###### Abstract
The main challenges in a thermal numerical analysis are related to the heat transfer within the concrete slab. It is therefore necessary to properly define the variation of thermal properties of the concrete with temperature, the thermal contact between the steel decking and the concrete, and the effect of moisture transport [13]. [PERSON] [14] carried out parametric studies to investigate the most important factors affecting the temperature distribution within a slab. The results showed that the thermal conductivity of the concrete, the moisture content, and the convection value on the fire-exposed side were the most influential parameters. However, steel decking was not considered and aspects such as its geometry or the emissivity of the steel were not taken into account. [PERSON] [15] simplified the thermo-mechanical model of composite slabs by sharing nodes between the steel decking and the concrete, assuming a continuity of temperature at the interface. Recent works emphasize the significance of development thermal analysis to address these problems. In 2017, [PERSON] et al. [16], from the NIST, performed a numerical study based on detailed and reduced-order models of heat transfer in composite slabs. In addition, a parametric study using the detailed model was conducted to evaluate the effect of some parameters on temperature such as thermal boundary conditions, thermal properties of materials, and slab geometry. The temperature-dependent emissivity was also considered. The results of the proposed model for emissivity showed better agreement with experimental results than those calculated from the standard EN 1994-1-2 [17], hereafter referred to as Eurocode 4. [PERSON] et al. [18] developed and validated 3D finite element models to evaluate the thermal performance of composite slabs. This study included an air gap between the steel decking and the concrete to simulate the thermal effects on the slabs induced by the debonding between the steel decking and the concrete. With regard to structural analysis, the main challenges include modelling the effects of material and geometric nonlinearities produced by the temperature. In addition, during fire exposure, the difference between the thermal expansion of steel and concrete causes debonding. Previous research works mention this phenomenon, which increases the thermal resistance in the interface between steel decking and concrete due to the air gap [18; 19; 20]. Most of the existing works define a thin layer between steel decking and concrete and assign thermal properties similar to air [21; 22]. [PERSON] et al. [23; 24] proposed new advanced 3D calculation models capable of simulating the debonding effect of steel decking from concrete topping. The models consider the existence of an air gap between these materials, acting as a thermal resistance on the temperature field through the thickness of the slab. The numerical results show a good approximation to the experimental results. However, when a simultaneous solving of the heat transfer and stress analysis problems is required, it is not possible to ignore the mechanical performance of the contact between the steel decking and the concrete. This procedure causes non-realistic behavior, and nonlinear contact must be considered in the thermo-structural analysis [23]. The original contribution of this work is to experimentally validate a nonlinear finite-element modelling approach for thermo-structural analysis of LWC composite slabs in fire. In addition, the debonding effect between the steel decking and the concrete is considered. To simulate (debonding effect) the thermal gap conductance, a numerical model is developed, taking into account the effect of the temperature on thermal resistance at the contact interface. To validate this model, four types of concrete (three LWC and one NC) are cast in profile steel decking to manufacture pre-cast composite slabs. These composite slabs are tested in a furnace, applying a constant load and following the standards EN 13381-5 [25] and EN 1363-1 [26].