Ribbed floors consisting of equally spaced ribs are usually supported directly by columns. They are either one-way spanning systems known as ribbed slab or a two-way ribbed system known as a waffle slab. This form of construction is not very common because of the formwork costs and the low fire rating. A 120-mm-thick slab with a minimum rib thickness of 125 mm for continuous ribs is required to achieve a 2-hour fire rating. A rib thickness of greater than 125 mm is usually required to accommodate tensile and shear reinforcement. Ribbed slabs are suitable for medium to heavy loads, can span reasonable distances, are very stiff and particularly suitable where the soffit is exposed.
Slab depths typically vary from 75 to 125 mm and rib widths from 125 to 200 mm. Rib spacing of 600 to 1500 mm can be used. The overall depth of the floor typically varies from 300 to 600 mm with overall spans of up to 15 m if reinforced, longer if post-tensioned. The use of ribs to the soffit of the slab reduces the quantity of concrete and reinforcement and also the weight of the floor. The saving of materials will be offset by the complication in formwork and placing of reinforcement. However, formwork complication is minimised by use of standard, modular, reusable formwork, usually made from polypropylene or fibreglass and with tapered sides to allow stripping.
For ribs at 1200-mm centres (to suit standard forms) the economical reinforced concrete floor span ‘L’ is approximately D x 15 for a single span and D x 22 for a multi-span, where D is the overall floor depth. The one-way ribs are typically designed as T-beams, often spanning in the long direction. A solid drop panel is required at the columns and loadbearing walls for shear and moment resistance.
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