Length of Bar
|
Straight
Bar
|
Two Hooks
= 9d + 9d = 18d |
No bend
|
l + 18D
|
Bent
Up at one End only
 |
Two Hooks
= 9d + 9d = 18d |
One bend bent at an angle 45 = 0.42D |
l + 18D + 0.42D
|
Double
Bent up Bar
 |
Two Hooks
= 9d + 9d = 18d |
Two bends
bent at an angle 450 |
l + 18D + 0.42D + 0.42D
=l+18D+0.84D |
Overlap
of bars
 |
Two Hooks = 9d + 9d = 18d |
No bends
|
Overlap Length
=(40d to 45d)+18d |
3. Overlap Length / Lap Length in
Reinforcement:-
The standard length of Rebar is 12m.
Suppose the height of the column is 20 m. To purvey this requirement, two bars
of length 12m and 8m are overlapped (joined) with overlap length.
Overlap Length for compression members (columns) = 50d
The Overlap Length for tension members (beams) = 40d
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