GNGTS 2022 - Atti del 40° Convegno Nazionale

336 GNGTS 2022 Sessione 2.2 Non-linear time history analysis (NLTHAs) are performed using the software Seismostruct (Seismosoft 2022). Both material and geometrical non-linearities are included. Material non- linearities are accounted for using a force-based inelastic frame element (infrmFB) that adopts a diffused plasticity model using fibre-section discretization. A total number of 5 integration sections with 150 discretized fibres is used for each beam element. To simulate the rigid slab at each floor, “rigid diaphragm constraints” have been used. As described in the Seismostruct Manual, the simultaneous use of diffused plasticity model and rigid diaphragm constraint will generate very high fictitious axial internal forces in horizontal elements. To prevent this interaction, beams are released from the axial constraint through rigid links to which is given no axial stiffness and infinite stiffness in the other degrees of freedom, as described in Barbagallo et al. A Rayleigh damping is used, for which it is necessary to define two vibrational periods and a target damping factor for each of the two chosen modes. The two periods are chosen in order to avoid over damping of higher model and elongated modes due to plastic deformations and consist in 1.5 times the first vibrational period and the period that leads to a cumulative mass participation ratio of 90%. The target damping factor is 3% (ASCE 2014). The Hilber - Hughes – Taylor integration scheme is used. Seismic input and intensity measure selection. Fragility is usually evaluated through structural responses estimated via Non-Linear Time History Analysis (NLTHA). There are several procedures to develop fragility functions based on NLTHAs such as Incremental Dynamic Analysis (IDA) (Vamvatsikos and Cornell 2002), and Cloud Analysis (CA) (Luco and Cornell 2007; Jalayer et al., 2015, 2017). In this paper, the CloudAnalysis procedure is selected to develop fragility curves, considering a set of 62 unscaled ground-motion records, chosen from European Strong Motion Database. When adopting the cloud method for fragility evaluation a few points should be considered (Jalayer et al., 2017): • The records should cover a wide range for the selected IM; • A significant number of records (about 20-30%) should lead to an exceedance of the selected EDP capacity for the selected performance level; • Not more than 10% of records from the same event should be selected. To compose the fragility curves, each record is represented by intensity measures combined in the horizontal components. D.M. Boore et al. assess it is not precautionary to use raw geometric mean for combining the horizontal components of the records. Thus, they proposed a new bi- dimensional parameter of the intensity measures of ground motion records, called RotDpp, which it is used in this work for each intensity measure. Both structure independent and structure dependent scalar intensitymeasures are selected among the most referred in practice and literature: the structure independent ones (i.e. they do not depend on vibrational properties of the structure) are the Peak Ground Acceleration (PGA), Housner Intensity and the Cumulative Absolute Velocity (Reed and Kassawara, 1990). The latter is expressed in eq.(1): (1) One of the most used IMs is the spectral acceleration at first mode S a ( T 1 ). As the reference structure and NLTHAs are 3D, the IM should account for the main vibrational properties in both directions. Hence here spectral acceleration is evaluated at a mean period T 1 m evaluated as the average of the values of the fundamental periods in each direction (FEMA 2018), expressed in eq.(2):