Engineering and Construction

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Universal beams supporting precast prestressed floor units (Fig. 2.28) have some advantages over other forms of construction. Although of heavier construction  than comparable composite metal deck floors, this system offers the following advantages.

• Fewer floor beams since precast floor units can span up to 6–8m without  difficulty.
• No propping is required.
• Shallow floor construction can be obtained by supporting precast floor units on shelf angles or on wide plates attached to the bottom flanges of universal columns acting as beams (Slimfloor).
• Fast construction because no time is needed for curing and the development of concrete strength.

On the other hand the disadvantages are:

• Composite and diaphragm action is not readily achieved without  a structural floor screed.
• Heavy floor units are difficult to erect in many locations and require the use of a tower crane, which may have implications for the construction programme.

Fig. 2.28 Precast concrete floors

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Deep metal decks are normally used to create shallow floors e.g. Slimflor® and Slimdek® construction. The deep metal deck extends the span capability up to 9m; however, the deck and/or support beams may require propping during concreting.

An additional tensile reinforcement bar is provided within the  ribs of the deep decking to improve the load carrying capacity and fire resistance of the floor slab.

Although there are several variants internationally, Slimflor® construction in the UK comprises a universal column with a plate welded to the underside supporting the deep metal decking. Shear studs are shop-welded to the top flange of the beam to form a connection between the steel beam and concrete slab. Slimdek® construction is a technologically advanced solution comprising a rolled asymmetric beam (ASB) which supports the deep metal decking directly. Shear connection is achieved through the bond developed between the steel beam and concrete encasure. These features reduce material and fabrication content. Partial concrete encasement of the steel beam provides up to 1 hour inherent fire resistance.

The range of deep metal deck profiles (Fig. 2.25(b)) is more limited than for shallow decks, and those available carry similar attributes and  advantages. Some additional advantages of Slimflor ® and Slimdek® construction are:

• The shallow composite slab achieves excellent load capacity, diaphragm action and robustness
• There are fewer frame components to erect, saving construction time
• The shallow floor construction allows more floors for a given building height or
reduces the cost of cladding and vertical services, lift shafts, etc.
• It provides up to 1 hour inherent fire resistance and up to 2 hours using passive fire protection to the beam soffit only
• Large openings for vertical services can be formed in the floor slab without the need for secondary framing
• Services can be integrated between the decking ribs passing through web- openings in the beam
• ASBs achieve composite action without the need for shear studs
• Rectangular hollow section edge beams provide good torsional resistance and maintain the shallow floor depth
• Floor slabs can be used for fabric energy storage forming part of an environmentally sustainable building solution.

Figure 2.27 illustrates a typical beam layout at the building perimeter.

Fig. 2.27 Slimdek® floor arrangement

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Experience has shown that the most efficient floor arrangements are those using shallow metal decking spanning about 3–4.5m between floor beams. For these spans the metal decking does not normally require propping during concreting and the concrete thicknesses are near the practical minimum for consideration of strength and fire separation.

Steel studs are welded through the decking on the flange of the beams below to form a connection between steel beam and concrete slab. Concrete, which may be either lightweight or normal weight, is then poured on to the decking, usually by pumping, to make up the composite system. Shallow metal decking acts both as permanent formwork for the concrete and as tensile reinforcement for the slab. There are many types of steel decking available (Fig. 2.25(a)) but perhaps the most com- monly used is the re-entrant profile type, which provides a flat soffit and facilitates fixings for building services and ceilings.

Fig. 2.25 Metal deck profiles: (a) shallow deck (50–100mm); (b) deep deck (150–250mm)

Some of the advantages of composite shallow metal deck floor construction are:

• Steel decking acts as permanent shuttering,which can eliminate the need for slab reinforcement and, due to its high stiffness and strength, propping of the con- struction while the wet concrete develops strength.
• Composite action reduces the overall depth of structure.
• It provides up to 2 hours fire resistance without additional fire protection and 4 hours with added thickness or extra surface protection.
• It is a light, adaptable system that can be easily manhandled on site, cut to awkward shapes and drilled or cut out for additional service requirements.
• Lightweight construction reduces frame loadings and foundation costs.
• It allows simple, rapid construction techniques.

Figure 2.26 illustrates alternative arrangements of primary and secondary beams for a deck span of 3m.

Fig. 2.26 Alternative framing systems for floors: (a) long span secondary beams; (b) long span primary beams