Steps involved in design of a two way slab | IS 456: 2000?

In present scenario it is very easy to design slab. As in most of the cases we use some third party commercial software to design the structure. Even then I believe for a civil engineer it is very much important to be aware about the different steps involved in designing the two way slab as per the IS specification.

You can check this post to know about two way slab.

Steps to design a two way slab

Step 1 : Determine the effective span of the slab – dimension (lx X ly)

  • Assume an effective depth (d) on the basis of span to effective depth ratio (Table 1).
  • Then compute the value of overall depth (D).
  • Overall depth D = d + clear cover
  • Clear cover depends on the exposure condition (for mild exposure, clear cover = 20 mm and for moderate exposure, clear cover = 30 mm ).
  • Effective span (l) l = lx + width of support or lx + d —– A lesser of these two
Type of slabFor Fe250For Fe415
Continuous lx/40lx/32
Simply supportedlx/35lx/28
Table 1 as per IS 456:2000

Step 2 : Analysis – Calculate the moment

  • Consider the various loads due to
    • Self weight
    • Dead Load
    • Live load
  • Calculate the total load and factored load (1.5 * Total load)
  • Calculate the bending moment

$$ M_{x} = \frac{W_{ux} \times l_x^2 }{8} $$

$$ M_{y} = \frac{W_{uy} \times l_y^2 }{8} $$

This formulas are generalized in the Indian standard code:

$$ M_{x} = \alpha_x \times w_x \times l_x^2 $$

$$ M_{y} = \alpha_y \times w_y \times l_y^2 $$

The value of αx and αy is given in Table 27 of the Indian standard code.

Step 3 : Determine the area of reinforcement ( Ast )

Design the reinforcement as per the bending moment. Follow the same procedure as followed in one way slab design.

Check

The minimum reinforcement in each direction should not be less than

  • 0.15% of the Ast for Fe-250 steel
  • 0.12% of the Ast for Fe-415 & Fe-500 steel.

The max. spacing of bars should not be more than

  • 3d or 300 mm whichever is smaller for main steel
  • 5d or 450 mm whichever is smaller for distribution steel

You can design the two way slab using our android application.

Slab design

  • RCC Slab Design is a free app for designing one way and two way reinforced concrete slab systems as per Indian Standards.
  • RCC Design and detailing could be performed for ten different boundary conditions specified in IS456:2000
  • Option to save the design projects in local storage.
  • Detailed calculation steps presented for verification and validation.

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