The often-used term ‘composite metal deck’ is slightly misleading, as the metal decking on its own cannot be classed as a composite construction element. It’s the combination of the metal decking (with tensile strength) acting together with a concrete slab (with compressive strength), connected by shear studs to a steel frame that forms the basic principle of composite steel deck flooring systems.
Design of the composite deck, slabs and beams must be considered in two separate stages.
In the construction stage, the metal decking and the beams need to be adequately designed to resist the temporary loading from wet concrete, reinforcement and concreting operations without the need for mid-span propping. Use of good construction practice will prevent detrimental loading from the heaping of wet concrete on the decking.
In the composite stage, or final condition, ‘in service’ imposed and dead loads and fire resistance will need to be considered for all structural elements.
Metal deck profiles have indentations and embossments rolled into them, which serve to create a mechanical interlock between the concrete and the deck itself, enhancing the shear bond between them. The re-entrants and ribs also effectively act to keep the deck and the composite slab as one unit.
The fully cured concrete slab, acting together with the tensile strength of the decking, becomes a composite slab. Transverse reinforcement of the concrete ‘flange’ over composite beams is usually provided by steel mesh and/or additional bars but in locations where the decking spans perpendicular to the beam centre line, the deck can also be considered as transverse reinforcement.
A beam is considered composite when it acts together with the slab and deck via the shear connectors and its load carrying capacity and stiffness are increased as a result. There are three basic types of composite beam.
The one most commonly used with composite metal decking is the downstand Universal Beam section. It’s referred to as ‘downstand’ because the composite slab sits on the top flange, connected to it by thru-deck welded studs or shear connectors. The studs do not act simply to fix the deck to the supporting beams. As stated above, the decking can contribute to the transverse reinforcement of the concrete flange over the beam and it’s the action of the studs that enables this. Decking spanning perpendicular to the downstand beams can provide lateral restraint to the beams during construction.
The downstand beams can also be used with precast concrete hollowcore slabs or planks and a concrete topping. The slabs or planks stop short of the beam centreline to allow for placement of shear studs. The studs would normally be shop-welded to the beams in this case.
Composite beams can also be part of a shallow floor system, minimising storey height. The shallow floor is achieved by placing the slabs and beams within the same zone, with either metal decking or precast concrete slabs bearing onto wide bottom flanges. Either shallow or deep decking may be used. Shallow floor construction can inherently achieve composite interaction between the beams and slab and has the advantage of a flat soffit to facilitate distribution of services.
So why use composite construction?
It has distinct advantages over non-composite in-situ reinforced concrete slab construction. These include:
- No requirement for traditional removable formwork and its associated forest of props.
- Overall reduction in slab self-weight and therefore in loading on the steel frame and foundations with resultant reduction in materials.
- Reduction in the amount of reinforcement required.
- Rapid installation, even on complex steel layouts.
- Provision of a safe working platform and cover for following trades immediately after installation.
- Enhanced structural stability with metal decking contributing to lateral restraint of the frame and tensile reinforcement of the slab and, in some cases, to diaphragm action that transfers wind loads back to the structure.
The Eurocodes for design of composite steel members and composite steel and concrete structures are BS EN 1993-1-3 (Eurocode 3) and BS EN 1994-1-1 (Eurocode 4). The equivalent British Standards are BS 5950-4 and BS 5950-6.
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