What Is Slab?
A slab is constructed to provide flat surfaces, typically horizontal, in building roofs, floors, bridges, and other types of structures. The slab could be supported by walls, by reinforced concrete beams normally cast monolithically with the slab, by structural steel beams, either by columns or from the ground.
A slab is a plate element having depth (D), very small as compared to its length and width. A slab is used as floor or roof in buildings, carry distribution load uniformly.
Type of Slab b
Slab May Be
- Simply Supported
Type of slabs based on support conditions are:
- One Way Slab
- Two Way Slab
- Flat Slab Resting Direction on a Column Without Beam
- Grid Slabs or Waffle Slab
- Circular Slab and Other Shapes
What Is a One Way Slab?
According to IS 456:2000, The ratio of longer span(l) to shorter span(b) which is (L/B) greater than 2 is known as One way slab. In practical, One way slab is supported by only two parallel beams or walls. Normally we don’t use one way slabs often.
One Way Slab
The most straightforward routine structural element for illustration of design provisions from the Code is that the one way slab.
A one way slab is defined for functions of the book as a flexural member with thickness small relative to other dimensions, supporting (gravity) loads applied normal to and directly above its surface, a span in one direction between parallel supports, and fortified for flexure in this direction only.
For purposes of analysis, one way slabs might be restrained to some degree in the supports or possibly unrestrained. A number of Code provisions reference to “flexural members,” including one and two way slabs, beams, girders, footings, as well as where bending is present together with the axial walls, load, and columns.
In general, when this code provision is intended to use to one way slabs, the term is going to be utilized in the sense of this definition herein.
If the is supported on two opposite sides, it is called a one way spanning slab. In this type of slab, loads are transferred on two opposite as per the above figure. A
If the slab is supported at four sides, and if Ly/Lx ≥ 2 one way spanning slab.
For any slab, if Ly = Lx, the slab has a tendency to bend in both directions. With an increase of Ly, the tendency of bending along Ly is reduced and that on Lx is increased.
When Ly/Lx ≥ 2, the slab bends only in X directions
When Ly/Lx ≥ 2, the slab is called a one way slab. In one way slab, the main reinforcement is provided along Lx ( Short Span)
What Is a Two Way Slab?
A two way street is a street that allows vehicles to travel in both directions. On most two way streets, especially main streets, a line is painted down the middle of the road to remind drivers to stay on their side of the road. Sometimes one portion of a street is two way and the other portion is one way.
The design considerations of wall supported two way slabs are similar to those pertaining to one way slabs.
The thickness of the slab is generally based on deflection control criteria, and the reinforcements in the two orthogonal directions are designed to resist the calculated maximum bending moments in the respective directions at the critical sections. [Additional reinforcement may be required at the corners of two way slabs in some cases, as explained later].
The slab thickness should be sufficient against shear, although shear is usually not a problem in two way slabs subjected to uniformly distributed loads.
If the slab is supported at all four edges and if Ly/Lx < 2,
The tendency of the slab is to bend in both directions. Such slab is called two way slab as per above figure c
In two way slabs, main reinforcement is provided along Lx as well as Ly direction
What Is the Flat Slab?
When the slab is directly supported on a column, without beams, it is known as a flat slab.
Flat slab is provided to increase the floor height and to permit a large amount of light which might be obstructed by the depth of beams.
What Is Grid Slab?
When the slab is supported on beams with column only on the periphery of the hall, the slab is called grid slab. Sometimes, in large halls, public places, marriage halls, auditoriums, etc. a large column free area is required.
In these cases, large deep beams may be permitted, but the columns are permitted only on the periphery.
Also, read: Procedure For Rcc Concrete
Analysis of Slab:
Slabs are primarily flexural members as beam and are analyzed and designed in the same manner as the beams. The analysis may be carried out as follows:
A strip of 1 m width of the slab is considered, and loads are found on this strip. This strip id analyzed as a beam 1 m width.
This is a semi empirical method of analysis based on yield line theory. The coefficients give in code may be directly used to analyze the slabs.
However, the redistribution of moments is not permitted in this case.
Yield Line Theory
This is a limit state design or collapse load method developed by Johanson.
Also, read: Core Cutter Method
Difference Between One Way Slab and Two Way Slab
|Sr No.||One Way Slab||Two Way Slab|
|1||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.|
|2||In one way slab, the load is carried in one direction perpendicular to the supporting beam.||In two way slab, the load is carried in both directions.|
|3||One way slab two opposite side support beam /wall||Two Way Slab four side mins all side supported beam /wall|
|4||One way slab is bend only in one spanning side direction while load transfer||Two way slab is bend both spanning side direction while load transfer|
|5||One way slab is bend only in one spanning side direction while load transfer||In two-way slab, the crank is provided in four directions.|
|6||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.|
|7||In one-way slab, the load is carried in one direction perpendicular to the supporting beam.||In two-way slab, the load is carried in both directions.|
|8||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.|
|9||Chajja and Varandha are practical examples of one-way slab.||Whereas two-way slabs are used in constructive floors of the Multistorey building.|
|10||In 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||Main Reinforcement is in provide short span due to banding.||Main Reinforcement is in provide short span due to banding|
|12||Ly/Lx ≥ 2 one way slab spanning.||Ly/Lx < 2 two way slab spanning|
|13||One way slab near about 100mm to 150mm based on the deflection.||two way slabs is in the range of 100mm to 200mm depending upon|
|14||one way slab economical near about 3.5 m.||Two way slab may economical for the panel sizes near about 6m x 6m.|