DESIGN OF SLAB

1) one-way slab

• The ratio of longer span to shorter span is greater than 2.

• 𝑙 𝑦/𝑙𝑥> 2

• Any slab which is supported on two sides is also considered as one way slab.

• The bending in this slab takes place along shorter span.

• Min. amount of steel is provided in the form of distribution bars along longer 

span is prevent shrinkage & cracking.

2) Two –way slab

• The ratio of longer span to shorter span is less than or equal to 2.

𝑙 𝑦/𝑙𝑥≤ 2

• Any slab which is supported on more than two sides is known as Two way 

slab.

• Main steel bars are provided along both spans.

• The deflection in two way slab is parabolic in nature while the deflection in one 

way slab is considered to be const after some distance.

Design of one –way slab

• Effective span 

➔ same as in beam design.

Simply supported beam Leff = 𝑙o+ 𝑊/2+ 𝑊/2

OR

 𝑙o + D

 Cantilever = Leff = 𝑙o+ 𝑑/2

Width < 600mm ➔ simply supported

Width ≥ 600mm → 𝑙o+ 𝑑/2, 𝑙o

• Clear cover 

➔ Min. clear cover = 20mm.

Max. dia of bar

θmax not greater than thickness of seak/8

θmax not greater than 𝑜𝑣𝑒𝑟𝑎𝑙𝑙 𝑑𝑒𝑝𝑡ℎ(𝑑)/8

Longitudinal R/F or main R/F

(a) Min. Ast = 0.15% of gross area of slab = 0.15% bD (Mild Steel)

= 0.12% %of gross Area of slab = 0.12% bD (HYSD bars)

(b) Max Steel

Max. ASt = 4% of grass Area of slab 

Max. ASt = 0.04 bD

Distribution steel

1) The main purpose of distribution bars is to control the affect of shrinkage & 

creep.

2) These bars are not considered for load carrying but these are considered for 

effective distribution of loads on main steel.

Distribution steel

Ast = 0.12% of bD

Max. spacing of bars

a) For Main bars  

i) 3d 

ii) 300mm 

Which is lesser

b) for distribution bars

1)5d

2) 450 mm

Which is lesser

Two Way slabs

𝑙y/𝑙𝑥≤ 2

• Design of two way slab is based upon distribution of moments along two 

directions ( Along x axis & y axis).

• Those distributed moments can be calculated by.

Where, ∝x & ∝y are moment coefficients.

• Their values depend upon is 456:2000.

• Depth of slab is calculated on the basis of max Bending moment & area of 

steel is calculated for both moments.

NOTE: Redistribution of moments in beams are upto 30% (max. value)