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.
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.
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.
- 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 mm.
- 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.
Waffle Slab Construction Procedure
The construction of waffle slabs can be done by three ways as follows.
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.
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.
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.
Formwork tools required in the construction of waffle slab are:
- Waffle pods
- Horizontal supports
- Vertical supports
- Wall connectors
- Cube junctions
- Hole plates
- Steel bars
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.
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.
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.
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.
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.
- 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.
- 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.
- 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.
- 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.
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.
- The services provided in the waffle arrangement without proper maintenance may causes damages to the slab.
- Skilled workers are required during its construction.
- 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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