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.


Flat Slab Figure 1-1

1. 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




2. What is column capital & drop panel?


Flat Slab Figure 2_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.



3. How to create a flat slab in nGen


3.1. Determine the thickness of the flat slab

flat slab_modeling_2(1)



[ACI 318M-14]

Table - Minimum Thickness of Non-Prestressed Two-Way Slabs without Interior Beams(mm)


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



3.2. Create the flat slab in nGen


How to Create Flat Slab in nGen Example

Member > Flat/Plate Structure > Flat Slab

❶ Select Member Set.

❷ Select Story Set.

❸ Select Thickness of 0.2m.

❹ Select ‘by Draw’ as the creating type.

❺ Click the corner point of target area.

❻ Click ‘Apply’ or push Enter key.



3.3. How to determine the size and thickness of the drop panel.


  1. How to determine the size of the drop panel

Flat Slab_Reference 2. Section 8.2.4 of ACI 318M-14Flat Slab_Figure 5. Drop Panel ModelFlat Slab_Figure 6a. Determining Size and Thickness of Drop Panel Model





  1. How to determine the thickness of the drop panel
    Flat Slab_Determining Size and Thickness of Drop Panel Model

The two conditions below must be satisfied.

i) The thickness of drop panel below shall not be greater than one-fourth of the face of the column.


Flat Slab_Reference 3. Section of ACI 318M-14

ii) The thickness of the drop panel shall project below the slab at least one-fourth of slab thickness.


Flat Slab_Reference 4. Section 8.2.4 of ACI 318M-14

So, the thickness of the drop panel is assumed to be 0.40m (≤ ds + Max. hd = 0.2m+0.25m).



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

Subscribe S.O.S Newsletter

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.