# Fire Tests of Load-Bearing, Light-Steel-Framed Wall Systems Insulated with Polyurethane Foam
## 1 Introduction
This means that the FRR of LSF constructions can be significantly increased by adding multiple layers of wallboards rather than just increasing the thickness of the wallboard, due to the staggered positions of the wallboard joints of the two applied layers which are often the weak point on wallboards. When the first joint on the exposed layer opens, underneath is usually a continuous layer which offers protection for a certain period of time. Due to the current trend in the construction industry to improve the energy efficiency of new and existing buildings, the use of thermal insulation in LSF structures is mandatory. Depending on the location of the insulation, LSF structures are defined as: (a) cold-framed structures, where the insulation is located in the cavity, (b) warm-framed structures, where the insulation is located entirely on the exterior and (c) hybrid structures, which are a combination of cold- and warm-framed structures [1; 15]. Thermal insulation positioned in the cavity of load-bearing LSF structures tends to lower FRR, as shown in the studies of [PERSON] and [PERSON] [9], [PERSON] and [PERSON] [16; 17] and [PERSON] and [PERSON] [18]. Due to the low thermal conductivity of the insulation, a thermal barrier is formed, which leads to non-uniform heating of the LSF members [19]. This creates a high temperature gradient between the exposed and unexposed sides of the LSF structure. Since one side of the steel structure is heated much more than the other, structural failure of the load-bearing structure occurs. Studies on this topic have shown that externally insulated load-bearing LSF assemblies have better fire performance [8; 20], but the disadvantage of such structures is the more complicated assembly process and the greater thickness of the element which makes the net floor area of such buildings smaller. A literature review revealed that most studies focused on the FRR of LSF constructions insulated with non-combustible (glasswood, rockwood and cellulose fibres in most cases [1; 15]) thermal insulation, because organic polymeric insulation materials tend to increase fire intensity. However, the use of these materials is problematic where there is a risk of condensation and increased exposure to moisture. If water penetrates the system, these materials absorb the moisture, which increases the thermal conductivity of the material, reduces its insulating properties and potentially increases the risk of corrosion of metal elements in LSF systems. With respect to organic combustible thermal insulation, [PERSON] et al. [21] investigated LSF walls with PCM-mat in the cavity and concluded that such materials lined with fire-rated wallboards do not contribute to the fire load. LSF system with other combustible insulation materials, especially polymers, are scarcely studied for their FRR although because the low thermal resistance value they could provide good thermal properties at ambient temperatures. In recent decades, rigid polyurethane foam (PUR) in particular has been used together with other materials to obtain composites with low weight, good heat resistance, high toughness and ductility, high impact resistance, efficient sound insulation and excellent mechanical properties [22]. On the other hand, when used in high temperature environments, PUR foams are very combustible polymers with rapidly spreading flames, high thermal emissions and smoke generation [23; 24; 25]. When PUR foam is exposed to elevated heat, the chemical bonds generally break down, producing volatile gases that ignite when combined with oxygen. During this process, the PUR first softens and then decomposes, which manifests itself as charring. Due to the increased heat during combustion, the chemical links are further broken down and ignite until only char remains. The ignition and decomposition of PUR foam has been intensively studied and well described in the relevant literature [23; 24; 25; 26].