What Is Slab?
In the structural system of buildings, slabs are an important element of the floor system. Slabs usually transfer loads such as gravity loads, dead loads, and live loads to a vertical framing system such as columns. Slabs are used as floor and ceiling in various buildings.
Slabs cover a large area of concrete in a building. So during an earthquake, various external forces are attracted to the surface of the slab.
Slabs are represented by different designs in different types of compositions. Such as to obtain flat surfaces in a particular building and as a deck slab of a bridge. The slab is usually formed by the structure of a reinforced concrete beam.
The two main types of slabs used in the construction of a building are widely used as one-way slabs and two-way slabs. In this article provides information about the difference between a one-way slab and a two-way slab.
Function of Slab
Depending on the design as per the requirement of the structure, some of the main functions of the slab are shown below.
- The slab provides a platform to work on the structure.
- The one-way slab or two-way slab method is properly used to transfer the load on the slab.
- The slab provides a flat surface to work with easily.
- The design of multi-story building has been made possible by slabs.
- The slab can be used as a sound, heat as well as a fire insulator.
- The slab helps to keep the room private.
Type of slabs
Based on support conditions,
- One Way Slab
- Two Way Slab
- Flat Slab Resting Direction on a Column Without Beam
- Grid Slabs or Waffle Slab
What Is One-Way Slab?
A one-way slab is designed in such a way that the load on the beam carries in one direction. One way slab is usually used for a short span.
According to IS 456: 2000, the ratio of long span (L) to short span (B) in one way slab is greater than 2.
Longer Span / Shorter Span ≥ 2.
One-way slabs can be constructed only on two parallel beams or walls in building construction. One-way slabs are commonly used for short spans in a building.
Reinforcement for one-way slabs is arranged by the main reinforcement in a short span and distribution reinforcement in a long span.
Some examples of one-way slabs include cantilever slabs, chajja, and verandas.
The supported slab on each of the two walls is always supported by one slab.
A slab supported by less than 4 sides in a slab of Structure in a building can be designed as a one-way slab.
All cantilever slabs are a simple example of a one-way slab.
What Is Two Way Slab?
Two-way slabs are supported from all four sides. The load is transferred in both directions through this slab. The main reinforcement for the structure of a two-way slab is given in both directions.
A two way slab is designed when the ratio of long span to short span is less than two. This type of slab carries the load in two directions.
The thickness of the two away slab depends on the deflection control criteria. In this type of slab, the reinforcement in the directions is designed to resist the bending moments.
Two-way slabs are commonly used to suit the needs of multi-story and commercial buildings.
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What Is Flat Slab?
A flat slab is a type of reinforced concrete slab. There are no beams and girders as support for the formation of a flat slab. In this type of flat slab, the load is transferred directly to the concrete columns.
Flat slabs are used by engineers for the construction of the building as required in different situations.
Some of the advantages of the flat slab are as follows,
Due to the flat slab can be properly planned for partition walls.
Using a flat slab for a building can give the owner the opportunity to choose a room layout as needed.
Using a flat slab in a building eliminates the need for false ceilings.
The floor height can be increased as no beams are used in the flat slab.
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What Is Grid Slab?
For a large room slab, a slab is formed over a number of beams on the column at the corner end. This type of slab structure is called a grid slab.
Sometimes grid slabs are designed for large column-free areas in large halls, public places, auditoriums, marriage halls, etc. For the formation of this type of slab, deep depth may be allowed for the column to column main beam.
Analysis of Slab
The structural design of the slab is the same as that of the beam. The slab is analyzed as follows,
The 1 m wide strips of the slab is analyzed as a base. The load on this base is calculated. Thus, a 1 m strip of the slab is analyzed by assuming a dummy beam as 1 m.
The slab is analyzed by yield line theory. The analysis of the slab can be directly relied on as a coefficient as shown in the code.
The calculation of redistribution of moments is not taken into account for slab analysis.
Yield Line Theory
Yield line theory is a method of analysis based on the final bending moment for the edge of a fall for a slab.
At collapse loads, the slab starts to crack because the slab is usually under-reinforced.
Reinforcement Details In Slab
The design of reinforcement in the slab is used to safely distribute the forces coming on the slab to the column.
One Way Slab Reinforcement Details
- In a one-way slab, the load is transferred from one side to the other. In this type of slab, the maximum load will be transmitted in a long span. Therefore it is necessary to give proper reinforcement on the long span side.
- So in a one-way slab to provide adequate support to the long span side, the main reinforcement is placed parallel to the short span by proper spacing, and the distribution reinforcement is placed on the long side according to the correct spacing.
- Therefore, one-way slabs are preferred over the short span of the slab. Where main reinforcement is given on long-span due to bending. Reinforcement of one Way Slab is given as shown in the figure.
Two Way Slab Reinforcement Details
- In a two-way slab, the applied external load is transferred to both sides in a long span and short span.
- In two way slab, the main reinforcement is laid on both sides. In this type of slab, the load on both sides is evenly distributed.
- In RCC two Way slab each of the four sides is held on the beam by main reinforcement. A load of a two-way slab is transferred through structural elements such as beams.
Types of Loads Acting on Slab
There are different loads on the slab. This is shown as follows
- Wind load
- Live load
- The dead load of the slab
- Earthquake loads
- Snow load
Difference Between One Way Slab and Two Way Slab
1. One Way Slab Vs Two Way Slab: Definition
- In a one-way slab, the length of the long span is greater than the ratio of the short span to two.
- In a two-way slab, the length of the long span is less than or equal to the ratio of the short span.
2. One Way Slab Vs Two Way Slab: Support
- One-way slab Only two opposite sides are supported by a beam or wall.
- The two-way slab is supported from all four sides.
3. One Way Slab Vs Two Way Slab: Transfer load
- One Way Slab, the utmost load is transmitted by long span.
- In Two Way Slab load is transferred in both span (long span and short span).
4. One Way Slab Vs Two Way Slab: Require Reinforcement
- A one-way slab requires less reinforcement than a two-way slab.
- Two-way slabs require more reinforcement than one-way slabs.
5. One Way Slab Vs Two Way Slab: Bending Moment
- In one way slab, a bending moment occurs in a short span in only one direction one-way slab, a bending moment occurs in a short span in only one direction.
- Bending moment in two-way slab occurs in both direction longer span and shorter span.
6. One Way Slab Vs Two Way Slab: Shape
- The shape of a one-way slab is usually rectangular.
- The shape of a two-way slab is usually square.
7. One Way Slab Vs Two Way Slab: Direction of Load Transfer
- In one way slab, the load is transferred in one direction.
- In a two-way slab, the load is transferred in both directions.
8. One Way Slab Vs Two Way Slab: Cost
- One-way slabs are economically less expensive than two-way slabs.
- Two-way slabs are economically more expensive than one-way slabs.
9. One Way Slab Vs Two Way Slab: Deflection
- Deflection in a one-way slab gives it a cylindrical shape.
- Deflection in a two-way slab makes it look like a bowl.
10. One Way Slab Vs Two Way Slab: Thickness
- One way slab thickness can be designed from 4 inches to 6 inches (100 mm to 150 mm) depending on deflection and span.
- The thickness of the two-way slab can be designed from 4 inches to 8 inches (100 mm to 200 mm) depending on the deflection and span.
11. One Way Slab Vs Two Way Slab: Reinforcement Placing
- In one way slab, main reinforcement is placed in a short span and distributions reinforcement is placed in long span.
- In two way slab, main reinforcement is placed in short span and long span.