CALCULATE THE EFFECTIVE DEPTH OF THE SLAB

Generally, we take the unit for slab thickness in mm, meter or inches depending upon the requirement to show in drawing and measuring purpose. The minimum thickness of the slab we consider is 125 mm, according to IS 456:2000. The thickness of the slab depends on the load on the slab. Here we have taken slab thickness for 2 to 3 story house as 150 mm. It is also used on the ground floor and as a footing slab. Before starting off the topic! If you are landing here in this page first then do read about How to calculate RCC column size and Beam size quickly, it will make your understanding more clearer.

WHAT IS SLAB?

The slab definition in civil engineering is simply given as a horizontal concrete surface which used for making flat part of structures like ceiling, roof deck and floors. It is an essential part of the construction which transfer all the loads on a supporting element such as beam, column, and wall. 

DEPTH OF SLAB

The slab depth we can calculate with the help of span to effective depth ratio. But before calculating the depth of the slab, we will decide which types of concrete slab is suitable for our structure. We must opt the right kind of slab according to the load and span of column support; then we can go as per the procedure of calculating the depth of the slab. 

TYPES OF SLAB

There are different types of slabs:

1. ONE-WAY SLAB: This slab is supported from two sides. Mostly, we use it for 3- 6 m span and suitable for a live load of 3 – 5 KN/m2. For aesthetic purposes, you can use the ceiling slab bottom of the concrete slab.

2. TWO- WAY SLAB: This type of slab has support from all four sides. Generally, we use a two-way slab for 6 – 9 m span and suitable for a live load of 3 -6 KN/m2. 

3. RIBBED SLAB: It is also called a one-way joist slab, used for medium and heavy live load ranges 4 – 6 KN/m2 and 6 – 9 m span is suitable for this slab. The thickness of the slab is usually taken as 50 – 100 mm, which is supported by reinforced concrete tapered ribs. Spacing between ribs should not be more than 750 mm. 

4. WAFFLE SLAB: Also known as grid slab, these slabs are generally employed for medium and heavy live load ranges 4- 7 KN/m2. And 9 – 15 m span is suitable for these slabs. The load-carrying capacity of waffle is more than other slabs.

5. FLAT SLAB WITHOUT DROP PANEL AND COLUMN HEAD: This slab directly takes support by columns or walls instead of beams. It is used as post-tensioned slabs too. We use flat slab for 3 -5 KN/m2 live loads and suitable for span 6 – 8 m, but span for the prestressed flat slab is between 8 – 12 KN/m2.

6. FLAT SLAB WITH DROP PANEL AND COLUMN HEAD: This slab takes support by column, column head or drops panel without beams. It is used as post-tensioned slabs too. The loads directly transfer to the columns. We use this slab for spans between 6 – 9 m and 4- 7 KN/m2 live loads.

7. HARDY SLAB: These slabs help to fill in the portion of other slabs. Made up of hardy bricks, they are used in places where the temperature is very high. It is generally not being practised in India. This slab thickness in mm is nearly considered as 270 mm.

8. BUBBLE DECK SLAB: This slab is interesting in its composition as it is composed of plastic bubbles. It is used for a longer span, and decrease the number of columns and constructed without beams or ribs. It provides more strength to the slab, less in weight and also saves the concrete.

9. HOLLOW CORE SLAB: It is used for prevention of road and also used with car park development and offices. The standard width of the slab is 120 mm, and thickness in between 150 mm to 500 mm.

10. COMPOSITE SLAB: In this slab, we use the profiled steel sheet as a permanent shuttering of a reinforced concrete slab. Therefore, it is called a composite slab. The thickness of this slab in mm is between 50 – 60 mm.

11. PRECAST SLAB: Precast slab is made in a manufacturing plant then transported to the construction site for erection. It is used for a longer span up to 15 m. 

ONE-WAY SLAB– It is supported on two opposite sides by beams for carrying the loads. If the ratio of longer span to shorter is greater than two, then, it is called a one-way slab.

Table no.1 Primary value for different types of slab

Total Depth =     Effective depth + Ø/2 + Clear cover

 D = d +Ø/2 +clear cover

Note: This effective depth formula is also applicable for flat slab.

For M.F. (Modification Factor)

(As per IS 456 : 2000 page no. 37 & 38 fig. 4 & 5)

Where, 

d = Effective depth of slab

D = Total depth of slab

fs = Steel stress of service load

Ø = Diameter of bar

Example-

If size of simply supported slab is 3 m x 6.5 m & grade of steel is Fe 415

Then Ly = 6.5 m & Lx = 3 m

fs = 0.58 x 1 x 415

fs = 240 N/ mm2

Assume Pt = 0.4 %

M.F. = 1.3

So,
d = 3000 / ( 20 * 1.3 )

d   = 115.38 mm ≈ 120 mm

D = d + Ø /2 + clear cover

Consider Diameter of bar = 10 mm

D = 120 + 10 / 2 + 20

D = 145 mm ≈ 150 mm

D = 150 mm

Total depth of slab is 150 mm

TWO- WAY SLAB – it is supported on all sides by beams for carrying the loads. If the ratio of longer span to shorter is equal to or less than 2 so, it is called a two -way slab.

As per IS 456: 2000 clause no 23.2.1

Table no. 2 – Basic value of different types of slab

Lx / Ly <= 2

Where,

LY = longer span

LX = shorter span

There are two conditions for calculating the depth of slab according to the load:

  • If Shorter span ≤ 3.5 m and L.L. < 3 KN/m2

Then,

For Fe – 250 or Mild steel

Effective depth formula

Example-

If size of simply supported slab is 6 m x 5 m & grade of steel is Fe 415

Then Ly = 6 m & Lx = 5 m

      6 / 5 = 1.2 ≤ 2   (This is a two-way slab)

  d   = 500 / 35 * M.F * 0.8 (As per IS 456 : 2000 clause no 23.2.1)

fs = 0.58 x 1 x 415

fs = 240 N/ mm2

Assume Pt = 0.4 %

M.F. = 1.3

So,

  d   = 5000 / ( 35 * 1.3 * 0.8 )

  d   = 137.36 mm ≈ 140 mm

 D = d + Ø /2 + clear cover

Consider Diameter of bar = 10 mm

D = 140 + 10 / 2 + 20

D = 165 mm

Total depth of slab is 165 mm

  • If Shorter span > 3.5 m and L.L. > 3 KN/m2

Then, for basic value you go to table no. 1

effective depth formula

Example-

If size of simply supported slab is 6 m x 5 m & grade of steel is Fe 415

Then Ly = 6 m & Lx = 5 m

    6.5 / 5  = 1.2 ≤ 2 (This is a two-way slab)

  d   = 5000 / 20 * M.F (As per IS 456 : 2000 clause no 23.2.1)

And  (As per IS 456 : 2000 page no. 37 & 38 fig. 4 & 5)

fs = 0.58 x 1 x 415

fs = 240 N/ mm2

Assume Pt = 0.4 %

M.F. = 1.3

So,

  d   = 5000 / 20 * 1.3

  d   = 192.3 mm ≈ 190mm

D = d + Ø /2 + clear cover

Consider Diameter of bar = 10 mm

D = 190 + 10 / 2 + 20

D = 215 mm 

Total depth of slab is 215 mm
You can calculate the maximum size of bar in slab from effective depth of slab which helps you to calculate the area of steel in r.c.c. slab. You can check out steel calculation formula for column beam and slab from dailycivil.com

Hope this information on calculating the RCC slab size will help you in preparation and study of structure analysis. If you have any doubts in solving problems of calculating the RCC column, beam or slab size or any fundamental topics related to structural analysis, then feel free to write to me! 

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