Assessment of the inter-story isolation technique applied to an existing school building

Inter-story Isolation System (IIS), which combines the functions of seismic isolation and mass damping, is becoming increasingly attractive to mitigate the seismic risk of buildings. This technique is mainly applied in high-rise and multi-purpose buildings, promoting architectural freedom, and representing a sustainable housing solution in densely populated countries. An interesting but still little exploited use of the IIS concerns the addition of isolated stories on top of existing buildings which, beyond increasing the volume and functionality of the buildings, can also reduce their base shear forces and inter-story drifts when the IIS is properly designed. To this end, some design methods are available in the literature based on both frequency and time domain approaches. However, research studies focusing on real case studies are still very few. This paper presents an ideal application of the IIS to an existing two-story RC school building, with the aim of raising it by one story while improving its seismic performance. Firstly, the optimization study of the IIS parameters (frequency and damping ratios) is presented, which is based on a time-history (TH) parametric analysis using the equivalent lumped-mass model, and the optimal solutions are compared with those from other general approaches in frequency domain. Then, the effectiveness of the optimal IIS in improving the seismic performance of the school building is evaluated by comparing the TH responses of the associated 3D finite element models, with and without IIS, under a set of spectrum- compatible bidirectional natural records. Ultimately, the results obtained confirm the effectiveness of the IIS and promote it as a potential seismic retrofit technique, even if specific case-by-case assessments are required.