Effect of the fire action on the seismic isolation system for existing reinforced concrete buildings

Seismic isolation is a design strategy adopted for the protection of both new and existing structures from the damaging effects of earthquakes. It is achieved through the use of elastomeric isolators, which, in case of fire, can suffer permanent damage, seriously compromising the bearing capacity of the isolation system. This study proposes a new methodology to assess the effect of high temperatures on the elastomeric isolators and the loss of efficiency of the isolation system after a fire event. Two possible interventions with seismic isolation for an existing reinforced concrete structure with multi-storey frames were considered to apply the methodology. The evaluation of the isolator thermal response for 30 and 60 minutes of exposure to the standard fire curve was used for quantifying the damage induced by the fire. This damage level was defined starting from the thermo-mechanical properties of the elastomeric layers damaged by fire, considering different critical temperature thresholds. The estimated degradation was used to evaluate the seismic performance of both isolators and structure in different design configurations, also in a multi-risk scenario that foresees the occurrence of a seismic event after a fire. It was found that the most critical case is the one in which the critical temperature is equal to 70 °C, since after 30 min of exposure to fire the isolator is not able to sustain the vertical loads due to the superstructure. Therefore, the design of a proper passive fire protection system was proposed, able to drastically reduce the temperature in the isolator, mitigating the fire damaging effect.