## What Is Slab?

**A slab** is constructed to provide flat surfaces, typically horizontal,** in building roofs, floors, bridges, and other types of structures**. The slab could be supported by **walls, by reinforced concrete beams normally** cast monolithically with the slab, by structural steel beams, either by** columns** or from the ground.

A slab is a plate element having depth (D), very small as compared to its length and width. A slab is used as floor or roof in buildings, carry distribution load uniformly.

**Type of Slab b**

**Slab May Be**

**Simply Supported****Continuos****Cantilever**

Also, read: What Is a Bar Bending Schedule | Preparation as Per Bs 4466 | Tolerances as Per Bs 4466

**Type of slabs based on support conditions are: **

**One Way Slab****Two Way Slab****Flat Slab Resting Direction on a Column Without Beam****Grid Slabs or Waffle Slab****Circular Slab and Other Shapes**

## What Is a One-Way Slab?

**One Way Slab**

The most straightforward routine structural element for illustration of design provisions from the Code is that the one-way slab.

A one-way slab is defined for functions of the book as a **flexural member** with thickness small relative to other dimensions, sup-porting (gravity) loads applied normal to and directly above its surface, a span in one direction between parallel supports, and fortified for flexure in this direction only.

For purposes of analysis, one-way slabs might be restrained to some degree in the supports or possibly unrestrained. A number of Code provisions reference to “flexural members,” including one- and – two-way slabs, beams, girders, footings, as well as where bending is present together with the axial walls, load, and columns.

In general, when this code provision is intended to use to one-way slabs, the term is going to be utilized in the sense of this definition herein.

If the is supported on two opposite sides, it is called a one-way spanning slab. In this type of slab, loads are transferred on two opposite as per the above figure. **A **

If the slab is supported at four sides, and if** Ly/Lx ≥ 2 **one way spanning slab.

For any slab, if **Ly = Lx, **the slab has a tendency to bend in both directions. With an increase of **Ly, **the tendency of bending along **Ly **is reduced and that on **Lx **is increased.

When **Ly/Lx ≥ 2**, the slab bends only in **X**-directions

When **Ly/Lx ≥ 2**, the slab is called a one-way slab. In one-way slab, the main reinforcement is provided along **Lx **( Short-Span)

Also, read: Total Station in Surveying | Operations | Advantage & Disadvantage | Types

## What Is a Two-Way Slab?

The design considerations of wall-supported two-way slabs are similar to those pertaining to one-way slabs.

The thickness of the slab is generally based on deflection control criteria, and the reinforcements in the two orthogonal directions are designed to resist the calculated maximum bending moments in the respective directions at the critical sections. [Additional reinforcement may be required at the corners of two-way slabs in some cases, as explained later].

The slab thickness should be sufficient against shear, although shear is usually not a problem in two-way slabs subjected to uniformly distributed loads.

If the slab is supported at all four edges and if **Ly/Lx < 2,**

The tendency of the slab is to bend in both directions. Such slab is called two-way slab as per above figure **c**

In two way slabs, main reinforcement is provided along **Lx** as well as **Ly** direction

Also, read: Principle of Plane Table Surveying Methods |Equipment | Error | Advantage | Limitation

## What Is the Flat Slab?

**Flat Slab**

When the slab is directly supported on a column, without beams, it is known as a flat slab.

Flat slab is provided to increase the floor height and to permit a large amount of light which might be obstructed by the depth of beams.

## What Is Grid Slab?

**Grid Slab**

When the slab is supported on beams with column only on the periphery of the hall, the slab is called grid slab.

Sometimes, in large halls, public places, marriage halls, auditoriums, etc. a large column-free area is required.

In these cases, large deep beams may be permitted, but the columns are permitted only on the periphery.

Also, read: Procedure For Rcc Concrete

## Analysis of Slab:

Slabs are primarily flexural members as beam and are analyzed and designed in the same manner as the beams. The analysis may be carried out as follows:

### Elastic Analysis:

A strip of 1 m width of the slab is considered, and loads are found on this strip. This strip id analyzed as a beam 1 m width.

### Code Coefficients:

This is a semi-empirical method of analysis based on yield line theory. The coefficients give in code may be directly used to analyze the slabs.

However, the redistribution of moments is not permitted in this case.

### Yield Line Theory

This is a limit state design or collapse load method developed by Johanson.

Also, read: Core Cutter Method

## Difference Between One Way Slab and Two Way Slab

One Way Slab | Two Way Slab |

One way slab two opposite side support beam /wall | Two Way Slab four side mins all side supported beam /wall |

One way slab is bend only in one spanning side direction while load transfer | Two way slab is bend both spanning side direction while load transfer |

Main Reinforcement is in provide short span due to banding | Main Reinforcement is in provide short span due to banding |

Ly/Lx ≥ 2 one way slab spanning | Ly/Lx |

One-way slab near about 100mm to 150mm based on the deflection | two-way slabs is in the range of 100mm to 200mm depending upon |

one way slab economical near about 3.5 m | Two-way slab may economical for the panel sizes near about 6m x 6m. |

Also, read : What Is Raft Foundation | Type of Footing | Detail of Raft Footin

**Design Considerations: **

### One Way Slab

#### 1. Effective Depth (d)

**For deflection control **

**L/d = 20 X M.F**

**M.F. Modifiction factor from— IS: 456, p.38.Fig-4****Assume % steel 0.3 to 0.6%**

**Fs = 0.58 Fy X (Ast requierd / Ast Provied)**

Initially assume that **Ast reqierd** = **Ast Provided **

Fy = 250 N/ Sq.mm —– Fs = 0.58 X 250 = 145 N/ sq.mm.

Fy = 415 N/ Sq.mm —– Fs = 0.58 X 415 = 240 N/ sq.mm.

Fy = 500 N/ Sq.mm —– Fs = 0.58 X 500 = 290 N/ sq.mm.

#### 2. Effective Span:

ClearSpan + d

c/c of Supports

Whichever is smaller ——– as per IS 456-2000 P. 34, CI 22.2.a

#### 3. Reinforcement Requirements

**Minimum reinforcement **

For Fe-250 Pt = 0.15 % of total C/s area (d x D)

For Fe-415 Pt = 0.12 % of total C/s area (d x D)

For Fe-500 Pt = 0.12 % of total C/s area (d x D) ——– as per IS 456-2000 P. 48, CI 26.5.2.1

**Maximum diameter (Sp 34)**

**For minbar:**

- Plain bars———–10 mm Ø min dia

Deformed bars—–8 mm Ø min dia

**For Distribution bars:**

- Plain bars———–6 mm Ø min dia

Deformed bars—–6 mm Ø min dia

**4. Check for Cracking**

**For Min Steel:**

3d ——— Where. d = Effective depth

300mm

Spacing should not exceed smaller these two values.

**For Distribution steel:**

5 d

450mm

Spacing should not exceed smaller these two values. ——- IS: 456-2000, P.46

**5. Check for Deflection:**

Allowable **L/d = 20 X M.F.**

- M.F is Obtained from IS:456-200 P-38 Fig 4

Find actual, L/d

If Actual **L/d** < allowable **L/d** ———- Ok

#### 6. Check for Development Length (Ld)

IS 456-2000,P.44, Cl. 26.2.3.3 C

**Ld should be ≤ 1.3 (M1/V) + L0**

Where

**Ld = (Ø.σs / 4 τ bd )—————–σs = 0.87 fy** As per IS 456-200, P.42

50 % of steel is bent up near support. Therefore find M.R for 50 % of steel only

**M1 = M.R. for 50% steel support**

**V = Shear force at the support **

**L0 = Sum of anchorage beyond the center of support**

** d**

**12 Ø**

Take **L0** as the smaller of two values.

Also, read: What is Chain Surveying (Principal, Procedure, Method, Instrument)

### Two Way Slab

#### 1. Effective Depth (d):

**For deflection control****L/d = 35 X M.F X 0.8****M.F. Modifiction factor from— IS: 456, p.38.Fig-4****Assume % steel 0.3 to 0.6%**

**Fs = 0.58 Fy X (Ast requierd / Ast Provied)**- Initially assume that
**Ast reqierd**=**Ast Provided** - Fy = 250 N/ Sq.mm —– Fs = 0.58 X 250 = 145 N/ sq.mm.
- Fy = 415 N/ Sq.mm —– Fs = 0.58 X 415 = 240 N/ sq.mm.
- Fy = 500 N/ Sq.mm —– Fs = 0.58 X 500 = 290 N/ sq.mm.

#### 2. Effective Span:

- ClearSpan + d
- c/c of Supports

Whichever is smaller ——– as per IS 456-2000 P. 34, CI 22.2.a

#### 3. Load Calculations:

- Total Load = D.L. + F.L. + L.L.
- Dead load of slab = (d x 25)
- Floor Finishing load = (as floor finishing near 1 kn/sq.mm)
- Live load = ( as per calculation)

- Factor Load = 1.5 x Total load

#### 4. Mid Span Moment:

Corners not held down conditions is given as per IS: 456-2000 P-90 CI D-2

**Mx = ax . w . lx . lx**

**My = ay . w . lx. lx**

**ax** and** ay** coefficientare obtained from IS: 456 table -26, fig 10.3 shoe nine separate possible arrangement for two way restrained slab.

#### 5. Effective Depth of Flexure:

**Mu = 0.138 . fck . b.d.d**

Heaer find d

Mu = Sp 16 P 10 Table C

Fck = strength of concrete

b = 1 m area required load

#### 6. Reinforcement in Mild Strip :

Along **Lx **

**Pt= 50 (fck/fy)x( 1-√(1-(4.6xMu/Fck b.d.d)))**- fck = strenth of concrete
- fy = 415 N/Sq.mm
- Mu = Sp 16 P 10 Table C =
**0.138 . fck . b.d.d= Mx**

Along **Lx **

**Pt= 50 (fck/fy)x( 1-√(1-(4.6xMu/Fck b.d.d)))**- fck = strenth of concrete
- fy = 415 N/Sq.mm
- Mu = Sp 16 P 10 Table C =
**0.138 . fck . b.d.d= My**

#### 7. Check for Cracking:

Along **Lx:**

3d ——— Where. d = Effective depth

300mm

Spacing should not exceed smaller these two values.

Along **Ly:**

3d ——— Where. d = Effective depth

300mm

Spacing should not exceed smaller these two values.

#### 8. Check for Deflection:

Allowable **L/d = 35 X M.F.X 0.8**

- M.F is Obtained from IS:456-200 P-38 Fig 4

Find actual, L/d

If Actual **L/d** < allowable **L/d** ———- Ok

#### 9. Check for Development Length:

IS 456-2000,P.44, Cl. 26.2.3.3 C

**Ld should be ≤ 1.3 (M1/V) + L0**

Where

**Ld = (Ø.σs / 4 τ bd )—————–σs = 0.87 fy** As per IS 456-200, P.42

50 % of steel is bent up near support. Therefore find M.R for 50 % of steel only

**M1 = M.R. for 50% steel support**

**V = Shear force at the support **

**L0 = Sum of anchorage beyond the center of support**

** d**

**12 Ø**

Take **L0** as the smaller of two values.

Also, read: Test for Compressive Strength of Brick | Water Absorption | Dimensions Test