Deep deck composite floors

Deep deck composite slabs are about 200-300mm thick with a maximum span of about 8m.

Deep deck systems also use profiled steel sheeting, but the profiles are much deeper - typically about 210-225mm - and are spaced at about 600mm centres. Because of the increased depth, the decking is much stronger and stiffer, and significantly longer spans - up to about 6.5m (unpropped) and 9m (propped) - can be achieved.

Deep deck floors are normally supported on the bottom flanges of the supporting beams.

To avoid excessive overall construction depths, the decking is designed to be supported on the bottom flanges of the supporting beams. Standard end pieces are used to close the voids at the supports, thereby containing the wet concrete during casting.

The voids between the ribs of a deep deck floor can be used to accommodate services.

Further compression of the construction depth can be achieved by running services between the ribs of the deep deck floor structure. In most cases the service runs can be continued through holes cut in the web of the supporting beams without detriment to the structural performance.

The supporting structure for deep deck composite floors is normally in the form of slim floor beams.

Because of the need to support the deep deck on the bottom flange and integrate the slab thickness within the beam depth, the supporting beams are generally of a somewhat different shape from conventional steel floor beams.

Slim floor beams are relatively shallow and are asymmetric with a wide bottom flange

In order to minimise construction depth the depth of the supporting beams should be approximately the same as the slab thickness. Slim floor beams are therefore typically about 300mm deep. This restriction on depth potentially limits the bending strength and stiffness of the beam, and this is compensated for by using thicker flanges.

The installation of the decking is facilitated by adopting a wider flange.

Standard slim floor beams are now available in a limited range of sizes. Alternatively they may be specially fabricated.

Slim floor beams may be specially fabricated, but a new section - the Asymmetric Slimflor Beam (ASB) - has been specially developed and is now manufactured as part of the standard range for this purpose. Fabricated beams are often based on Universal Column (UC) sections with an additional steel plate welded to the bottom flange to provide the increased width. Slim floor beam sections can also be fabricated from individual plates to form the flanges and web.

Slim floor construction, especially with the Asymmetric Slimflor Beam, reduces or eliminates the need for fire protection.

Because most of the cross-section is embedded in the concrete of the floor, slim floor beams heat up relatively slowly in the event of fire. The only part of the cross-section exposed to the fire is the lower face of the bottom flange, and in many cases it may be possible to leave even this unprotected. This is particularly the case with the Asymmetric Slimflor Beam which has a thicker web specifically to improve fire survival.

Composite beam action is often not possible with slim floor construction, except with the Asymmetric Slimflor Beam.

Because the slim floor beam and slab are arranged within the same depth it is more difficult to achieve effective composite action between the two elements, and the beams may therefore need to be designed as non-composite. However, the Asymmetric Slimflor Beam incorporates a specially designed pattern on its top flange to act as a key for the concrete of the floor slab. This allows composite action to develop, and the bending strength is therefore improved.