# Fire Tests of Load-Bearing, Light-Steel-Framed Wall Systems Insulated with Polyurethane Foam
## 1 Introduction  
Light-steel framing (LSF) is a drywall system that offers high architectural flexibility, lower construction and transportation costs, reduced weights, the possibility of recycling and reuse, and at the same time high mechanical strength and stability [1, 2]. LSF systems are manufactured as both load-bearing and non-load-bearing construction elements and are available in a wide range of compositions. The disadvantages of these systems are the lack of adaptability in situations where on-site adaptation is required, the low thermal mass, the higher maintenance costs, the limited number of storeys, the risk of corrosion of the metal elements due to condensation and possible air and water infiltration, the high thermal conductivity of the steel parts and fire behaviour [3]. The system consists of three main components: cold-formed steel structure (studs and tracks), sheathing boards (wallboards) and thermal insulation, each of which has its own function as an integral part of the system [1]. Other materials are necessary, such as screws, membranes for waterproofing and air tightness, and finishing layers. The cold-formed steel structure forms the skeleton of the system and provides stability and rigidity, which is protected by the sheathing. The thermal insulation serves to prevent heat loss and provide the necessary thermal comfort. The fire resistance of LSF wall systems is an important factor in preventing the spread of fire and eventually the collapse of the building in case of fire occurrence. Although it can be determined by fire testing or modelling, in most cases, it is determined by fire tests in which a specimen is subjected to a standardized fire load, usually represented by the ISO standard curve [4]. According to the test results, the specimen is assigned a fire resistance rating (FRR) based on the failure time evaluated according to the three criteria of structural stability, insulation and integrity [5, 6]. Structural failure of load-bearing LSF systems under fire conditions is primarily due to the reduction in the mechanical properties of the steel frame, where at a temperature of 550 \({}^{\circ}\)C, only about 60% of the original yield strength of the steel is maintained [7]. The FRR of LSF wall systems isaffected by many factors, such as configuration (number and type of applied wallboards, type and position of insulation material), geometry (stud spacing) and load ratio [8; 9]. Of the factors mentioned, the type of wallboards and the type and position of insulation play a decisive role [10]. The wallboards protect the steel frame from direct fire exposure and delay temperature development [5]. Elevated thermal properties of various wallboards that can be used in LSF structures have been studied as an indication of their behaviour under fire exposure. As an example, [PERSON] and [PERSON] [11] studied gypsum boards (GBs), calcium silicate boards (CaSiBs), magnesium oxide boards (MgOBs), perlite boards (PBs), and structural plywood (PW), while [PERSON] et al. [12] investigated GBs, PCM-gypsum boards (PCMBs), magnesium sulphate boards (MgSO4 Bs) and fibre cement boards (FBs). The results revealed that GBs exhibited the least mass loss when exposed to elevated fire temperatures, which has the potential to maintain a higher FRR of LSF structures than those with other type of wallboards. Furthermore, the addition of fibres in GBs could potentially lead to better FRR by preventing cracking during fire exposure [13]. Other benefits of GBs are ease of fabrication and the widespread availability of the primary material for its production [14]. [PERSON] and [PERSON] [9] determined that a layer of 15.9 mm thick GB on an uninsulated load-bearing wall gives an FRR of 35 min, while two layers of 12.5 mm thick GB give an FRR of 100 min.