Secrets behind Flat Slab and Waffle or Ribbed Slabs are way more interesting than actually seeing one. This answer will be easy if I make you understand with a Building Structure. Considering a Building structure (STRUCTURALLY) we generally found the following components
- Slabs supported on Beams
- Beams supported on Columns
- Columns supported on Foundation
- Foundation supported on Soil
It is interesting over here to note that the flow of load also occurs according to points mentioned above through the structural members starting from the point 1 through point 5. By this I mean to say that Load first comes directly on slab,then it goes to the beam as it is supporting the slab, then from beam it goes to column as column is supporting the beam, then from column it goes to the foundation as foundation is supporting the column then ultimately it goes to the soil on which foundation rests. This phenomenon is known as the Load Dispersion criteria of a structure.
Now in general all cases we see that slabs are monolithic cast with the beams, that means Beams and Slabs are not two separate member, but two different structural members in a single body. This is the regular slab.
This type of slabs when its beams are also interconnected with grids of beams in close spacing, these type of beams are like ribs to the slabs, as it is supporting the slab above. and such slabs are known as the Ribbed Slab. It resembles with Wafer. That means in this case Load flows from slab to Beam then from Beam to Column and so on.
Ribbed or Waffle slabs are mainly used to break the slabs into small panels structurally so that the slab can sustain the vibrations easily. This type of slabs are suitable for Laboratories, Hospitals, Stations are other places where there are chances of vibrations due to machinery operation.
Now, there are many cases, where mainly due to improvement of the appearance, Slabs are directly connected to the beams, without any beams on the slab. Here the load flows from the slab to the column then to the foundation and so on. This type of slab where there are no beams placed and the slab is directly supported on columns with or without column heads are known as the Flat Slab.
This type of slabs are mainly seen in areas like Entrance of commercial and Institutional Buildings like Cinema Halls, Shopping Mall etc.
Main advantage of Flat Slab
As compared with the Ribbed Slab is that, Flat Slab has much better appearance as beams are omitted.
Main disadvantage of Flat Slab
As compared with the Ribbed Slab is that, due to absence of beams Flat Slabs are more prone to failure in Shear and also if columns are not sufficiently closely spaced then the slab as it is not being subdivided with supports of beams the effective slab panels are much bigger in size on plan, due to above facts Slabs thickness increases much and also the amount of reinforcement increases manifold, making the whole structure much expensive affair than regular slabs and in many cases than waffle or ribbed slabs.