Full-scale experimental investigation of external-insulated cold-formed steel load-bearing walls under fire conditions

Abstract This paper presented a detailed experimental investigation on three full-scale external-insulated cold-formed steel (CFS) load-bearing wall specimens under ISO 834 fire exposure. The load ratio of each specimen was 0.55. The results showed that using autoclaved lightweight concrete (ALC) board as the external-insulation was more reasonable than the configuration of rock wool cavity-insulation and rock wool external-insulation under fire conditions. The specimens S2 and S3, which adopted ALC board as the external-insulated sheathing and calcium-silicate (CS) board as the base-layer sheathing on both sides, demonstrated desirable thermal insulation and integrity in fire. The corresponding fire resistance was longer than 3 h, meeting the highest requirement of the fire resistance rating specified in Chinese codes. Moreover, the fire-induced structural failure of specimen S3 was significant different from that of the non-cavity-insulated specimen of S1, which adopted the rock wool as the external insulation and was lined with gypsum plasterboards (GP) and CS boards on either side. This was attributed to the constraint effect of different sheathing configurations on the out-of-plane deflection of the wall studs. At the final stage of fire experiment, specimen S1 experienced significant out-of-plane deflection towards the furnace and the local buckling occurred on the cold flanges, which eventually led to the flexural–torsional buckling of the wall studs towards furnace. For specimen S3, the out-of-plane deflection was invisible through the experiment and the structural failure tended to be described as the local buckling of the whole cross section of wall studs. Additionally, compared with specimen S2, the cavity depth of specimen S3 was increased from 89 mm to 150 mm by increasing the stud web depth. It was found that for the non-cavity-insulated CFS walls, the increased cavity depth had insignificant effect on the temperature rise of wall cavity, wall studs and the ambient surface, if the possible change of heat transfer path caused by the cracking and falling-off of fire-side sheathing was not considered.