two way slab

Two Way Slab

In two way slab, the ratio of the longer span to the shorter span is less than two i.e.

 Longer Span/Shorter Span <2

The load is carried in both directions in a two-way slab. So, the main bars are provided in both directions.

Two-way slabs are extensively used in the multi-storey and commercial buildings.

 

 

Types of Two Way Slab

There are 3 types of two way slab. They are:

 

1. Two Way Solid Slab ( with beams)

This slab can be used for a wide variety of loading systems. It is suitable for small to large scale constructions.

 

2. Two Way Waffle Slab (with beams)

This slab system is used for office buildings (low rise), warehouses, parking structures, etc.

 

3. Two Way Waffle Slab (with Integral Beams)

This slab system is provided with a greater span as compared to the solid slab. It is suitable for modular constructions.

 

 

Reinforcement Detailing of Two Way Slab

 

two way slab

Figure: Reinforcement Detailing of Two Way Slab

The reinforcement detailing of a two-way slab is shown in the figure above. As shown, the ratio of longer to shorter span is less than two and main anchored bars are provided in both directions.

 

Advantages of Two Way Slab

a. Availability of flat ceiling because of the absence of beam in the middle of the cell.

b. More availability of the height of the room.

c. Increase in the aesthetic beauty of the room.

d. Reflection of light is not obstructed.

 

 

Disadvantages of Two Way Slab

a. Less economical as compared to one way slab.

b. More complex to design and construct.

c. The requirement of skilled workers for construction.

 

 

Steps in Two Way Slab Design

a. Choose layout and type of slab. Type of slab is affected by construction as well as architectural considerations.

b. Choose the slab thickness that is sufficient to control deflection. Also, check if the thickness is sufficient for shear.

c. Choose the design method:

  • Direct Design Method: Determine positive and negative slab moments using coefficients.
  • Equivalent Frame Method: Determine positive and negative slab moments using elastic frame analysis.

d. Determine the distribution of moments across the slab width on the basis of geometry and beam stiffness.

e. Allocate a part of the moment to beams, if present.

f. Design reinforcement for moments from steps d and e. Steps c – f needs to be done for both principal directions.

g. Check shear strengths at the columns.

 

 

Example of Design

Mostly, the direct method is used for two way slab design. So we have provided you with example PDF of the direct method.

Direct Method ExampleDownload

 

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