# 12 Difference Between One way and Two way Slab

Table of Contents

## 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.

## 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.

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 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

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

## One way SlabReinforcement details

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 SlabReinforcement 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.

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Also, Read

What is Development Length? – Complete Guide

What is Lap Length? How to Calculate it? – Complete Guide

Difference Between Plinth Level, Sill Level and Lintel Level

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

Difference Between Pre Tensioning and Post Tensioning

### 24 thoughts on “12 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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10. 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 ?!

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• You have to take the slab’s clear span (Length & Width) to calculate a one-way or two-way slab.

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11. If A Building Has Canteliver All Round Is It A One Way Or Two Way Slab

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• Usually, all the Cantilever slabs are designed as one-way slabs.

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