## 4 Discussion  
The results presented in the paper indicated expressly, then, that the proposed method of system analysis is an effective and fast tool. Of course, it still needs development; for example, it should be checked for the structure under 'true' load (loaded by wind, snow, live load, etc.). The intention of the author is to cover this topic in one of the following papers.  
The correctness of the method was checked using Truss 1, for which the system reliability analysis of the system was previously performed using a full cut-sets method. The results were identical using both methods (reduced vs. full cut-sets).  
Using the new method enabled us to conduct the reliability analysis with two other types of trusses with an additional tie (blocked additional degree of freedom). The first one was the structure with a blockade of free thermal elongation (Truss 2), and the second one was the structure with intermediate support in the half of the bottom chord span (Truss 3).  
Analysis of both types of structures (Truss 2, 3) allowed us to find a few interesting facts. First, in the case of Truss 2, it is obvious that such a type of support will generate huge values of effects of action, so under fire influence, such a structure will be unreliable. Surprisingly, this structure is also not safe without a thermal load; before developing the fire (0 min), the reliability index was equal only 0.84. According to Eurocode 0 [38] in the persistent design situation, this index should not be less than 3.8. In fact, there are no normative guidelines on the value of this index for accidental situations, but according to the same papers [39; 40] it should be less than in the case of persistent design situations. Such a low reliability index for Truss 2 is the result of the fact that the found cut-sets were connected in a series way, which was surprising, because this structure is statically indeterminate. Such a behavior of the truss, which is highly statically indeterminate, is enormously strange, so it needs further detailed research. What is more in the case of Truss 2, although the reliability of single elements was quite high, the reliability of the whole structure was not high enough (in situation without fire influence). This fact points clearly to how important the system analysis is.  
What is also worth being underlined is the fact, then, that the introduction of an additional tie in Truss 2 caused the significant change in the values of reliability indices in the external cross-traces (0 min). In the case of elements 20 and 31 (according to Figures 4 and 5), the \(\beta\) index decreased from 3.88 to 1.25 (Truss 1 compared to Truss 2).  
Interesting facts were also found during the analysis of Truss 3, with intermediate support. At the beginning of the fire duration, it seemed, then, that such a structure is very reliable because during 15 min of fire duration, the reliability was equal to 1.0. Inthe 20 th minute of fire duration, the reliability index was very high (\(\beta=6.22\)). It is worth noting that after the first change in the static scheme, this index decreased very rapidly (\(\beta=0.34\)). Theoretically, at this moment, the further computation should be stopped because the structure was not reliable, but computation was continued to find out any other dependences. In the 30 th minute of the fire duration, the reliability index increased again. It was at the point where the structure got a new static scheme, which represents the series system. It is some paradox, because it seems, then, that the structure with a mixed system should be more safe then these with a series systems; however, the described phenomenon was observed by the author several times in the previous research [37; 41].