The load-bearing capacity of corrugated steel without fire protection is 15–30 minutes depending on the structure. The steel sheet bends but the load-bearing capability remains for a significant amount of time.
When there is no insulation used on top of the corrugated steel sheet, the heat goes through the metal and dissipates upwards and the steel temperature rises slower.
When corrugated steel is used as a load-bearing structure for the roof construction and insulation is installed on top of the corrugated steel board, the temperature of the metal rises very quickly. This is why you need to take into account the insulation amount above the corrugated steel board when designing the fire protection thickness below the corrugated steel construction.