Effects of thermal spalling on the fire resistance of earthquake-damaged reinforced concrete structures

Abstract

An investigation is performed here on three reinforced concrete (RC) structures designed for peak ground acceleration of 0.30 g and life safety level of performance. The rebars of beams are placed in two configurations – a one-layer distribution and a two-layer distribution where each layer includes 50% of the required rebars. Based on the Tabas earthquake acceleration (occurred in 1978 in Tabas city, Iran), nonlinear dynamic analyses are performed to determine the seismic damage. The structural fire analyses are then performed based on the Eurocode 1 parametric fire curve to determine the fire-resistance ratings (FRRs) of the damaged structures. The structural fire analyses are performed for two separate scenarios – when thermal spalling is considered, and when it is ignored. Under the ‘no thermal spalling’ scenario, the results indicate that, although post-earthquake fire (PEF) loading causes some deterioration to the FRRs of the structures, it does not make the structures collapse. However, when thermal spalling is considered under the one-layer distribution; the results show that all the three structures collapse rapidly. Interestingly, when the rebars are placed according to the two-layer arrangement, the PEF analyses show that no collapse occurs. As PEFs often create chaotic situations with resulting difficulties of timely access for rescue teams, adopting a two-layer arrangement in the design process can make a significant improvement to FRRs; thus, reducing the associated risk. The suggestion here is also applicable as a functional strategy to improve FRRs of RC structures under fire-alone scenarios when thermal spalling is a concern.