Transcript
Hello Everyone
Rashedul Kabir 10.01.03.051
Two Way slab
What’s two way slab?
A rectangular slab is supported by beams at all sides ( length /width ) < 2
What’s two way slab ?
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
Why Two way slab design adopted ?
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.
Why two way slab design adopted ?
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.
Why Two way slab design adopted ?
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.
TYPES OF TWO WAY SLAB
Flat Slabs
Waffle Slabs
Flat Plates Two-way Edge-supported Slabs
Two-way Column Supported Slabs
Two-way Edge-supported Slabs
Two-way Edge-supported Slabs
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 .
Two-way Column-supported Slabs
Two-way Column-supported Slabs
Flat Plate These slabs do not have beams between the columns, drop
panels or column capitals. Usually, there are spandrel beams at the edges.
Two-way Column-supported Slabs
Flat Plate 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.
Two-way Column-supported Slabs
Flat slabs These slabs do not have beams but have drop panels or
column capitals.
Two-way Column-supported Slabs
Flat slabs This system is appropriate for heavier loads encountered
in office or industrial buildings.
Two-way Column-supported Slabs
Waffle slabs For long span construction, there are ribs in both the
spanning directions of the slab. This type of slabs is called waffle slabs.
Two-way Column-supported Slabs
Waffle slabs This is attributed to the considerable reduction in
dead load as compared to other floor systems.
Two-way Column-supported Slabs
Waffle slabs 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.
Analysis
Analysis 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 .
Analysis
Analysis Equating the center deflection of the short & the long strips
gives
Consequently ,
So, larger share of load carried in the short direction.
AnalysisIf both support are simply supported then ,
Wa = Wb = W/2
If length are equal then ,
la= lb
Then maximum moment ,
For elastic plate ,
Assuming torsion ,
AnalysisCorner panel : 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.
Analysis
Corner panel :
Top = parallel to diagonal
Bottom = perpendicular to diagonal
Design Two empirical methods are mainly established for
analysis and design . The first of these methods is the Direct Design
Method which has to meet certain limiting conditions.
The second method is called the Equivalent Frame Method, which has no restrictions on its use. But this method is rather complex.
End of presentation
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