Assessment of composite slabs
A composite steel deck floor is designed in bending as either a series of simply supported spans or a continuous slab. Strength in fire is ensured by the inclusion of reinforcement. This can be the reinforcement present in ordinary room temperature design; it may not be necessary to add reinforcement solely for the fire condition.
In the fire condition it is normal, although conservative, to assume that the deck makes no contribution to overall strength. The deck does however play an important part in improving integrity and insulation. It acts as a diaphragm preventing the passage of flame and hot gases, as a shield reducing the flow of heat into the concrete and it controls spalling. It not normally necessary to fire protect the exposed soffit of the deck.
In fire the reinforcement becomes effective and the floor behaves as a reinforced concrete slab with the loads being resisted by the bending action. Catenary action may develop away from the edges of the floor with the reinforcement then acting in direct tension rather than bending. Slab failure occurs when the reinforcement yields.
Two methods are available for the design of composite metal deck floors, both of which are described in the Steel Construction Institute publication, The Fire Resistance of Composite Floors with Steel Decking. These are the fire engineering and the simple method.
In the fire engineering method it is assumed that the plastic moment capacity of the floor can be developed at elevated temperatures and that redistribution of moments takes place in continuous members. The hogging and sagging moment capacities of the slab are calculated via temperature distributions based on extensive fire testing covering periods of up to four hours. These are then compared with free bending moments for both internal and end spans at the required fire resistance period and the design adjusted as necessary to ensure that the floors meet the required criteria.
The simple method consists of placing a single layer of standard mesh in the concrete. Guidance is available on maximum loads, reinforcement size and position and also allowable span and support conditions.
In practice the simplified method will almost invariably lead to the use of less reinforcement than the fire engineering method. The fire engineered method however allows greater flexibility in reinforcement layout, loading and achievable fire resistance times. Typically the use of the fire engineering method will result in thinner slabs.
Lightweight concrete is a better insulator and thus loses strength less rapidly in fire than normal weight concrete. Hence lightweight concrete floors tend to be thinner than normal weight alternatives.
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. (Figure 49) Details are given in the Steel Construction Institute publication The Fire Resistance of Composite Floors with Steel Decking.
Figure 49:Composite Steel Deck Floor with Unfilled Voids.
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.