PSI - Issue 44

Marco Gaetani d’Aragona et al. / Procedia Structural Integrity 44 (2023) 1052–1059 M. Gaetani d’Aragona, M. Polese, A. Prota / Structural Integrity Procedia 00 (2022) 000–000

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Figure 3 (a) shows the pushover curves in terms of base shear (V b ) vs top displacement ( D top ). The sole response in the longitudinal direction is analyzed. Dashed lines (JF + BCF) represent the expected building response, obtained by considering the complete beam-column sub-assembly failure, also accounting for infill-frame interaction. If brittle failure for columns is not activated (continuous line, JF + no BFC), the initial lateral stiffness is not sensibly varied, while the post-peak behavior evidences a slight increase in the base-shear capacity, and area under curve (i.e., dissipation capacity). This is ascribable to a lower number of columns failing in flexure-shear due to local infill-frame interaction. If brittle failure for columns is considered while the beam-column contribution is neglected (no JF + BCF, dotted line), the post-cracking stiffness significantly increases along with the peak base shear. In fact, for the GLD building, the behavior of the beam-column sub-assembly is mainly governed by the shear failure of joint panels. Thus, if JF is neglected, it is expected a substantial increase in frame global lateral resistance, and a higher dissipation capacity under seismic loads. If infill-induced failures of columns and the behavior of beam-column joints are neglected (dash-dotted line), the full capacity of the original frame can be exploited, and the lateral capacity of the frame significantly increases, while the post-peak behavior is characterized by a more gradual degradation, leading to higher dissipation under reverse loadings. Further, square markers represent the occurrence of global failure, according to the definition of GLC and SSC whichever comes first. Figure 3(a) evidences that the most influential parameter on the lateral drift capacity is the possible development of brittle failure in beam-column joints. The initial lateral stiffness of the pushover curve mainly depends on the uncracked stiffness of masonry panels, and does not vary if brittle failures in columns or beam-column joints are considered or not. Instead, brittle failures affect the first point of the curve in which nonlinearity is attained, along with the peak shear and post-peak behavior of the frame. In this case, the attainment of first non-linearity mainly depends on the simulation of joint failure, since it corresponds to a significant reduction of the sub-assembly capacity. The infill-frame interaction, instead, moderately influences the post-peak behavior, while influences the post-peak slope. In fact, most of the columns exhibit flexure shear failures, thus the maximum ductility of the column is primarily affected, which traduces into a change in global behavior after lateral peak response. Figure 3(b) compares the behavior of the as-built configuration (considering brittle failures in beams, columns and beam-column joints also including infill-frame interaction) with those obtained considering Fiber Reinforced Polymer wrapping strategy (FRP-S, dotted line) and Reinforced Concrete Jacketing of columns (RCJ-S, dashed line). When FRP-S is employed, the brittle failure of RC members is avoided, and the full lateral capacity of the frame can be exploited. In this case, the FRP-S allows to reach an increased base shear capacity of about 48% and increase in lateral displacement capacity of 53%. When RCJ-S is considered, a simplified design process is employed, and the jacketing of columns is designed to not exceed specified interstory drift ratios for a seismic intensity level corresponding to Damage Limit State, defined according to current Italian seismic design regulations (DM2018). To satisfy the RCJ-S design process, an increase in lateral story stiffness between 90% and 110% (from upper to base story) is required in the longitudinal direction. Every column in the perimeter frames (except for corner columns) needs to be jacketed to fulfill the design criterion, and the structural intervention needs to be applied from 1st to 5th story. In this case, the presence of jacketing leads to a stiffer response with respect to the as-built configuration, and the effect is more evident after the occurrence of first cracking for masonry infills. Due to the delaying of brittle failures in perimeter frame columns (i.e., RC elements for which occurs infill-frame interaction), a softer slope in post-peak behavior is evidenced, and a substantial increase in the ultimate displacement corresponding to global collapse (97%). 4. Conclusions The Stick model is a Multi-Degree of Freedom system composed of a series of lumped masses concentrated at the story level connected in series by nonlinear springs reproducing the interstory behavior. The Stick allows to predict the building seismic performances in terms of Interstory Drift Ratios and Peak Floor Accelerations under specific ground motions via Nonlinear Response History Analysis. Since the accuracy in prediction is strongly affected by the proper calibration of the nonlinear spring behavior, this paper introduces a simplified procedure relying on the basic hypothesis of shear-type behavior for the generation of interstory backbones proposed in a previous work by the authors. The procedure allows generating Stick models to simulate the behavior of masonry-infilled RC multi-story buildings designed and constructed in different periods,