Experimental and numerical study on thermal-structural behavior of steel portal frames in real fires

This paper presents experimental and numerical investigations on the collapse behavior of a 12 m × 6 m steel portal frame exposed to fire. A real fire test is conducted with a 4 m × 6 m fire compartment at the corner of the frame. Extensive thermal and structural responses of the frame are measured and presented. The experimental results show that the fire compartment collapses after 20-min heating with a critical column temperature of 850 °C. It is found that the measured gas temperatures are higher than the ISO fire, but lower than the parametric fire specified in EN 1991-1-2, indicating the underestimation of the thermal exposure for standard fires and unrealistic estimation for parametric fires. For steel temperatures, the prediction from parametric fires is in a reasonable agreement with the measurements by predicting a similar maximum temperature at a delayed time. The prediction from standard fires significantly underestimated the steel temperatures by up to 300 °C. The significant non-uniform gas temperature distribution along the frame height shows a two-zone model with a temperature gap of 200 °C. The maximum temperature of the members beyond the fire compartment exceeds 500 °C due to the spread of hot smoke. A numerical model is established and validated against experimental results. A potential outward collapse mode of the frame is predicted. It is suggested that a more realistic description of fire scenarios is still needed for a performance-based structural fire design, based on a better consideration of ventilation conditions and thermal properties of boundary enclosure.