3. A rectangular slab is supported by beams at all sides
 ( length /width ) < 2


4. 
A Circular slab is also
a two way slab

In general a slab which is not falling in the category of one
way slab , considered as two way slab.


So
load is carried in both direction
main reinforcement is placed in both direction

5. 

When (length /width ) < 2 the one way slab design go beyond
effectiveness.
We know, one way slabs are designed to transfer their loads to only two
opposite support walls. But when (l/b<2)
two opposite support walls can’t take the load ,so a criteria is required to
transfer the load to four supporting walls.

6. 
In case of one way slab we all know that ,when loaded such
slabs bend into cylindrical surface.
 But in case of two way slab when loaded such slabs bend
into a dished surface rather than a cylindrical surface.

7. 


That means at any point the slab is curved in both direction
& since bending moment are proportional to curvature &
moments also exist in both direction .
To resist this moments the slab must be reinforced in two
direction , by at least 2 layers of bars , perpendicular ,
respected to two pairs of edges .
The slab must be design to take a proportionate share of the
load in each direction.

8. Two-way Edgesupported
Slabs
Two-way
Column
Supported Slabs
Flat Plates
Flat Slabs
Waffle Slabs

10. 

It is the simplest type of two way slab .
Here the slab or slab panel is supported along its four edges
by relatively deep , stiff, monolithic concrete beams or walls
or steel girders .

12.  These slabs do not have beams between the columns, drop
panels or column capitals. Usually, there are spandrel beams
at the edges.

13.  This type of two-way slabs is suited for light loads.

Use of flat plates is limited by their punching shear capacities
where large thickness is required in case of heavy loads or
large spans, thus rendering the system uneconomical.

14.  These slabs do not have beams but have drop panels or
column capitals.

15.  This system is appropriate for heavier loads encountered
in office or industrial buildings.

16.  For long span construction, there are ribs in both the
spanning directions of the slab. This type of slabs is called
waffle slabs.

17. 
This is attributed to the considerable reduction in
dead load as compared to other floor systems.

18. 
Waffle slab construction consists of orthogonal sets of ribs
with solid parts at the columns, as shown in figure . The ribs
are formed with fiber glass or metal dome forms.

20. 
In fig let us assume , it is consist of 2 sets of parallel strips ,
in each of 2 direction , intersecting each other.

21. 
By using second-order differential equation , for the
maximum bending moment we found the deflection curve
& maximum deflection equation .

22. 
Equating the center deflection of the short & the long strips
gives
Consequently ,

23. If both support are simply supported then ,
If length are equal then ,
Then maximum moment ,
For elastic plate ,
Assuming torsion ,

24. 
The twisting moments which usually consequence only at
extreme corners of a two way slab system , where they tend to
crack the slab at the bottom along the panel diagonal & at the
top perpendicular to the panel diagonal.

So special reinforcement is required .

Provided at the extreme corner in both the top & the bottom
of the slab , distance is equal to 1/5 th of the longer span.

25. Top = parallel to diagonal
Bottom = perpendicular to diagonal

26. 
Two empirical methods are mainly established for
analysis and design .
 The first of these methods is the
which has to meet certain limiting
conditions.
 The second method is called the
, which has no restrictions on its use. But this
method is rather complex.