Fire response of steel frames retrofitted against progressive collapse

A large share of the existing building stock was built before the implementation of design standards against progressive collapse and is in need for retrofitting. Retrofit measures are typically designed with a threat-independent approach, involving the removal of one or more structural elements. This approach has been widely applied to deal with events such as impacts or blasts (short-duration events), but may not be adequate for fire scenarios (long-duration events) due to peculiar phenomena such as restrained thermal expansion and degradation of mechanical properties. This is particularly relevant for steel structures, which are sensitive to thermal attack and are typically designed as low-redundant systems, and used in large industrial or strategic buildings. This paper investigates the fire response of a 9-storey steel moment-resisting frame before and after progressive collapse retrofitting. Three retrofit measures designed considering the removal of the central ground storey column are investigated, including a bracing system, a roof-truss, and the concrete encasement of critical columns. Parametric fires, different compartment scenarios, and various loading levels are considered. The retrofit measures are shown to improve the fire response of the structure by preventing or delaying collapse. Finally, the effectiveness and suitability of the measures in different circumstances are discussed.