A composite steel-concrete slab consists of a concrete topping cast on the top of a profiled steel deck. Normally, the concrete is reinforced with an anti-crack mesh positioned on the upper part and individual reinforcing bars placed within the ribs. Composite slabs play an important role in the overall stability of buildings during a fire, and must be designed in accordance with regulations and standards. Usually, this structural element is rated on the basis of standard fire tests using the standard fire curve ISO 834. The fire resistance should be determined according to three different criteria, namely Load Bearing (R), Integrity (E) and Insulation (I). The Annex D of the EN 1994-1-2 presents a simplified calculation method for the determination of the fire resistance (I) of composite slabs subjected to standard fire exposure from below. During the last two decades, no revisions were made to this method, and there are no proposals for changes in the design formulae for the next version of the EN 1994-1-2. This investigation presents the development of numerical thermal models for three-dimensional analysis of composite slabs under fire conditions in ANSYS and MATLAB. During fire exposure, the steel deck heats up rapidly, expands and may separate from the concrete topping. In order to simulate debonding effects, an alternative thermal model is utilized, presenting an air gap with constant thickness between the steel deck and the concrete topping. The results of the numerical simulations are validated against the results of experimental fire tests, and compared with the simplified calculation method. A new equation is proposed for the calculation of the fire resistance (I) of composite slabs.
