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The paper investigates the effects of site-amplification and soil-foundation-structure interaction (SFS) on the fragility curves of a real unreinforced masonry (URM) structure. The building is inspired to a school with an irregular T-shape plan and is modelled according to the equivalent frame approach. The structure was ideally placed on a stiff rock outcrop (case FB A) or settled on four different soft soil profiles, including its real foundation subsoil. In the latter cases the base of the structural model was assumed as fixed (FB C) or endowed with springs (CB C) simulating the soil-foundation dynamic impedance. To evaluate the fragility curves, the corresponding structural models were analysed through non-linear dynamic analyses under 136 input motions, propagated in 1D soil models reproducing the selected profiles for FB C and CB C. The results were compared in the paper in terms of: -fragility curves calculated for five damage levels (defined to be conceptually consistent with those of the EMS98 scale) and expressed as a function of different intensity measures of the input motion; - average damage derived as a weighted average of the probability of failure associated to all the damage levels. The comparison among the results of the different models highlights the expected increase of damage due to site effects and a beneficial effect of soil-structure interaction, mainly due to the increment of damping associated to the additional energy dissipated by the soil-foundation system. Finally, the average damage of FB C and CB C models were compared with those obtained from the fragility curves of a fixed base system in which site effects are considered through the conventional coefficients proposed in Codes (e.g. by the Eurocode 8), that neglect SFS. This latter approach is that followed in the current practice. The comparison demonstrates that the Code-based approach underestimates the fragility especially at high damage levels.

Questo articolo presenta e discute i risultati ottenuti nell’ambito del progetto di ricerca SISIFO volto a calcolare le funzioni di impedenza di una fondazione superficiale dalle registrazioni del comportamento dinamico del prototipo in scala reale di oscillatore semplice su fondazioni superficiali EuroProteas. Il prototipo è collocato presso il sito strumentato Euroseistest in Grecia su alternanze di argilla e sabbia deformabili, la cui rigidezza si riduce a pochi metri di profondità . A tal fine, sono state eseguite prove di rumore ambientale, vibrazioni libere e forzate in un'ampia gamma di frequenze e ampiezze di interesse per l'ingegneria sismica. Le funzioni di impedenza sperimentali sono state poi confrontate e validate con le soluzioni analitiche maggiormente utilizzate in letteratura. Infine, è stato realizzato un modello numerico attraverso il software SAP2000 in cui la deformabilità del terreno di fondazione è stata introdotta mediante molle elastiche, la cui rigidezza è stata calibrata considerando sia le impedenze analitiche che sperimentali. La frequenza fondamentale ottenuta dall’analisi modale del modello la cui interazione con il terreno di fondazione è stata definita a partire dalle impedenze sperimentali riproduce la frequenza sperimentale meglio di quello in cui le molle sono calibrate con le funzioni analitiche. Tali risultati mostrano dunque l'efficacia dell'utilizzo di funzioni di impedenza sperimentali nelle simulazioni numeriche, soprattutto quando i parametri equivalenti del terreno risultano difficili da definire, come nel caso in esame.

The cross interaction between two closely-spaced shallow foundations has been numerically investigated under static conditions and low-amplitude dynamic loading through a finite difference procedure implemented in the FLAC3D code. The motivation towards this research topic may be found in the increasingly growth of urbanization in modern metropolitan areas worldwide and in the urgent need for seismic requalification of vulnerable historical centres in ancient towns, where buildings are placed very close each other. Even though a disconnection between the superstructures exists, the response of their foundations (and that of the buildings themselves) may be coupled due to the continuity of the soil underneath. Consequently, the overall system response may be different from that predicted with reference to the simple scheme of an isolated footing or building. In the performed numerical study, the soil-foundation impedance functions of a single foundation were firstly compared to well-known literature closed-form solutions to validate the procedure from a numerical viewpoint. The analyses were then repeated with adding another footing identical to the first one and placed at a varying distance from it. The comparison between the response of the master foundation when modelled as isolated and as part of a group shows that the dynamic impedances of the footing in-group may remarkably change with respect to those of the isolated foundation. In particular, the stiffness (real part) increases at a frequency depending on footing-footing distance and the damping ratio increases or decreases depending to the type of imposed oscillation.

La risposta dinamica di una struttura può essere influenzata dall’interazione cinematica e/o inerziale con il sottosuolo. L'interazione inerziale, in particolare, comporta una modifica della frequenza fondamentale e del fattore di smorzamento, con effetti postivi o negativi rispetto alla condizione a base fissa, in funzione sia della tipologia di struttura e terreno sia delle caratteristiche dell'input sismico. Nella nota è descritta una procedura semplificata per la valutazione della variazione del periodo indotta dalla deformabilità del sottosuolo nei casi di geometria complessa della fondazione e sottosuolo stratificato. È inoltre descritta la sua applicazione a un edificio scolastico in muratura che era oggetto di monitoraggio permanente dall’Osservatorio Sismico delle Strutture, poi severamente danneggiato dal terremoto del Centro Italia nel 2016 e infine oggi demolito. I risultati ottenuti sono incoraggianti sull’affidabilità della procedura proposta ottenendo frequenza prossime a quelle sperimentali e ai valori dinamici stimati su un modello 3D a telaio equivalente a base deformabile della scuola

Gli edifici in muratura sono spesso caratterizzati dalla presenza di un piano interrato coincidente con la fondazione della struttura. La nota valuta l’effetto della presenza del piano interrato sulla risposta dinamica del sistema terreno-fondazione-struttura (SFS). Sono state eseguite delle analisi dinamiche su modelli bidimensionali completi SFS, con il metodo delle differenze finite, variando la rigidezza del terreno ed il numero di piani dell’edificio. La variazione della frequenza rispetto al valore a base fissa è espressa in termini di variabilità della risposta dinamica dell’edificio in funzione di un parametro adimensionale rappresentativo della rigidezza relativa terreno-struttura, definito in modo da tenere conto della presenza del piano interrato e del sottosuolo stratificato. Poiché i modelli considerati sono piuttosto ricorrenti nel patrimonio edilizio italiano ed europeo, la procedura proposta potrebbe essere ampiamente estesa a casi simili soprattutto nella valutazione del rischio sismico a scala territoriale.

The paper deals with the results of a multidisciplinary study aimed at assessing the seismic vulnerability of two historical buildings in a picturesque medieval village in Southern Italy, taking into account the interaction among structure, soil and underground cavities. The subsoil model accounted for nonlinear visco-elastic hysteretic behavior, as calibrated on the basis of past and recent geotechnical and geophysical investigations. A strain-softening constitutive model was adopted to simulate the nonlinear behavior of masonry, according to a macro-mechanical approach. Some of mechanical parameters were calibrated against experimental data resulting from uniaxial compression tests performed on single-leaf masonry prisms. 2D nonlinear dynamic analyses were performed with the FLAC code on soil-cavity-structure models by applying a reference input motion compatible with the design acceleration spectrum specified by the Italian code of practice. The building performance and its ability to restore the pre-event conditions were respectively assessed in terms of maximum and residual lateral drifts of the main walls and foundations.

Historical urban centers are often characterized by cavity systems beneath the foundations or underground stories of masonry buildings. This may have an impact on the expected damage induced by earthquake excitations. This paper presents the nonlinear dynamic analysis of a historical multistory building made of stonemasonry load-bearing walls and horizontal diaphragms, taking into account the complex interaction between the foundation system and soil. The aim of this research is to evaluate whether and how the presence of an underground floor and cavity influences the Soil-Foundation-Structure (SFS) interaction. Dynamic analyses of 2D SFS models were performed with the finite difference method (FDM), by varying geotechnical and structural properties such as the soil stratigraphy, the basement depth and the number of building stories. The main results of 234 dynamic linear elastic analyses are presented in a dimensionless format. A new equivalent wave parameter was properly defined to take into account the presence of underground level and cavities in SFS interaction. Relationships between the most important parameters controlling the interaction phenomena are proposed. Since the geometric layouts considered in this study are rather recurrent in the Italian and European built heritage, the proposed procedure might be widely extended to comparable cases.

The Central Italy earthquake sequence nominally began on 24 August 2016 with a M6.1 event on a normal fault that produced devastating effects in the town of Amatrice and several nearby villages and hamlets. A major international response was undertaken to record the effects of this disaster, including surface faulting, ground motions, landslides, and damage patterns to structures. This work targeted the development of high-value case histories useful to future research. Subsequent events in October 2016 exacerbated the damage in previously affected areas and caused damage to new areas in the north, particularly the relatively large town of Norcia. Additional reconnaissance after a M6.5 event on 30 October 2016 documented and mapped several large landslide features and increased damage states for structures in villages and hamlets throughout the region. This paper provides an overview of the reconnaissance activities undertaken to document and map these and other effects, and highlights valuable lessons learned regarding faulting and ground motions, engineering effects, and emergency response to this disaster.

The paper presents the main results of 1D/2D site response numerical analysesat 4 sites selected among the 137 municipalities of the Seismic Microzonation study which hasbeen carried out after the 2016 Central Italy seismic sequence. Complex site effects at ridge/slope crest and at the edge of slope covers lead to 2D aggravation factors in the range 2-3 withrespect to ground motion predicted with simple 1D analyses. A systematic comparison carriedout for all 137 municipalities, between computed and Italian code NTC18 amplification fac-tors highlighted that the code is generally conservative at low-to-medium periods and over-conservative at medium-to-high periods. A strategy for the use of microzonation site responseanalyses in support of seismic design for the reconstruction process is finally suggested .

L’articolo riassume i risultati di uno studio multidisciplinare applicato alla valutazione della vulnerabilità del costruito storico a Sant’Agata de’ Goti, un suggestivo borgo medievale del nostro Paese, edificato su una rupe tufacea caratterizzata dalla frequente presenza di cavità ipogee. Lo scopo di questo lavoro è stato, in particolare, la messa a punto di un approccio metodologico per la valutazione dell’interazione tra edificio, cavità e sottosuolo in condizioni sismiche. In prima applicazione, è stato considerato un edificio storico in muratura adibito a seminario, caratterizzato dalla presenza di ambienti ipogei. Il modello di sottosuolo è stato calibrato basandosi sulla geometria delle cavità sottostanti e sulla raccolta di dati geotecnici e geofisici disponibili nell’area. Sono stati considerati quattro schemi di calcolo con complessità crescente, con l’obiettivo di individuare l’influenza dei principali fattori morfologici e stratigrafici. Dal confronto tra i risultati ottenuti sono stati individuati gli effetti di geometria e posizione delle fondazioni, nonché delle cavità sottostanti, sulle azioni sismiche e sui cinematismi della struttura.