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The paper presents a new simplified mathematical model for predicting the structural seismic response of buildings. The model, denominated Seismic Model from Ambient Vibrations (SMAV), is based on the experimental modal parameters identified from ambient vibration and only a few information about the geometry and the structural typology of the building. After a short review of the Multi Rigid Polygons model, recently illustrated and validated in an other paper by some of the authors, that allows to estimate the seismic participation factors of the experimental modes also for buildings characterized by complex shaped plan and structural irregularity along the height, the attention is focused on a new stochastic approach for modeling the seismic response of masonry buildings. In particular this new approach aims to take into account the non linearity occurred during a seismic event so that the nonlinear behaviour of the building is considered by reducing its modal frequencies according to the response amplitude. The reduction of the natural frequencies, extracted by Operational Modal Analysis from ambient vibrations, is computed according to specific probabilistic curves: the Frequency Shift Curves (FSCs). This curves provide the percentage reduction of the natural frequencies as a function of the maximum roof drift reached during the strong motion and they are obtained, for some specific masonry typologies, through a Monte Carlo analysis carried out using a simple mechanical model of a masonry panel with geometric and mechanical parameters that vary according to their probabilistic distributions. The seismic response of the building is then computed through a linear equivalent analysis in which an iterative algorithm updates the resonant frequencies according to the specific FSC curve. The concept of structural serviceability index (IOPS), expressing the probability of the building to remain operational, is also introduced. Finally, a comparison between the seismic response computed by the model and the experimental seismic response of the Pizzoli town hall, a masonry existing building endowed with a permanent accelerometer monitoring system, is illustrated.

Site effect studies are currently mostly based on seismic noise measurements, processed using methods like horizontal-to-vertical spectral ratios (HVSR) to estimate dominant frequencies and maximum amplifications, or methods based on cross-correlation between stations like spatial autocorrelation (SPAC) and seismic interferometry. However, HVSR and SPAC assume that the underlying medium has a 1D structure and that seismic noise consists mostly of surface Rayleigh waves. We present a detailed analysis of seismic noise records at two sites in Euroseistest (Mygdonian basin, northern Greece) where two normal faults have created lateral heterogeneity in the subsoil structure. Site GRA is located near fault F3, with a larger throw than fault F2, close to site FRM. Our results show the effects of these irregularities. Dominant frequency results for both sites become spatially incoherent close to the fault. In the case of site GRA, a range of wavelengths is missing in the recorded seismic noise field. These effects extend up to some fraction of the affected wavelengths in the case of fault F3. The effect of lateral variations on seismic noise measurements will be lost in gross spatial averaging of the results or simply the failure of HVSR or SPAC methods will be observed.

In the present work, a combination of single station and multi-sensors array measurements of ambient noise have been performed in the urban area of Chania and its southern basin, in order to determine the main subsurface geophysical model properties and assess the local site-effect on seismic motions. Furthermore, Horizontal to Vertical Spectral Ratio on Earthquake recordings (HVSRE) has been employed to validate the ambient noise results. Simulation of ambient seismic noise has been also used in an effort to cross-validate the experimental methods applied. The results indicate a highly-varying subsurface model due to the complex geological setting. Horizontal to Vertical Spectral Ratio (HVSR) reveals a low frequency resonance peak associated with the seismic contrast of the overall Quaternary/Neogene sediment cover with the underlying bedrock. A second higher HVSR frequency peak, usually disregarded in standard HVSR analysis, is observed for most HVSR data. This peak is found to be associated with the strong velocity contrast of the low velocity Quaternary sediments and the underlying stiffer Neogene deposits, using both empirical correlations and appropriate modelling. The available array measurements verify the clear separation of these Quaternary deposits and the underlying Neogene sediments in the southern Chania basin, as well as the more loose surficial Neogene formations and the underlying stiffer layers within the main city complex. Simulated ambient noise signals processed with the same HVSR settings verify the findings of the experimental approaches. Earthquake recordings processed with the HVSRE method show a good agreement with the expected amplification pattern of the final geological-geophysical structural model of the broader Chania area.

In this paper, a simple analysis is done to explain the observed increase of resonance frequency of City-Hall building in Grenoble (France), a 12-story reinforced concrete building. This period corresponds also to the observed variation of the resonance frequency of the Grenoble’s sedimentary basin. The postulated hypothesis is that the frequency increase reflects the stiffness increase of the soil–structure system related to the cold period that hit Western Europe in 2012. To explore this hypothesis we have processed continuous recording during the early 2012 recorded at the roof level and at a close free-field accelerometric station. The variation of site effect is monitored by the horizontal-to-vertical spectral ratio of seismic noise, and the variation of apparent and system frequencies of the building by the random decrement technique. Apparent frequency is computed by deconvolution method between roof and basement. The maximum freezing penetration is 0.75 m and the horizontal relative motion stiffness of the foundation is strongly sensitive to the modification of the upper soil layer. The results suggest a variation (

After the seismic events of the 20th and 29th of May 2012 in Emilia (Italy), most of the monumental and historic buildings of the area were severely damaged. In a few structures, partial collapse mechanisms were observed (e.g. façade tilting, out-of-plane overturning of panels…). This paper presents the case-study of the bell tower of the Santa Maria Maggiore cathedral, located in Mirandola (Italy). The dynamic response of the structure was evaluated through operational modal analysis using ambient vibrations, a consolidated non-destructive procedure that estimates the dynamic parameters of the bell-tower. The dynamic tests were carried out in pre-intervention and post-intervention conditions in order to understand the sensitivity of dynamic measurements to safety interventions. Furthermore, a comparative study is made with similar cases of undamaged masonry towers up to the 6th mode. Finally, an investigation on the state of connections and of the building itself is carried out via FE model updating.

A 3-storey residential building with OSB-sheathed light-frame timber walls and timber-concrete composite slabs was subjected to dynamic excitation in two different stages of construction. The experiments consisted of measurements of the accelerations resulting from forced horizontal vibrations which were excited by means of a hydraulic actuator shaking a mass of 940 kg. The exciter was positioned on the second floor of the building and rigidly anchored in the slab. It excited the building in its two main directions to vibrations with frequencies within a range of 0.2–14 Hz. Natural frequencies, modal damping ratios and mode shapes of the building were estimated using accelerations of the building measured in the four external corners of each upper floor. For increasing displacement amplitude (1/8000 mm → 0.7 mm) the natural frequencies decreased moderately (−15 %) whereas the modal damping ratios increased markedly (+100 %). Modal damping ratios computed using ambient vibration data were generally smaller than modal damping ratios obtained from forced vibration tests, which suggests that modal damping for the levels of vibration in interest in design might be significantly underestimated if based on results from ambient vibration tests. However, the mode shapes computed using ambient vibration data turned out to be reliable. The differences in dynamic properties assessed in the different stages of construction were much smaller than expected. Compared to the natural frequencies estimated by the designer by applying a simple single degree of freedom model, the measured natural frequencies were considerably higher. This may be attributed to the impact of non-structural internal walls and walls with openings not accounted for in the designer’s model.

Over the years, the belief that the first mode of vibration governs the seismic response of shear-type frame structures has been widely accepted and proved to be effective for preliminary structural design. Indeed, most of the actual seismic design procedures are based on drift profiles which are typically an approximation of the shape of the fundamental mode of vibration. In this paper, an analytical study on the dynamic properties of multi-storey shear-type frames is carried out with the purpose of precisely identifying the contribution of the modes of vibration to the seismic response of such structures, both in terms of maximum inter-storey displacement profiles (which govern the beams and columns maximum actions) and maximum inter-storey velocity profiles (which govern the viscous dampers maximum forces, of fundamental importance for building structures equipped with additional viscous dampers). A new parameter, referred to as Seismic Modal Contribution Factor, which represents the contribution of the generic mode to the seismic response of the structure, is introduced. With respect to the well-known Modal Contribution Factor, grounded on the concept of modal static response, the Seismic Modal Contribution Factor explicitly takes into account also the dynamic nature of the response due to earthquake excitation. The Seismic Modal Contribution Factor could be a meaningful parameter to be implemented in a professional structural design software and used in conjunction with the common modal participating mass ratios to identify the number of modes to be included in the analyses.

The seismic performance assessment of existing masonry buildings involves many uncertainties, whose impact can be reduced to some extent by using non-destructive in-situ tests of such buildings, at least when destructive in-situ tests, which can provide more reliable results, cannot be performed. In this paper the extent of the potential beneficial effects achievable by calibration of a structural model of a building to its experimentally estimated vibration periods has been investigated. This was done by performing measurements of ambient and forced vibrations on an old two-storey masonry building, and by then assessing its seismic performance using a simplified nonlinear method. The results of numerical investigations revealed that the natural vibration periods of such buildings can be reproduced with sufficient accuracy, although it is possible that they will be overestimated or underestimated by analysts by up to around 40 %. This means that the accuracy of the prediction of the intermediate results of the seismic performance assessment of any particular building can be significantly increased by calibration of the structural model. Additionally, the beneficial effects of such calibration were observed even in the case of the final outcome of the nonlinear analysis, which is expressed through the near-collapse limit state capacity in terms of the peak ground acceleration.

In recent years, many studies have been focused on the use and effectiveness of passive islotaing devices for reducing the effect of seismic ground motion on buildings. Among the available methods, one consists in isolating the structure using rubber bearings, the solution certainly the most accomplished and having the most feedback. In this study, we focused on the case of Martinique earthquake (Mw=7.4) of 29 November 2007, recorded by accelerometric stations installed at the Centre de Decouverte des Sciences de la Terre (Martinique), a base-isolation building with rubber bearings. Several earthquakes are used in this paper, from moderate to strong ground motion. Ambient vibration modal analysis is first described in order to understand the elastic response of the building. The earthquake data are then interpreted, in particular to understand the mechanism of vibration of the structure and its comparison with the experimental modes previously estimated using ambient vibrations.

The presence of peats and high compressibility inorganic clays within alluvial fills on the left-side tributaries of the Tiber River, close to Rome’s historic center in Italy, is well documented in literature. Nevertheless, few literature data exist until now on the dynamic properties of these deposits by undisturbed bore-hole samples. The Galba test-site was set up to characterise dynamic properties of the alluvial deposits by using lab-tests as well as to derive velocity profiles by seismic noise measurements. These were performed in the Giustiniano Imperatore area located in the Grottaperfetta valley, about 2 km south of Rome’s historic centre. The alluvial deposits filled a paleovalley excavated in the bedrock during the Wurm glacial (18–20 ky). The stratigraphic setting of the alluvial body was reconstructed along three geological cross-sections by means of the available logs; seven lithotecnical horizons can be distinguished within the alluvial body, some tens of meters thick, based on both log-stratigraphic data and in-site geotechnical tests. These horizons include peaty layers (T) and high compressibility inorganic clays (AGI), which characterise the alluvial deposits in the Grottaperfetta valley. They do not have direct correlation with the alluvial horizons which constitute the alluvial body of the main Tiber valley in Rome’s historical centre. These alluvial horizons which are distinguished and characterised at the Galba test-site can be regarded as typical of other lateral valleys of the Tiber River in Southern Rome. They are characterised by the presence of similar high compressibility clayey deposits as well as peaty layers up to some meters thick. Undisturbed samples were also obtained at the Galba test-site for dynamic testing via resonant column and cyclic torsional shear tests. In order to attribute dynamic properties to the alluvial body at the Grottaperfetta valley, an extrapolation process was performed based on a detailed engineering-geology model of the alluvial body which was reconstructed along three transversal geological sections of the valley using bore-hole data. Three subsoil profiles, considered representative of the geological setting along the three reconstructed transversal cross sections, were analysed by seismic noise measurements performed specifically to derive S-waves velocity profiles. The results obtained show a very low velocity (

The problem of seismic characterization of rock-mass or stiff-soil outcrops by the use of passive seismic prospecting techniques is discussed. Difficulties in the application of this kind of procedure in the presence of rugged morphology, high surface wave velocities and low ambient vibration powers, typical of stiff-soil/rock-mass sites are examined. A methodology to face these problems is here proposed, which is based on the strict synergy of detailed geologic surveys and application of robust seismic prospecting techniques, jointly considering single-station and multi-station tools. The application of this approach for the characterization of a number of sites belonging to the Italian Accelerometric Network is described, focusing on two representative case histories.

In recent years, seismic noise array measurements have emerged as a promising way for estimating Vs profiles and characterizing the corresponding soil structure. Although methods based on analysis of seismic noise are simpler, cheaper and faster than conventional soil penetrating techniques, array deployment requires the use of several stations which are not always available for all the research groups. Broadband stations are expensive, but even short-period sensors can require a significant economical effort as it is necessary to use several. In this paper, the results from broadband stations and vertical geophones using the f-k method have been compared. Circular arrays with both types of instruments were installed north and south of the Himalayan Frontal Thrust (HFT) also known as Mohand Thrust, with different soil characteristics. The comparison carried out in terms of dispersion curves shows an excellent agreement between broadband stations and geophones recordings in the frequency range of applicability of the arrays used. These results indicate the possibility of using seismic refraction equipments for the f-k analysis in the frequency band of geotechnical engineering interest. This has potentially great importance since the same equipment is indicated to be applicable for both seismic noise measurements as well as for active reflection seismic surveys.

International audience; Over recent years there has been a growing interest in the building frequency analysis for earthquake and structural engineering fields using ambient vibrations. Simultaneously, velocity measurements with LASER remote sensing techniques have gained more interest for several applications. This paper details the comparison of the frequency analysis obtained using sensitive velocimeter sensor and coherent LIDAR (Light Detection and Ranging) sensor, and applied to one RC existing building. Ambient vibrations recordings were processed using the Frequency Domain Decomposition method for defining the frequencies and mode shapes of the building target, while LASER remote sensing approach used coherent LIDAR method for velocity and frequency measurement. The results of the two systems are discussed. A good agreement is observed, which let us conclude on the ability of the coherent LIDAR to assess the frequency of existing buildings for structural and earthquake engineering fields at long range and without any retroreflector on the structure.

Horizontal-to-vertical spectral ratios (HVSR) of ambient vibrations measured in the ancient town of Ston (Croatia) on 99 locations, are shown to be well matched to the theoretical ones computed for body-waves as well as for the surface waves. This match is poorer for sites on the slopes of nearby hills. The ratios of measured peak horizontal ground acceleration during the damaging earthquake in 1996 (M L = 6.0) and the ones obtained using empirical attenuation laws is approximately equal to the mapped value of the dynamic amplification factor determined on the basis of observed HVSR in the vicinity of the accelerometric station. The HVSR of the accelerogram is very similar to the HVSR of the ambient noise. The damage to the building stock in the old town centre caused by the earthquake series of 1996 is closely related to the estimated soil amplification and its fundamental frequency. More measurements in buildings are needed to arrive at confident conclusions about possible soil-structure resonance.

The city of Ljubljana is located in one of the three areas with the highest seismic hazard in Slovenia, and it is also the most densely populated. Site effects due to Quaternary sediments, which fill the up to 200 m-deep basin, are characteristic of the whole city area, but they can be especially strong in the southern part of Ljubljana, which is built on very soft lacustrine deposits. Existing microzonation studies of the city are inadequate, since there is a lack of borehole, geophysical and earthquake data. The microtremor horizontal-to-vertical spectral ratio (HVSR) method was therefore applied to a 200 m dense grid of free-field measurements over an area of 45 km2 (1,223 measuring points) in order to assess the fundamental frequency of the sediments. The main difficulties in microtremor measurement arose from high levels of traffic and industrial noise, and from underground structures. Experimental conditions which can influence data quality, such as strong wind and water saturation of soil, were analysed. Very clear HVSR peaks were obtained in the entire southern part of the city, whereas in the northern part the site response is in general lower due to lower impedance contrast of gravel with the bedrock. The iso-frequency map of sediments shows a distribution in the range of 0.9–10 Hz. In the southern part of Ljubljana, sediment frequency correlates well with the thickness of soft sediments known from geophysical investigations and sparse drilling. Average amplitude of the HVSR peaks is considerably higher in the southern part (6.7 ± 2.4) than in the northern part (4.0 ± 2.0) of the city, indicating a high impedance contrast of lacustrine sediments with the bedrock. Microtremor measurements were also performed inside 122 buildings of various heights. We focused on important public buildings and selected blocks of flats and houses. To assess the longitudinal and transverse fundamental frequencies of each building, amplitude spectra and the spectral ratio between the upper floor and the basement were analysed for both directions. When one of these frequencies is close to a nearby free-field fundamental frequency, a potential soil-structure resonance is present. This was found in 12 of the measured buildings. Three of them are tall residential buildings (from 10 to 15 floors) with a fundamental frequency of 2–3 Hz, and nine of them are low-rise buildings (from 3 to 5 floors) with a fundamental frequency ranging from 3 to 4.5 Hz. Using the relationship between fundamental frequency and height, the typical height of buildings that might cause soil-structure resonance can be estimated at a given sediment frequency obtained from free-field measurements.

Continuous broad-band measurements of vibrations on the 3rd floor of the building of the Department of Geophysics, Faculty of Sciences and Mathematics (DGFSM) in Zagreb provided high quality data that enabled monitoring of fluctuations of the building’ s basic dynamic parameters (fundamental frequencies and the corresponding damping). It was found that nonlinear behaviour is present even for the small strains induced by ambient vibrations, which was manifested in measurable reduction of fundamental frequency and increase of damping for larger excitation levels. These effects were also observed in response of the structure to two small local earthquakes which occurred during the observation period. Correlation of the observed long-term wander of dynamic parameters with the change of meteorological elements revealed notable correspondence of the variations of the fundamental frequency and the relative air humidity (which was related to soil saturation, thus causing variability of soil-structure interaction). The damping was found to vary synchronously with the air temperature. The results point to the importance of considering all aspects of nonlinearity of building response even for small strains.

Plovdiv is the second largest city in the Republic of Bulgaria. A large part of the city is located on Holocene alluvial sediments and the oldest neighborhoods are situated on syenitic rock outcrops. We believe that local site effects may be an important contributor to the destruction caused by earthquakes. The primary objective of this study was to estimate quantitatively the local site effects in the central area of Plovdiv in terms of fundamental site frequency and amplification factor. Another important objective was to see how these correlate with the geological structures underlying the city. Measurements of the seismic noise at more than two hundred regularly placed points were made in the central area of the city. The H/V spectra were then calculated and analyzed to determine the spatial distribution of the fundamental site frequency and the amplification factor. The results exhibit very good correlation with the local geology. They were also compared with an intensity map from the strong 1928 Plovdiv earthquake. The comparison clearly demonstrates that the local site effects were the main factor in the destruction of buildings—the zones where the most damage was observed are also the zones where we have low fundamental site frequencies and high amplifications. Similarly the areas with high fundamental site frequencies and low amplification factors cover the neighborhoods where less damage has been observed. This study may form a basis for a more comprehensive and systematic microzonation study in Plovdiv.

The dynamic characteristics of two representative R/C bridges on Egnatia Odos motorway in Greece are estimated based on low amplitude ambient and earthquake-induced vibrations. The present work outlines the instrumentation details, algorithms for computing modal characteristics (modal frequencies, damping ratios and modeshapes), modal-based finite element model updating methods for estimating structural parameters, and numerical results for the modal and structural dynamic characteristics of the two bridges based on ambient and earthquake induced vibrations. Transverse, bending and longitudinal modes are reliably identified and stiffness-related properties of the piers, deck and elastomeric bearings of the finite element models of the two bridges are estimated. Results provide qualitative and quantitative information on the dynamic behavior of the bridge systems and their components under low-amplitude vibrations. Modeling assumptions are discussed based on the differences in the characteristics identified from ambient and earthquake vibration measurements. The sources of the differences observed between the identified modal and structural characteristics of the bridges and those predicted by finite element models used for design are investigated and properly justified.

Local S-wave velocity-depth profiles are a key factor in seismic hazard assessment, as they allow the amplification potential of the sedimentary cover to be evaluated. Ambient seismic noise is mainly composed of surface waves, and therefore contains vital information about the S-wave velocity structure, allowing polarization or dispersion curves to be obtained from single station or array noise recordings. At two sites in the area of Cologne, Germany, the extended spatial correlation method was applied to such recordings and apparent phase velocity curves in the frequency range of interest for earthquake engineering were obtained. Using this data, a linearized inversion, the simplex downhill method, and a genetic algorithm yielded similar S-wave profiles. However, the latter method is recommended since it is less dependent upon a good starting model. Importantly, the presence of low-velocity layers in the Cologne area made it necessary to consider in the frequency range of interest higher modes in the inversion procedures. Finally, independent information on the total thickness of the sedimentary cover permitted the estimation of a 2D S-wave velocity profile crossing the Cologne area. Here, the H/V ratio inversion using 20 single-station noise recordings was used, with the results in good agreement with a geological profile.

Throughout their long history, the towns of Lod and Ramle have been severely affected by strong earthquakes. The last destructive earthquake occurred on July 11, 1927 and caused the destruction of large parts of these cities, reaching a seismic intensity of VIII–IX on the MSK scale. Such a high intensity from a relatively distant earthquake (about 70 km) of magnitude 6.2 is likely to be the result of local site effects of the sedimentary layers that may have significantly enhanced earthquake ground motions.

This special issue of the Bulletin of Earthquake Engineering contains 13 paper devoted to ambient noise measurements on soil and buildings. Five of the paper are the outcome of a NATO Science for Peace Project named “Assessment of Seismic Site Amplification and Seismic Building Vulnerability in the FYR Macedonia, Croatia and Slovenia” whose aim was to bring together geologist, seismologist and engineers and perform field measurement of dynamic characteristics of large sets of buildings and their foundation soils, trying to integrate interpreted measurement data into microzonation maps of urban areas. Around this core I collected other papers by authors that were invited to participate to project meetings and to another initiative that stemmed out of the project, that is a NATO Advanced Research Workshop entitled “Increasing Seismic Safety by Combining Engineering Technologies and Seismological Data”. The ARW produced also a book (Mucciarelli et al. 2009) that provided a detailed state of the art and new ideas for the future, making it a complement to this special issue devoted to original experimental and numerical results. Two papers of this issue were later contributed by authors that became aware of the initiative and decided that their work was fit in this framework, confirming the ongoing interest on this subject. The papers cover issues starting from the classical use of microtremors to study soil properties (Herak et al. 2010; Gosar and Lenart 2010) consider new theoretical models of seismic noise (Albarello and Lunedei 2010), discuss the stability of noise measurement to invert velocity profiles (Endrun et al. 2010) and include the study of possible resonance between soil and structures (Gosar et al. 2010). To this last aim, the knowledge of the dynamic properties of building is essential. It is very interesting that two independent papers, one from the SfP project (Gallipoli 2010) and one submitted later (Oliveira and Navarro 2010) came to the same conclusion analysing different sets of building in different countries: the period