## 3 Result and Discussion
These results demonstrate that the lower the factor, the less heated up the element, which produces an advantageous effect on steel's mechanical properties in fire. It should also be emphasised that differences in the results  
Figure 6: Resistance decrease in truss members under fire conditions (insulation 2 cm): (**a**) I 100 tension members 2–5, (**b**) IPE 140 compression members 9,10, (**c**) RHS 50X50X5 compression members 21,24, (**d**) RHS 50X50X5 tension members 20,25.  
obtained for different types of insulation tend to grow as the fire develops. An adequate selection of fire-proofing insulation is of key importance for those members that require a high fire resistance class. The diagrams in Figure 6 indicate that the use of 2 cm fire insulation makes it possible to reach the fire resistance class R30 for compression and class R60 for tension members of the truss. This result means that elements of bottom chord and tension members are able to withstand 60 min of exposure to fully developed fire, whereas compression elements are able to withstand only 30 min. The fire resistance class of the whole structures is equal to the lower fire resistance class of any single element. So, with the assumed insulation, the structure reached the R30 class. The analysis of the graphs in Figure 6 clearly indicates a large capacity reserve in the tension elements. For instance, regarding the tension elements of the bottom chord, when 2 cm-thick vermiculite and gypsum insulation is used, the capacity of elements No. 2-5 after 60 min of fire duration is 172 kN, whereas the effect of action is 53 kN. That gives a reserve of 225%. A similar situation holds for the most stressed elements of the latticework, No. 20 and 25. After 60 min of the fire, their capacity is 208 kN while their effect of action amounts to 46 kN. Thus, the capacity reserve is 352%. Conversely, the performance of the compression elements is quite different. Figure 6 again illustrates this result. It can be seen that the most stressed elements of the top chords, No. 9 and 10, have a capacity of 36 kN and an effect of action of 54 kN. As a result, their capacity is exceeded by 33%. For the compression elements of the latticework, No. 21 and 24, their capacity exceedance is 9.5%. On the basis of Figure 6, we can see that the commonly used method of constant insulation thickness is uneconomical. This paper proposes a differentiation of insulation thickness on truss elements. The best results were reached for the insulation of vermiculite with gypsum, so this material was used in the next analysis. The decrease of resistance during fire for different thicknesses of insulation is shown in Figure 7.  
According to charts in Figure 7, the thickness of fire insulation was altered. For the top chord it was increased to 3 cm; for bracing elements it was increased to 2.5 cm. The insulation thickness of the  
Figure 7: Resistance decrease in truss members under fire conditions depending on fire insulation thickness: (**a**) I 100 bottom chord, (**b**) RHS 50X50X5 bracing elements, (**c**) IPE 140 top chord. tension elements of the bottom chord was decreased to minimum value of 1.5 cm (Table 3) to reduce the costs of fire protection. Finally, the structure reached the R60 fire resistance class.