PSI - Issue 78

Laura Giovanna Guidi et al. / Procedia Structural Integrity 78 (2026) 2154–2161

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3.3. Global behavior: shear distribution at the base Referring to the stiffer global model with infill panels, the distribution of shear forces at the base is analyzed. Modal analysis results indicate that, for over 200 modes considered, the cumulative participating mass reaches approximately 82% in the X-direction and 75% in the Y-direction. The seismic response spectrum adopted for the site (Pomigliano d’Arco), corresponding to the Life Safety Limit State (SLV) with a return period (TR) of 712 years and a use class III, includes a maximum spectral acceleration (Se/g) of approximately 0.449, as defined in the design spectrum.

Fig. 6 Acceleration response spectra at SLV and sear distribution at the base for horizontal force in both plan directions

The horizontal forces applied at the base of the structure derive from (1) e (2), as percentages of te overall eight (W): F h,X = 0.82 W g ∙ S a (T) = 0.82 W g ∙ 0.44g= 0.36W (1) F h,X = 0.75 W g ∙ S a (T) = 0.75 W g ∙ 0.44g= 0.33W (2) Under the X-direction earthquake, the shear distribution is relatively balanced ( Fig. 6 ): Block 1 experiences the highest demand (47% of Fx), which is consistent with its greater mass and longitudinal flexibility. Block 2 contributes 19%, followed by Block 3 with 22%, and Block 4 with 12% of the total shear force. Under the Y-direction earthquake, the response becomes highly concentrated: Block 1 absorbs as much as 54% of the vertical shear force Fy, and Block 3 takes on 40%. The remaining blocks are significantly less engaged, with Block 2 contributing only 4% and Block 4 just 2%. Despite this uneven distribution, the presence of pedestrian bridges and horizontal connections allows the complex to behave as an integrated structural system. However, the contribution of each unit is strongly influenced by its relative stiffness and geometric position within the overall layout. Each structural unit was individually modelled using the professional software CDSwin. Given that the frame systems in each unit are essentially arranged along a single direction — transversely — the seismic action was considered only along the direction of the frames for each building unit. The analysis reveals that the base shear forces obtained from the global models and those from the individual-unit models sometimes show significant discrepancies. The most notable difference occurs in Body 1, where the base shear in the Y-direction is significantly higher in the SAP2000 global model compared to the CDSwin model. This is due to the presence, in the SAP2000 model, of a stairwell structure that alone absorbs more than 2,000 kN of horizontal force — an element that is not included in the CDSwin models. As a result, a substantial portion of the seismic forces is absorbed by components that do not exist in the local models, which explains why, despite the significant differences in base shear, the internal forces in the main frames are still comparable. 4. Seismic assessment: preliminary results Preliminary checks under vertical loads yielded a positive response for the building in its current condition, confirming the absence of critical issues with respect to ordinary gravitational actions. The analyses were performed using finite element modeling with the CDSwin software. A parallel analysis was also conducted using three-dimensional models