Easy Step by Step process of Two way slab design as per IS-456

Easy Step by Step process of Two way slab design as per IS-456

Finding difficulties on Two way slab design?well,Here is a complete step by step process of Two way slab design.It is easier than ever to learn the complicated Two way slab design process in a simplest way with images.
Before starting the design process of two way slab,let us learn some basic thing that will help you in understanding easily.


Two way slab design(IS-456):

  • what is two way slab?

  •   when the ratio of longer span to shorter span is less than 2,then the slab is called as two way slab. [(Ly/Lx)<2]

  • Total depth of slab = {effective depth + clear cover + (Dia of bar that will be used)/2}

  • You have to know the edge condition of the slab for which you are designing. Edge conditions says the position of the slab like where the slab is located(eg: 1 edge of a slab can be discontinuous and other 3 edges of that slab can be continuous,etc).

  • Ast = amount of steel ,
  • Pt= percentage of steel,


First you ensure that the slab is a two way slab.
just check Ly/Lx <2.


calculation of depth-

effective depth (d) = { effective length(Lx)/(Basic value*modification factor) }

[basic values are given in Is code 456 (clause no-23.2.1)],
[ the values of the modification factor can be taken from the range of 1.2 to 1.5.]

Table for basic values

After calculating effective depth, next is Overall Depth.
Assuming, clear cover for slab is 15 mm , 10 mm dia of bar is used.

Overall depth(D) = {d + 15+(10/2)}


Load calculation-

In case of load calculation, you have to estimate - (a) dead load , (b) live load , (c) load of wall
(Note:if any wall is situated on the slab then you have to consider the load of that wall which is called as load of wall).

Dead load
=(effective depth of slab*length*width*unit weight of concrete)=(d*1*1*25)Kn/m

[ Note: Here we are calculating Load per meter,therefore we are taking length =width=1 meter.
 and unit weight of concrete=25 kn/cu.m]

(Don't forget to multiply total calculated weight by the factor of safety)


Moment calculation-

From now just give a little bit more ATTENTION.

 For two way slab design,we will get 4 different values of moment.
(Note:For shorter span one is negative and another is positive,similarly for longer span one is negative and other one is positive,Thus total 4 no. of moment)

To calculate these 4 Moment,we have to find out 4 Bending moment coefficient from Is-456(clause D-1.1 and 24.4.1)

table for coefficient factor

From that table you will get 4 coefficient with respect to your (Ly/Lx)value.


Moment =(∝*w*Lx^2) [where ∝ is coefficient that you got from table,w is the factored load,Lx is shorten span length)

[TIP: By putting 4 different  bending moment coefficient  you will get 4 different moment.]


Depth check-

In a simple form, To Understand just know that there is one equation by which we can confirm whether the assumed depth is safe or not.

Mu lim = (K * Fck *b *d^2)..............eqn (i)

Mu lim = actual moment capacity that the slab can resist,

Fck = characteristic strength of concrete,

K = 0.149 for Fe 250 steel
    = 0.138 for Fe 415 steel
    = 0.133 For Fe 500 steel.

  • Just Replace the (Mu lim) by the value`of Maximum moment (M max) you got from the above 4 values of Bending moment.
  • look carefully at the units while putting the values.
From , eqn (i) you will get the value of d.

Now see,If the value of d is less than the effective depth which you have calculated from the Step 1,then design is safe.
Otherwise, you have to increase the effective depth(from Step 1) by some extent and check the depth again.


Calculation of Pt-

By following Step 3. you get 4 values of bending moment.

Now by this 4 values of moment we will get 4 values of percentage of steel.

Formula is given below to find the value of Pt.
percentage of steel



calculation of  area of steel Ast-

From Step 5 you get 4 values of Pt.
similarly,you will get 4 values of Ast by a simple formula which is given below-


Spacing calculation-

The simple formula for spacing is-
 spacing =  {(As * 1000) / Ast}mm
As=cross section of 10 mm dia bar as we have assumed 10 mm dia bar to be used in Step 1,
As we are taking breath = 1000 mm=1 meter,thus b = 1000 mm,
Ast= you got 4 values of Ast. JUST LOOK TO STEP 6.

Here you will get 4 different values of spacing.

[Note; Never provide the spacing of reinforcement bar more than 300 mm].


We have calculated the following data:

  • factored load
  • four values of bending moment(2 positive & 2 negative)
  • four values of percentage of steel(Pt)
  • four values of area of steel(Ast)
  • four values of spacing(Sp)
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I hope you liked this step by step process of two way slab design.for any query/question/suggestion about two way slab design please leave a reply.

Easy Step by Step process of Two way slab design as per IS-456 Easy Step by Step process of Two way slab design as per IS-456 Reviewed by Sk rafiqul islam on August 04, 2018 Rating: 5

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