Passive Fire Protection of Cold-Formed Steel under In-Fire and Post-Fire Conditions

This study investigates the influence of an intumescent fire-protective coating on the in-fire and post-fire tensile behavior of cold-formed S235 and S355 steels. A total of 246 tensile coupon tests were conducted on 3.0-mm-thick coated and uncoated specimens from 23 °C to 1000 °C using an ISO 834-based heating protocol and natural air cooling. The novelty lies in directly comparing in-fire and post-fire tensile-property retention of protected and unprotected CFS across the same broad temperature range. The coating formed an insulating char above 300 °C and progressively degraded at higher temperatures. Under in-fire conditions, maximum coating contributions reached 65% in yield strength and 124% in ultimate tensile strength at a nominal furnace temperature of 700 °C, and 34% in ultimate elongation at 1000 °C. Post-fire benefits were smaller. Only one water-based intumescent coating was investigated; therefore, the findings are limited to the tested coating system and coupon-scale conditions.