## 1 Introduction  
In slim floor (or shallow floor) systems, the beams are integrated in the thickness of the slab, thus reducing the height of the floor. The first slim floor system appeared in England during the last decade of the nineteenth century, as the Jack arch floor system [1], made of double-T section steel beams with flat masonry arches in-between. Although this flooring system was very popular [2], improvement has been made by replacing the masonry with the concrete material, which provides better behavior under horizontal dynamic loads [3]. Nowadays, there are different types of slim floor beam made of steel and concrete [4]. While in all existing configurations, the steel profile is embedded in the concrete slab, the shape of the steel profile cross-section can vary from rectangular [5, 6, 7] to double-T shapes [8, 9, 7, 10, 11]. However, the steel double-T cross-section remains the most widely used. A common and economical slim floor beam configuration was developed by Arcelor-Mittal in the 1990s, consisting of a double-T profile with a welded plate underneath (SFB, slim floor beam) or of an asymmetric double-T profile with the bottom flange wider than the upper flange (IFB, integrated floor beam), as shown in Figure 1[7]. The composite behavior is ensured by a concrete dowel, placed perpendicular to the steel profile web and embedded in the concrete poured in situ (Figure 2). Other asymmetrical double-T steel beams can be realized by direct manufacturing (ASB type, Figure 3a) [12], by welded plates (Figure 3b), or by reducing the width of the top flange of a European H-profile (SB type, Figure 3d). Slim floor systems present a series of advantages, among which are their inherent fire resistance, as the bottom flange is the unique part directly exposed to fire. Extensive experimental research has been conducted in the last decades [13; 14; 15], which demonstrated an enhanced fire resistance compared to the classical composite beams, without additional fire protection. The fire resistance of slim floor beams can be determined by numerical simulations or by experimental tests. Despite their popularity, the standards do not yet comprise a regulated method for the fire design of such elements. The Eurocodes offer the possibility to compute a mean value of temperature on the entire cross section of an element, based on its section factor. This method cannot be applied in case of slim floor  
Figure 1: Asymmetrical slim floor beam: (a) with a wider plate welded below the bottom flange slim floor beam (SFB); (b) with a wider bottom flange integrated floor beam (IFB) [7]. Figure 3: Asymmetric double-T steel cross sections as slim floor beams: (a) ASB; (b) welded; (c) IFB; (d) SB. Figure 2: Composite slim floor beam (CoSFB) with dowel reinforcement–composite behavior [7]. beams, as the steel profile is embedded in concrete, thus resulting in temperature variation along the height. [PERSON] and [PERSON] [15] proposed a simplified method for calculating the temperature in the steel profile (on lower flange and along the web height) and in the rebars above the bottom flange. This method was developed based on numerical simulations of an IFB system, subjected to an ISO fire. [PERSON] et al. [16] proposed an improved equation to calculate the temperature in the reinforcement bars placed longitudinally. The set of equations was obtained from isotherms of the EN 1992-1-2 [17], used there for the computation of temperature on concrete element cross-sections. A correct evaluation of the rebars temperature is very important, especially for high fire resistance requirements. The temperature evolution on the lower plate and lower flange of an SFB type was analyzed by [PERSON] et al. [18] and [PERSON] [19]. They noticed a positive influence of thermal resistance at the interface between the lower flange and the plate welded beneath [20]. Recently, [PERSON] et al.