Types of Slab in structure
A reinforced concrete slab is a crucial structural element and is used to provide flat surfaces(floors and ceilings) in buildings. On the basis of reinforcement provided, beam support, and the ratio of the spans, slabs are generally classified into one-way slab and two-way slab. The former is supported on two sides and the ratio of long to short span is greater than two. However, the latter is supported on four sides and the ratio of long to short span is smaller than two.
Varying conditions and stipulations ask for the selection of appropriate and cost-effective concrete slab, keeping in view, the type of building, architectural layout, aesthetic features, and the span length. Concrete slabs, therefore, are further classified into one-way joist slab, flat slab, flat plate, waffle slab, hollow core slab, precast slab, slabs on grade, hardy slab, and composite slab.
1. One-Way Slabs on Beams
Cast in situ method is used to construct one-way slabs on beams which involves fixing of forms followed with the installation of reinforcements, and finally the pouring of fresh concrete.
One-way slabs on beams are most suitable for spans of 3-6m, and a live load of 3 to 5KN/m2. They can also be used for larger spans with relatively higher cost and higher slab deflection. Additional form-work for the beams is however needed.
2. One-way joist slab (Ribbed slab)
It consists of a floor slab, usually 50 to 100mm thick, supported by reinforced concrete ribs (or joists). The ribs are usually tapered and are uniformly spaced at distances that do not exceed 750mm. The ribs are supported on girders that rest on columns.A one-way joist concrete slab is suitable for spans of 6-9m and live loads of 4-6KN/m2. Because of the deep ribs, the concrete and steel quantities are relatively low, but expensive formwork is needed.
3. Waffle Slab (Grid slab)
It is a type of reinforced concrete slab that contains square grids with deep sides. Waffle slab construction process includes fixing forms, placement of pods on shuttering, installation of reinforcement between pods, installation of steel mesh on top of pods, and pouring of concrete.
Grid slabs are suitable for spans of 9-15m and live loads of 4-7KN/m2. Formwork, including the use of pans, is quite expensive.
4. Flat Plates
Flat plates can be constructed as one-way or two-way slabs and it is directly supported by columns or walls. It is easy to construct and requires simple formworks.
Flat plates are most suitable for spans of 6 to 8m, and live loads between 3 and 5KN/m2. Added to that, the range of spans for prestressed flat plates is between 8-12m, and it can also be constructed as post-tensioned slabs.
The advantages of adopting flat plates include low-cost formwork, exposed flat ceilings, and faster construction. Flat plates have low shear capacity and relatively low stiffness, which may cause noticeable deflection.