Important Point

## 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?

According to IS 456:2000, The ratio of longer span(l) to shorter span(b) which is (L/B) greater than 2 is known as **One way slab**. In practical, **One way slab** is supported by only two parallel beams or walls. Normally we don’t use **one way slabs** often.

or

**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, supporting (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?

A **two** **way** street is a street that allows vehicles to travel in both directions. On most **two** **way** streets, especially main streets, a line is painted down the middle of the road to remind drivers to stay on their side of the road. Sometimes one portion of a street is **two** **way** and the other portion is **one** **way**.

or

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

Sr No. |
One Way Slab |
Two Way Slab |

1 | The one way slab is supported by a beam on two opposite side only. | The two way slab is supported by the beam on all four sides. |

2 | In one way slab, the load is carried in one direction perpendicular to the supporting beam. | In two way slab, the load is carried in both directions. |

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

4 | 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 |

5 | One way slab is bend only in one spanning side direction while load transfer | In two-way slab, the crank is provided in four directions. |

6 | If L/b the ratio is greater than or equal 2 or then it is considered a one-way slab. | If L/b the ratio is less than 2 then it is considered a two-way slab. |

7 | In one-way slab, the load is carried in one direction perpendicular to the supporting beam. | In two-way slab, the load is carried in both directions. |

8 | The deflected shape of the one-way slab is cylindrical. | Whereas the deflected shape of the two-way slab is a dish or saucer-like shape. |

9 | Chajja and Varandha are practical examples of one-way slab. | Whereas two-way slabs are used in constructive floors of the Multistorey building. |

10 | In one-way slab quantity of steel is less. | In two-way slab quantity of steel is more as compared to the one-way slab. |

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

12 | Ly/Lx ≥ 2 one way slab spanning. | Ly/Lx < 2 two way slab spanning |

13 | 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 |

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

### What Is Slab?

A concrete slab is a common structural element of modern buildings, consisting of a flat, horizontal surface made of cast concrete. Steel-reinforced slabs, typically between 100 and 500 mm thick, are most often used to construct floors and ceilings, while thinner mud slabs may be used for exterior paving.

### One Way Slab and Two Way Slab

The **one**–**way slab** is supported by a beam on **two** opposite side only. The **two**–**way slab** is supported by the beam on all four sides. In **one**–**way slab**, the load is carried in **one** direction perpendicular to the supporting beam. In **two**–**way slab**, the load is carried in both directions.

### One Way Slab

**One way slab** is a **slab** which is supported by beams on the two opposite sides to carry the load along **one** direction. The ratio of longer span (l) to shorter span (b) is equal or greater than 2, considered as **One way slab** because this **slab** will bend in **one** direction i.e in the direction along its shorter span

### One Way Slab Design

**One**–**way slab** is a type of concrete **slab** in which loads are transferred in **one** direction to the supporting beams and columns. Therefore, the bending occurs in only **one** direction. The **design** of **one**–**way slab** is simple and can be carried out easily. **Slabs** are used to provide flat, useful surfaces.

### Two Way Slab Design

**Two Way Slab Design** by Direct **Design** Method as per ACI 318-11. The one-**way slab** is deflected in one **way** direction and primary reinforcement is placed in one direction whereas the **two**–**way slab** deflect in **two** directions and primary reinforcement placement is in **two** directions.

### One Way Vs Two Way Slab

In **one**–**way slab**, the load is carried in **one** direction perpendicular to the supporting beam. In **two**–**way slab**, the load is carried in both directions. The deflected shape of the **one**–**way slab** is cylindrical. Whereas the deflected shape of the **two**–**way slab** is a dish or saucer-like shape.

### Slab Reinforcement

In one way **slab** main **reinforcement** is parallel to shorter direction and the **reinforcement** parallel to longer direction is called distribution steel. In two way **slab** main **reinforcement** is provided along both direction. **Slabs** could be simply supported, continuous or cantilever.

### What’s a Two Way?

**wo**–**way** means moving or working in **two** opposite directions or allowing something to move or work in **two** opposite directions. The bridge is now open to **two**–**way** traffic.

### Difference Between One Way Slab and Two Way Slab

In **one**–**way slab**, the load is carried in **one** direction perpendicular to the supporting beam. In **two**–**way slab**, the load is carried in both directions. The deflected shape of the **one**–**way slab** is cylindrical. Whereas the deflected shape of the **two**–**way slab** is a dish or saucer-like shape.

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**Suggested Read –**

- How to Structural Design a Building/House Step by Step Part-1 (One Way Simply Support Slab)
- How to Structural Design a Building/House Step by Step Part-2 (Two Way Simply Support Slab)
- How to Structural Design a Building/House Step by Step Part-3 (Slab Beam Design)
- How to Structural Design a Building/House Step by Step Part-4 (Column Design)
- Test for Compressive Strength of Brick | Water Absorption | Dimensions Test
- What Is Raft Foundation | Type of Footing | Detail of Raft Footin

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