# 10 Difference between One way slab and Two way slab

Table of Contents

## What is a slab?

A slab is a flat, horizontal plane element in which the thickness is considerably smaller than its span and width. It is the most common bending or flexural member used in buildings, water tanks, and in bridge decks, etc.

The slabs are designed and constructed to support the load imposed on them such as walls, furniture, people, etc. They transfer these loads to the beams (or) walls supporting them.

## Types of slab

Slabs are classified into different types based on their,

### i) Shapes

Slabs can be designed and constructed in different shapes such as,

• Square,
• Rectangular,
• Circular,
• Trapezoidal etc.

The square and rectangular slabs are commonly used in building systems because of their simplicity. Circular slabs are commonly used in structures like water tanks, water treatment tanks, etc. The trapezoidal slabs are commonly used in the construction of combined footing to provide support for multiple columns.

### ii) Support conditions

Based on the end support of the slab, the slabs are classified as,

a) Simply supported slab – It is a slab that is supported on its two opposite edges and there is no additional intermediate support between them.

b) Cantilever slab –It is a slab in which one end is fixed and the other end is hanging freely beyond horizontal projection and creates an overhanging structure. It is generally used for sunshades and balconies.

c) Continuous slab – It is a slab that is spanning across multiple beams and columns. A slab which is having at least two spans or 3 supports is considered as a continuous slab.

### iii) Load distribution

Based on the dimension of the slab, the distribution of load from slab to beam varies. It is of two types namely,

• One way slab and
• Two way slab.

We’ll discuss these types in detail below.

### iv) Special types

a) Ribbed slabs – A slab that consists of closely spaced concrete ribs to increase the load-carrying capacity and to reduce weight.

b) Waffle slabs – It is a slab with concrete ribs (or) joists running in two directions forming a grid-like waffle pattern. This slab is much more stronger than the solid slabs.

c) Flat slabs – Flat slabs are generally multi-span slabs that rest directly on columns without any beams. This can increase the headroom available and provides flexibility in architectural design.

## What is one way slab?

The slabs that are supported on two opposite sides (or) on all four sides with a ratio of long span to short span exceeding 2 are referred to as one-way slabs.

If (Longer span/Shorter span) > 2, then it is a one way slab.

For example, let’s consider the following slab

Now, that you know what is one way slab, let’s explore some of its features and properties.

i) Load distribution

In nature, objects tend to adopt the easiest path to accomplish their tasks. For instance, rainwater naturally forms a round or oval shape because it is the simplest form to take.

In a similar manner, concrete structures also have a tendency to seek the path of least resistance.

In the case of a one-way slab, the load naturally chooses the easiest route for transfer, which happens to be along the shorter span. Therefore, the load instinctively follows the path of least resistance, which is through the smaller span in a one-way slab. It is called a one-way slab because the load distribution happens in one direction.

ii) Since the transfer of load happens along the shorter span, there is a bending along the shorter span direction.

iii) The main reinforcement is provided along the shorter span to counteract the bending moment produced. Since there is no bending moment along the longer span direction, we don’t need to provide main reinforcements, it is enough to provide secondary reinforcement along the longer span.

iv) One way slab only experiences deflection along the shorter span, while the longer span remains relatively straight.

v) The deflection shall be somewhat higher than in two way slabs.

vi) Since there is no need for main reinforcement along the longer span direction, the quantity of steel used in one way slab is less when compared to a two way slab.

## What is two way slab?

The slabs that are supported on all four sides with a ratio of long span to short span equal to (or) less than 2 are referred to as two-way slabs.

If (Longer span/Shorter span) 2, then it is a two way slab.

For example, let’s consider the following slab

Now, that you know what is two way slab, let’s explore some of its features and properties.

i) Load distribution – In two way slab the load is distributed to the supporting beams in both directions. The load is distributed as triangle and trapezoidal patterns.

ii) Since the transfer of load happens along both directions, the bending moment also developed along both directions. However, if both spans are not equal, then the magnitude of the bending moment developed along the shorter span is larger.

iii) Here, we provide main reinforcement along both directions and there is no need for secondary reinforcement.

iv) Two-way slab experiences deflection along both directions with a combination of upward and downward deflections.

v)  The deflection shall be lower than in one way slabs due to the flexural rigidity of the slab in a mutually perpendicular direction.

vi) The quantity of steel used in the two way slab is higher than the quantity of steel used in one way slab.

## Difference between one way slab and Two way slab

Now that you have learned the concepts. Let’s summarize their difference in a table format.

## FAQ

### What is the purpose of providing main and secondary reinforcement in the slab?

The main purpose of providing the main reinforcement in the slab is to transfer the bending moment developed at the bottom of the slab to the supporting wall or beam.

The main reinforcement is

• Placed along the shorter direction in one way slab
• Placed along both directions in two way slab

The main purpose of providing secondary reinforcement is to resist cracking, shrinkage, and temperature changes. The secondary reinforcement is also known as Distribution reinforcement.

Now that you learned all the concepts, why not try some quizzes?

## Quiz

Welcome to your One way & Two way slab Quiz.

1.
If the ratio of the longer span to the shorter span is equal to 3, then the slab should be designed as

2.
Which type of slab experiences deflection along both directions with a combination of upward and downward deflections?

3.
In a one-way slab, where is the main reinforcement provided?

4.
Which type of slab requires a higher quantity of steel reinforcement?

5.
What is the purpose of providing secondary reinforcement in the slab?

Hope you understand everything you need to know about One way Slab and Two way Slab. If you find this article helpful, We would appreciate your feedback in the comment section. Your comments will surely make our day 😊.

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### 2 thoughts on “10 Difference between One way slab and Two way slab”

1. Samuel Taiwo Olaide

Really enjoyed the way you have broken down the topic. Quite interesting and scintillating. 😜
Kindly treat topic on bottom, near face, top etc bars in reinforcement arrangement in slab, and what are their functions.?

1. I’m glad you enjoyed this article.

Regarding your question,

The slab is a flexural member which means it experiences both compression and tensile forces.

Concrete is excellent at handling compression forces but tends to be weaker against tensile forces. So we have to introduce another material which is steel reinforcement, to take care of the tensile forces.

As the slab undergoes loading, the middle zone of the slab sags downwards, creating compression forces on the top and tensile forces on the bottom. Meanwhile, the edge zone undergoes hogging action due to support reaction, creating tensile forces on the top and compression forces on the bottom.

Therefore, the reinforcement should be placed near the top, at the edge zones, and should be placed near the bottom, at the mid zones, for effectively distributing the compression and tensile forces of the slab.

I hope this explanation clarifies why these specific reinforcement placements are essential in slab construction.

If you have any more questions, feel free to ask!