Performance of reinforced concrete structures subjected to fire following earthquake

Fire following earthquake (FFE) is a serious threat to structures that are partially damaged in a prior earthquake potentially leading to a quick collapse of the structure. The majority of standards and codes for the design of structures against earthquake, however, ignore the possibility of FFE and thus buildings designed with those codes fail swiftly when exposed to fire after earthquake. A sequential structural analysis based on FEMA 356 is performed here on the Immediate Occupancy (IO) and the Life Safety (LS) performance levels of two reinforced concrete frames. The frames are first subjected to an earthquake load with a Peak Ground Acceleration (PGA) of .30 g. This is followed by a fire analysis, using ISO 834 and natural fire curves. The time taken for the structures weakened by the earthquake to collapse under these fires is then found through a robust numerical analysis. As a benchmark, fire-only analyses are also performed for undamaged structures. The results show that earthquake-weakened structures are more vulnerable to fire than undamaged structures, to the extent that the fire resistance of the damaged structures can decline to about a third of the original undamaged structures. The results also show that the fire resistance of the frame exposed to the natural fire differs from that of the frame exposed to the ISO 834 fire. This is due to the inclusion of parameters such as dimensions of the compartment as well as thermal properties of the combustible materials and the size and position of opening in the natural fire model, which does not exist in the ISO 834. Whilst the investigation is conducted for a certain class of structures (regular buildings, reinforced concrete frames, 3 stories), the results confirm the need for the incorporation of FFE into the process of analysis and design, and provides some quantitative measures on the level of associated effects.

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