The underside of a waffle slab, showing the grid like structure
Awaffle slab ortwo-way joist slab is aconcrete slab made ofreinforced concrete withconcrete ribs running in two directions on its underside.[1] The namewaffle comes from the grid pattern created by the reinforcing ribs. Waffle slabs are preferred for spans greater than 40 feet (12 m), because, for a given mass of concrete, they are much stronger than flat slabs, flat slabs with drop panels, two-way slabs,one-way slabs, and one-way joist slabs.[2]
Section of a waffle slab including beam, ribs, and column head
A waffle slab is flat on top, whilejoists create a grid like surface on the bottom. The grid is formed by the removal ofmolds after the concrete sets. This structure was designed to be more solid when used on longer spans and with heavier loads.[3] This type of structure, because of its rigidity, is recommended for buildings that require minimal vibration, likelaboratories andmanufacturing facilities.[3] It is also used in buildings that require big open spaces, liketheatres ortrain stations.[2] Waffle slabs are composed by intricateformwork, and may be more expensive than other types of slabs, but depending on the project and the quantity of concrete needed it may be cheaper to build.
A waffle slab can be made in different ways but genericforms 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[clarification needed] andsteel bars.[4] 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).[4]
Precast: The slabs are made somewhere else and then brought to the site and assembled.[4]
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.[4]
Different guides have been made for architects and engineers to determine various parameters of waffle slabs, primarily the overall thickness and rib dimensions. The following are rules of thumb, which are explained further in the accompanying diagrams:
Diagram shows slab and rib width with rules of thumb formula
Slab depth is typically 75 mm (3 in) to 130 mm (5 in) thick.[5][3] As a rule of thumb, the depth should be1⁄24 of the span.[5]
The width of the ribs is typically 130 mm (5 in) to 150 mm (6 in), and ribs usually have steel rod reinforcements.[5]
The distance between ribs is typically 915 mm (3 ft).[3]
The height of the ribs and beams should be1⁄25 of the span between columns.[3]
The width of the solid area around the column should be1⁄8 of the span between columns. Its height should be the same as the ribs.[3]Diagram showing waffle slab rib and Beam Heights rule of thumb formulas.
Diagram shows the width of the column head with rule of thumb formula
Greater quantities of formwork materials are needed, which can be very costly
Waffle slabs are thicker than flat slabs, so the height between each floor must be greater to have enough space for the slab system and other building services
Waffle slabs are preferred for flat topographical areas not sloped sites[4]
