## 4 Discussion  
### Assessment of the FE Model  
The choice of the heat convection coefficient (h\({}_{\rm c}\)) on the fire-unexposed side of the door heavily affects the modelling--the final calculated temperature on the fire-unexposed side decreased by about 50 \({}^{\circ}\)C when h\({}_{\rm c}\) was increased from 0 to 12 W (m\({}^{2}\) K)\({}^{-1}\)[13]. The Eurocode 1 [24] recommends the use of values 4 or 9 W (m\({}^{2}\) K)\({}^{-1}\), when the effects of heat transfer by radiation are considered. The literature shows that h\({}_{\rm c}\) ranging from 4 to 10 W (m\({}^{2}\) K)\({}^{-1}\) are used in order to fit the experimental results [2; 9; 10]. In this work the numerical results could not properly fit with the experimental data using either of the recommended values, as exemplified in Table 2. Noticeably, the Eurocode 1 recommended values could reasonably fit well to the data obtained on a door with different construction geometry (compare to caption of Figure 10), as observed from the analysis in the preliminary study [14]. This clearly shows the need for the appropriate and independent definition of the boundary conditions. With this aim, the methodology reported in Section 2.2.2 has been proposed to calculate the T\({}_{2}\) temperature and hence h\({}_{\rm c}\), using the geometry of the door and the physical properties of the employed materials as input data--this approach leads to a self-consistent procedure. Moreover, a criterion for the numerical validation of the results is proposed to confirm the reliability of the calculation. Noticeably, the presented self-consistent procedure can also properly fit to the results of the door analyzed in the preliminary study (data not reported for brevity) [14]. Convection and radiation phenomena inside the door are irrelevant [11] due to the use of relatively dense materials (d \(\geq\) 130 kg m\({}^{-3}\)) [31; 32]. The value of 25 W (m\({}^{2}\) K)\({}^{-1}\), recommended by Eurocode 1 [25] for the convection coefficient on the fire-exposed side of the door, is quite consistent with the high temperature inside the furnace (945 \({}^{\circ}\)C) and clearly does not depend on the door employed. Emissivity factors ranging from 0.3 to 1.0 are adopted [9; 10], mostly giving no justification. Clean carbon steel features an emissivity factor of 0.3, while the value of 0.7 is recommended in Eurocode 3 [15], consistent with the presence of surfaces oxides [33; 34]. To avoid a source of possible flame propagation, unpainted doors are generally used for the FTP fire test, with a film of surface oxide--accordingly, a value of 0.7 is used and recommended for the calculation of the equivalent convection parameter (H). Figure 9: Transversal temperature profile at the final stage of the fire test (60 min) along the line passing through measuring point 5 (see black line in Figure 1b): comparison between foam and rock-wood model. The use of constant convective coefficients is certainly appropriate for the fire-exposed side of the door as the final temperature is achieved very rapidly and it is subjected to uniform heat. For the fire-unexposed side, the use of variable convective coefficients might be appropriate; yet the results could be numerically assessed, suggesting that a realistic model of the FTP test is achieved. As a matter of fact, the calculated convective coefficient is some sort of an average value that keeps track of the initial and \"final\" temperature on the door surface. Even if a simplified approach is adopted, the choice appears to effectively describe the fire resistance of fire doors in the FTP test, thus providing an efficient tool with predictive capabilities, useful for innovation. The structural analysis of the door appears less critical as it depends on the constraints between the door and the frame and on the thermal load. The constraints are directly identified by the technical design of the door. Further, the structural analysis is dominated by the carbon steel structure, as the insulation material does not contribute significantly [2].