What is a slab & its types?
In any building the most common structural element of
construction to create a flat surface such as floor & roof is called slabs.
The depth of the slab is very less compared to its effective length. Different
loads act on a slab, such as the dead load of the slab, live load on a slab,
super-imposed dead load, earthquake load. There are different kinds of slabs
based on their support conditions.
Types of slabs:
- One-way Slab – Slabs whose longer dimension
exceeds two times the shorter dimension is termed as a one-way slab. They are
supported by beams on two opposite sides. In one-way slabs the imposed loads
are transferred to the two opposite faces of the slab & in one direction,
hence the name one-way slabs. In one way slab, the main reinforcements run
along the shorter edges while distribution reinforcements run along the
transverse direction. The most common example of a one-way slab is a balcony
(generally). The load distribution is illustrated in the following figure.
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| Load Distribution in One-Way Slab |
- Two-way Slab – Slabs whose longer dimension does
not exceed two times the shorter dimension is termed as a two-way slab. They are
supported by beams on all four sides. In two-way slabs the imposed loads are
transferred to the two opposite faces of the slab & in both the direction,
hence it is called two-way slabs.
Generally, floor slabs in a multi-storeyed building is a two-way slab. In Two-way slab, the bending moment is maximum at the center of the slab and the
larger moment invariably develops along the shorter span. When a slab is simply supported on all four sides and is
subjected to transverse loading, the bending effect in two main directions of
loading causes the corner at the supports of the slab to curl and lift up. This
bending moment effect in two directions gives a torsional effect to the slab,
therefore during designing the corners are restrained.
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| Load Distribution in Two-Way Slab |
- Cantilever Slab – Slabs that are supported on
one side only are termed as cantilever slabs. In cantilever slab, the main
reinforcement runs along the shorter span and from the top, as the main moment is
generated at the top supports. Horizontal projections of a building are
generally cantilever slab.
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| Load Distribution in Cantilever Slab |
- Flat Slab – Places where the clear height is an
important factor or where the use of beams restricts the usability of the
space, to avoid beams flat slabs are used. More suitably it is a multi-spanned
slab supported by columns directly without the use of beams. There are
different types of flat slabs such as (a) Slabs without drops & column head (b) Slabs without drops & column with
column head (c) Slab with drop & column head.
Bending Moment in a slab:
The bending moment in a slab depends upon the following
criteria:
- Support conditions of the slabs, also known as
edge condition. (Like free, fixed continuous)
- The short & long span (Lx and Ly)
- Type of loading on the slab (Uniformly
distributed load, Concentrated load, etc).
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