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Suspended floors

A suspended concrete floor is a floor slab where its perimeter is, or at least two of its opposite edges are, supported on walls, beams or columns that carry its self weight and imposed loading. The floor spans between supports and will normally deflect under load to a dimension that is limited by the design used. The prestressing of a concrete floor slab can eliminate its usual deflection below support level. This is achieved either by pre-tensioning, where precast floor units are used, or by post tensioning with steel wire tendons incorporated in an in-situ concrete slab.

Suspended floors can be constructed in three basic forms:-

1. as wholly of reinforced in-situ concrete,
2. as a floor consisting of reinforced or prestressed precast concrete units, usually spanning in one direction,
3. as a floor comprising reinforced or prestressed precast concrete units overlaid by an in-situ concrete layer formed in such a way that it acts compositely with the precast concrete units.

Another form of composite construction is that of an in-situ concrete slab cast on and integrally with a profiled metal decking supported usually on the beams of a steel framed building. Some aspects of the construction of this type of slab and associated design considerations are given in Concrete Society Technical Report 75, Composite concrete slabs with steel decking.

Where a concrete floor slab is to be built on weak ground of poor supporting characteristics, it may be supported on concrete pile foundations or ground beams to prevent excessive slab settlement under imposed loading. These slabs will have to be designed and reinforced as suspended floors.

Acknowledgement: The Concrete Society

Concrete Bookshop - Members receive 40% discount on Concrete Society publications

Suspended concrete floors: maximum size of pour allowable and location of construction joints

Reynolds's reinforced concrete designers handbook (Hardback)

TR75 Composite concrete slabs using steel decking