A waffle slab is a two-way joist slab system reinforced with concrete ribs constructed in the perpendicular direction of each other. These are laid in equal depth having a uniform rib appearance. It is called a waffle as it’s plain upright and holes underneath giving the appearance of waffles. It is usually used where large spans are required to avoid columns interfering with space. Its main function is to withstand higher loads and longer distances. They are also used where there is a restriction on the depth of the beams to obtain a clear height. Waffle slab is stiffer and lighter slabs compared to other slab types such as Hollow core ribbed Slab, Dome roof, Pitch roof slab, Post tension slab to name a few.
Advantages of waffle slabs
- Reinforced with mesh or steel bars
- The volume of concrete required is less
- Separate excavation for the beam is not required.
- Faster construction
- Excellent vibration control
- Thermal mass adherence
- Vertical penetrations between ribs are easy.
- The load carrying capacity is greater than the other types of slabs
- Greater load bearing
- Bigger spans can be achieved with less material
- Forms can be implemented with wood, concrete or steel
- Vertical penetration between ribs is easy
- Suitable for flat areas
- Use of less concrete and steel
- Provides good finishes and robustness
- Can Be Used As Both Ceiling And Floor Slab.
Waffle slab types
Waffle slab type can be categorised as per construction method and shape of the slabs
The waffle slab classification based on construction methods:
- Waffle slab with Solid Heads – solid heads at the joints with columns. This provision is made to bear the shear requirements faced during the loading conditions.
- Waffle slab with Band Beams – a wide solid beam is provided along the centre line of the column which will facilitate the structure to have a uniform depth.
Waffle slabs are classified into the following types based on the shape of the slabs (PVC sheets):
- Triangular – pod structure
- Square-pod structure
Factors determining the design of waffle slab
- The total area required for the waffle slab
- The thickness of waffle slab
- Waffle slab overall depth
- The distance between two ribs in a row
Waffle Slab Construction Method;
The construction of waffle slabs can be done in three ways however the basic method is the same in each:
- In-situ- this method involves placing formwork and pouring concrete over it. As per the design and the size required slab is cast on site
- Precast- in this method slab panel casting is done elsewhere and then inserted and joined with proper reinforcement and concrete pour.
- Prefabricated- steel bars are used for this method and reinforcement is provided in the slab panels. Slabs are manufactured elsewhere and then taken to the site and assembled. They do not require internal reinforcement on the site.
Application method steps;
- Formwork preparation
- Lay in the formwork elements
- Place your waffle pods on shuttering
- Arrange horizontal and vertical support elements by the connectors
- Working from one corner, lay out the waffle pods and spacer in a grid pattern within the formwork, as specified in the footing design
- Use cube junctions to fix the corners of pods with the framework
- Place reinforcement bars into position on the spacers in between the waffle Pods
- After fixing the formwork, reinforcement is placed in the two directions
- Lay top mesh, following design specifications and fasten where necessary.
- Pour the concrete and vibrate well, ensuring intersecting ribs are filled
- After hardening, pods and frameworks are removed from the bottom.
- Stacking is provided in the end
Applications of waffle slab;
It is used where massive span slabs are to be constructed in areas where the number of columns is small. Its application includes commercial and industrial buildings such as Airports, Hospitals, Auditoriums etc.
Waffle slabs can carry a greater amount of load compared with conventional concrete slabs. Discussed above are different advantages, types and construction methods of waffle slabs. In an upcoming series of articles, we will cover other slab types.
Analysis of Waffle slabs construction and advantages