Two way slab by Rashedul kabir

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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


Flat Plate These slabs do not have beams between the columns, drop

panels or column capitals. Usually, there are spandrel beams at the edges.


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.


Flat slabs These slabs do not have beams but have drop panels or

column capitals.


Flat slabs This system is appropriate for heavier loads encountered

in office or industrial buildings.


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.


Waffle slabs This is attributed to the considerable reduction in

dead load as compared to other floor systems.


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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Two way slab by Rashedul kabir

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