Fire resistance of composite steel deck floors

Composite steel deck floors consist of a profiled steel deck with a concrete topping. Included in the concrete is some light welded mesh reinforcement which acts to control cracking, to resist longitudinal shear and, in the case of fire, to act as tensile reinforcement. Indentations in the profiled deck allow the concrete and steel to bond and share load. Composite action between the supporting beams and the concrete is created by welding shear studs through the deck onto the top flange of the beam.

In the fire condition, practice amongst designers is to make the conservative assumption that the steel deck makes no contribution to overall strength. The deck does however play an important part in maintaining 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 is not normally necessary to fire protect the exposed soffit of the deck.

BS 5950 Part 8[2] provides a table of recommended concrete thicknesses to satisfy the insulation criterion for common periods of fire resistance. The minimum thickness of concrete required to satisfy the insulation requirements is shown for trapezoidal and re-entrant decks when subjected to a BS 476 Part 20[1] fire test. The insulation depth depends on the type of profile, and it is the concrete cover to the main crest of the deck for trapezoidal profiles and the full slab depth for re-entrant profiles.

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 steel deck floors designed to BS 5950 Part 4[3] for fire. Both are described in the Steel Construction Institute guide P056. These are the fire engineering and the simplified 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 fire resistance 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 welded fabric reinforcement 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.