# Fire Resistance of Ultra-High-Strength Steel Columns Using Different Heating Rates
## 1 Introduction  
Ultra-High-Strength Steel (UHSS) is gaining recognition in the construction industry for its potential applications in building structures and sustainable construction. UHSS can be used in the construction of high-rise buildings for critical structural elements such as columns and beams. Its exceptional strength contributes to the use of lighter and slender elements, reducing the self-weight of the building structure. Designers may explore the use of UHSS in optimised structural design, creating aesthetically pleasant and efficient structures. As research and development in materials science progresses, the use of UHSS in building construction becomes prevalent, offering new possibilities for efficient and resilient building design. The use of UHSS can lead to a great deal of reduction in the consumed mass and CO\({}_{2}\) production, still increasing the overall load-bearing capacity of the structures. The use of UHSS also leads to longer-span elements and can handle more complex shapes. UHSS allows for thinner structural members and reduces the overall weight of the structure, leading to cost savings in material and labour. Usually, UHSS presents higher durability (more corrosion resistance), due to the existence of specialchemical elements, extending the service life of the structure. On the other hand, UHSS is typically more expensive than mild steel and can present some weldability issues.  
Under compressive load, the UHSS columns may experience failure modes due to the brittle behaviour of the material as well as global buckling, depending on the geometry and slenderness. The lower ductility, when compared to mild steel sections, has an undermining effect on the global performance of the whole structure, especially under certain extreme events, but, in general, UHSS cold-formed CHS columns have improved buckling resistance, with the possibility to use the curve \"b\" from EN1993-1-1. The future generation of prEN1993-1-1 [1] considers the possibility of using the design curve \"c\" for HSS cold-formed CHS columns, when using grade S700.  
While the fundamental principles of structural analysis remain consistent, the design of columns made from UHSS may require modifications to traditional design formulas developed for normal-strength steel. This is because the mechanical properties of UHSS, including its higher strength and reduced ductility, can influence the behaviour of columns under different loading conditions.  
Over the past decade, different columns comprising ultra-high-strength steel sections have been proposed and successfully tested at room temperature [2; 3; 4]; however, only a reduced number of studies have been developed to test these materials under elevated temperatures [5; 6; 7; 8]. At elevated temperatures, UHSS presents a different microstructure, and the reduction factors at elevated temperatures are not similar to mild steel. A higher reduction in strength at elevated temperatures is observed, and the fire response significantly differs from that of mild steel [9].  
However, currently, the European standard prEN 1993-1-2:2023 [10] considers only steels with a strength of up to 700 MPa for compression and tension at elevated temperatures. Therefore, simulating this material in fire scenarios becomes crucial for understanding the risks associated with fire exposure conditions.