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SLABS : One-way slab and Two way slab

SLAB:

Slabs are structural elements, made of concrete, which are used to form floors, roofs, and ceilings in buildings. Slab carry uniform loads which are distributed through beams, columns, walls on which it is supported.
Slabs may have a solid uniform thickness or with ribs running from one or two directions.

Slabs differentiate as per the method of support used. They are as given below:
One-way slab: Spanning one way between the support walls or beams.
Two-way slab: Spanning two way between the support walls or beams.

Types of Slabs:
ONE-WAY SLAB
This type of slab has to support on two opposite sides, therefore, the action i.e load is only in one direction which also means the load transfer direction is perpendicular to the support beams. 

The thumb rule to identify one-way slab is as follow:
If a slab is supported on all the 4 aspects but the ratio of longer span (l) to shorten span (b)  is more than 2, then the slab may be considered as a one-way slab. Because due to the huge difference in lengths, the load isn't transferred to the shorter beams. The main reinforcement is furnished in the handiest one path for one-way slabs. 

One way slab is further differentiated into the following type:

Ribbed Slabs:
It is a system that consists of a chain of parallel reinforced concrete T beams framing into reinforced concrete girders. The slab is the flange of the beam and the extended part is the web. This prolonged component is known as ribs. It is economical for medium spans with small to medium loads.



TWO-WAY SLAB:
Two-way slabs are the slabs that can be supported on four aspects and the ratio of longer span (l) to shorter span (b) is less than 2. In two-manner slabs, the load might be carried in each the guidelines. So, predominant reinforcement is supplied in both paths for 2-way slabs.

Two-way slabs are of the following types:


Waffle Slabs: 
It's a two-way slab reinforced by using ribs in two dimensions. Able to hold heavier loads and span longer than ribbed slabs.


Flat slabs: 
Slabs of uniform thickness which are bend and reinforced in two directions and are supported directly by columns without beams.
Also read: Thumb Rules for Civil Engineers

Flat slabs with drop panel:  

Drop panel is a region in the flat slab which is provided to avoid a shear punch in the slab which generally occurs in and around the area of the column. It is generally  suitable for heavily loaded span.

LOAD TRANSFER ON ONE-WAY & TWO-WAY SLAB:
Difference between One-way slab and two-way slab:


S.No
One-way Slab
Two-way Slab
1
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.
2
In a one-way slab, the crank is provided in two directions.
In a two-way slab, the crank is provided in four directions.
3
In 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.
4
In 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.
5
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.
6
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.
7
While designing 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 two-way slab we provide more steel hence the depth of slab decreases, as a result, the thickness of two-way slab is less as compared to the one-way slab.
8
The 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.
9
In 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.
10
In a 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
Terrace and Varandha are practical examples of one-way slab
Whereas two-way slabs are used in constructive floors of the Multistory building.

In practices, 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 
  4. Length of span

Reinforcement details of One way Slab:


Here L/B = 10/4 = 2.5 ≥ 2
From the above drawing it is clear that L/B is greater then 2, hence it is a one-way slab. So the main bar or cranked bars are provided in shorter span and distribution bars are provided in the longer span.

Reinforcement details of Two way Slab:

Here L/B = 6/6 = 1 < 2
From the above 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 a longer span as well as shorter span also.
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