# The Impact of Support Type on the Reliability of Steel Trusses Subjected to the Action of a Fire
## 4 Discussion  
The objective of the article involved analyzing the impact of the method of supporting a plane steel truss on its behavior under fire conditions. The reliability index \(\beta\) was adopted as the measure of safety of a structure in this accidental design situation. It was determined using the system analysis. It involved a truss with an additional intermediate support (truss \"A\") and a truss in which the manner of support blocked the freedom of thermal deformations of the bottom chord (truss \"B\"). In the case of truss \"A\", until the 20 th minute of the fire, the probability of failure amounted to zero. This means that the truss had very little strain. In such a case, it is not possible to calculate the reliability index. For truss \"A\", the first value of the reliability index was not reached until the occurrence of a change of the static scheme of the structure. Although this value was initially relatively high, it dropped rapidly along with the development of the fire. Before the 30 th minute of the fire, there was a second change in the static scheme, resulting in the conversion of the structure into a mechanism. Such behavior of the structure is quite surprising, since it turns out that the structure, which was over-dimensioned in the basic situation, does not necessarily have to be safe under the fire conditions. With the assumptions adopted for the description of a fire and fire insulation, the analyzed truss did not even reachR30 fire resistance class. An even more dangerous phenomenon can be observed in the case of truss \"B\". The blockade of free thermal deformations in the bottom chord leads to the generation of high axial forces under the impact of temperature. In the case of the analyzed truss, they lead to a change of the static scheme already after the fourth minute of fire duration. From that moment on, the decrease in the value of the reliability index of the structure is very rapid, which causes it to become lower than what is required before the sixth minute of the fire. In this case, there is no fire resistance class to speak of. The results corresponding to truss \"0\", which differs from trusses \"A\" and \"B\" only in the manner of support, are listed for comparative purposes only. In this case, three degrees of freedom are taken away (for trusses \"A\" and \"B\"--four). It turns out that, for the fire conditions, the selection of truss \"0\" proves to be the safest. The performed analysis suggests that the introduction of any additional bonds to the structure in the case of a fire-related design situation is dangerous. However, this conclusion should be verified, which the authors intend to do in future publications. The results produced for truss \"B\" are not surprising. On the other hand, in the case of truss \"A\" an advantageous effect was to be expected involving the unloading of the bottom chord, while an additional support decided about considerable deterioration of the fire resistance of the structure (compared to truss \"0\"). Subsequent changes of the static scheme of truss \"A\" suggest that this support limited the possibility of free elongation of central cross-braces and the middle post, which decided about the generation of considerable compressive axial forces leading to an exceeded ULS in these elements. It should be concluded that the behavior of a steel structure under the conditions of a fire is difficult to predict. A detailed fire analysis should be performed to ensure its safety. It is not enough to just check whether the limit state of bearing capacity is fulfilled in the minute corresponding to the desired fire resistance class. It is recommended to analyze the entire course of the fire, since there is high likelihood of the occurrence of changes of the static scheme along with its development. It is necessary to calculate the moment in time in which this happens and continue calculations for a structure with a new static scheme starting from this point.