3. HISTORICAL DEVELOPMENT
• Flat slabs were originally invented in the U.S.A in year
1906.
• This was the start of these type of construction.
• Many slabs were load-tested between 1910-20 in U.S.A.
• 1914 Nicholas proposed a method of analysis of these slab
based on simple statics, this method is know as direct
design method.
4. BASIC DEFINITION OF FLAT
SLAB
The term flat slab means a reinforces concrete slab with or
without drop, supported generally without beams, by
column with or without flared column heads.
5. APPLICATION OF FLAT SLAB
In the case of high rise building thinner slabs are required so
that additional floors can be added.
The distance that be spanned by post-tensioned slabs exceeds
that of reinforced constructions with the same thickness.
For increasing span lengths so as to increases the usable
unencumbered floor space in buildings.
For diminishing the number of joints in the structure.
For the speedy construction of the project.
The amount of steel required is much less than in normal RCC
structure.
The moulds can be used No. of times as per the demand.
Due to reduce beam section the load transferred to foundation
is less compared to that of RCC structure.
The structure is crack free as the whole structure is in
compression.
Large span of slab can obtain easily.
6. ADVANTAGES OF FLAT SLAB
Floor to floor height reduction
Faster construction
Early formwork stripping
Water resistant properties
Saving in materials.
Reduced foundation load
Greater column free areas
Architectural freedom
Reduced construction costs
7. LIMITATION OF FLAT SLAB
Careful handling of prefabricated components such as
concrete panels or steel and glass panels is required.
Attention has to be paid to the strength and corrosion-
resistance of the joining of prefabricated sections to avoid
failure of the joint.
Similarly, leaks can form at joints in prefabricated
components.
Transportation costs may be higher for voluminous
prefabricated sections than for the materials of which they
are made, which can often be packed more compactly.
8. CLASSIFICATION OF FLAT
SLAB SYSTEM
1. Solid flat slab(or flat plate)
2. Solid flat slab with drop panels
3. Solid flat slab with column heads
4. Banded flat slab
13. METHOD OF
CONSTRUCTION SYSTEM
The flat slabs can be cast-in-situ (cast-in-place). Else,the
slabs can be precast at ground level and lifted to the final
height. The later type of slabs is called lift slabs.
1. POST TENSIONING SYSTEM
2. PRE TENSIONING SYSTEM
16. PURPOSE TENSIONING
SYSTEM
To reduce the deflection.
To reduce the punching shear.
Reduced structure depth
To Greater column free areas.
Reduces the required number of columns and foundations.
To Increase load bearing capacity.
To bear seismic forces.
18. ANLYSIS OF FLAT SLAB
The steps of analysis slab is
1. Determine the factored negative (Mu–) and positive moment (Mu+)
demands at the critical sections in a slab-beam member from the analysis of
an equivalent frame. The values of Mu– are calculated at the faces of the
columns. The values of Mu+ are calculated at the spans. The following
sketch shows a typical moment diagram in a level of an equivalent frame
due to gravity loads.
19. ANLYSIS OF FLAT SLAB
2. Distribute Mu– to the CS and the MS. These components are
represented as Mu,–CS and Mu,–MS, respectively. Distribute Mu+ to
the CS and the MS. These components are represented as Mu, +CS and
Mu, +MS, respectively.
20. ANLYSIS OF FLAT SLAB
3.If there is a beam in the column line in the spanning direction, distribute
each of Mu,–CS and Mu, + CS between the beam and rest of the CS.
4) Add the moments Mu,–MS and Mu, +MS for the two portions of the MS
(from adjacent equivalent frames).
5) Calculate the design moments per unit width of the CS and MS.
21. Code provision for flat slab
As per IS-456 : 2000
1.Thickness of flat slab
24. Direct design method
Limitations
Slab system designed by the direct design method shall
fulfill the following condition.
a. There shall be minimum of three continuous spans in
each direction.
b. The panels shall be rectangular, and the ratio of the
longer span to the shorter span within a panel shall not
be greater than 2.0.
c. It shall be permissible to offset column to a maximum of
10 percent of span in the direction of offset not with
standing the provision in (b).
25. Direct design method
d) The successive span length in each direction shall not
differ by more than one-third of the longer span. The
end spans may be shorter but not longer than the
interior span , and
e) The design lived load shall not exceed three times the
design dead load.
26. Design steps for flat slab
1. Check preliminary dimension
2. Check for applicability of DDm
3. Divide the slab with frame in X and Y directions and obtain
dimension of X and Y frames.
4. Analysis the interior and exterior panel.
a. Longitudinal distribution
b. Transverse distribution
5. Estimate the design moment in the external column
6. Estimate the design moment in the internal column
7. Design for shear
8. Detailing should be done as per code requirement.
32. CONCLUSIONS
As per Indian code we are using cube strength but in
international standards cylindered are used which gives higher
strength than cube.
Drops are important criteria in increasing the shear strength of
the slab.
Enhance resistance to punching failure at the junct ion of
concrete slab & column.
By incorporating heads in slab, we are increasing rigidity of
slab.
In the interior span, the total design moments (Mo).
The negative moment’s section shall be designed to resist the
larger of the two interior negative design moments for the span
framing into common supports.
33. CONCLUSIONS
According to Indian standard (IS 456) for RCC code has
recommended characteristic strength of concrete as 20, 25, and
30 and above 30 for high strength concrete. For design purpose
strength of concrete is taken as 2/3 of actual strength this is to
compensate the difference between cube strength and actual
strength of concrete in structure. After that we apply factor of
safety of 1.5. So in practice Indian standard actually us es 46%
of total concrete characteristic strength. While in International
practice is to take 85% of total strength achieved by test and
then apply factor of safety which is same as Indian standard so
in actual they use 57% of total strength.
Pre fabricated sections to be integrated into the design for ease
of construction.
34. References
Indian standards 456,875.
S.P 16.
Advanced Reinforced Concrete Design-P.C Varghese.
A.K. Jain - Limit state design of Reinforced concrete.
Reinforced concrete design - S.unnikrishna Pillai, Devdas
Menon
S.Ramamrutham & R. Narayan - Design of Reinforced
concrete Structures.
V.N, Vazirani & S.P Chandola – Hand book of civil
Engineering.
35. During the mini project I visited lanco hills site under
which I was done the supervision of IT 6 block in
which I was saw the different types of flat slab and
construction details ,arrangement of formwork ect.
