Difference Between One Way Slab and Two Way Slab

Difference Between One Way Slab and Two Way Slab | What Is Slab | What Is a One Way Slab | What Is a Two Way Slab

What Is Slab?

The slab is an important structural element that is constructed to create horizontal or flat surfaces such as roofs, floors, decks, and ceilings. A slab has generally some thickness and it is supported by other structural elements like RCC columns, RCC beams, walls, or the ground surface.

Difference Between One Way Slab and Two Way Slab
One Way and Two Way Slabs

The slab is made up of cement concrete with top and bottom surfaces are parallel or near so. The depth of a concrete slab is very small compared to its span or length.

There are two major types of slabs used in building construction one-way slabs and two-way slabs. In this article, we will discuss what is the Difference Between One Way Slab and Two Way Slab


What Is a One-Way Slab?

According to IS 456:2000, the slab is called one way slab when the ratio of longer span(L) to shorter span(B) which is (L/B) greater than 2. In general practices, a One-way slab is supported by two parallel walls or beam.

One Way Slab
One Way Slab

The flat surface of a mainly uniformly loaded slab deforms in a cylindrical surface in which the curvature therefore the bending moment develops in only one direction, such a slab is known as the one-way slab.

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

Verandah is the best example to recognize the one-way slab, where the slab is traversing in the shorter direction with main reinforcement and the distribution of reinforcement placed in the opposite direction.

Read More: How to Calculate Steel Quantity For Slab Excel Sheet


What Is Two Way Slab?

Two-way slabs are slabs that are supported on all four sides. In this slab, the load will be transferred in both directions, thus main reinforcement is provided in both directions for slabs. In the two-way slab, the ratio of longer span to shorter spans less than two, which carries the load by flexing in two perpendicular directions.

Two Way Slab
Two Way Slab

In the case of the two-way slab, both the longer span and shorter span deflects in dish or saucer shape.

In this kind of slab, shear forces and large moments are initiated in the vicinity of the column. Hence, the thicknesses of slabs are provided more nearby the column.

These types of slabs are mainly used on the floors of multistoried buildings.


What Is Flat Slab?

A flat Slab is an economical type of slab supported through concrete columns rather than the use of a beam.

Flat slabs have a wide range of uses. Various Industrial Structures, Hotels, parking garages, Warehouses, ramps, and tall buildings are the components where they can be used.

It serves the purpose of permitting a large amount of light obstructed by the depth of beams, by increasing the floor height.


What Is Grid Slab?

Grid Slabs are supported on the beams with columns only on the edge of the hall.

 In places such as large halls, auditoriums, public places, marriage halls, etc. column-free area is required on large scale, hence to serve this purpose large deep beams may be allowed but the periphery is the permitting element.


Analysis of The Slab

Slabs are analyzed and designed in the same manner as the beams. The reason is that the slabs are primarily flexural members as that beams. The analysis is carried out as follows:

Elastic Analysis:

1 m width strip of the slab is considered and analyzed. Loads acting on this strip are calculated. This strip is analyzed as a dummy beam 1 m in width.

Code Coefficients:

Being a semi-empirical method its analysis based on the Yield Line Theory. The coefficients provided in the code can be trusted directly to analyze the slabs.

Care should be taken that the redistribution of moments is not permitted in this case.

Yield Line Theory:

Yield line theory is a limit state design or collapse load method or limit state design developed by Johnson.

Read More: Load Calculation on Column, Beam & Slab


Significance and Function of Slab

Depending upon the requirement and the design of the structure, the slab can be utilized for some major functions and has significance as follows:

  • In the reinforced concrete (RCC) construction, the slab is a widely used structural element forming the floor and the concrete slab is the plane element having a depth D much smaller than its span or width L.
  • Slab bears the loads acting on it and properly transfers the forces either in a one-way or two-way mechanism.
  • A slab provides a flat surface or a walking surface.
  • Slab gives support to the loads acting on the structure.
  • The slab also constructs a divider between the consecutive floors thereby providing privacy to the occupants of the building.

Mechanism of Load Transfer In Slab

The forces transfer from slab to beam occur in one way or two ways. The whole system completely depends on the geometrical dimensions of the slab.

If slabs may be supported by columns only, in this case, two-way action will prevail.

If the ratio of long side / short side > 2 it is considered as one-way slab, and if longer side to shorter side ratio < 2 then it is considered as a two-way slab.

The load transfer mechanism from slab floor to supporting elements for one-way slab and two-way slab is shown in the figure.


Reinforcement Details In Slab

Reinforcement arrangement in a slab is the main parameter to understand the system of forces and safely distribute the loads coming on the slab.

One Way Slab Reinforcement Details

Commonly a One-Way Slab, as one side is greater than the other one, the utmost load will be transmitted by the larger side. Accordingly, it is necessary to contribute sufficient support to the larger side.

One Way Slab Reinforcement
One Way Slab Reinforcement

To provide sufficient support to the extended side, the main reinforcement bars are placed parallel to the shorter side and the distribution bars are placed on the longer side which does not support transmitting the load.

So, a one-way slab is chosen where main bars or cranked bars are provided on the shorter side of the slab due to bending. Distribution bars (straight bars) provided on the longest side as shown in the figure.


Two Way Slab Reinforcement Details

In the 2 way slab, as the loads are acting in both the direction i.e. longer and shorter direction.

Two Way Slab Reinforcement
Two Way Slab Reinforcement

In this type of slab, the main reinforcement bars are laid in both directions. The loads held up by two sides in this type of slabs are equal.

You can recognize the main bars are placed on both sides. The RCC slab is held up by beams on every four sides and the loads are upheld by structural components such as beams along with both directions.

Slab with reinforcement steel in both directions is more practicable and supportive than one-way reinforcement slabs.


Types of Loads Acting on Slab

There are different load that act on a slab,

  • The dead load of the slab
  • Live load
  • Floor finish load
  • Snow load in the case of roof slab
  • Earthquake loads

Read More: What Is Plinth | What Is a Plinth Beam | What is Height of Plinth | What is Plinth Protection | Purpose of Plinth Protection


Difference Between One Way Slab and Two Way Slab

Sr. No.One-Way SlabTwo-Way Slab
1.The utmost load is transmitted by the larger side.The loadings are imposed and held up in both directions i.e. longer and shorter directions.
2.Bending moment occurs only in one direction i.e. shorter direction.Bending moment occurs in both directions i.e. shorter and longer.
3.One way slabs have less steel content.Two way slabs have more steel content as compared to one way slabs.
4.The main reinforcement bars are placed parallel to the shorter side and the distribution bars are placed on the longer side which won’t support transmitting the loads.In this type of slab, the main reinforcement bars are placed in both directions i.e. shorter and longer.
5.The slab is supported by other structural elements such as beams on the two different sides to bear the load and transmit it along one direction.A RCC slab is upheld by beams on all four sides and the loads are supported by the structural elements such as beams along with both directions.
6.The ratio of the longer span (ly) to the shorter span (lx) must be equal to or greater than 2.The ratio of the longer span (ly) to the shorter span (lx) must be less than 2.
7.It is economical near about 3.5 m.It is practicable for the panel sizes of approximately 6m x 6m.
8.In designing one-way slabs, we placing less steel so the depth of the slabs increases, therefore the thickness of the one-way slabs is larger than that of two-way slabs.In designing a two way slab, we placed more steel so the thickness of the slabs is reduced, hence the thickness of the two way slabs is less than that of the one-way slab.
9.The deflected structure is cylindrical.The shape of deflected structure is like a dish or saucer.
10.Example: Cantilever slabs, Chhajas, and verandahs.These kinds of slabs are mainly utilized on the creative flooring of multi-story buildings.

FAQs

What Is Slab?

The slab is a common structural component of buildings, consisting of a flat, horizontal surface made of cast concrete. Slab bears the loads acting on it and properly transfers the forces either in a one-way or two-way mechanism.

One Way Slab and Two Way Slab

In the One Way Slab, the utmost load is transmitted by the larger side. In the Two Way SlabThe loadings are imposed and held up in both directions i.e. longer and shorter directions. The amount of steel Steel content required in one-way slab is less as compared to a two-way slab.

One Way Slab

One Way Slabs are supported on two different and opposite sides by the rigid supports, carrying the imposed load by flexing in a direction perpendicular to that of supports.
It essentially acts as a shallow beam with a large width hence, Verandah is the best example to recognize the one-way slab.

One Way Slab Design

Designing a one-way slab is a simple task and can be carried out easily. Arranging reinforcement in one way slab is the main parameter to understand and design as per the system of forces.

Two Way Slab Design

Two Way Slabs can be accurately Designed by Direct Design Method as per ACI 318-11. Deflection of Two-way slab is in two directions along with primary reinforcement placement. Here the main reinforcement bars are laid in both directions to tackle two-way deflection.

One Way Vs Two Way Slab

The basic difference between One Way and Two Way Slab is that the bending moment occurs only in one direction i.e. the shorter direction in One Way Slab while in Two Way Slab bending moment occurs in both directions i.e. shorter and longer. The deflected structure is cylindrical in one way and the country shape of the deflected structure is like a dish or saucer in Two Way Slab.

Slab Reinforcement

Reinforcement is laid parallel to the shorter direction and the reinforcement parallel to the longer direction is distribution steel in one way slab. While talking about reinforcement in Two Way slab it is laid in both the direction.

What’s a Two Way?

Something which is bi-directional is known as Two Way. Working or allowing something to move in two opposite directions is said to be Two Ways. For example, The bridge is now accessible for two–way traffic.

Difference Between One Way Slab and Two Way Slab

Difference Between One Way Slab and Two Way Slab:
Bending moment occurs only in one direction i.e. the shorter direction in One Way Slab while in Two Way Slab bending moment occurs in both directions i.e. shorter and longer. The deflected structure is cylindrical in one way and the country shape of the deflected structure is like a dish or saucer in Two Way Slab.


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