Deck voids

Research has shown that filling the gaps between the raised parts of the deck profile and the beam top flange in composite construction is not always necessary. The upper flange of a composite beam is so close to the plastic neutral axis that it makes little contribution to the bending strength of the member as a whole. Thus, the temperature of the upper flange can often be allowed to increase, with a corresponding decrease in it’s strength, without significantly adversely affecting the capacity of the composite system.

Gaps under decking with dovetail profiles can remain unfilled for all fire resistance periods. The larger voids which occur under trapezoidal profiles can be left open in many instances for fire ratings up to 90 minutes, although some increase to the thickness of protection applied to the rest of the beam may be necessary, shown below.  Details are given in The Steel Construction Institute publication The Fire Resistance of Composite Floors with Steel Decking, shown below.

Designers should take care that gaps are filled where the beam forms part of the compartment wall to ensure the integrity of the compartment. In the rare case where non-composite metal deck construction is used, the gaps must always be filled.