Waffle slab explained. What is Ribbed Slab? Advantages and Disadvantage of Waffle Slab

Waffle slab explained. What is Ribbed Slab? Advantages and Disadvantage of Waffle Slab

Waffle slab, also known as ribbed slab, is a structural component with a grid-like arrangement on the bottom surface and is plain on top. The top of a ribbed slab is often thin, whereas the lower grid lines are usually ribs arranged perpendicular to each other and of equal depth. The waffle slab is reinforced in two directions.

All of the ribs come from the column heads or beams. The depth of the ribs is the same as the depth of the column head or beam. It is more stable and suitable for bigger span slabs or foundations because to the ribs and double reinforcing.

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What are the Characteristics of Waffle Slab or Ribbed Slab

Waffle slabs are best used on level surfaces.

In comparison to others, the amount of concrete used is quite small.

The waffle slab’s reinforcement comes in the shape of mesh or individual bars.

In the case of a waffle slab, no separate beam excavation is necessary.

The slab’s bottom surface resembles waffle, which is achieved by employing cardboard panels or pods, etc

The recommended waffle slab thickness is 85 to 100 mm, with a maximum overall depth of 300 to 600 mm.

The beams or ribs in a waffle slab are typically 110 to 200 mm wide.

Rib spacing should be between 600 and 1500 mm.

Reinforced waffle slabs can be built for spans up to 16 meters, however prefabricated waffle slabs are preferred for spans longer than that.

Waffle slabs are less expensive than reinforced rafts and footing slabs and have better shrinkage resistance.

Waffle slabs only require 70% concrete and 80% steel from the concrete and steel required for the strengthened raft.

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What is the  Construction Procedure of Waffle Slab or Ribbed Slab

Waffle slabs can be made in three different methods, as shown below.

In-situ

Precast

Prefabricated

In-situ waffle slabs are made by pouring concrete on the site or in the field with the right setup.

Precast waffle slabs are made up of slab panels that are cast somewhere and then connected together with adequate reinforcement before being filled with concrete.

The third option, a prefabricated waffle slab, is the most expensive of the three. In this case, the slab panels are reinforced while being cast with some tension. As a result, they do not require any interior site reinforcement.

In order to construct a waffle slab in situ, formwork will be required to support the slab. In the case of a waffle slab, however, some additional tools are required for the form work.

What are the formwork tools are required for the fabrication of a Waffle Slab or Ribbed Slab:

  • Pods of waffle
  • Supports that are horizontal
  • Supports that are vertical
  • Connectors for the walls
  • Intersections of cubes
  • Plates with holes
  • Clits
  • Bars of steel

The horizontal and vertical supports are placed first, then the connectors secure them in place. Wall connectors are used at the edges to connect the wall to the slab. Small beam connectors link the horizontal beam supports, forming a square-like structure in which pods will be put.

waffle-slab

The pods are typically constructed of plastic, and they come in a variety of sizes and shapes. The size of the pod is determined by the requirement and the length of the span. A large number of pods is necessary for a longer span. For one whole slab, the same size should be used.

Similarly, beam connectors and cube junctions come in a variety of sizes to accommodate varied pod sizes.

Cube joints are used to connect the frame construction to the corners of pods. When securing the formwork, reinforcement is inserted in the slab’s two directions, and concrete is poured into the gaps, which are referred to as ribs after they have hardened.

A thin concrete slab is placed on top, and pods and frames are removed from the bottom after it has hardened. As a result, the bottom surface has a waffle-like form.

What are the advantages of Waffle Slab or Ribbed Slab Construction

Waffle slabs are utilized for larger span slabs or floors and when there is a restricted number of columns required.

Waffle slabs have a higher load carrying capacity than other slab types.

They offer structural stability as well as an attractive look. As a result, it is used in airports, hospitals, temples, and churches, among other places.

Waffle slabs can be built of concrete, wood, or steel. Concrete waffle slabs are chosen for commercial structures, while the other two are favoured for garages, decorative halls, and other applications.

Because to the two-directional reinforcement, it has an excellent vibration control capacity. As a result, controlling vibrations caused by crowd movements is beneficial for public structures.

Waffle slabs are lightweight and use less concrete, making them cost-effective.

With proper supervision, building a waffle slab is simple and quick.
Since the amount of concrete and steel required is minimal, a light framework is sufficient for the waffle slab.

Lighting, plumbing pipes, electrical wiring, air conditioning, insulating materials, and other services can be given within the depth of the waffle slab by drilling holes in the waffle bottom surface. Holedeck is the name given to this system.

What are the Disadvantage of Waffle Slab or Ribbed Slab

Because of the vast quantity of pods required and some unique techniques, the formwork tools required are highly expensive.

As a result of the increased floor height, the number of floors is lowered.

The services supplied in the waffle configuration may cause damage to the slab if they are not properly maintained.

During construction, skilled workers are required.

They are not appropriate for locations with a slope. If there is a slope, it must be filled or excavated to make the area level. Good soil should be used for soil filling.

Because of their light weight, they are not appropriate for heavy winds or cyclonic locations.

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