# Fire Tests of Load-Bearing, Light-Steel-Framed Wall Systems Insulated with Polyurethane Foam
## 4 Discussion  
### Overall Behaviour of Tested Specimens  
The results presented separately for each LSF specimen show similar behaviour and mode of failure during standard fire exposure. At the beginning of the test, the gypsum fibreboards protected the steel frame from a temperature rise for some time. In addition, the low thermal conductivity of the PUR foam contributed to low heat transfer through the specimen before it began to decompose. Once the free and physically bound water evaporated from the GFBs, they became susceptible to cracking, and parts of boards fell off, allowing heat to penetrate the combustible cavity insulation and steel members. For the specimens with double GFBs (specimens P1 and P2), the sudden temperature rise on the steel members started after about 50-60 min (Figures 6a,b and 7a,b), while the additional board on the specimen with triple GFBs prolonged the steep temperature rise for an additional 30 min (Figure 8a,b). When the ignition temperature for the PUR insulation was reached (between 200 \({}^{\circ}\)C and 300 \({}^{\circ}\)C), the additional heat generated by combustion contributed to the temperature rise on the steel sections and maintained the high temperatures even after the burners in the furnace were turned off. Nevertheless, the heat generated by the PUR combustion was not sufficient to raise the furnace temperature (Figures 6a,b-8a,b). Throughout the test period, all tested specimens showed no signs of integrity and insulation failure. As mentioned earlier, according to previous studies on the load-bearing cavity insulated LSF wall exposed to fire test [16; 17; 18], structural failure is usually the predominant failure mode due to the non-uniform heating of the steel members. In accordance with EN 1365-1 [33], structural failure occurs when the displacement reaches the value h/100 mm (height/100) or when the displacement rate reaches a value of 3 h/1000 mm/min. This means that a displacement of 30 mm or a displacement rate of 6 mm/min must be achieved. Figure 10a shows the comparison between the average measured displacements for all tested specimens, while Figure 10b shows the comparison of the average displacement rates. During the time of unloading, the two structural failure criteria were not reached but a trend of steep increase can be observed. Since the specimens were manufactured with non-standard dimensions and unloaded earlier than the above criteria were reached, it is not possible to give an official FRR. However, the results shown  
Figure 9: Post test observations for specimen P2: (**a**) exposed side of upper part of test specimen; (**b**) exposed side of lower part of test specimen; (**c**) node 6; (**d**) node 7.  
in Figure 10,b suggest that specimens P1 and P2 could achieve a minimum FRR of 60 min and specimen P3 of minimum 90 min, which is sufficient for use in residential and commercial buildings. Post-test inspection of the tested specimens revealed that the cavity insulation was completely burned, leaving only layers of charcoal (Figure 9), while the steel members around nodes in central studs (N6 and N7 according to Figure 1) were deformed in all three specimens (Figure 9,d). It was also noted that the GBFs were still attached to the steel structure at the top and near the bottom of the specimen. This led to the conclusion that the failure of the exposed GBF was localized and probably occurred about 1 m from the bottom of the specimen. At this location, the fire from the furnace entered the cavity, resulting in decomposition of insulation. This is confirmed by the results of the infrared thermography monitoring and thermograms taken on specimen P1 at the 67 th and 76 th minutes of testing, Figure 11.  
Thermographic measurements showed that the predominant heat transfer occurred in the lower third of the specimen, which is consistent with the temperature-time curves shown in Figure 6,b.