## 3 Result and Discussion
Due to the fact that the structure is statically determinate, during the whole fire analysis, internal forces remain the same as in the basic situation. The computations, aimed at finding axial forces, strains, and displacements, are run with the MES3D program [32; 33; 34]. The computations proceeded in two steps. The first step (20 \({}^{\circ}\)C) produced the specification of the state of the structure in a basic design situation. In the second step, temperature increments related directly to time steps, are taken into account. The analysis was repeated every second. The simplest and most effective way to protect steel structures in the event of a fire is to isolate them from the influence of high temperature using the following protection [35]:  
* Thermally activated (reactive) agents, e.g., swelling coatings;
* Thermally passive means, e.g., spray masses and plate claddings;
* Hybrid means combining plate claddings and spray masses. Thermal properties of insulating materials change as fire develops [36; 37; 28; 38]. However, the Polish standard [29] allows adopting them as fixed values. In this paper, fire analysis of the structure was carried out using three different types of thermal insulation with a thickness of 2 cm:  
* Protection with vermiculite fireproofing mortar;
* Protection with vermiculite mortar with gypsum (high density);  
Figure 2: Stress–strain relationship for steel S275. * Protection in the form of vermiculite boards with cement, as contour insulation (for all types of cross-sections) and as box insulation (only for I-sections). The characteristics of the selected fire-proofing materials are listed in Table 3.  
Sprayed-on coatings are the most popular type of fireproofing insulation because of the many advantages they offer, including low thermal conductivity, low self-weight, cost-effectiveness, and simple installation [28; 37]. Sprayed-on fireproofing pastes are produced in the form of dry mix, the ingredients of which are [35]: cement- or gypsum-based binder, aggregate (e.g., vermiculite, perlite, mineral fibre, or mineral (rock) wool granulate), and modifiers. Sprayed-on pastes can be applied in wet or dry technology. Currently, the most commonly used is a dry technology, consisting of transporting a pneumatically dry mix, which is combined with water or a liquid binder at the outlet end of the sprayer. Wet technology is less frequently used, i.e., mechanical application of water-mixed mixture using pump-spray units in a manner similar to machine plastering [2; 35]. The all wet process involves dirt and wetting the surroundings. Fireproof spraying is performed based on the thickness in one or several layers, which are most often 2 or 3 layers. The thickness of the insulation from the spray masses ranges from 15 to 60 mm. The fire resistance class obtained with the participation of spraying fire-retardant masses is R15-R240. Fireproofing protection of steel structure members can be provided using the following types of boards [35]: rock wool, gypsum plasterboard, magnesium oxide-based, those that have gypsum or cement binder, those with cement and lime binders with reinforcement (most often a glass fibre one), and various types of filler materials. Board insulations can be installed using different designs, namely contour or box configurations. In the first design, boards are fixed in such a way that the protected element cross-sectional shape is maintained. The other configuration involves the formation of the cuboid box, inside which the protected section is built-in. Board claddings are aesthetically appealing, and they usually do not need additional finishing, but they provide a more expensive fire protection option compared with sprayed-on or intumescent coatings. Additionally, the time necessary for installation is much longer than for fireproofing coatings. That contributes to higher investment costs and extends the duration of the construction works [1]. Board protection, applied as single or multilayer cladding, allows one to achieve fire resistance class R15-R240 [35].