2. What is Waffle Slab?
• A waffle slab or two-way joist slab is a concrete slab made
of reinforced concrete with concrete ribs running in two directions
on its underside.
• The name waffle comes from the grid pattern created by the
reinforcing ribs.
• Waffle slabs are preferred for spans greater than 40 feet (12 m), as
they are much stronger than flat slabs, flat slabs with drop panels,
two-way slabs, one-way slabs, and one-way joist slabs.
• This type of reinforcement is common on concrete, wood and metal
construction. A waffle slab gives a substance significantly more
structural stability without using a lot of additional material.
• This makes a waffle slab perfect for large flat areas like foundations
or floors.
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3. Appearance :
• The top of a waffle slab is generally smooth, like a traditional
building surface, but the underside has a shape reminiscent of a
waffle.
• Straight lines run the entire width and length of the slab, generally
raised several inches from the surface.
• These ridges form the namesake square pockets of the entire length
and width of the slab.
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4. Where this system is used ?
• Waffle slab can be used as both ceiling and floor slab. They are used in
the areas where less number of columns are provided, i.e. it is basically
used in the areas which has huge spans.
• Waffle slab is not used in typical construction projects. They are used
for specialized projects that involve clean rooms, spaces requiring
seclusion from low frequency vibration or those needing low floor
deflections.
• The concrete waffle slab is often used for industrial and commercial
buildings while wood and metal waffle slabs are used in many other
construction sites.
• This form of construction is used in airports, parking garages,
commercial and industrial buildings, bridges, residences and other
structures requiring extra stability.
• The main purpose of employing this technology is for its strong
foundation characteristics of crack and sagging resistance. Waffle slab
also holds a greater amount of load compared with conventional
concrete slabs.
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6. Waffle slab characteristics are defined by the following parameters:
• Total slab edge (H)
• Height of the lightening formwork or lightening blocks (h)
• Spacing between rib axes (e)
• Basic thickness of the ribs (b), though in waffle slabs with system
formwork they have a truncated pyramid-shaped core of variable
thickness.
• Thickness of the compression layer (c)
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7. Design & Production of Waffle Slab
Production :
• Waffle slabs can be casted by placing he pods on he formwork itself or
they can be available pre-casted.
• Concrete waffle slabs can be purchased and shipped to the
construction site as prefabricated or precast sections, or they can be
poured on-site.
• If prefabricated products are not in the budget, consider purchasing
casts for the concrete waffle slabs.
• The pre-casted pods are then placed directly on the site and then
provided with reinforcement and filled with concrete.
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8. Design :
• There are three basic designs for concrete slabs that improve the
strength-to weight ratio. For each design, the top surface is flat while
the underside is modulated with either a corrugated, ribbed or waffle
design.
• Corrugated slabs are created when concrete is poured into a wavy
metal
form. This shape prevents the slab from sagging.
• Ribbed slabs add strength in one direction, while the concrete waffle
slab
design adds strength in perpendicular directions. Concrete slabs
can be
reinforced with rebar for additional strength.
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9. Services Provided in Waffle Slab
Due to the holes in the waffle system following services can
be provided:
• Air-conditioning
• Plumbing
• Lighting
• Insulation Materials
• Wiring, etc.
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10. Construction Process:
• A waffle slab can be made in different ways but generic forms are
needed to give the waffle shape to the slab. The formwork is made
up of many elements: waffle pods, horizontal supports, vertical
supports, cube junctions, hole plates, clits and steel Bars.
• First the supports are built, then the pods are arranged in place, and
finally the concrete is poured.
• This process may occur in three different approaches, however the
basic method is the same in each:
In situ: Formwork construction and pouring of concrete occur on
site, then the slab is assembled (if required).
Precast: The slabs are made somewhere else and then brought
to the site and assembled.
Pre-fabricated:The reinforcements are integrated into the slab
while being manufactured, without needing to reinforce the
assembly on site. This is the most expensive option.
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11. Construction Technique
1. Arranging the 2. Fixing the 3. Fixing the 4.Providing
Formwork connectors framework horizontal connector
5. Placing the pods 6. Fixing pods to the 7. Removing framework
connector
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13. Advantages:
• Waffle slabs are able to carry heavier loads and span
longer distances than flat slabs as these systems are
light in weight.
• Suitable for spans of 7m – 16m ; longer spans may be
possible with post tensioning.
• It is also economical as the amount of concrete and steel
is reduced as compared to flush slabs. Saving 15%
concrete and 10% steel as compared to traditional T
beams.
• These systems are light in weight and hence
considerable saving is ensured in the framework as light
framework is required.
• Coffered underside is usually left exposed for visual
appearance.
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14. Disadvantages:
• Construction requires strict supervision and skilled
labour.
• The casting forms or moulds required for precast units
are very costly and hence only economical when large
scale production of similar units are desired.
• Headroom is reduced , hence increased storey height.
• Due to waffle ceiling , it creates problem in lighting
facilities and hanging pipes or ducts.
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16. Lodytel Communication Development Centre, Spain
It was the first building to use the Hole deck pods for
waffle system.
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17. Chattrapati Shivaji Terminal,India
It is recognized for its innovative column design which
also consists waffle design.
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18. Case Study – Chattrapati Shivaji International
Airport, Mumbai
• Long span roof of 70,000 square meters
• Roof supported by 30 columns, column distance of 64 meters
in the N-S direction and 34 meters in E-W direction.
• By increasing the depth of trusses near the columns and
running trusses, large spacing of cantilevers of 40 meters along
the perimeter is achieved.
• Total truss depth = 4 meters only
• The columns are also designed to serve as hoist mechanism,
entire roof was constructed without any tower cranes.
• The structure prioritizes modular construction for economy and
also accelerates construction schedule.
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