Contents:

**Basics of Reinforced Concrete Slab Design**

Slabs are generally designed on the assumption that they consists of a number of beams of breadth ‘one meter’.

**1. Effective Span of Slab**

The effective span of a simply supported slab shall be taken as the lesser of the following:

- Distance between the centers of bearings,
- Clear span plus effective depth

**2. Thickness of Slab**

The following table gives the maximum values of the ratio of span to depth.

Type of slab | Ratio of span to depth |

Simply supported and spanning in one direction | 30 |

Continuous and spanning in one direction | 35 |

Simply supported and spanning in two directions | 35 |

Continuous and spanning in two directions | 40 |

Cantilever slabs | 12 |

**3. Reinforcement for Slab**

Minimum reinforcement in either direction shall be 0.15 percent of total cross-sectional area. Main reinforcement which is based on the maximum bending moment shall not be less than 0.15 per cent of the gross sectional area. The pitch of the main bars shall not exceed the following:

- Three times the effective depth of slab, and
- 45 cm.

**Distribution bars** are running at right angles to the main reinforcement and the pitch shall not exceed

- Five times the effective depth of slab, and
- 45 cm.

The diameter of main bars may be from 8 mm to 14 mm. for distribution bars, steel 6 mm or 8 mm are generally used.

**4. Reinforcement ****Cover **

The minimum cover to outside of main bars shall not be less than the following:

- 15 mm and
- Diameter of the main bar.

## 5. **Concrete Slab Design Procedure**

**Steps to be followed in the design of slab**

- Assuming suitable bearings (not less than 10cm), find the span of the slab between the centers of bearings.
- Assume the thickness of slab (take 4 cm per metre run of the span).
- Find the effective span which is lesser of (i) distance between centres of bearings, and (ii) clear span and effective depth.
- Find the dead load and the live load per square meter of the slab.
- Determine the maximum bending moment for a one meter wide strip of the slab.

The maximum bending moment per meter width of slab,

Where, w = total load intensity per square meter of the slab.

- Equate the balanced moment of resistance to the maximum bending moment

Find the effective depth ‘d’ from the above equation.

- Calculate the main reinforcement per metre width

For M15 concrete, lever arm = 0.87 d

Spacing of bar =

**Design of Continuous Slab**

Suppose a slab is supported at the ends and also at intermediate points on beams, the maximum sagging and hogging moments to which the slab is subjected to due to uniformly distributed load, can be computed as follows:

Let = intensity of dead load per square metre

= intensity of live load per square metre.

Bending moment due to dead load and live load may be taken as follows (IS: 456 – 2000)

Bending Moment | At middle of end span | Over support | At middle of interior support | Over interior support |

Bending Moment due to dead load | ||||

Bending moment due to live load |

## Steven McCrary

in ratios of span to thickness, is the loading=dead load only?

## Madeh Izat Hamakareem

This table is not related to loads.

BS8110 provides a table for span to depth ratio which is based one total unfactored dead and imposed load.

For instance, for 5KN/m^2 imposed on simply supported slab , D/L =29, 10KN/m^2, D/L=25, and 20KN/m^2, D/L=22.

if the depth of slab is 0.4m only then hw to assume its thickness? pls reply

sir ,why shorter dimension of the slab is chosen to decide the % of the main reinforcements being provided for the corresponding bending moment

thanks this site very help full.

thanks admin,upload more thngs it helping me

hello

yaar kuch naya socho……….

the diameter of main is too small.

Will be cont……..

give me a suggetion to provide a slab for a room of 40' x 15' without beams.

Design the slab as one way simply supported slab

I conceive this website holds some rattling excellent info for everyone : D.

only for civil…

friend i m civil eng.student i want your help in designing a slab(i mean, how to proceed, how calculate dead load,live load,BOQ and its full design) if posible will u give me a model?

am a civil eng. Student. I have a project to design a classroom block. Kindly assist on how to go about it.

This method is obsolete. Why don’t u post it for limit state method of design? I dont think designers practically use Working stress method any more!!!

## Gopal Mishra

Sure, that will also be posted…