Make your slab construction strong with Ribbed (Waffle) Slab System

Ribbed floors are made with ribs paced uniformly. The columns are used to provide support to the directly. The ribs are categorized as ribbed slab and waffle slab. In ribbed slab, there is one-way spanning and in a waffle slab, there is two-way ribbed system.

Due to the high formwork cost and degraded fire rating, this type of construction generally is not recommended. A 120-mm-thick slab containing a lowest rib thickness of 125 mm for continuous ribs is necessary to attain a 2-hour fire rating. A rib thickness exceeding 125 mm is generally essential to adjust tensile and shear reinforcement. Ribbed slabs are ideal for medium to heavy loads. They can extent reasonable distances. They are very inflexible and can be used specifically where the soffit is uncovered.

Slab depths usually differ from 75 to 125 mm and rib widths from 125 to 200 mm. There is Rib spacing of 600 to 1500 mm. The complete depth of the floor generally differs from 300 to 600 mm with overall extents of up to 15 m when reinforced, extended when post-tensioned.

The application of ribs to the soffit of the slab minimizes the quantity of concrete and reinforcement as well as the weight of the floor. The complexity in formwork and arrangement of reinforcement will offset the saving of material. By applying standard, modular, recyclable formwork, the complexity in formwork can be reduced. These are generally formed with polypropylene or fibreglass and with tapered sides to facilitate stripping.

For ribs at 1200-mm centres (to fit standard forms) the economical reinforced concrete floor span ‘L’ is about D x 15 for a single span and D x 22 for a multi-span, where D denote the overall floor depth. The one-way ribs are usually designed as T-beams, frequently stretching in the long direction. A concrete drop panel is essential at the columns and loadbearing walls toward shear and moment resistance.

Pros:

• Savings on weight and materials
• Long spans
• Beautiful soffit appearance if uncovered
• Cost-effective if reusable formwork pans are applied
• Vertical penetrations among ribs are smooth.

Cons:

• Depth of slab among the ribs may check the fire rating
• Necessitates special or proprietary formwork
• Larger floor-to-floor height
• It becomes tough to manage large vertical penetrations

Ref: civildigital.com& Image Courtesy: alibaba.com