PSI - Issue 64

Sabatino Di Benedetto et al. / Procedia Structural Integrity 64 (2024) 983–990 S. Di Benedetto / Structural Integrity Procedia 00 (2019) 000 – 000

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The SAP2000 model has been modified, and the linear static analyses have been carried out again, employing the same load combinations used in evaluating the seismic performance of the as-built structure. The D/C ratios of the bracings have been determined to be 1. The columns, however, have been checked using the ultimate strength of the bracing diagonals, taking into account the most unfavourable combination of axial force and bending moments (Eqs. 1-3): = , +1.1 Ω , (1) = , +1.1 Ω , (2) = , +1.1 Ω , (3) In this study, however, it is assumed that 1.1 Ω=1.5 . With this assumption, and for ease of calculation, the loads impacting the columns are assessed when the dampers reach their ultimate strength through linear static analyses. The seismic action is amplified by 50%, and the resistances assigned to the diagonals of the first and second levels are set at 690 kN and 353 kN, respectively. The strength and stability check for the most stressed column could be defined equal to 0.91. Regarding the DL checks, linear static analyses have revealed a maximum interstorey drift of 0.67%, which is below the 1% limit specified by regulations. Consequently,  E is equal to 1 and according to the Guidelines for the classification of seismic risk of buildings D.M. 58 (2017), the retrofitted structural configuration falls into Seismic Risk Class A. As mentioned earlier, modifications in the geometric properties of the bracings necessitated a re-evaluation of the connections between the diagonals and the columns. Upon review of the previous configuration, it was determined that the connection at the first level did not meet the verification criteria, prompting the need for modifications in its construction detail. Additionally, it should be noted that the maximum tensile force in the diagonals induces shear actions at the base nodes, resulting in unsatisfactory results in the checks for screw pullout and base plate shear. To fix this issue, the plan involves retrofitting the base nodes by modifying them from "exposed" nodes to "embedded" nodes. Given that the shear force transmitted to the base node is 618 kN, the strategy includes increasing the thickness of the foundation plinth by 20 cm. Furthermore, a new structural model has been defined in this scenario, featuring bracings equipped with non linear link elements designed only for axial tensile actions. Specifically, the dampers are positioned at the bracing diagonals, and their mechanical behaviour can be represented using a perfectly plastic elastic link. Fig. 6 illustrates the outcomes of the non-linear static analyses, showing the pushover base-shear-roof displacement curves associated with the Modal and Masses distributions along ±X and ±Y directions. It is evident that the retrofitted solution offers increased resistance and ultimate displacement compared to the existing configuration. Additionally, a more uniform activation of the dampers along the height of the building has been observed.

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Fig. 6. Pushover curves of the structure in its existing (E) and retrofitted (R) configurations along X (a) and Y (b) directions.