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Steel/concrete suspended floors

As an alternative to concrete precast units (see Structural elements/Suspended floors/Composite suspended floors), profiled steel decking may be used as permanent formwork with in-situ concrete (often lightweight aggregate concrete) to form a composite floor slab. This construction method is mainly used in steel frame, high rise buildings, though can be used with any type of frame. The profiled steel provides all the tensile reinforcement for the completed slab; nominal steel fabric is provided to control shrinkage and thermal cracks in the in situ concrete, and to give some continuity over supports.

A recent innovation has been the use of either steel or macro synthetic fibres in the concrete as a replacement for the nominal steel fabric. (Before omitting all the fabric, the designer should satisfy himself that all structural aspects are adequately catered for. Fibres are only used to replace the nominal fabric; any additional reinforcement required for structural purposes, e.g. U-bars for edge beams, will still be required. Reinforcement will also be required to provide continuity through any construction joints in the slab.) The approach has been justified by work commissioned by SCI (the Steel Construction Institute) for specific combinations of fibres and decking. To justify the use of fibres, tests were carried out to determine their effect on the transverse shear resistance of composite slabs and the capacity of the welded studs. In order to determine the performance in fire, tests were carried out on full-scale specimens. The range of application of the results has been extended to other slab thicknesses and spans by the use of a fire engineering model, allowing suppliers to provide appropriate design tables for 60, 90 and 120 minutes fire resistance.

Further information on the use of fibres in composite floor slabs may be found in Concrete Society Technical Report 63, Guidance for the design of steel-fibre-reinforced concrete and Technical Report 65, Guidance on the use of macro-synthetic-fibre-reinforced concrete. Further information on the construction and associated design considerations is given in Concrete Society Technical Report 75, Composite concrete slabs with steel decking.

Acknowledgement: The Concrete Society

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TR63 Guidance for the design of steel-fibre-reinforced concrete

TR65 Guidance on the use of macro-synthetic fibre reinforced concrete

TR75 Composite concrete slabs using steel decking