How to design Flat Slab?

August 24, 2021


A flat slab is a two-way reinforced concrete slab that usually does not have beams and girders, and the loads are transferred directly to the supporting concrete columns.


The flat plate is a two-way reinforced concrete framing system utilizing a slab of uniform thickness, the simplest of structural shapes. The flat slab is a two-way reinforced structural system that includes either drop panels or column capitals at columns to resist heavier loads and thus permit longer spans.


Advantages of Flat Slab

1.  Flexibility in room layout

   - Partition walls can be placed anywhere.

   - Offers a variety of room layouts to the owner.

   - False ceilings can be omitted.

2. Reinforcement placement is easier

   - As reinforcement detailing of the flat slab is simple, it is easier to place

3. Ease of Framework installation.

   - Big table framework can be used in flat slab

4. Building height can be reduced.

   - Floor height can be reduced and consequently the building height will be reduced.

   - Approximately 10% of the vertical member could be saved.

   - Foundation load will also reduce.

5. Less construction time

   - Use of a big table framework helps to reduce construction time


What is column capital & drop panel?


flat slab_modeling_1Uses of column heads (capital): 

⋅ Shear strength of flat slab is increased by using column heads.

⋅ Column heads reduce the clear or effective span, and therefore, reduce the moment in the flat slab floor.


Uses of drop panels:

⋅ Drop panels increase the shear strength of flat slab floor.

⋅ Drop panels increase flat slab's negative moment capacity.

⋅ Drop panels reduce deflection by stiffening the flat slabs.

How to determine the thickness of the flat slab


flat slab_modeling_2(1)


When using fy=420Mpa,

In case of flat slab with drop Panel, Slab thickness = Ln/36 = 7.0m/36 = 0.194m → Apply0.20m

* I will set the drop panels in this example

In case of flat slab without drop Panel, Slab thickness = Ln/ 33 = 7.0m / 33 = 0.212m → Apply 0.22m










flat slab_modeling_2(2)

How to determine the size of the drop panel.




Please download the white paper of Structural Design Guide for Flat Slabs below to see the full contents. 

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About Author
Young-il Seo | Principal Structural Engineer

Young-il has over 13+ years of experience in building design, especially high-rise buildings with column reduction analysis, plant structures, pushover analysis, health monitoring, and vibration control projects. Since 2016, he is planning and providing technical supports for midas building products such as midas Gen, nGen, and Design+.

E-BOOK Structural Design Guide
for Flat Slabs

This Design Guide provides a comprehensive guide for Flat Slab design
including design examples.

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