# Seismic Isolation Devices for Protecting RC Buildings: The Frangipane School in Reggio Calabria
## 5 Retrofit Intervention (MIS Configuration)
### Seismic Demand Assessment  
In conclusion, the comparison between the seismic performance of the fixed-base, non-isolated (NB) building system and the base isolated (IB) Frangipane school building system under seismic actions was further quantified in terms of maximum seismic demands in load-bearing RC members.  
The numerical analysis for the previously defined seismic combinations gave evidence of typical behaviours that can be overall summarized in structural performances of the isolated system characterized by:  
* a significant reduction in stress characteristics in terms of bending moment and shear design actions due to input seismic loading for all the load-bearing components of the RC superstructure;
* but also a significant reduction in the expected inter-story drift, with most of the horizontal displacement demand due to input seismic actions measured at the level of the base isolators rather than on the elevation of the building system itself.  
Such an outcome and structural behaviour in seismic conditions for the IB system compared to the NB configuration can also be perceived from the numerical examples reported in Figures 17 and 18.  
In Figure 17, the typical distribution of bending moment in RC columns can be seen for longitudinal RC frames in the \(x\)-direction. It is worth noting, as also expected that the typical bi-triangular distribution is qualitatively similar for the two configurations. Besides, the presence of seismic isolators manifests in a bending moment reduction, both at the ground level and at the upper story levels, which is in the range of 70-80%, compared to the NB system (see Figure 17b).  
In terms of drift demand, further numerical comparisons are proposed in Figure 18, with evidence of the major modification in the structural behaviour of NB and IB systems, respectively. In this case, attention is given to the story drift demand and inter-story drift ratio as a function of the story level of each centre of mass.  
From Figure 18a, it is thus worth noting that the NB configuration is typically characterized by a story drift demand which is proportional to the first vibration shape of the structural system (see also Figure 9a). As far as the drift demand is addressed for the building system equipped by base isolators (IB), it is possible to note that most of the measured horizontal displacement is lumped at the level of isolators themselves, and the story drifts demand for the superstructure is less pronounced. Such an observation confirms the mostly rigid-body motion of the superstructure under seismic events and is in line with the fundamental vibration shape reported in Figure 16a.  
When the inter-story drift ratio is also examined for both configurations, typical results, as in Figure 18b can be found.  
For the IB configuration, the inter-story drift demand was generally measured in the range of \(-\)70-90% of the corresponding demand for the NB system, with most of the inter-story demand at the levels of seismic isolators.  
Figure 17: Example of numerically calculated bending moment in RC columns for longitudinal frames in the \(x\)-direction (Midas): (**a**) non-isolated (NB) or (**b**) isolated (IB) building system (values in kNm).