Early-warning methods for fire-induced collapse of single span steel portal frames

The accidents of firefighter injuries caused by the fire-induced collapse of steel structures occur frequently, putting an important issue on establishing early-warning systems for such collapse. This paper presents early warning methods for predicting collapse of single span steel portal frames under fire based only on real-time measurement of the displacement and velocity of apex and eaves. A three-dimensional finite element model verified by numerical simulation of physical tests was adopted to simulate the collapse behaviour of steel portal frames. Analysis indicated that the displacement and velocity of apex and eaves have a close relationship with collapse state. Moreover, the collapse mode of the burning frame can be predicted through analysing the direction of displacements. Through parametric studies, the early-warning points that can reflect the collapse mode and collapse trend of steel portal frames are determined. Then the three-level early-warning methods applicable to different collapse modes are proposed with dividing the collapse process of steel portal frames into several stages, including: safety stage, 1st early-warning stage, 2nd early-warning stage, 3rd early-warning stage and collapse. In order to quantitatively evaluate the collapse risk of the burning frame, the normalised occurrence time of each early-warning level is determined using Monte Carlo Method. Finally, the feasibility and reliability of the proposed methods are validated by two natural fire tests on steel portal frames.