# The Impact of Support Type on the Reliability of Steel Trusses Subjected to the Action of a Fire
## 1 Introduction  
The article discusses the issue of fire safety of steel truss structures. The objective of the publication is to check the impact of the support type of a structure on its reliability under fire conditions. It is common knowledge that the introduction of bonds that hinder the freedom of thermal deformations to structures may have disastrous results during the action of high temperature. Additional cross-sectional forces induced due to the impact of high temperature constitute a direct cause of failures of steel structures with the blockade of thermal deformations. This is what happened in the case of the Chemko-System hall in Rudna Wielka in Poland. The purlins constituted statically indeterminate beams with multiple members that were connected to main girders in a nondisplaceable manner. Such a connection resulted in considerable limitation of the displacement of purlins along the lengthwise axis, which caused the generation of considerable compressive forces, making the purlins bend and twist [1]. Scientific publications address the problem of blockade of free deformations under the conditions of elevated temperature, also from a theoretical standpoint. The article [2] analyzed two beam types: a cantilever and one fixedly mounted at both of its ends. Numerical simulations and experimental tests were performed for both types of structures under the load of temperature. In the summary, the authors point out that, when deformations are blocked, if an element is slender, it can buckle before reaching its yield point. A similar comparison is presented in article [3]. The problem of the generation of additional forces due to the impact of temperature when thermal deformations are blocked is addressed in publication [4], using the example of a stainless-steel beam.  
The structures, which were considered in the paper, represent roof beams and therefore the truss model was applied. It is a simplification and it is not always so obvious that the structure works as the truss. The interesting analysis is contained in the papers [5; 6], where the author analyses the truss bridges and considers what is a better solution--whether to model them as a truss or as a frame structure.  
The main subject of the presented article involves the reliability of structure. Currently, it is a rapidly developing branch of science with well-established theoretical background. Numerous methods of reliability analyses can be named, including approximation methods (FORM, SORM) [7; 8; 9; 10], simulation methods (Monte Carlo, Importance Sampling) [11; 12; 13; 14], artificial neuron networks [15; 16], as well as the system method used in this paper. For this method, one can list several classic positions of the global literature, which provide a detailed description of both the theoretical bases and more complicated issues [17; 18]. In recent years, the system reliability method has been used when analyzing the safety of structures under persistent design situation [19; 20; 21; 22]. The authors have successfully applied system analysis to the case of an accidental design situation--during a fire load [23]. So far, different methods of the reliability analysis were used in most publications in order to estimate the safety of structures under the conditions of a fire [24; 25].  
An unquestionable advantage of the system method is the fact that it accounts for mutual relationships between elements. Therefore, it is perfectly suitable for the analyzed aspect, where the impact of the degree of restraint of a structure on its safety is investigated under fire conditions. Unfortunately, as static indeterminacy increases, the application of this method faces a number of difficulties, related primarily to the identification of kinematically admissible failure mechanisms (KAFM). The authors have already addressed this problem in article [26], where a six-member truss is used as an example to present the method of applying the system analysis to estimate the reliability of a structure under fire conditions.