What Is A Waffle Slab Or Two-Way Joist Slab?
What Is A Waffle Slab Or Two-Way Joist Slab?
A waffle slab is a type of reinforced concrete slab that has concrete ribs running in two directions on its underside. This creates a grid pattern, hence the name “waffle.” These slabs are used for spans greater than 40 feet and are stronger than other types of slabs such as flat slabs, flat slabs with drop panels, two-way slabs, one-way slabs, and one-way joist slabs.
Waffle slabs are flat on top and have a grid-like surface on the bottom created by the removal of molds after the concrete has been set. They are often used in buildings that require minimal vibration such as laboratories and manufacturing facilities, and in buildings with large open spaces such as theaters or train stations.
Waffle slabs may be more expensive to build due to the intricate formwork required but they may also be more cost-effective depending on the specific project and the amount of concrete needed.
Types Of Waffle Slab
There are two types of ribbed slab systems: one-way and two-way. A one-way ribbed slab system consists of small reinforced concrete T beams connected with girders that are supported by building columns.
The T beams are formed by placing steel pans at a constant spacing and casting concrete between them to create ribs with the slab also being cast to become the flange of the T beam. A two-way ribbed slab system or waffle slab is designed to reduce the weight of traditional full-concrete slabs.
It features a matrix of voids surrounded by orthogonal ribbing forming a dome-shaped structure that is suitable for large-spanning slabs. The voids in the waffle slab reduce the dead load as the width of the slab is less than a flat slab.
Waffle slabs are also called waffle slabs because they resemble waffles with rows of beams running underneath them. They are suitable for installing electrical appliances and have an aesthetic appearance.
Advantages Of A Waffle Slab Or Two-Way Joist Slab
There are several benefits to using a waffle slab floor system:
- It is effective at reducing vibrations in buildings due to its two-way joist reinforcements.
- The system allows for longer spans with less material making it cost-effective and environmentally friendly.
- Some people may find the waffle pattern visually appealing.
- It has a higher load capacity compared to traditional one-way slabs.
- Forms for the waffle slab can be made from wood, concrete or steel.
Disadvantages Of A Waffle Slab Or Two-Way Joist Slab
There are several disadvantages to using a waffle slab;
- More formwork material is required which can increase the cost of the project.
- Waffle slabs are thicker than flat slabs so there needs to be more space between floors to accommodate the slab system and other building services.
- Waffle slabs are best suited for flat rather than sloped sites.
Construction Process For A Waffle Slab Or Two-Way Joist Slab
There are several ways to create a waffle slab, but they all involve using formwork to give the slab its waffle shape. This formwork consists of various elements, including waffle pods, horizontal and vertical supports, cube junctions, hole plates, clits, and steel bars. The process typically involves constructing the supports, arranging the pods in place, and pouring the concrete.
There are three main approaches to this process: in situ, precast, and prefabricated. In situ involves constructing the formwork and pouring the concrete on site, and then assembling the slab if necessary. Precast involves creating the slab offsite and then transporting it to the construction site for assembly. Prefabricated involves integrating the reinforcements into the slab during manufacturing, eliminating the need for on-site reinforcement. This is the most expensive option.
