Waffle slab or ribbed slab is a structural component with a flat top surface and grid-like bottom surface containing perpendicular ribs. It has two-directional reinforcement and is used for large spans to avoid many interior columns. The waffle shape is formed by placing pods on the formwork before pouring concrete, leaving ribs containing reinforcement. Waffle slabs provide stiff, lightweight structures suitable for areas requiring low deflection and vibration control like airports and hospitals.

1. Waffle Slab or Ribbed Slab
Pritesh Parmar
D.D.University,Nadiad

2. What is Waffle Slab or Ribbed Slab?
✘ Waffle slab or ribbed slab is a structural component which is
plain on its top and contains grid like system on its bottom
surface.
✘ The top of ribbed slab is normally thin and the bottom grid
lines are generally ribs which are laid perpendicular to each
other with equal depth.
✘ Waffle slab has two directional reinforcement.
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3. What is Waffle Slab or Ribbed Slab?
✘ A waffle slab is a type of slab with holes underneath, giving
an appearance of waffles.
✘ It is usually used where large spans are required (e.g
auditorium) to avoid many columns interfering with space.
✘ Hence thick slabs spanning between wide beams (to avoid
the beams protruding below for aesthetic reasons) are
required.
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4. What is Waffle Slab or Ribbed Slab?
✘ Since the tensile strength of concrete is mainly satisfied by
the steel bar reinforcement, only the “ribs” containing the
reinforcement are kept where the remaining ‘unused’
concrete portion below the neutral axis is removed, to
reduce the self-weight of the slab.
✘ This is achieved by placing clay pots or other shapes on the
formwork before casting of the concrete.
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5. What is Waffle Slab or Ribbed Slab?
✘ All the ribs are directed from column heads or beams. The
depth of ribs maintained is as same as depth of column head
or beam.
✘ Because of the ribs and double reinforcement, it is more
stable and recommended for larger span slabs or
foundations.
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6. What is Waffle Slab or Ribbed Slab?
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7. What is Waffle Slab or Ribbed Slab?
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8. What is Waffle Slab or Ribbed Slab?
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9. Purpose Of Waffle Slab:
✘ Waffle slabs provide stiffer and lighter slabs
than an equivalent flat slab.
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10. Purpose Of Waffle Slab:
✘ They provide low floor deflections. It has good
finishes and robustness. Fairly slim floor depth
and fire resistant.
✘ Excellent vibration control.
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11. Characteristics of Waffle Slabs :
✘ Waffle slabs are generally suitable for flat
areas.
✘ Volume of concrete used is very less compared
to others.
✘ The reinforcement in the waffle slab is
provided in the form of mesh or individual bars.
✘ Separate excavation for beams is not required
in case of waffle slab.
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12. Characteristics of Waffle Slabs :
✘ The bottom surface of slab is looks like waffle
which is obtained by using cardboard panels or
pods etc.
✘ The thickness of waffle slab recommended is
85 to 100 mm while the overall depth of slab is
limited to 300 to 600 mm.
✘ The width of beams or ribs provided in waffle
slab are generally 110 to 200 mm.
✘ Spacing of ribs recommended is 600 to 1500
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13. Characteristics of Waffle Slabs :
✘ Reinforced waffle slabs can be constructed for
the span up to 16 meters while beyond that
length prefabricated waffle slab is preferable.
✘ Waffle slab is good against shrinkage and it is
lower than stiffened rafts and footing slabs.
✘ Waffle slab requires only 70% of concrete and
80 % of steel from the concrete and steel used
for stiffened raft.
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14. Waffle Slab Construction Procedure:
The construction of waffle slabs can be done by
three ways as follows.
✘ In-situ
✘ Precast
✘ Prefabricated
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15. Waffle Slab Construction Procedure:
✘ In-situ waffle slabs are constructed by pouring
concrete in the site or field with proper
arrangements.
✘ In case of precast waffle slab, slab panels are
casted somewhere and they are joined
together with proper reinforcement and
concrete is filled.
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16. Waffle Slab Construction Procedure:
✘ The third case, prefabricated waffle slab is
costliest than the other two methods.
✘ In this case, reinforcement is provided in the
slab panels while casting with some tension.
✘ Hence, they do not need internal reinforcement
in the site.
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17. Formwork tools required in the
construction of waffle slab are:
✘ To construct a waffle slab in-situ conditions,
formwork should be necessary to support the
slab.
✘ But some special tools are required for the
form work in case of waffle slab.
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18. Formwork tools required in the
construction of waffle slab are:
✘ Waffle pods
✘ Horizontal supports
✘ Vertical supports
✘ Wall connectors
✘ Cube junctions
✘ Hole plates
✘ Clits
✘ Steel bars
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19. Formwork tools required in the
construction of waffle slab are:
✘ Horizontal support and vertical supports are
arranged first and they are fixed in position by
the connectors.
✘ At the edges wall connectors are used to
provide connection between wall and slab.
✘ The horizontal beam supports are connected
by small beam connectors which form square
like shape in which pods are going to be
placed.
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20. Formwork tools required in the
construction of waffle slab are:
✘ The pods are generally made of plastic and
they are available in different sizes and
different shapes.
✘ Size selection of pod depend upon the
requirement and span length.
✘ For longer span large number of pods are
required.
✘ Same size should be used for one complete
slab.
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21. Formwork tools required in the
construction of waffle slab are:
✘ Similarly beam connectors and cube junctions
are also available in different sizes based on
the suitability of pod sizes.
✘ Cube junctions are used to fix the corners of
pods with the frame work. After fixing the
formwork, reinforcement is placed in the two
directions of the slab and then concrete is
poured in the gaps which are called as ribs
after hardening.
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22. Formwork tools required in the
construction of waffle slab are:
✘ Thin concrete slab is provided on the top and
after its hardening pods and frameworks are
removed from the bottom.
✘ Thus, the waffle like shape appears at the
bottom surface.
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23. Benefits of Waffle Slab Construction :
✘ Waffle slabs are used for larger span slabs or
floors and used when there is limited
requirement for number of columns.
✘ The load carrying capacity of waffle slab is
greater than the other types of slabs.
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24. Benefits of Waffle Slab Construction :
✘ They provide good structural stability along
with aesthetic appearance. Hence, it is
constructed for airports, hospitals, temples,
churches etc.
✘ The waffle slab can be made of concrete or
wood or steel among those concrete waffle
slab is preferred for commercial buildings and
other two are preferred for garages, decorative
halls etc.
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25. Benefits of Waffle Slab Construction :
✘ It has good vibration control capacity because
of two directional reinforcement. So, it is useful
for public buildings to control vibrations
created by movements of crowd.
✘ Waffle slabs are lightweight and requires less
amount of concrete, hence it is economical.
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26. Benefits of Waffle Slab Construction :
✘ Construction of waffle slab is easy and quick
with good supervision.
✘ Concrete and steel volume required is small,
hence, light framework is enough for waffle
slab.
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27. Benefits of Waffle Slab Construction :
✘ Several services like lighting, plumbing pipes,
electrical wiring, air conditioning, insulation
materials etc. can be provided within the depth
of waffle slab by providing holes in the waffle
bottom surface. This system is called as
Holedeck.
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28. Holedeck :
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29. Drawbacks of Waffle Slab :
✘ Formwork tools required are very costly
because of large quantity requirement of pods
and some special tools.
✘ The floor height should be more hence number
of floors are reduced.
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30. Drawbacks of Waffle Slab :
✘ The services provided in the waffle
arrangement without proper maintenance may
causes damages to the slab.
✘ Skilled workers are required during its
construction.
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31. Drawbacks of Waffle Slab :
✘ They are not suitable for sloped areas. If there
is slope area, the area must be leveled with
filling or by excavating. For soil filling, good soil
should be used.
✘ They are not suitable against high winds or
cyclonic areas because of their light weight.
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32. Uses And Applications Of Waffle Slab:
✘ It is used where vibration is an issue and where
large span slabs are to be constructed i.e areas
having less number of columns.
✘ For example airport, hospitals, commercial and
industrial buildings etc & where low slab
deflections and high stability are required.
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Thanks!
Any questions?
You can find me at:
priteshparmar5.6.1998@gmail.com