Ambient vibrations of age-old masonry towers: results of long-term dynamic monitoring in the historic centre of Lucca

The paper presents the results of an ambient vibration monitoring campaign conducted on so-called Clock Tower (Torre delle Ore), one the best known and most visited monuments in the historic centre of Lucca. The vibrations of the tower were continuously monitored from November 2017 to March 2018 using high-sensitivity instrumentation. In particular, four seismic stations provided by the Istituto Nazionale di Geofisica e Vulcanologia and two three-axial accelerometers developed by AGI S.r.l., spin-off of the Istituto Nazionale di Astrofisica, were installed on the tower. The measured vibration level was generally very low, since the structure lies in the middle of a limited traffic area. Nevertheless, the availability of two different types of highly sensitive and accurate instruments allowed the authors to follow the dynamic behaviour of the tower during the entire monitoring period and has moreover provided cross-validation of the results

Ambient vibration recording on the Maddalena Bridge in Borgo a Mozzano (Italy): data analysis.

This paper reports on a vibration measurements campaign performed on the medieval Maddalena Bridge, also known as the "Devil's Bridge", in Borgo a Mozzano (Italy), one of the most fascinating in Italy. This 11 th century masonry bridge, supported by four circular arcades, crosses the Serchio River for about one hundred meters. Information on the dynamic response of the structure have been obtained through a wholly nondestructive technique, by measuring the environmental vibrations affecting the structures. A monitoring system has been fitted on the external surface of the bridge in order to evaluate its dynamic response to vibrations originating in the adjacent railway and two nearby roads. The natural frequencies and mode shapes of the structure and the corresponding damping ratios have been obtained by analyzing the recorded data using different techniques of Operational Modal Analysis. Lastly, a finite-element model of the bridge has been calibrated to fit the experimental data

Results from the temporary installation of a small aperture seismic arrayin the Central Apenninesand its merits for local event detectionand location capabilities

In order to evaluate the detection and localisation improvement of a small aperture array in the Northern Apennines, we installed an irregularly spaced test configuration in the vicinity of Città di Castello (CDC) for a period of two weeks. The experimental array consisted of nine 3-component stations with inter-station distances between 150 m and 2200 m. Seismic data were digitised at 125 Hz and telemetered to a mobile acquisition, processing and storage centre. The data could only be recorded in trigger mode. The peculiarity of the test array installation was the exclusive use of 3-component sensors at all array sites, which also allowed beamforming for S-phases on the horizontal components. Since the altitudes of the single array sites differed considerably among each other, for f-k analysis and beamforming an elevation correction was included. During the two weeks of operation about 20 local earthquakes with magnitudes ML<2.6, 1 regional, and several teleseismic events were recorded. In addition to these events, the array occasionally triggered on coherent noise-signals generated by local industrial activity. The data analysis was performed by means of f-k analysis and beamforming, providing wavenumber characteristics of the incident plane wave. Typical apparent velocities were determined to be 4.8 km/s and 6 km/s for Pg-phases and ~10 km/s for Pn-phases. We observed local seismic events, which occurred just beneath the array. In these cases wavefronts with unusual high apparent velocities, similar to those found for the Pn-phase, were observed. Since no continuously recorded array data were available, we extrapolated the lower detection magnitude threshold as a result of the SNR improvement due to array beamforming. Compared to the actual detection threshold of MT ~1.6 reached by the national seismic network in this area, a nine element array would improve this value up to MT ~ 0.8

MICROTREMOR MEASUREMENTS IN PALERMO, ITALY: A COMPARISON WITH MACROSEISMIC INTENSITY AND EARTHQUAKE GROUND MOTION

The city of Palermo is an appropriate test site where the efficiency of microtremors in predicting ground motion properties during earthquakes can be checked. Palermo is a densely populated city with important historical heritage and was object of previous studies. Areas of local amplification of damage were identified in downtown Palermo using historical macroseismic data. Moreover, aftershocks of the September 6, 2002, earthquake (Mw 5.9, 40 km offshore) provided a dataset of seismograms that quantify spatial variations of ground motion. The availability of more than 2000 boreholes in the city allowed a reconstruction of the 3D structure of surface geology, indicating that all the higher damage zones correspond to sediment-filled valleys. The high variability of the surface geology is mostly due to the presence of two filled river-beds of about 150 m width. In the framework of the SESAME project (Seismic EffectS assessment using Ambient Exctations, funded by the European Union), 90 microtremor measurements were performed across several profiles crossing the soft sediment bodies. The measurement points were intensified close to the valley edges (every 20 m), according to our geological reconstruction. H/V spectral ratio on ambient noise (HVSR) show significant variations along each profile: as soon as the transition stiff to soft is crossed, a typical spectral peak exceeding a factor of 3 in amplitude appears in the HVSR. The peak falls between 1 and 2 Hz and, along each profile, the peak disappears as soon as the other edge of the valley is crossed. These results indicate that microtremors are sensitive to the presence of large impedance contrasts of deep soft soil, at least in the Palermo area, with an important implication: the HVSR method seems to be able to recognize conditions potentially favourable to the occurrence of higher damage even when local geological characters are masked by the urban growth. However, we were not able to establish a quantitative correlation between microtremor properties and ground motion (or damage) amplification

Local variability of the ground shaking during the 2009 L’Aquila earthquake (April 6, 2009—Mw 6.3): the case study of Onna and Monticchio villages

The 2009 Mw 6.3 L’Aquila event caused extensive damage in the city of L’Aquila and in some small towns in its vicinity. The most severe damage was recognized SE of L’Aquila town along the Aterno river valley. Although building vulnerability and near-source effects are strongly responsible for the high level of destruction, site effects have been invoked to explain the damage heterogeneities and the similarities between the 2009 macroseismic field with the intensities of historical earthquakes. The small village of Onna is settled on quaternary alluvium and suffered during the L’Aquila event an extremely heavy damage in the masonry structures with intensity IX–X on the Mercalli-Cancani-Sieberg (MCS) scale. The village of Monticchio, far less than 1.3 km from Onna, is mostly situated on Meso- zoic limestone and suffered a smaller level of damaging (VI MCS). In the present paper, we analyze the aftershock recordings at seismic stations deployed in a small area of the middle-Aterno valley including Onna and Monticchio. The aim is to investigate local ampli-fication effects caused by the near-surface geology. Because the seismological stations are close together, vulnerability and near-source effects are assumed to be constant. The wave- form analysis shows that the ground motion at Onna is systematically characterized by large high-frequency content. The frequency resonance is varying from 2 to 3 Hz and it is related to alluvial sediments with a thickness of about 40 m that overlay a stiffer Pleistocene substrate. The ground motion recordings of Onna are well reproduced by the predictive equation for the Italian territory.Published783-8072T. Sorgente SismicaJCR Journalreserve

Sesame Project - Deliverable D08-02 - WP02 H/V technique : experimental conditions - Final report on Measurement Guidelines

In the following we report the final results for WP02-Measurement Guidelines. This work was conducted under the framework of the SESAME Project (Site Effects Assessment Using Ambient Excitations, EC-RGD, Project No. EVG1-CT-2000-00026 SESAME), Task A (H/V technique), Work Package 02 (WP02 – Measurement Guidelines).European Commission – Research General Directorate Project No. EVG1-CT-2000-00026 SESAMEPublished4.1. Metodologie sismologiche per l'ingegneria sismicaope

Preface Special Issue: Lessons from the past: the evolution of seismic protection techniques in the history of builings

“Grecae magnificentiae vera admiratio extat templum Ephesiae Dianae CXX annis factum a tota Asia. In solo id palustri fecere, ne terrae motus sentiret aut hiatus timeret, rursus ne in lubrico atque instabili fondamenta tantae molis locarentur, calcatis ea substravere carbonibus, dein velleribus lanae”. With these words, Gaius Plinius Secundus, better known as Pliny the Elder, in his Naturalis Historia (XXXVI, §95), described the Temple of Artemis at Ephesus, one of the largest temples built by Greeks and one of the seven wonders in the ancient world. Recent historic studies allowed pointing out that the foundations were composed by a continuous stone plate separated from a thin layer of marshland by means of a layer of clay mixed with charcoal and ashes [Carpani, 2017]. Nowadays those layers are interpreted as a pioneering base isolation system.PublishedSE3455T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa

Dynamic behaviour of the carillon tower in Castel San Pietro Terme, Italy

The paper presents the experimental investigations conducted on the carillon tower of the Santissimo Crocifisso Sanctuary in Castel San Pietro (Bologna, Italy) and the analysis of data collected by velocimeters and accelerometers installed on the structure. The main goal is to assess the effects of the swinging bells on the dynamic behaviour of the tower. The paper’s novelty relies on the kind of structure monitored, and the approach followed. The structure is a rare example of a carillon tower, subjected to a careful measurement campaign never carried out before. Moreover, the experimental results are complemented by numerical simulations of the dynamic behaviour of the tower subjected to the action of a swinging bell

A new numerical procedure for assessing the dynamic behaviour of ancient masonry towers

The paper presents a new numerical procedure for the modal analysis of ma- sonry structures implemented in the finite element code NOSA-ITACA, which models masonry as a nonlinear elastic material with zero tensile strength and infinite or bounded compressive strength. The procedure, which allows for taking into account the effects of the stress field and crack distribution within a structure on its natural frequencies and mode shapes, is ap- plied to assess the dynamic behaviour of a medieval bell tower in Lucca. Data recorded by the monitoring system installed on the tower have been processed via suitable modal identifi- cation algorithms, which have enabled calculating the tower’s modal frequencies, mode shapes and damping ratios. Then, the tower has been analysed via the NOSA-ITACA code and model updating techniques applied to fit the data measured on the tower. The results of the standard modal analysis have been compared to those of modal analyses conducted on the structure subjected to its own weight while taking into account the corresponding crack distribution.PublishedCOMPDYN 2017 6 th ECCOMAS Thematic Conference on Computational Methods in Structural Dynamics and Earthquake Engineering M. Papadrakakis, M. Fragiadakis (eds.) Rhodes Island, Greece, 15–17 June, 20174T. Sismologia, geofisica e geologia per l'ingegneria sismic

Experimental and numerical investigations on the seismic behaviour of the San Frediano bell tower in Lucca

This paper presents a study on the seismic response of the San Frediano bell tower in Lucca. The tower’s ambient vibrations were continuously monitored for about one year, form October 2015 to October 2016. During this monitoring campaign, many seismic events were recorded on the tower and the most relevant turned out to be the Amatrice earthquake, which struck Central Italy on 24 August 2016. The paper begins with a review of the experimental results obtained on the tower’s dynamic behaviour. Then, a finite element numerical model of the tower is presented and validated via model updating, by assuming the tower’s constituent materials to be unable to withstand tensile stresses. The experimental records of the Amatrice earthquake are also included in the analysis to assess the dynamic behaviour of the finite element model under ambient vibration. Finally, a numerical simulation is performed of the tower subjected to the Lunigiana earth-quake, recorded in Fivizzano (Tuscany) on 21 June 2013: the results of the dynamic analysis are presented and discussed