PSI - Issue 78

Franco Braga et al. / Procedia Structural Integrity 78 (2026) 2176–2183

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indicating forward directivity at high frequencies (low periods), while the N-S component (with a propagation direction almost parallel to the fault) shows a signal clearly indicating fling step, with more intense pulses at low frequencies. Finally, in Fig. 3c, the high intensity and concentration of the vertical acceleration component within a narrow frequency range are evident. The analysis reported above clearly indicates that, for the town of Amatrice, the event of August 24, 2016, was a near-fault (or near-source) earthquake. 3.3. Near-Fault Earthquakes in Technical Codes The most advanced international seismic codes contain within them, for several decades, specific indications for "near fault" earthquakes and in particular: - Uniform Building Code 1997 (ICBO 1997): considers two "near-source" factors (Na and Nv) depending on the type of seismic source and its distance from the area in question; - ATC-40 (ATC,1996): the same factors prescribed in UBC 1997 are used; - ASCE/SEI 7-10 (ASCE, 2010): the need to consider the effects related to "near-source" earthquakes is prescribed, also giving specific prescriptions regarding the direction to be considered for each pair of horizontal components for sites falling in areas less than 5 km away from active faults; - GB 50011-2010 (Ministry of Construction; 2010): in the “National Standard of the People’s Republic of China - Code for Seismic Design of Buildings” the “near field” effects have to be considered for structures less than 10km away from active faults. For structures less than 5 km away, the effects are taken into account by amplifying the vertical and horizontal seismic actions by a factor of 1.5, while for buildings located between 5 and 10 km from the fault the same factor is reduced to a value of not less than 1.25 - New Zealand NZS 1170 – part 5 and commentary (NZS, 2004): Near-fault factor (NZS, 2004, Clause 3.1.6, C3.1.6): it is the code that in a more analytical and detailed manner deals with the definition of actions for “near fault” earthquakes. In particular, the commentary drawn up in support of this legislation highlights how in the event of a "near-fault" earthquake, in the "low period" range, the intensity of the vertical spectrum can equal or even exceed that of the horizontal spectra. On the other hand, it is essential to point out that, in the definition of the seismic action of the project, both the European and the Italian codes do not make any reference to the distance between the source of the expected earthquake and the site where the structure in stands, ignoring, in fact, the “near fault” earthquakes. European seismic code (UNI EN 1998-1) only indicates that "different (elastic response) spectra (from those proposed) can be defined in the national annex, if we take into account the deep geology". The NTC18 do not refer to the aforementioned distance when speaking of elastic response spectra, while, dealing with the possible use of time histories of the ground motion generated by simulating the source mechanism and propagation, they provide extremely reduced prescriptions. As for the characterization of the vertical component of the design acceleration, UNI EN 1998-1 invites to consider response spectra obtained by scaling those relating to the horizontal components through the spectral ratio V / H, constant in any interval of the periods, assumption, as seen , absolutely unjustified for shallow earthquakes. The NTC18 provide an approach similar to that proposed by UNI EN 1998-1, and therefore show the same limits when referring to shallow earthquakes. 4. The structures of the buildings 4.1. Geometric characteristics of reinforced concrete frames The buildings were constructed with a reinforced concrete frame structure (with pillars, perimeter beams, and internal beams arranged, according to construction practice of the time, only along the long side of the building, and hollow block reinforced concrete slabs with a total height of 20 cm) according to a design drawn up in 1973, in accordance with the then-current seismic code (Law 1684/62). The following figures show the plans of the various decks constituting the buildings; the inter-story heights were 2.8 m for the first span and 2.90 m for the subsequent spans, while the maximum height of the attic was 2.05 m.