BBS (bar bending schedule )of flat slab and estimation of Steel quantity,in this topic we know about BBS of flat slab and estimation of steel (reinforcement) quantity used in slab and Thumb Rule for Steel in flat slab and types of slab: flat slab, one way slab and two way slab.
We know that slab is very important work which transfer load safely to beam and from to column and load acting on column safely to the bed of soil.
When we start our project and Foundation column and brickwork and Beam when it is completed than and most important work to make design of slab and how to make BBS of flat slab and estimation of Steel quantity and how much quantity of reinforcement required.
If you calculate quantity of steel required for slab you will give order and purchase Steel quantity according to requirement, as you know extra amount of Steel get corrosion when laying in open environment that is damage of reinforcement
so we should buy steel according to requirement and BBS of flat slab help you to know how much quantity of steel required for your roof slab work and for your project. in this topic we discuss about BBS of flat slab. Let us discuss about roof slab
What are types of roof slab?
● Roof slab :- roof slab is structural design which is put over horizontal surface on brick wall and beam and column. It is horizontal structure made of heavy concrete. In structural designing slab is of three types
1) flat slab :the horizontal structure slab which is supported directly by the column or brick wall is known as flat slab. It has no wall beam, dead load and live load of flat slab directly transfer to column or brick wall and then to foundation of footing and then to bed of soil.
.2) one way slab :– the slab which is supported by beams on two opposite sides only in single direction that is one way slab. That’s why one way slab bend in one direction and load acting on it that is dead load and live load distributed only in two opposite sides in single direction.
If the ratio of longer span and shorter span is equal or greater than 2 then we should adopted one way slab. And it will bend in one direction that is longer direction of Span. Mostly one way slab have two parallel beam which carry load acting on it by the slab.
● 3)two way slab :- in two way slab it is supported by beam on all four sides and load carried in both direction. And distribute the load in all four sides in equal manner.
That’s why in two way slab it’s bend in both direction and resistance is provided from all four sides of beam to withstand with gravitational load acting on it by dead load and live load.
In two way slab ratio of longer span and shorter span is less than 2 then we adopted two way slab.
Steel (reinforcement) used in flat slab
As we know flat slab is supported directly by the column so it has no use of crank bar. Two types of bar provided for flat slab 1st straight bar having higher dimension used as main bar and provided at bottom of slab in shorter direction.and 2nd straight bar having lesser dimension used as distribution or cross bar and provided at top of slab over main bar in longer direction.
Steel (reinforcement) used in one way slab
As we know there is a two types of bar used in slab one is main bar that is also known as crank bar and second one is distribution bar.
In one way slab main bar that is crank bar used only in one direction of shorter span in bottom of slab and cross bar that is distribution bar is straight bar putting over main bar in longer span.
Steel (reinforcement) used in two way slab
In two way slab main bar that is crank bar used in bottom of slab in both direction of longer span as well as shorter span. And that is cross bar used in top of slab that is straight bar used over main bar.
BBS ( bar bending schedule ) of flat slab
Note:- this type of flat slab is outdated and not used in construction process and in structural design but in rural areas it will be provided due to low economical condition for the roof cast of building.
In given diagram there is cross sectional area containing x axis along the horizontal direction and the y axis along the vertical direction.
There is two types of Steel provided in flat slab work first is main bar which will be provided along Y axis and second one is distribution bar which will be provided along X axis.
● in this topic we have to determined following
1) number of main bar used along y axis
2) number of distribution bar used along x axis
3) cutting length and effective length of single main bar used along y axis
4) cutting length and effective length of single distribution bar used along x axis
5) estimation of reinforcement requirement in BBS of flat slab
Suppose we have given following data
Length of slab (X)= 3000 mm ( x-axis)
Length of slab (Y)= 4000 mm ( y -axis)
Diameter of main bar Dm = 10mm
Diameter of distribution bar Dd=8mm
Clear cover at all side C = 25 mm
Thickness of slab H= 150 mm
Spacing between main bar Sm = 125 mm
Spacing between distribution bar Sd= 150 mm
●1) number of main bar used toward y axis and shorter span
Nm= number of main bar ?
Sm = spacing = 125 mm
Width along Y = 4000 mm
We have formula to calculate number of main bar
Nm =( width_2 cover)/spacing)+1
Nm =( Y_2C/Sm) +1
Nm = (4000mm_2×25)/125mm+1
Nm = (4000_50)/125+1
Nm = 31.6+1 we round of 31.6 is equal to 32
Nm = 32 +1 = 33 nos
●2)number of distribution bar used toward X axis in longer directions
Nd= number of distribution bar ?
Sd = spacing = 150 mm
Width along X = 3000 mm
We have formula to calculate number of distribution bar
Nd =( length_2cover)/spacing+1
Nd =( X_2C/Sd) +1
Nd = (3000mm_2×50)/150mm+1
Nd = 2950 mm/150mm +1
Nd= 19.66 +1 we round of 19.66 as 20 so
Nd = 20 +1 = 21 nos
calculate cutting length of single piece of main bar in flat slab
Cm1 = cutting length of single piece of main bar
Cm1 = effective length + development length
Note. Actually this is Flat slab so they have no use of crank bar or bent Up Bar only straight bar is provided as main bar and distribution bar and it has provided with developmental length with the the joint section of column and slab.
Effective length = length of slab along x axis_2 cover
Development length Ld=40d where d is diameter of main bar and if we use M20 grade of concrete and Fe415 grade of Steel.
We have formula for calculating cutting length of single piece of main bar
Cm1= (length _ 2 cover) + Ld
Where we have
Length of span along X axis = 3000 mm
Cover = 25 mm
Cm1 = (3000 _2×50) + 40×10
Cm1 = (3000_50) + 400mm
Cm1 = 2950 +400 mm
Cm1 = 3350 mm
Cm1 = 3350 mm = 3.350 meter
calculate total cutting length of main bar in flat slab
Cm = total cutting length of main bar
Cm = No.of nos × cutting length of one main bar
Cm = Nm × Cm1
Cm = 33 × 3.350 meter = 110.55 meter
weight calculation of main bar in bbs of flat slab
We have formula for the weight calculation of main bar in Kilogram per metre
Weight = (D^2/162)×L
We have Dm = diameter of main bar is 10 mm and length is 110.55 meter
Weight =[ (10×10)/162]×110.55 kg/m
Weight of main bar Wm = 68.24 kg
calculate cutting length of single piece of distribution bar in bbs of flat slab
Cd1 = cutting length of single piece of distribution bar
Cd1 = effective length + development length
Effective length = length of footing_2 cover
Note. Actually this is Flat slab so they have no use of crank bar or bent Up Bar only straight bar is provided as main bar and distribution bar and it has provided with developmental length with the joint section of column and slab.
We have formula for calculating cutting length of single piece of distribution bar
Cd1= ( length of y axis _ 2 cover) + Ld
Development length Ld=40d where d is diameter of distribution bar and if we use M20 grade of concrete and Fe415 grade of Steel.
Where we have
Width of slab y = 4000 mm
Cover = 25 mm
Dd=8mm diameter of distribution bar
Cd1 = (4000 _2×25) + 40×8mm
Cd1 = (4000_50) + 320 mm
Cd1 = 3950 +320mm
Cd1 = 3950 +320 mm
Cd1 = 4270 mm = 4.270 m
calculate total cutting length of distribution bar in bbs of flat slab
Cd = total cutting length of distribution bar
Cd = No.of nos × cutting length of one piece distribution bar
Cd = Nd × Cd1
Cd = 21 × 4.270 meter = 89.67 meter
●8) weight calculation of distribution bar
We have formula for the weight calculation of distribution bar in Kilogram per metre
Weight = (D^2/162)×L
We have Dd = diameter of distribution bar is 8 mm and length is 89.67 meter
Weight =[ (8×8)/162]×89.67 kg/m
Weight of distribution bar Wd = 35.43 kg
calculate total requirement of reinforcement for the BBS of flat slab
Total weight = Wm + Wd
Total weight = 68.24 kg + 35.43 kg
Total weight = 103.67 kg
You should also visits:-
calculating Thumb Rule for steel for flat slab
If we want to know quantity of steel required for 1 square feet
We have dimension= 3000×4000mm
Which is area = 3m ×4 m =12m2
Now converting square metre into square feet
1m2 = 10.764 sq.ft
12m2 = 12×10.764 sqft = 130 sq.ft
130 sq.ft flat slab we need about 104 kg steel
For 1sq.ft = 104/130=0.8kg
For 100 sq.ft we need = 80 kg
1) Thumb Rule for main bar
For 1sq ft = 68.24/130 =0.53 kg
For 100 sq ft = 53 kg
2) Thumb Rule for distribution bar
For 1sq ft = 35.43/130 =0.27 kg
For 100 sq ft = 27 kg
Ratio = quantity of distribution bar/ quantity of main bar
Ratio = 27/53 = 1:2 it means if you want to buy Main bar 10 mm dia and distribution bar of 8mm dia we keep in mind main bar is just double of distribution bar.