A rectangular slab is supported by beams at all sides
( length /width ) < 2
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.
load is carried in both direction
main reinforcement is placed in both direction
When (length /width ) < 2 the one way slab design go beyond
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.
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.
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.
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 .
These slabs do not have beams between the columns, drop
panels or column capitals. Usually, there are spandrel beams
at the edges.
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.
These slabs do not have beams but have drop panels or
This system is appropriate for heavier loads encountered
in office or industrial buildings.
For long span construction, there are ribs in both the
spanning directions of the slab. This type of slabs is called
This is attributed to the considerable reduction in
dead load as compared to other floor systems.
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.
In fig let us assume , it is consist of 2 sets of parallel strips ,
in each of 2 direction , intersecting each other.
By using second-order differential equation , for the
maximum bending moment we found the deflection curve
& maximum deflection equation .
Equating the center deflection of the short & the long strips
If both support are simply supported then ,
If length are equal then ,
Then maximum moment ,
For elastic plate ,
Assuming torsion ,
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.
Top = parallel to diagonal
Bottom = perpendicular to diagonal
Two empirical methods are mainly established for
analysis and design .
The first of these methods is the
which has to meet certain limiting
The second method is called the
, which has no restrictions on its use. But this
method is rather complex.