Quantitative evaluation of progressive collapse process of steel portal frames in fire

This paper presents quantitative investigations on the collapse behavior of steel portal frames exposed to standard fires. The complete collapse process is divided into safe, alert, dangerous and collapse stages. These four stages are to warn firefighters to make timely reactions to stay, ready to evacuate, and must evacuate, respectively. Numerical models are established and validated against fire tests on a full-scale steel portal frame. Based on the key displacements of the heated columns and rafters, the four stages are quantitatively determined in terms of visible phenomena and warning times. It is found that one should be ready to evacuate when the heated rafter shows large deflections, and must evacuate when the heated column moves back to its initial position. The 1 h, 2 h, 3 h fire rating of protected steel portal frames can be used to estimate the ready-evacuate, must-evacuate and collapse warning time, respectively. The findings from this preliminary study aim to provide references for firefighters to make wise decisions to evacuate timely and safety from the fire scene.