# Influence of Internal Pressure on Hollow Section Steel Members in Fire
## 1 Introduction  
Structural steel hollow section members are extensively utilized in civil engineering due to their numerous advantages, such as their lightweight nature, excellent mechanical performance, favourable geometry for corrosion protection, and aesthetic appeal. Various shapes of hollow sections, including rectangular (RHS), circular (CHS), and elliptical, are produced from different types of steel materials, including carbon, high-strength, and stainless steel, which have a significant impact on their mechanical behaviour. Steel is a material that is sensitive to environmental temperature changes, leading to degradation in material properties and thermal expansion of steel structures at high temperatures. Consequently, steel structure members may experience a loss of stiffness and strength. In recent years, extensive research has been conducted to investigate the mechanical performance of steel structures in fire conditions, with a particular focus on the behaviour of structural hollow sections. Hollow sections can be used in isolated members, such as beams and columns, as well as structural systems with numerous members combined. Tubular geometric configuration makes hollow sections especially efficient for compression members where buckling capacity is decisive. Research studies have addressed the performance of hollow sections made of different types of steel materials in fires. Design rules for carbon steel have been well established and formulated in EN 1993-1-2 [1]. [PERSON] et al. [2] compared design methodologies of rectangular hollow sections and square hollow sections (SHS) at elevated temperatures. In several works [3; 4; 5; 6; 7] the buckling of steel hollow section columns at fire temperatures was studied numerically. [PERSON] and [PERSON] [8] experimentally compared stainless steel vs. carbon steel restrained tubular columns in fire tests and assessed the applicability of EN 1993-1-2. Hollow sections are widely used in various truss structures, inboth 2D and 3D systems. The procedure for truss design involves checking of members and joints. As the material properties change, the interaction of joint and member behaviour at elevated temperatures is a complicated process. The mechanical behaviour of tubular joints, such as T-joints and K-joints of CHS, RHS, and SHS, has been extensively studied in recent years [9; 10; 11; 12; 13; 14; 15; 16]. However, there are only a few works that study trusses regarding the interaction of several members and joints. In [17], the results of the numerical parametric study of a welded steel tubular truss indicate that due to the truss undergoing large displacements at elevated temperatures, some truss members experience a large increase in member forces. An analytical method to calculate increases in truss compressive brace member forces has been proposed and validated. In [18; 19], as a result of the experimental study of a full-scale CHS truss, the failure modes were clarified. The presented references demonstrate that the effects of elevated temperatures on the residual material properties and resistance of various structural components and joints of tubular sections have been thoroughly studied. However, the closed inner space of hollow sections presents challenges at elevated temperatures, as the expansion of air may produce internal pressure. Corrosion protection is of utmost importance for exposed steel structures. Special attention needs to be given to the inner surface of hollow sections since it cannot be easily inspected and treated. In trusses and columns, which are common structural elements, the hollow sections are usually required to be hermetically sealed by welding to prevent air humidity leakage. Galvanization is employed for structures that require a higher level of corrosion protection. It is essential to follow the guidelines of galvanization, which specify that sealed hollow sections must have venting holes for safety reasons, allowing the escape of air [20].