Progressive collapse of steel structures exposed to fire: A critical review

A state-of-the-art review is presented on design and research related to progressive collapse of steel structures under fire conditions. The influence of load ratios, strength of structural members, fire scenarios on the collapse behavior of structures is comprehensively reviewed. The corresponding mitigation measures and quantitative assessment methods are summarized. The existing experimental works are limited to low-rise buildings, plane frames or scaled models due to cost and safety issues, while numerical works involve high-rise buildings, threedimensional models, full scales and various influencing factors. It is found that these influencing factors as well as fire protections and bracing systems significantly affect whether, how and when a structure collapses. Whether a structure collapses in fire essentially depends on the location of fire, heating rates, peak temperatures and duration of fire. Three collapse modes of global lateral drift collapse, global downward collapse and combined lateral-downward collapse are concluded for steel framed structures in fire. Three-dimensional models are recommended to make an accurate prediction of collapse modes of structures exposed to various fire scenarios. The determination of collapse time of structures under fire is a complicated process, and it is suggested to divide it into safe, alert, dangerous and collapse stages. This can provide three collapse warning levels of stay in, ready to evacuate and must evacuate. Further work is still needed on fire experiments considering collapse purpose, large structural scale and realistic fire conditions (cooling phase and fire spread) to achieve the real fire-induced collapse mechanism of structures and validate numerical models.