## 4 Conclusions  
System reliability analysis seems to be an appropriate tool to assess the reliability of the steel plane frame in a fire design situation. The behavior of the structure during fire was examined in detail, but the probabilistic model was quite simple, so only the most possible failure paths were considered. It is important to note that in the analyzed examples, only a few values were treated as random; a better assessment would be possible in the case of a more detailed probabilistic description. In such a case, the identification of the most probable failure path is difficult, or even impossible, to identify. Then, the task becomes more complex, so computational methods have to be applied. The more the failure paths are analyzed, the better the reliability assessment that could be obtained. Nevertheless, the possibilities of creating plastic hinges in frames are unlimited, so some strategies of failure path limitations have to be developed. Finally, it should be underlined that although in building structures some benefits of high redundancy are known, each case should be analyzed individually. Figure 9: The displacements of selected nodes of frame B in the following minutes of fire duration: (**a**) in the horizontal direction; (**b**) in the vertical direction. Although the presented analyses have initial character, some conclusions may be drawn. First of all, the system reliability analysis for the individual collapse mode can be conducted easily. In this article, the most typical failure modes for frame structures were analyzed. It is obvious that, in fact, possible failure paths are unlimited, but it seems (based on a review of the literature) that inward, outward, or mixed collapse modes are the most important to assess the reliability of the frame structure. This hypothesis will be checked in the author's future work. The analyzed portal frame is a very simple structure, but it is necessary to examine the basic structure in depth to continue research. Such an approach may help to identify the typical behavior and dependencies during the analysis of more complex structures, as they can be easily overlooked. During computation, the typical behavior of the steel frame was observed. Under temperature load, the girder elements elongated due to the additional forces that are typically generated under fire load. Also, these forces were the reason for the unburdening of the beam, which can be observed in Figures 6b and 9b. Figure 6b indicates that this effect disappears between the 35 th and 40 th minutes, which is a direct result of the decrease in reduction factor for yield strength (see Table 4). The decrease in the reliability indices in following minutes of fire duration is the result of the increase in effect of action and the decrease in bearing capacity, and this is typical behavior that anyone can expect. Nevertheless, it is noteworthy that during temperature load (especially in statically indeterminate structures), force redistributions are observed. Each failure mode consists of a few elements; some of them may be temporarily unburdened, which can result in an increase in the reliability index in the following minutes of fire duration. Such a situation was observed for both the analyzed frames during inward collapse mode analysis (M2, Figure 5 and M2, Figure 8). It is generally known that designers may benefit from the redundancy of the structure. This fact was also confirmed by the authors of [1; 2] in the context of steel trusses; the unreliable element in the fire did not decide the failure of the whole structure, but a new static scheme was created. The currently conducted research did not enable us to find a similar dependency. On the contrary, for a portal frame, the creation of one plastic hinge leads directly to the failure of the whole structure. The research presented confirms that the reliability analysis may be helpful during the design of frame structures for fire situations. This enables anyone to predict the possible activity of the structure during the temperature load.