## 1 Introduction  
Steel offers many advantageous properties, including high strength and high resistance. Those properties make steel an almost ideal structural material. However, under fire conditions, steel properties deteriorate with exposure to elevated temperatures. This deterioriation leads to a decrease in the resistance of individual structural elements, which may result in a failure or collapse of the structure. In a fire situation, the main priority is to protect human life. Next, the structure should be protected. These two goals are closely related. Improper protection of the building often prevents efficient evacuation of people, which may result in a calamity. Therefore, when a structure is designed, it must be provided with an appropriate fire resistance class to give people a long enough time to escape. All measures should be taken to reduce the risk of a structural collapse [1; 2; 3]. Then, it is necessary to ensure that after the fire, the structure can still be brought back into use with minimum necessary repairs. One of the most catastrophic structure disasters in history was the collapse of the Twin Towers of New York City's World Trade Center. Media coverage created a public belief that the towers collapsed due to the impact of an aircraft. Numerous studies and expert opinions produced after the disaster, however, show that this impact was only an indirect cause. The impact of the aircraft caused a fire, which became a direct cause of the destruction of the towers [4; 5; 6; 7; 8; 9]. Fires in the tall and high-rise buildings had happened before, but the September 11 th disaster was the only one in which the steel skyscraper collapsed completely. Some technical solutions that work perfectly well under normal conditions become a major disadvantage during a fire. In extreme situations, steel structures exposed to fire can disintegrate very quickly. This happened in Luban, Poland, in July 2012, as a result of a fire in the production facility. The steel building, designed and made from thin-walled steel elements with a thickness not greater than 3 mm, collapsed in just 17 min [10]. The use of cold formed structural elements has become more common in the construction industry during the past decades due to the various advantages cold formed structures offer, such as weight reduction of the structural system. The thin-walled sections differ from hot-rolled sections in terms of failure modes. In addition to local and global buckling, distortional buckling is also possible. The slenderness of steel members is a major factor in determining their failure mode at elevated temperatures. Short, stocky elements typically have a slenderness ratio less than 40, while for long, slender elements this value is greater than 120. The stocky columns exhibited local buckling as their failure mode, while longer, more slender columns displayed global buckling accompanied by limited local buckling. Problems related to the description of failure modes have been the subject of discussion in many works. Articles that deserve special attention include those in [11; 12; 13]. The fire safety of steel structures does not only refer to buildings. Fires of bridges do not happen often, but if they occur, they produce serious consequences. The effects associated with a bridge being put out of operation are felt by a significant number of residents of the area and often the entire economy of the region. That was the case after the fire of the Lazienkowski bridge in Warsaw in February 2015. An average of 95,000 vehicles travelled daily over the bridge (about 18% of vehicular traffic on all Warsaw bridges). The initial expert report of the bridge's technical condition showed degradation and a loss of carrying capacity in, among other elements, the steel structure of the bridge, resulting from elevated temperatures. [14]. On February 20, 2015, the city authorities decided to dismantle the damaged steel structure of the bridge and mount a new one on the surviving pillars.