Slabs are statically indeterminate elements, therefore, redistributing the stresses applied to them. This ability makes them highly safe against bending and shear failure. Cantilever slabs do not dispose of this property as they are statically determinate elements and therefore need special care in their construction.

Slabs are surface plane elements that bear loads transverse to their plan

Depending on their support conditions, slabs are classified as follows
One-way slabs (simply supported) They are those supported on two out of four, opposite sides like S1 of the bottom example.
Two-way slabs They are those supported on all four sides like S2, S3 of the bottom example
Cantilever slabs They are those with a fixed support on only one out of four sides, like S4 of the bottom example.
Two-way slabs supported on three edges They are those supported on three out of four sides, like S5 of the bottom example.
Two-way slabs supported on two adjacent edges They are those supported on two adjacent sides, like S6 of the bottom example.

Hypothetical formwork plan, where the arrows indicate the supported sides of slabs

Slab behaviour

As regards to support, the safest slab is the two-way slab and the most vulnerable one is the cantilever slab. This happens because, in a two-way slab, potential loss of one support results in a two-way three support slab, while in the case of a cantilever slab potential support loss results in collapse.

The slab with the bilateral continuity (a) behaves in the best way. Next comes the slab with the unilateral continuity (b) while last comes the slab with no continuity (c)

One–way ribbed slab

A ribbed slab with a total thickness of 300 mm may have a self-weight equal to 3.75kN/m², which corresponds to the self-weight of a 150 mm thick solid slab.

Ribbed slabs (waffle slabs) (Zoellner, sandwich) can be used for large spans, like the one shown in the figure

Two–way ribbed (waffle) slab 

The square voids are being shaped either using lightweight fillings like polystyrene or using plastic moulds. The latter has significant constructive advantages (qualitative, fast construction) thus providing economical solutions.

Usually, two-way slabs have square voids between the ribs because the demand for adequate bending strength is equal in both directions
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