Slabs in Construction: Types, Functions, and Key Differences Between One-Way and Two-Way Slabs

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

1. Simply Supported 
2. Continuos 
3. Cantilever 

Type of slabs based on support conditions are: 

1. One Way Slab
2. Two Way Slab
3. Flat Slab Resting Direction on a Column Without Beam 
4. Grid Slabs or Waffle Slab
5. 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.

or

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.

or

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?

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?

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.

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:

Elastic Analysis:

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.

Code Coefficients:

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.

Difference Between One Way Slab and Two Way Slab

Frequently Asked Questions (FAQ) About Slabs

What Is a Slab in Construction?

A slab is a flat, horizontal structural element used in construction to provide surfaces such as floors, roofs, and bridges. It is typically supported by beams, walls, or columns and carries loads uniformly across its surface.

What Are the Different Types of Slabs Based on Support Conditions?

Slabs can be classified based on support conditions into:

1. One-Way Slab: Supported by two parallel beams or walls.
2. Two-Way Slab: Supported by beams or walls on all four sides.
3. Flat Slab: Supported directly by columns without beams.
4. Grid Slab: Supported by beams with columns only on the periphery of the hall.
5. Circular and Other Shapes: Slabs in non-standard shapes for specific architectural needs.

How Do You Define a One-Way Slab?

According to IS 456:2000, a one-way slab is defined as a slab where the ratio of the longer span (L) to the shorter span (B) is greater than 2 (L/B ≥ 2). It is supported by parallel beams or walls and bends primarily in one direction.

What Is the Difference Between a One-Way Slab and a Two-Way Slab?

1. One-Way Slab: Supported by beams on two opposite sides, bending in one direction only. It is efficient for longer spans in one direction.
2. Two-Way Slab: Supported by beams or walls on all four sides, bending in both directions. It is used for more square or nearly square panels where loads are distributed in both directions.

What Is a Flat Slab, and Why Is It Used?

A flat slab is a type of slab that is supported directly by columns without the use of beams. It is used to create a clean ceiling appearance, increase floor height, and allow for larger openings and more natural light in buildings.

What Is a Grid Slab?

A grid slab is a type of slab supported by a grid of beams, with columns typically located only around the periphery of the space. It is often used in large spaces such as auditoriums or public halls where a large column-free area is needed.

How Are Slabs Analyzed in Structural Design?

Slabs are analyzed using methods similar to those for beams:

1. Elastic Analysis: Treats a strip of the slab as a beam for load calculations.
2. Code Coefficients: Uses coefficients from building codes for simplified analysis.
3. Yield Line Theory: A limit state design method that models potential failure lines in the slab.

How Thick Should a Slab Be?

The thickness of a slab depends on its type and function:

1. One-Way Slab: Typically ranges from 100mm to 150mm.
2. Two-Way Slab: Generally ranges from 100mm to 200mm.

What Are the Main Reinforcements Used in Slabs?

1. One-Way Slab: Main reinforcement is placed along the shorter span (Lx).
2. Two-Way Slab: Reinforcement is provided in both directions (Lx and Ly) to handle bending in both directions.

What Are Some Practical Examples of One-Way and Two-Way Slabs?

1. One-Way Slab: Common in chajja (overhangs) and verandahs.
2. Two-Way Slab: Typically used in the construction of multi-storey buildings and large commercial spaces.