
11 Apr Estratto da: Terremoti in Italia centrale, lo studio della sequenza sismica alcune considerazioni
L’intensità, la durata e la vastità del territorio colpito dalla sequenza sismica che ha investito il centro Italia tra agosto ed ottobre del 2016 porta alla memoria eventi catastrofici che hanno segnato l’Italia e hanno dato impulso determinante allo sviluppo della moderna ingegneria sismica, ci si riferisce in particolare ai terremoti del Friuli 1976 (sequenza lunga alcuni mesi con terremoto di magnitudo massima pari a 6.4) ed Irpinia 1980 (sequenza durata vari mesi con terremoto di magnitudo massima pari a 6.9).
La sequenza sismica è stata imponente e a quattro mesi dal primo terremoto ha superato le 40500 scosse: circa 880 terremoti di magnitudo compresa tra 3 e 4, 50 quelli di magnitudo compresa tra 4 e 5, 5 terremoti di magnitudo maggiore o uguale a 5.
La sequenza è iniziata con due forti terremoti il 24 agosto 2016 a distanza di un’ora l’uno dall’altro e con epicentro rispettivamente ad Accumoli e Norcia. Il terremoto più potente del 24 agosto è stato di magnitudo M 6.0 ed ha duramente colpito le zone del centro Italia coinvolgendo quattro regioni Lazio, Umbria, Marche ed Abruzzo. Un territorio in gran parte montano e rurale caratterizzato da parchi naturali e da un basso livello di urbanizzazione, con l’80% degli edifici residenziali costruiti prima del 1971 (secondo dati ISTAT 2011) e con diversi insediamenti di epoca medievale (con un patrimonio culturale di rilievo), tra cui spiccano i centri di Amatrice e Norcia.
La sequenza che sembrava andare affievolendosi ha invece ripreso vigore con altri due forti terremoti il 26 ottobre 2016 anche questi temporalmente molto ravvicinati e con epicentro nel Maceratese. Il terremoto più potente della sequenza sismica è stato registrato il 30 ottobre 2016 di magnitudo momento M 6.5 ed epicentro Norcia. Per avere un’idea della sconvolgente intensità di quest’ultimo evento, basti pensare al fatto che ha indotto decine di chilometri di fratture nel suolo e ribassamenti dello stesso di circa 70 cm (è il caso di Castelluccio di Norcia, Blog INGVterremoti 02/11/2016), oltre alla ricomparsa di un torrente nei pressi di Norcia che era scomparso in seguito al sisma del 1979. I terremoti di ottobre, in particolare quello del 30 ottobre, hanno colpito ancora più duramente le zone del centro Italia già fortemente danneggiate dai sismi di agosto ed hanno ulteriormente ampliato le aree investite da accelerazioni sismiche rilevanti.
Danneggiamento atteso
Il verificarsi di un evento sismico di elevata intensità in un determinato luogo rappresenta un evento di estrema rarità, per il quale, il consolidato approccio semiprobabilistico agli stati limite, consente alle strutture di superare il limite elastico ammettendo danneggiamenti anche tali da rendere più conveniente una successiva demolizione/ricostruzione dell’edificio, pur conservando la capacità di sopportare i carichi verticali. Gli eventi sismici a cui ci si riferisce sono quelli che sollecitano le costruzioni allo Stato Limite di Vita (SLV), per il quale le vigenti NTC 2008 assegnano una probabilità di superamento PVR = 10 %.
I terremoti che hanno colpito il Centro Italia possono considerarsi come esempi di questi eventi di estrema rarità, ed hanno sollecitato le costruzioni allo SLV ed oltre. Perciò il danneggiamento atteso sulle nuove costruzioni progettate secondo NTC 2008 poteva essere costituito “da rotture e crolli dei componenti non strutturali ed impiantistici e significativi danni dei componenti strutturali cui si associa una perdita significativa di rigidezza nei confronti delle azioni orizzontali; la costruzione conserva invece una parte della resistenza e rigidezza per azioni verticali e un margine di sicurezza nei confronti del collasso per azioni sismiche orizzontali” (cit. §3.2.1 delle NTC 2008). Il danneggiamento atteso dunque, come già osservato, può anche rendere vantaggiosa la demolizione/ricostruzione dell’edificio.
Per le costruzioni antecedenti le NTC 2008, il livello di danno atteso può anche essere maggiore di quanto sopra esposto, dato che i moderni metodi di progettazione e criteri antisismici non erano ancora stati sviluppati.
Critiche risultano le costruzioni progettate prima del 1974, anno in cui è stata pubblicata la Legge 02/02/1974, n. 64 (ancora in vigore), che rappresenta in sostanza la legge quadro della normativa sismica italiana. Problematiche sono anche le costruzioni progettate in epoche antecedenti il DM 16/01/1996 per le quali le metodologie di progetto e le conoscenze in merito alla pericolosità sismica erano ancora limitate, lacune queste solo parzialmente superate con il DM 16/01/1996 stesso.
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