Ribbed or waffle slab is a slab system which consists of series of parallel reinforced concrete T beams framing into reinforced concrete girders. The slab is the flange of the beam and the extended part is the web. The extended part is known as ribs. The spacing between the ribs should be in general 20-30 inch. The ribs are tapered in cross-section in its lower part.
A one-way joist floor slab consists of a series of small, reinforced concrete T beams that are connected with girders that in turn carried by the building column. T beams are known as joists which are formed by setting steel pan at a constant spacing. Concrete is cast between those spacing to make those ribs and in this way, the slab also cast and the slab becomes the flange of T beam.
The system was designed to decrease the weight of traditional full-concrete slabs. The dome-shaped forms create a matrix of voids surrounded by orthogonal ribbing, producing a two-way configuration very suitable for large-spanning slabs. Voids between all the domes reduce the dead load as the width of that portion of the slab is less than a flat slab. This type of slab is known as waffle slabs because they look like waffles with rows of beams running underneath them. Waffle slab shows a ceiling which is suitable to install all the electrical appliances and can give a better outlook thus increasing the aesthetic factor.