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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5% of the value assumed for designing new buildings in the same site. In both cases, checks are not satisfied both for the steel bracings and for the shear walls at the base. This means that the current structure is not even able to bear the 5% of design seismic force. The result is in accordance with the original structural design of the building: it refers only to gravity load bearing capacity, without taking into account the effects of seismic actions. At the same time, structural behaviour from dynamic analysis of the current building shows a lack of capacity in counteracting any seismic force: this can be expressed through a seismic vulnerability index, z E , closed to zero.

Fig. 5. Shear distribution at the base for seismic force in x-direction (a) and y-direction (b).

4. The hybrid retrofit solution

The existing structure has not been designed to bear seismic loads. In this case, the upper steel frames are very deformable, not resisting the forces of this type. In this case, the main strengthening measures refer to the R.C. components. The preventive actions include the strengthening of the existing shear walls at the upper levels, cutting them below the floating plane to provide the whole structure for the introduction of the isolation system; the structural consolidation of the SAP-type slabs, by casting a 6 cm-deep reinforced-concrete slab, collaborating with the lower layer through a chemical anchor: in this way a rigid diaphragm is obtained at each floor level, improving seismic behavior of the overall structure.

Fig. 6. Hybrid retrofit solution: introduction of new shear walls and base isolation system