Difference Between One way and Two way Slab

What is the Slab?

In reinforced concrete construction, the slab is an extensively used structural element forming floors and roofs. A concrete slab is the plane element having a depth d much smaller than its span and width

It may be supported by reinforced concrete beams, by masonry wall or directly by column. The difference between one way and two way slab are as follows.

Types of Slabs

Slabs are classified according to the system of supports as:

  1. One-way slabs,
  2. Two-way slabs,
  3. Flat slabs supported directly on the column without beams,
  4. Circular and non-rectangular slabs,
  5. Grid or Waffle slabs.

What is a One-way Slab?

Slab supported only on two opposite sides by rigid supports carry the load by flexure in the direction perpendicular to the supports.

The plane surface of a predominantly uniformly loaded slab deforms into a cylindrical surface in which curvature and hence bending moment develops only in one direction such slabs are called a one-way slab.

One way slab essentially acts as shallow beams with a large width. The slabs supported on all four sides also behave as one-way slabs if the longer span ratio to shorter span is greater than two.

Difference Between One way and Two way Slab
Deformation of One way Slab

Also, ReadWhat is Concrete Cover? – Clear Cover, Nominal Cover and effective Cover.

What is Two-way Slab?

The rectangular slabs supported on two adjacent, three or four edges, and with the ratio of longer to shorter span less than two, which carry the load by flexure in two perpendicular directions are called two-way slabs.

Difference Between One way and Two way Slab
Deformation of Two way Slab

The slabs which are supported directly on the column without beams or girders are called flat slabs

In this type of slabs, large moments and shear forces are produced in the columns’ vicinity. Therefore, the slabs are thickened in the area enclosing the column. 

The columns are flared at the top to reduce stress due to shear force and negative bending moment. These arrangments are referred to as drop panels and column heads, respectively. 

When the spans of the slab are not large and loads are not excessive, both the panel drop and column head may be omitted. 

Such a slab is called a flat plate. A flat slab with a recess formed(by removable and filler blocks) on the soffit so that it comprises a series of ribs in two directions called waffle slabs

The slabs supported on a series of intersecting long grid beams is called slab grid.

How do we calculate a one way or two-way slab?

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

(Longer span / Shorter span) L / b ≥ 2

If the L/b ratio is less than 2 then it is considered a two-way slab.

(Longer span / Shorter span) L / b < 2

Also, Read – Bar Bending Schedule (BBS) – Importance, Advantages, Preparation

Difference Between One way Slab and Two way Slab

Difference Between One way and Two way Slab
Load transfer Mechanism of one way and Two way slab
S.No         One-way Slab          Two-way Slab
1If L/b the ratio is greater than or equal to 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.
2In a one-way slab, the main reinforcement is provided in a short span and distribution reinforcement is provided in a long span.In a two-way slab, the main reinforcement is provided in both directions.
3In a one-way slab, the crank is provided in two directions.In a two-way slab, the crank is provided in four directions.
4The one-way slab is supported by a beam on two opposite sides only.The two-way slab is supported by the beam on all four sides.
5In a one-way slab, the load is carried in one direction perpendicular to the supporting beam.In a two-way slab, the load is carried in both directions.
6The 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.
7Chajja and Varandha are practical examples of one-way slabWhereas two-way slabs are used in constructive floors of the Multistorey building.
8While designing a one-way slab we provide less steel hence the depth of the slab increases, as a result, the thickness of the one-way slab is more as compared to the two-way slab.While designing a two-way slab we provide more steel hence the depth of the slab decreases, as a result, the thickness of the two-way slab is less as compared to the one-way slab.
9The one-way slab is economical up to a span of 3.6 meters.Whereas the two-way slab is economical for the panel sizes up to 6m × 6m.
10In one-way slab quantity of steel is less.In a two-way slab quantity of steel is more as compared to the one-way slab.
11In a one-way slab, bending is only in one direction i.e. in a shorter span.In a two-way slab, bending is in both directions.

In practice, the choice of the one-way slab and two-way slab for a particular structure will largely depend upon the 

  1. Economy 
  2. Buildability 
  3. Loading condition and 
  4. length of the span 

Also, Read How to Calculate Cutting Length of Stirrups For Beam and Column?

One way Slab Reinforcement details

Difference Between One way and Two way Slab
One way Slab
Difference Between One way and Two way Slab
One Way Slab
one way slab and two way slab

L/B = 10/4 = 2.5 ≥ 2

From the above drawing, it is clear that the L/B ratio is greater than 2, hence it is a one-way slab. So the main bar or cranked bars are provided in a shorter span and Distribution bars are provided in the longer span.

Two way Slab Reinforcement details

L/B = 6/6 = 1 < 2

From the below drawing, it is clear that the L/B ratio is less than 2, hence it is a two-way slab, that’s why main bars or cranked bars are provided at longer span as well as shorter span also.

Difference Between One way and Two way Slab
Two way Slab
Difference Between One way and Two way Slab
Two Way Slab
one way and two way slab

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Also, Read

What is Development Length? – Complete Guide

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Pre-stressed Concrete – Definition, Methods, Advantages, Disadvantages

Difference Between Pre Tensioning and Post Tensioning

14 thoughts on “Difference Between One way and Two way Slab”

  1. I am little confused.
    Slab mean ? total of house slab or segment in between column.
    Can roof of the floor is called a single slab
    Or
    A roof consist of multiple Slab ?
    I am not an engineer , drawing for my house on 30×45 feets area. and confused due to one-way and two way slab destinations.
    IF full roof is a slab and I have columns and beams in left and right and center then what kind of slab it will be for above mentioned area.

    Reply
    • yes, slab means the total of the house slab but, we design it as the segment between the beams and columns according to the size of rooms ( one way or two way). Your house whole slab may be a combination of one way and two-way slab depend upon rooms size. The topmost slab is called the roof slab and the topmost slab of the building that is accessible to residents is called terrace slab.

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  9. Dear friends, I have a question about calculating The one-way slab my question is do I have to take only the clear span length, or do I have to take it with the thickness Of the column too ?!

    Reply

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