## 3 Result and Discussion
In accordance with Equations (2) and (3), the temperature increment in steel members also depends, among others, on the section factor. For sprayed-on contour board insulation, this factor is equal to the perimeter to the sectional area ratio. For box insulation, this factor is computed from formula [29]:  
\[\frac{A_{p}}{V}=\frac{2(b+h)}{A_{S}} \tag{11}\]  
where \(b\) is I-section width, \(h\) is section height, and \(A_{s}\) is the cross-sectional area of the profile. The highest section factor is found for the I100 section, used in the bottom chord. The value is equal to 349.057 m\({}^{-1}\) for sprayed-on and contour board insulation. The value of box board insulation is reduced to 283.019 m\({}^{-1}\). The top chord sections, namely IPE140, with box insulation show an exposure factor of 259.756 m\({}^{-1}\) and 335.976 m\({}^{-1}\) for other types of insulation. With respect to the bracing elements, made from square tubing RHS50x50x5, the exposure factor equals 214.204 m\({}^{-1}\). \begin{table}
\begin{tabular}{l c c c} \hline \hline \multirow{2}{*}{**Fire-Proofing Insulation Material**} & \multirow{2}{*}{**Unit Mass \(\rho_{p}\) kg/m3**} & \multicolumn{2}{c}{**Thermal Conductivity**} & \multicolumn{1}{c}{**Specific Heat \(C_{p}\),**} \\  & & \(\lambda_{p}\)**W/(mK)** & **J/(kgK)** \\ \hline vermiculite & 350 & 0,12 & 1200 \\ vermiculite (or perlite) and gypsum & 650 & 0,12 & 1100 \\ vermiculite boards (or perlite) and cement & 800 & 0,20 & 1200 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Thermal properties of selected fire-proofing materials [35]. Figure 3: Temperature of steel members in the successive minutes of the fire (insulation 2 cm): (**a**) I 100 bottom chord, (**b**) IPE 140 top chord, (**c**) RHS 50 \(\times\) 50 \(\times\) 5 bracing elements. Figure 4: Decrease in modulus of elasticity under fire conditions (insulation 2 cm): (**a**) I 100 bottom chord, (**b**) IPE 140 top chord, (**c**) RHS 50X50X5 bracing elements. The truss in Figure 1b is statically determined. A serial reliability system is suitable for such a structure. In this case, the failure of one of the most stressed elements is equivalent to a failure of the entire structure. Therefore, the resistance analysis includes the most stressed elements of the truss. Tension members of the bottom chord from number 2 to 5 have the same resistance. The most stressed of the compressed top chord were elements 9 and 10. Next, the resistances of compressed members  
Figure 5: Decrease in yield strength under fire conditions (insulation 2 cm): **(a)** I 100 bottom chord,**(b)** IPE 140 top chord, **(c)** RHS 50X50X5 bracing elements. Analysing Figures 3-6, it is easy to notice that the best results were obtained by vermiculite with gypsum insulation. With this type of protection, steel members are heated up to the least extent value (Figure 3), which corresponds to a slower decrease in the modulus of elasticity (Figure 4) and yield strength (Figure 5) and, consequently, produces a milder reduction in resistance in the successive minutes of the fire (Figure 6). Slightly worse results were obtained using sprayed-on vermiculite insulation (Figures 3-6). In terms of fire analysis, this means density is one of the major parameters. The higher the density, the better fire-proofing properties a material has. Another important property is thermal conductivity; conversely, the material should have the lowest possible thermal conductivity value. In this analysis, the worst results were observed when the insulation from vermiculite and cement boards was applied. The boards had the highest density of all materials under consideration, but their thermal conductivity was over 50% higher relative to the sprayed-on insulation (Table 3). With vermiculite and cement boards, considerable improvements in results could be found using box insulation configuration instead of the contour one, which lowers the section factor.