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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.

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.

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 .

Event Overview and Organization of Reconnaissance Activities,Overview of Reconnaissance Activities, Regional Faults and Seismic Source,Ground Motions, Landslides and Rockfalls, Performance of Building Structures and Damage Patterns,Performance of Bridge Structures, Other Infrastructure Systems, Lessons Learned and Next Steps

The paper analyses in detail and compares different interpretation procedures of snap-back and forced-vibration tests on a full-scale prototype founded on soft-soil, in order to assess their effectiveness for measuring the stiffness and damping of a shallow foundation under dynamic loads. Both properties were back-calculated through three alternative methods, i.e. through impedance functions computed in the frequency domain, or by interpreting force-displacement loops in terms of peak-to-peak amplitudes or of phase-shift. The damping was further calculated from the logarithmic decrement of free-vibration records. The resulting foundation stiffness and damping were observed to vary with the number of cycles and with the load frequency and amplitude. The comparison among the interpretation techniques revealed that the peak-to-peak approach fails when damping is high, because it neglects the delay between force and displacement.

The instability risk of underground cavities is issue of concern particularly for areas densely urbanized as the case of many historical centers in Italy. Among the possible causes of the cavity damages up to collapse, the proposed work considers a particular triggering factor that is the dynamic loading induced by an earthquake. Thus, the seismic vulnerability of shallow cavities in soft rock is addressed by proposing a methodology based on an extensive parametric two-dimensional finite element analysis. The proposed methodology is extended to a significant number of realistic cases, considering the effect of the variability of some predisposing and triggering factors on the cavity stability like roof thickness and cavity width, presence of overburden on the top of the rock layer, seismic signal intensity. The seismic vulnerability of the studied cases is assessed through the minimum Factor of Safety calculated during the whole length of the dynamic analysis. Finally, the results of the parametric study are rearranged into 'seismic stability charts for cavities in soft rock' proposed to be adopted as preliminary level of screening, to assess the seismic vulnerability of shallow underground cavities in soft rock. The results of screening enable to identify the critical zones that need a detailed analysis, in consideration of their possible interaction with above-ground structures, infrastructures and human activities.

The paper presents a state-of-the-art review of the current understanding of the seismic behaviour of tunnels. Emphasis is placed on recorded responses of actual tunnels during past earthquakes, as well as relevant experimental studies. In particular, the observed seismic performance of tunnels is initially discussed, by providing cases of reported damage during past earthquakes. This is followed by a review of the current monitoring systems for this infrastructure, which can be used to create future case studies based on actual recordings of the seismic response. Subsequently, the paper provides a summary of relevant experimental studies that took place in the centrifuge or on shaking tables, followed by a short discussion of current analytical models, simplified methods and numerical schemes for the seismic analysis and vulnerability assessment of tunnels. Throughout the presentation, the current overall gaps in understanding the seismic response of tunnels are identified in an attempt to stimulate further work in these areas by the research community., Tunnelling and Underground Space Technology, 99, ISSN:0886-7798, ISSN:1878-4364

The stability of ancient room and pillar cavities dug in seismic sites was modified by the occurrence of earthquakes during the time. In such complex systems, the seismic-induced modifications of resistances and loads may lead to local improvements of the condition under gravity loads. The paper proposes a probabilistic evaluation of such effect, in the frame of the performance based seismic assessment. The procedure was applied to evaluate the safety and the resilience of a room and pillar cavity dug in a rocky hill in Naples, known as Fontanelle cemetery, after the maximum credible earthquake. Non-linear dynamic analyses were performed on a refined 3D model calibrated on the available experimental data. The safety of pillars and rooms was evaluated against two limit states, defined as a function of the Ratio between the Capacity and the Demand, CDR. The CDR of room and pillars computed after the seismic analyses were compared to their values before earthquake to check the capability of the cavity components to face the earthquake-induced modifications of the static equilibrium and consequently evaluate their resilience.

Past earthquakes revealed that underground tunnel structures are exposed to seismic risk and their seismic vulnerability is mainly function of the tunnel structure technology, the soil-tunnel interaction developing during the seismic shaking and the intensity of the seismic event. Each of these factors play an important role in terms of the probability of damage and loss of functionality due to the tunnel deformations in- duced by increments in internal forces. Currently, the fragility curves are among the most widespread methods for the rapid assessment of structural performance at different hazard levels, giving the probability of reaching a defined damage level with respect to a given level of seismic motion. In the present work, a recently developed approach to evaluate the seismic risk has been applied to circular tunnels, whereby the main focus is the expression of the failure annual rate through the convolution of the fragility of the system under investigation and the seismic hazard on site. In this way, a direct link be- tween the performance of the structure with its measuring parameter is established. The procedure has been adopted for some reference tunnel sections of the Metro Line of Naples.

The Central Italy earthquake sequence initiated on 24 August 2016 with a moment magnitude M6.1 event, followed by two earthquakes (M5.9 and M6.5) on 26 and 30 October, caused significant damage and loss of life in the town of Amatrice and other nearby villages and hamlets. The significance of this sequence led to a major international reconnaissance effort to thoroughly examine the effects of this disaster. Specifically, this paper presents evidences of strong local site effects (i.e., amplification of seismic waves because of stratigraphic and topographic effects that leads to damage concentration in certain areas). It also examines the damage patterns observed along the entire sequence of events in association with the spatial distribution of ground motion intensity with emphasis on the clearly distinct performance of reinforced concrete and masonry structures under multiple excitations. The paper concludes with a critical assessment of past retrofit measures efficiency and a series of lessons learned as per the behavior of structures to a sequence of strong earthquake events.

This paper demonstrates the applicability of a probabilistic performance-based approach to assess the stability of rock cavities under gravity and seismic loads, with reference to the case study of a Neapolitan room and pillar cave, located inside a rocky hill. An advanced 3D model was generated, importing in the numerical code the data acquired during a laser scanning-based geometrical survey. The careful reproduction of the on-site geometry was found to improve the reliability of the results of static and dynamic analyses; because (i) the load eccentricity induced by the irregular shape of the hill reduces the stability of pillars, (ii) the localization of the roof crack depends on the shape of the ceiling, and, furthermore, (iii) the global dynamic behaviour of the hill is affected by topographic effects . The safety factors of rooms and pillars computed after the seismic analyses were compared to their values before earthquake to highlight earthquake-induced modifications of the static equilibrium under gravity loads. Some considerations on the resilience of the cavity are reported based on the latter comparison.

The paper illustrates and compares simplified approaches to interpret the mechanisms of damage observed on rigid bodies in the cemetery of Amatrice, after the main shock (August 24, 2016, MW=6.0) of the Central Italy earthquake. The final goal of the work is to link the observed movements of the fallen objects to specific characteristics of the ground motion occurred at the specific site.