PSI - Issue 44

Maria Maglio et al. / Procedia Structural Integrity 44 (2023) 550–557 M. Maglio, R. Montuori, E. Nastri, V. Piluso / Structural Integrity Procedia 00 (2022) 000–000

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reorganization of the provisions and a redefinition of the ductility classes with limits of applicability in terms of seismic action for structural systems and materials. Some of the strength point of the new Eurocode draft is the introduction of new design rules specific for some structural typologies which have been since now neglected. Moreover, some specific provisions have been introduced for the seismic design of the aluminum alloy structures, by the studies of Piluso et al. (2019) and Montuori et al. (2020), following the same approach devoted to the carbon steel structures. The new Eurocode draft is divided in two parts: the prEN1998-1-1 (2021) is the material invariant part containing the general requirements for all types of structures for earthquake resistant design, it includes definition of the seismic action and prescription of the methods of analysis and verification; the prEN1998-1-2 (2021) in which the design rules are detailed for each structural typology and with reference to all the ductility classes. To satisfy the seismic performance requirements in the new structures according to prEN1998 (2021), reference must be made to the deformation capacity and cumulative energy dissipation capacity of the structure. So, three ductility classes are introduced, namely DC2 (Ductility Class 2) intermediate between the low DC1 (Ductility Class 1) and the high DC3 (Ductility Class 3). In this work the attention is focused on the DC2 Ductility Class in which the local overstrength capacity, the local deformation capacity and the local energy dissipation capacity are considered (prEN1998, 2021). As DC2 Ductility Class has the aim to design shallower structures in medium intensity seismic zones, it is not needed to exploit all the plastic resources of the structure and therefore develop a collapse mechanism of global type as defined by Piluso et al. (2019) and Montuori et al. (2022). However, brittle mechanism such as soft storey mechanism must be avoided. For this reason, the main purpose of the DC2 design statement is the prevention of soft storey mechanism. This work aims to carry out a critical analysis on the DC2 design rules for Moment Resisting Frames proposed in the new prEN1998 (2021) and with reference to the design statement finalized to the prevention of the soft storey mechanism and to all the other requirements that must be fulfilled to achieve the design goal. Finally, a numerical application to highlight the failure in the design objective of the DC2 procedure is reported. A pushover analysis is The prEN1998-1-1 (2021) provides the general rules and the definition of the seismic action to design structures for earthquake resistance. To measure the seismic performance of a structure, reference is made to its state of damage under a given seismic action, that can be referred to four limit states (LS). The Ultimate Limit States (ULS) are the LS of Near Collapse (NC) and the LS of Significant Damage (SD), instead the Serviceability Limit States are the LS of Damage Limitation (DL) and the Fully Operational LS (OP). The seismic action can be specified in terms of their return periods, , , depending on the specified limit states and the consequence class of structures or alternatively in terms of performance factors γ , for a given type of structure. For new structures is only required to verify the non-exceedance of the SD limit state, so the spectrum at the LS of NC is not reported, instead the two serviceability limit states spectra are following reported. The construction of the spectra (SD, DL and OP) for DC2 is shown below with reference to site category B and consequence class CC2. To represent the seismic action the reference parameters, for 5% damping and a reference return period , are the maximum spectral acceleration , corresponding to the constant acceleration range (plateau of the spectrum) and , the spectral acceleration at the vibration period = 1 . The return periods values of seismic action in years in Table 4.3 of EN1998-1-2 (2021) are reported in function of the consequence class, so for CC2: ( ) = 475 , ( ) = 60 . For the OP limit state, the return period is not present, so it was assumed like 30 years. The design horizontal forces are determined considering for the DC2 ductility class a seismic zone whose seismic intensity measure matches the maximum seismic action index allowed by prEN1998-1-1 (2021) according to the structural type and obtained by the following formula: = , 475 (1) where δ = 1.0 for CC2; = 1.3(1 − 0.01 ) is the short period site amplification factor (for site category B); = 1.0 is the topography amplification factor (for category B). The relevant Authorities or the National Annex must provide the reference acceleration parameters in terms of values defined on the mapped representation of the seismic hazard. But if not available , 475 value may be obtained by applying this formula: performed to check the actual collapse mechanism developing. 2. Definition of seismic action aligned with prEN1998-1-1 (2021)