PSI - Issue 44

Giuseppe Brandonisio et al. / Procedia Structural Integrity 44 (2023) 1316–1323 Giuseppe Brandonisio et al. / Structural Integrity Procedia 00 (2022) 000–000

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The diagram in Fig. 9 (a) shows a comparison between the cumulative mass percentage in the x-direction (continuous line) and the y-direction (dashed line) for the compared models. In the x-direction, that final design solution has a more regular structural behaviour: it can involve the 100% of the participating mass through the first two vibration modes (red curve). On the contrary, the structure at the current state (blue curve) needs 6 vibration modes to include the total seismic mass. In the y-direction, the isolated solution involves 100% of the participating mass through the first three vibration modes, while the current structure needs the contribution of 6 vibration modes.

Fig. 9. (a) Comparison between the cumulative mass percentage in x-direction and y-direction; (b) Comparison between the 1 st mode spectral accelerations of FB and BIS models.

The diagram in Fig. 9 (b) relates to the spectral acceleration associated with the first vibration mode of the fixed base structure (blue) and of the isolated solution (red). The effectiveness of BIS in terms of seismic demand is clear from the comparison. In fact, the spectral acceleration is reduced from S d (T 1, FB ) =0.192g of the current state (valued on the design spectrum, assuming q=2.0) to Sd(T 1, BIS )=0.045 g (value on the elastic spectrum), with a reduction of the spectral acceleration equal to 76%.

Fig.10. Shear distribution at the base of BIS-model for seismic force in x-direction (a) and y-direction (b)

Fig. 10 displays result in terms of seismic shear distribution at the base of BIS model, for seismic forces applied in both x-direction, Fig. 10 (a), and y-direction, Fig. 10 (b). Looking at the abovementioned figure and comparing it with results obtained from FB-model (Fig.5), it is clear the effectiveness of the hybrid solution applied to the monumental building, as valuable strategy for seismic hazard mitigation. This solution guarantees a significant reduction of the seismic action, being feasible with the original structural configuration of the building. 5. Conclusion The paper discusses a hybrid design strategy for seismic retrofit of an existing monumental building based on a preliminary seismic vulnerability assessment. The case study is an “unconventional” mixed concrete-steel structure originally designed for gravity loads. Discontinuities in terms of material and stiffness distribution occur along the