Slabs used in building works can be classified based on CONFIGURATION, STRUCTURAL ARRANGEMENT AND STRUCTURAL ACTION.
BASED ON CONFIGURATION, WE HAVE THE FOLLOWING TYPES OF SLAB:
1. Solid Slab: Solid slab is a slab with uniform thickness and is usually constructed of the same material throughout. It is the commonest type of slab used in buildings especially in residential areas and offices. It is also the easiest to design and construct and used when the span is ≤ 6m (See Figure 1).
2. Ribbed Slab: These types of slab are ribbed of certain portions (usually at their tension zones) in order to achieve reduction in their self-weight. They are often used in offices and buildings where large spans are expected. They can be whole concrete ribbed floor slab or ribbed floor with hollow pots in-fill. The floor consists of series of T-beams closely spaced in most cases between 400 and 600 mm. The slab is more or less designed as T-beams with thin concrete slabs as flanges. When they are used with clay pots, they are referred to as self-supporting floor and they can be of pre-cast units hoisted to position (See Figure 2). Advantages of ribbed slabs over solid slabs is significant reduction of self-weight and economical over large span. The disadvantage is limitation on the imposed loads it can support (≤ 3kN/m2).
3. Flat Slab: These are slabs supported directly by columns and not by beams or walls (see Figure 3). They are used where large spans and/or heavy live loads are required or where aesthetics would be pleasing. The heavy imposed loads on these slabs result in high shear stresses at the supports which may cause failure as columns tend to punch through the slab. One of the following measures are taken to reduce shear stresses:
a. Use columns with large diameters and or slabs with increased thickness.
b. Enlargement or flaring of column heads.
c. Thickening the slab at column positions by providing drop panels which may be rectangular or conical.
Advantages of flat slabs:
i. Since they are beamless, they allow light and air circulation around the premises.
ii. They offer reduced storey height.
iii. The formwork is easy to make.
4. Waffle slab: This is an extension of ribbed floor slab in which the slab is ribbed in two directions. This causes the formation of inverted pot-like hollow which serves as ceiling for the floor below (see Figure 4). This type of slab can support large live loads up to 5.0 kN/m2 and are commonly used in banking halls, markets, shopping plazas and libraries.
There are two type of waffle slabs viz:
1. Slab and beam waffle: In this type, the waffle terminate into an end beam of the same thickness or deeper.
2. Mushroom waffle: In this type, edge beams are absent but the ribs terminate into a solid slab capital around the columns. (See Figures)
BASED ON STRUCTURAL ARRANGEMENT, WE HAVE:
1. Simply Supported Slab: This type of slab does not have edge continuity usually linked by beams. They are common in overhead tanks.
2. Continuous Slab: This type of slab is common in real buildings. They have spans continuous over one or more of their edges (support).
3. Cantilever Slab: This type of slab has support at one edge and free at 3 other edges in most cases. They are often introduced for aesthetics or where space restriction is on the ground.
BASED ON THE STRUCTURAL ACTION, WE HAVE:
1. One-way spanning slab: A slab is a one-way spanning slab when the aspect ratio (Ly/Lx) that is the ratio of the longer span to the shorter span is greater than 2 (Ly/Lx ˃ 2). Also, the structural behaviour of the slab is in one direction only.
2. Two-way spanning slab: A slab is two-way spanning slab when the aspect ratio (Ly/Lx) is less than or equal to 2 (Ly/Lx ≤ 2). Also the structural behaviour is in two direction.
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