A slab can be a floor slab or a
roof slab. The live load acting on the floor slabs is greater than the roof
slabs. Thus, the floor slabs are heavily reinforced when compared to the roof
slabs.
Types of Loads on
a Slab:
Types of loads acting on a slab include:
1.
Dead load of the slab
2.
Live load
3.
Floor finish load
4.
Snow load in the case of roof
slab
5.
Earthquake loads
Types of Slabs:
1. Conventional Slab:
The
slab which is supported by beams and columns is called conventional slab. In
conventional slab the thickness of the slab is small but depth of the beam is
large. The load is transmitted from slab to beam and then from beam to column.
These
types of slabs are provided with reinforcement where the horizontal bar is
called main reinforcement and vertical bar is called distribution bar.
Conventional slabs are two types
(i)
One way slab (ii) Two way slab.
(i). One Way Slab:
One
way slab is a conventional horizontal slab rectangular in shape and supported
by beams on two of its four sides. The slabs are supported on beams opposite to
each other. Therefore, the deflected shape is cylindrical.
In
one way slabs the longer span is more than twice of the shorter span. The ratio
of longer span to the shorter span is greater than two. As a result, the
shorter span is subjected to bending.
The
main reinforcements are provided in the shorter directions to resist the
bending and the secondary reinforcements are provided in the longer direction.
(ii). Two Way Slab:
Two
way slabs are also conventional slabs mostly rectangular in shape and supported
on all the four sides on a beam. In two way slabs, the longer span is less than
twice the shorter span. The ratio of longer span to shorter span is lesser than
two. The deflected shape of the slab is like a dish or saucer shape.
In
this case, both the shorter and longer directions are subjected to bending.
Therefore, the main reinforcements would be provided in both directions based
on the magnitude of bending. The two way slabs are economical to a size of 6m x
6m. Also, the quantity of steel used in two way slab is more than the one way
slab.
Advantages
of Conventional Slabs:
·
i) Easy to construct.
·
ii) It does not require skilled labours.
·
iii) Easy to install electrical, mechanical and plumbing services.
·
iv) The formwork are simple.
Disadvantages:
·
i) Becomes very deep for higher loads.
·
ii) During the curing period, the formwork cannot be removed.
·
iii) The concrete in the tension zone has no structural performance but
it is provided to simplify the construction.
2.
FLAT SLAB:
·
Flat
slab is also known as beam-less slab because it is supported directly by
columns or caps. Here, the loads are transferred directly to the columns.
·
The
function of this type of slab is to provide a plain ceiling surface, which will
give the best diffusion of light. They are commonly used in hotels, parking
lots, commercial buildings or places where beam projections are not suitable
for height controls or even aesthetics view.
3.Waffle Slabs Or Grid Slabs:
Waffle
slabs are
lightweight slabs with hollow grid-like systems on its soffit. The hollow grid
system reduces the self-weight of the slab without compromising its structural
stability.
The
grids are spaced at equal intervals and they transfer the load from above to
the beams. The space between the ribs is sometimes treated as a beam and
reinforced to increase the flexural rigidity of the slab.
Due to the light weight, they can span to long distances with
ease. The waffle slabs may have a grid system or a girder system where the
bands of beam run throughout the slabs. The grids can be or any shape – square,
rectangular, triangular etc.,
Advantages Of Grid Slab:
·
i) Lighter sections and therefore less self weight leading to economic
foundations.
·
ii) Long spans can be constructed.
·
iii) The construction materials used for the slab can be saved.
·
iv) Very economical when modular coordination is followed for the form
works.
·
v) High load carrying capacity than conventional slabs.
·
vi) Less use of concrete leads to less CO2 emissions.
Disadvantages Of Grid Slab:
·
i) The reduced depth of the slabs between the grids increase the risk of
fire transmission.
·
ii) The construction of waffle slabs is also tedious since they are
demanding special and sophisticated formwork.
·
iii) The mechanical and electrical installations are very difficult to
install because of the complex design in the soffit.
·
iv) The height between the floors is increased and thus not economical.
·
v) It is not suitable for buildings subjected to sudden vibrations or
constant vibrations.
·
vi) Not suitable for regions with high wind velocities due to the low
self weight.
·
vii) Skilled workers are required.
·
viii) High maintenance.
·
ix) Not economical for small projects.
·
x) Difficult to repair.
4. Dome Slab:
This kind of slab is generally
constructed in temples, Mosques, palaces etc. And Dome slab is built on the
conventional slab. The thickness of Dome slab is 0.15m. Domes are in the
semi-circle in shape and shuttering is done on a conventional
slab in a dome shape and concrete is filled in shuttering forming dome shapes.
This is one of the types of concrete slabs.
5.SUNKEN SLAB:
This type
slab used below the washrooms to cover sewer pipes or WC pipes or other
equipment is called a sunken slab. Care should be taken to avoid leakage
problems as the water pipes are hidden below the ground.
Proper
waterproofing and treatment of the slab provided to prevent leakage or
moisture. After sending the sewer pipes in the slab, the slab is filled with
broken bricks or coal or suitable lightweight material.
The slab that
is provided below the normal level of the floor at a depth of 200mm to 300mm
and filled with pieces of broken brick is called a sunken slab.
or
The slab
that is provided above the normal floor level at a height of 200mm to 300mm and
filled with coal or pieces of broken bricks called the sunken slab.
6.PROJECTED SLAB:
A slab
that is fixed on one side and free on the other is called a projected slab or
cantilever slab. These types of slab are usually built in hotels, universities,
function halls, etc to use that area for picking or dropping up zone and for
unloading and loading area. This is one of the types of concrete slabs.
7. WAIST SLAB:
The Waist Slab is nothing more than a
normal slab with some inclination angle between the two supports. (Mainly used
on RCC stairs).
The Waist Slab rests from the beam to
the beam and the main stair reinforcement steel goes up to the middle of the
resting beam. The thickness of the waist slab depends on the length of the
stair flow. Usually the thickness is 5 ″ or 6 of.
8.Ribbed Slab:
Ribbed slabs are very similar to waffle slabs but often mistaken for being the same. A ribbed slab has wide grids or bands of beams in the soffit of the beam whereas, a waffle slab has deeper
corresponding parts.
The
ribbed slabs like waffle slabs are flexible, light in section, economical and
can be constructed for longer spans. They have the same advantages and
disadvantages of the waffle or grid slab.
Applications: Public
buildings, museums, galleries.
9.Hollow Deck Slab:
Hollow
deck slabs are special types of prefabricated concrete slabs that use the
fundamentals of the stress developed in a section for the design. The concrete
takes care of the compression while the steel takes care of the tension. There
is no specific use for the concrete in the tension zone other than to provide a
solid section.
Taking this principle into
account, the hollow deck slab does not provide concrete in the tension zone. It
is provided with minimum concrete to hold the steel reinforcements in place.
The hollow deck slab can be prestressed or non-prestressed.
Advantages:
·
i) Lighter sections and therefore less self-weight leading to economic
foundations.
·
ii) Long spans can be constructed.
·
iii) The materials used for the slab can be saved
·
iv) Prefabrication lead to faster construction.
·
v) It can be fully loaded immediately after placing.
·
vi) It gives a fuller and pleasing appearance from below.
Disadvantages:
·
Not suitable for regions with high wind velocities due to the low self
weight.
·
ii) Skilled workers are required.
·
iii) Connections are difficult and tedious in a precast structure.
·
iv) Cranes are required for the handling of the slabs.
·
v) It is time consuming if it is a Cast in-situ construction.
·
vi) Not economical for small projects.
·
vii) Difficult to repair.
Applications:
Bridges, wall panels, multi-storey
prefabricated buildings.
10.Arched Slab:
Arched
slabs are mostly found in bridges. Arches like domes, transfer the loads
through axial compression. The arches are provided in the bridges to resist the
wind loads acting on the bridge. In ancient times, arches were built in
masonry, but the modern developments are giving birth to reinforced curved arch
slabs.
11.Cantilever Slab:
Cantilever
slabs, as the name suggests are fixed at one end and free at the other ends.
The deflection in cantilever slabs is greater than the other slabs due to its
support condition. One great example of the cantilever slabs is Balconies.
The
main reinforcement is provided in the span that is protruding outside. The
secondary reinforcements are provided throughout the length of the slab. In RCC
work, cantilever slabs should not be extended more than 4 feet.
Advantages Of Cantilever Slab:
·
i) It improves the elevation of the building.
·
ii) It does not require any support on the other side.
·
iii) It is easier to construct.
Disadvantages Of Cantilever Slab:
·
i) It is not economical since it requires heavy sections.
·
ii) The deflection is more.
·
iii) The uplift at the fixed support has to be taken into account while
designing.
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