FLAT SLAB FLOORS
• It is made up of R.C.C slab.
• In this the thickness of slab is
• Directly supported on
• They are generally thicker
• Economical in heavy loads.
• Column spacing ranges between 5-8 meters.
ADVANTAGES OF FLAT SLAB
• Formwork required for this is very simple
• It gives a good appearance.
• It gives better lighting facilities.
• It is specially suited for the heavy live
• It also affords the convenience of hanging
pipes which is added advantage in
ESSENTIAL FEATURE OF
CONSTRUCTION OF FLAT SLAB
• Economy in construction cost.
• No false ceiling is required.
• Construction technique is simple.
• More clear headroom is available.
SPECIALLY USED UNDER
• Where large clear spaces are used.
• The end spans should not be more than the
• Where head room is limited.
• The ratio of length to breadth of panels
should not be more than 4:3.
• Where heavy loads are to be carried.
• Generally used for buildings such as
factories, mills, theatres, public buildings
REINFORCEMENT IN FLAT
1. Two-way system: In this reinforcement
is carried from column to column and at
right angles to one another.
2. Four-way system: In this reinforcement
is placed in four directions. Consists of
two bands of main steel from column to
column and other two bands placed
diagonally across the panel from column
DROP BEAM AND SLAB
• THIS FLOOR CONSTRUCTION CONSISTS OF A FLOOR
SLAB WHICH IS THICKENED BETWEEN COLUMNS IN
THE FORM OF A SHALLOW BUT WIDE BEAM.
• A DROP SLAB FLOOR IS OF ABOUT THE SAME DEAD
WEIGHT AND COST AS A COMPARABLE SLAB AND
• IT WILL HAVE UP TO HALF THE DEPTH OF FLOOR
CONSTRUCTION FROM TOP OF SLAB TO SOFFIT OF
• THIS DIFFERENCE WOULD CAUSE A SIGNIFICANT
REDUCTION IN OVERALL HEIGHT OF
CONSTRUCTION OF A MULTI-STOREY BUILDING
WITH APPRECIABLE SAVING IN COST.
DROP BEAM AND SLAB
• DROP SLAB FOOR HAS UP
TO HALF THE DEPTH OF
FROM TOP OF SLAB TO
SOFFIT OF BEAMS.
• FOR e.g.- ON A 12.0 SQUARE
COLUMN GRID THE
OVERALL DEPTH OF A SLAB
AND BEAM FLOOR BE
ABOUT 1.2 WHERE THE
DEPTH OF A DROP SLAB
FLOOR WOULD BE ABOUT
DROP BEAM AND SLAB
COLUMNS TO FORM
DROP BEAM AND SLAB
FROM ACI 318 BUILDING CODE
• FOR MONOLITHIC OR FULLY COMPOSITE CONSTRUCTION, A
BEAM INCLUDES THAT PORTION OF SLAB ON EACH SIDE OF
THE BEAM EXTENDING A DISTANCE EQUAL TO THE
PROJECTION OF THE BEAM ABOVE OR BELOW THE SLAB’
WHICHEVER IS GREATER, BUT NOT GREATER THAN FOUR
TIMES THE SLAB THICKNESS.
• WHEN USED TO REDUCE THE AMOUNT OF NEGATIVE
MOMENT REINFORCEMENT OVER A COLUMN OR MINIMUM
REQUIRE SLAB THICKNESS, A DROP PANEL SHALL PROJECT
BELOW THE SLAB AT LEAST ONE-QUARTER OF THE SLAB
THICKNESS BEYOND THE DROP AND EXTENT IN EACH
DIRECTION FROM THE CENTER LINE OF SUPPORT A
DISTANCE NOT LESS THAN ONE-SIXTH THE SPAN LENGTH
MEASURED FROM CENTER-TO-CENTER OF SUPPORTS IN
• A Waffle slab is a two-way
concrete slab reinforced by ribs in
• In this type of slab intermediate
beams are cast on a regular
square grid that gives the
underside of the floor appearance
of waffle , hence it is named as
• Waffle slab construction is used
for ceilings and floor
Benefits of Waffle Slab System
•Dead weight of the slab is reduced as
compared to flush slab of similar span
, as the intermediate beams of waffle
supports the thin slab above it.
In these systems Bulk of cement
concrete below the neutral axis is
removed , as concrete below the
neutral axis is considered as having no
tensile value, except to resist shear
stresses . Hence certain portion of the
concrete is removed , leaving a
sufficient number of ribs or concrete
joists at short intervals carrying
reinforcement to take care of shear
•Waffle slabs are able to carry heavier loads and span longer distan
than flat slabs as these systems are light in weight.
•Suitable for spans of 7m – 16m ; longer spans may be possible wit
•It is also economical as the amount of concrete and steel is reduc
compared to flush slabs. Saving 15% concrete and 10% steel as com
to traditional T beams.
•These systems are light in weight and hence considerable saving i
ensured in the framework as light framework is required.
•Coffered underside is usually left exposed..
Demerits of Waffle Slab System
•Construction requires strict supervision and skilled labour.
•The casting forms or moulds required for pre-cast units are very co
hence only economical when large scale production of similar units
•Headroom is reduced , hence increased storey height.
•Due to waffle ceiling , it creates problem in lighting facilities and h
pipes or ducts.
Details of Waffle Slab System
•Square metal or fiberglass dome form
available in 485mm and 760mm width
form 205mm to 510 mm depth in 51
increment. Larger size are also availab
•485mm dome with 125mm ribs create
610mm module and 760mm domes wi
rib creates 915mm module.
•Slab depth is usually taken between 7
115mm but by the thump rule it is take
•Tapered sides allow for easier remova
•It is suitable for span 7m – 16m.
•For greater shear strength and
moment –resisting capacity , solid
heads at the column supports are
formed by omitting dome forms ; size
depends on the span on the load
•For maximum efficiency , bays
should be square or nearly square as
•Waffle slab can be cantilevered in
two direction up to 1/3 of the main
•The waffle grid form is cast around
plastic or metal formers laid on the
timber centering , these formers are
screwed to the centering , so that
smooth finish of the soffit is left
Waffle Slab System Floor Construction
This type of floor is useful where
a widely spaced column grid is
necessary and the floor supports
heavy load because of the use of
intermediate cross beams.
•Ensure building site is level and all
plumbing pipes are fixed in place as
•Spread a layer of sand or metal dust
approximately 50 mm thick over
building area . Place plastic sheeting
and tape laps.
• Set up formwork, set
out Waffle Raft Pods in
a grid pattern using
which form all beams.
• Cut the Waffle Raft
Pods to fit around
plumbing pipes etc.
using sharp handsaw
or power saw. Place
reinforcing bars in
• Place steel mesh on top of
Waffle Pods and in the
• Pour concrete, part fill the
ribs to hold the Waffle Raft
Pod in position before
pouring 85 mm thick slab
with 25 mm minimum
cover over reinforcing
Benefits of the Waffle Slab System floor
over conventional slab floor system
1. Reduced building costs on sites containing :
reactive soils ,sand ,rock ,water tables
2. Less use of concrete .
3. Greater strength and stiffness over conventional slab systems
4. Savings in construction time over conventional slab systems.
5. Insulation benefits .
6. Effective termite protection.