PSI - Issue 64

Massimiliano Ferraioli et al. / Procedia Structural Integrity 64 (2024) 1025–1032 Ferraioli et al./ Structural Integrity Procedia 00 (2019) 000–000

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collapse during strong earthquakes. This has prompted the development of new seismic codes focusing on structural resilience, energy dissipation, and recentering capabilities to enable repairability after intense seismic events. In recent times, external retrofitting substructures have been developed to address diverse enhancement requirements, such as strength, stiffness, and ductility, which can be categorized into several types as outlined by Cao et al. (2022): 1) frame substructures; 2) frame-brace substructures; 3) rocking systems; 4) wall substructures. Within this framework, alternative retrofit approaches have been introduced as low-impact, reversible interventions, utilizing external diagonal grids known as "diagrids," forming a 3D lattice structure termed the "exoskeleton" (Di Lorenzo et al. , 2020). These strategies vary based on material (e.g., steel, concrete), geometry (e.g., 2D or 3D), orientation (e.g., parallel or orthogonal to the building façade), and seismic resistance system (e.g., concentric or eccentric braced frames, RC frames). While many studies in the literature have focused on designing exoskeletons to remain linear and elastic during seismic events, this approach often results in a stiffened retrofitted building, thereby amplifying seismic demands. To address this, this paper proposes coupling the exoskeleton with dissipative systems (dampers) or recentering shape memory alloy (SMA) dampers. The effectiveness of this retrofit solution is demonstrated through nonlinear time-history analyses under various earthquake scenarios, showing promising results in minimizing residual drift and enabling repairability. 2. Case study building An existing school building located in Vibo Valentia (Calabria, Italy) served as the reference structure. Constructed in 1962 under outdated seismic design regulations, it spans three stories and features an L-shaped floor plan measuring 17.70 x 35.50 meters. A full knowledge level (KL3) was attained due to comprehensive data collection and testing, including construction drawings, site surveys, soil investigations, and material testing. Material strength values (i.e., f cm =35.2 MPa for concrete, f ym =408 MPa for steel) were derived from these assessments for seismic analysis purposes. Soil classification as per the Italian Seismic Code (2018) categorizes it as ground type B. The nonlinear (pushover) analysis was conducted following the Italian Code (2018), employing two force distributions ("modal" and "uniform") and considering a 5% accidental eccentricity perpendicular to the direction of excitation. SeismoStruct (2023) software was utilized to model the reinforced concrete (RC) structural elements using inelastic force-based fiber elements (infrmFB). The cross-sections were discretized into fibers representing steel reinforcing bars, confined concrete within inner hoop layers, and unconfined concrete outside the hoops. Reinforcing steel bars were modeled with a bilinear hysteretic model, while confined concrete behavior followed the well-known Mander model (1988). Elastic behavior under shear and torsion was assumed for the sections. Limit states including Immediate Occupancy (IO), Damage Limitation (DL), Life Safety (LS), and Collapse Prevention (CP) are defined according to the Italian Code (2018). Ductile and brittle member capacities were assessed in terms of chord rotation and shear strength. Table 1 summarizes the seismic safety verification results, presenting the capacity of the existing building in terms of peak ground acceleration (PGA) and return period ( T r ) for various limit states, along with the corresponding safety index ( ζ ), representing the ratio between capacity and demand. Key deficiencies identified in the existing building include insufficient stiffness for IO and DL Limit States, poor shear capacity of brittle components, torsional effects, and inadequate member chord rotation capacity for the LS Limit State.

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Fig. 1. RC school building in Vibo Valentia. a) External view; b) FE model.