Ambient Vibration Analysis Research Articles

Ambient Vibration Analysis of Diversion Pipeline in Mount Changlong Pumped-Storage Power Station

This study analyzes the ambient vibrations induced while running the Mount Changlong pumped-storage power station (PSPS). The ground vibration data of the power station during its operation were acquired with vibration sensors. Different units were selected and compared under working conditions, and the conclusions were as follows: (1) Ambient vibrations induced by the running of units constituted the primary source of vibration, and they attenuated as the distance increased. (2) The vibration acceleration under pumping conditions was larger than that under power generation conditions, and the ground vibration acceleration increased with an augmentation of the power. (3) The running of adjacent units generated mutual interference, and the types of units were different, which led to complex variations in the spectrum maps. (4) The vibration acceleration of the lower flat tunnel was prone to surpassing the standard when the number of units running together exceeded three.

Directionally Effect Observation in Topographical Site-Effects at Feden-Sema Range (Guelma-Northern Algeria) from Ambient Vibration

The Eocene Feden-Sema Range is a topographical structure located in the complex area of Mechtat El-Ababsia (Guelma) characterized by topographical site effects from an experimental investigation using earthquake and ambient vibration. The structure has its maximum orientation toward the south and a half-circular shape. To study the directionality effects caused by this structure, two profiles of ambient vibration measurements were carried out on the maximum axe and through the range of 12 points. The analysis of the horizontal over vertical spectral ratio indicated that the fundamental frequency, f0, of the topographical structure is constant and around 3.0 Hz in all measurement points. The ambient vibration analysis in the 0.1–20 Hz frequency domain showed that f0 is strongly related to the polarization or directionality, where the maximum amplification is obtained at 45° azimuth from the north with an amplitude of 3.5 times compared to 0° at 2.5 times. The obtained result is of great importance in showing that the topographical site effects are not free from the directionality influence in the ground motion, which must be taken into consideration in the case of building construction orientation.

Modelling the seismic response of the Mýtina maar volcanic structure

In this study, we determine the properties of the maar volcanic structure using ambient vibration analysis. We performed several ambient vibrations surveys at Mýtina maar (West-Bohemia, Czechia) to map amplification by site-to-reference spectral ratios and to estimate average shear wave velocities by array methods. As the maar diatreme is formed by a deep concave body of broken rock, strong 3D resonances develop. The site-to-reference amplification factors reach a value of 20 in the central part of the maar. Using the noise data, we demonstrate possibility of mapping the lateral dimensions of the maar infills by ambient noise measurements. To understand these observations, we developed a simplified 3D viscoelastic velocity model and simulated synthetic ambient vibrations by the finite difference method. The simulated response fitted both, measured fundamental frequencies and the amplification levels. The maximum depth of the crater was estimated to be at least 800 m.Graphical

In situ seismic testing for experimental modal analysis of civil structures

This paper presents a novel excitation method for in situ experimental modal analysis (EMA) and compares its performance to ambient vibration analysis, also known as operational modal analysis (OMA). The method consists of base excitation through ground accelerations generated through a large mobile shaker, a unique form of building excitation which has not been studied to date. The controllable and repeatable nature of the excitation complements some limitations of OMA, including inconsistent excitation conditions and low energy at high frequencies. The method is applied to an instrumented eleven-story building in Austin, Texas, resulting in observations of significant energy input up to 50 Hz. The high input bandwidth is essential to characterizing high order modes which ambient sources do not excite. Using a five-minute set of multiple 20-s chirp excitations from 5 to 50 Hz in two directions (vertical and transverse), it is shown that the in situ seismic EMA yields modes which are not identified through standard ambient OMA. The differences in identification methods were most noticeable above 7 Hz, where vertical contributions accounted for a significant portion of the modal response. Above 11 Hz, ambient excitation was too weak for proper OMA, but EMA enabled the identification of high-order “local” modes, although their complete identification was limited by low sensor density. The primary benefit of OMA comes from processing time series of much longer duration (20 min), which reduced variation in most of the modal estimates, given the fairly consistent ambient excitation conditions during testing. Overall, in situ seismic testing is shown to be a viable option for forced vibration testing of very large civil structures. With refinements in the testing methods, the technique could be used as a complement to ambient vibration testing in contexts where the ambient excitation bandwidth is limited or quick, short-term tests are desired.

Observed Seismic Behavior of a HDRB and SD Isolation System under Far Fault Earthquakes

The behavior of a reinforced concrete building, seismically isolated with high damping rubber bearing (HDRB) and sliding devices (SD), observed during the most important events of the seismic sequence that struck central Italy from August 2016 to January 2017, is analyzed. Due to the epicenter distances, all the events had light effects at the site, thus the isolation system was not always put into action. A previous very low energy earthquake and the ambient vibration analysis are used for comparison. The study of the isolation system response is first carried out and the variability of the resonance frequencies with the input energy at the site is pointed out. These frequencies are quite close to those of the superstructure considered as fixed base. Small cracks were observed after the sequence in some partition walls of the building. The analysis of the superstructure was performed by means of a finite element model, assuming a non-linear model for the isolators, based on previous experimental data. The importance of a suitable decoupling between the superstructure and the ground and the contribution of the sliding devices under low energy earthquake is pointed out.

Imaging Subsurface Structure of an Urban Area Based on Diffuse-Field Theory Concept Using Seismic Ambient Noise

Single station ambient noise measurement in campaign mode has of late gained a huge popularity among geo-scientists. Herein, we present results of ambient vibration analysis, executed in a highly populated urban area covering 47 survey points. The resonance frequency estimates range from 0.5 to 3 Hz, as found from H/V. Taking H/V curve as input for retrieving subsurface information, we deploy diffuse field assumption (DFA) theory. The obtained shear wave velocity from the inversion of H/V curve through DFA approach provides evidence of the complex nature of the subsurface geological structures. Identifying six characteristic 2D cross-sections of the entire area, we attain prevalence of a low-velocity intermediate layer with velocity ranging from 128–192 m/s. On the contrary, a relatively high-velocity layer is also obtained (279–471 m/s) which can be treated as sedimentary deposits (may be for some sites as basin basement). The attained results, when extended to a 3D shear wave profile, tally excellently with estimated frequency distribution and its corresponding links with depth wise strata, accompanied by a topographical profile of the surveyed locations. All findings are comprehensively analyzed and interpreted as a proof of concept of implementation of DFA approach towards retrieving subsurface information.

Ambient Vibration Analysis on Large Scale Arrays When Lateral Variations Occur in the Subsurface: A Study Case in Switzerland

The ambient vibration analysis is a non-invasive and low-cost technique used in site characterization studies to reconstruct the subsurface velocity structure. Depending on the goal of the research, the investigated depth ranges from tens to hundreds of meters. In this work, we aimed at investigating the deeper contrasts within the crust and in particular down to the sedimentary-rock basement transition located at thousands of meters of depth. To achieve this goal, three seismic arrays with minimum and maximum interstation distances of 7.9 m and 26.8 km were deployed around the village of Schafisheim. Schafisheim is located in the Swiss Molasse Basin, a sedimentary basin stretching from Lake Constance to Lake Geneva with a thickness ranging from 800 to 900 m in the north to 5 km in the south. To compute the multimodal dispersion curves for Rayleigh and Love waves and the Rayleigh wave ellipticity angles, the data were processed using two single-station and three array processing techniques. A preliminary analysis of the inversion results pointed out a good agreement with the fundamental modes of Rayleigh and Love waves used in the inversion and a quite strong disagreement with the higher modes. The impossibility to explain at the same time most of the dispersion curves was interpreted as the co-existence, within the investigated area, of portions of the subsurface with different geophysical properties. The hypothesis was confirmed by the Horizontal-to-Vertical spectral analysis (H/V) which indicated the presence of two distinguished areas. The observation allowed a new interpretation and the identification of the Rayleigh and Love wave fundamental modes and of the S-wave velocity profiles to be reconstructed for each investigated zone. It results in two S-wave velocity profiles with similar velocities down to 15 km deferring only in their shallow portions due to the occurrence of a low velocity zone at a depth of 50–150 m at the centre of the investigated area.

Ambient vibration analysis on seismic arrays to investigate the properties of the upper crust: an example from Herdern in Switzerland

SUMMARYThe difficulty and the high cost to assess the subsurface properties led to the development of several geophysical techniques. Generally, the focus of a site study is the reconstruction of the S-wave velocity profile down to few tens to hundreds of metres (e.g. 30–300 m), but not the investigation of deeper structures, such as the transition to the crystalline basement. However, such deeper structures are of interest when seismic hazard products have to relate to a reference rock-velocity profile, for example in regional seismic hazard assessment and microzonation studies. To estimate the S-wave velocity profiles down to several kilometres, we study the potential of Rayleigh and Love waves at low (down to 0.1 Hz) and high (up to 20 Hz) frequencies using two seismic arrays of increasing size. The small array, with a maximum inter-station distance of 900 m and a recording time of 3 hr, was aimed at constraining the shallow subsurface down to about 350–400 m, while the big one, with a maximum inter-station distance of more than 29 km and 23 hr of recording had the goal to constrain the deeper structure. The arrays were deployed in northern Switzerland (east of the village of Herdern) within the Swiss Molasse basin, a sedimentary basin north of the Alps stretching from Lake Constance to Lake Geneva; its thickness increases from 800 to 900 m in the northeast to more than 5 km in the southwest. The seismic data recorded by the two arrays were analysed using the techniques developed for the analysis of small-aperture arrays. The results were inverted for the S-wave velocity profile in two steps: first, the Rayleigh and Love wave phase dispersion curves were inverted together. Secondly, the previous dispersion curves were jointly inverted with the measured Rayleigh wave ellipticity angle. The resulting S-wave velocity profiles are similar and show agreement with the available geological and geophysical data, confirming the potential of surface waves to investigate deep structures. Moreover, our analysis proves the feasibility of site characterization techniques to large arrays and the possibility to estimate the P- and S-wave velocity profiles down to 5 km, deeper than the contrast between Molasse basin and crystalline rock at around 2.1 km.

Damage assessment of Nepal heritage through ambient vibration analysis and visual inspection

Damage assessment of Nepal heritage through ambient vibration analysis and visual inspection

Relevance of rock slope deformations in local seismic response and microzonation: Insights from the Accumoli case-study (central Apennines, Italy)

Based on a National Government Decree, detailed seismic microzonation studies were carried out after the seismic sequence of Central Italy (2016–2017) in support of post-earthquake reconstruction. During these activities mass rock creep deformations of slopes were observed in the municipality of Accumoli (Latium Region), involving two urbanized areas on the left bank of the Tronto River. The geological and geomorphological surveys as well as the geomechanical characterization of rock masses revealed a deformation process of flexural toppling which involves the sub-vertical thick sandstone layers down to 50 m of depth. Geophysical investigations, consisting of downhole tests, active surface-wave testing and seismic ambient noise measurements, were carried out in the areas involved in the deformational processes. The results highlighted that the rock mass involved in the flexural toppling exhibits peaks of the horizontal-to-vertical spectral ratios (HVSR) in a frequency band ranging from 1.5 up to 2.5 Hz. In one of the two areas, H/V calculated on weak-motions recorded during the seismic sequence within the deforming rock mass, showed a clear peak in the H/V frequency range 1.5–2.0 Hz, the same range pointed out by the ambient vibration analysis. Due to local morphological, lithological and bedding conditions, such an evidence cannot be related to a simple 1D resonance due to the seismic impedance contrasts originated from the local stratigraphy. Nevertheless, it seems more reliable that the HVSR peaks can be related to the 3D volume of intensely jointed rock mass, in agreement with the field evidence of flexural toppling, as these peaks result confined within the deforming mass. In addition, the lack of a clear polarization of the particle motion in the seismic ambient noise dataset suggests that the observed effect should not be linked to the interaction of seismic surface waves with persistent discontinuities in an anisotropic rock mass. According to the National guidelines for seismic microzonation studies, these rock masses have not been identified as unstable, i.e. involved in conventional landslide processes, but have been identified as seismic amplification prone microzones with associated amplification factors of at least 1.5 in the period range 0.1–0.5 s. The identification of such microzones within areas involved in gravity-induced slope deformations is a significant prerequisite for planning strategies of post-earthquake reconstruction.

Modal parameter extraction from measured signal by frequency domain decomposition (FDD) technique

Purpose The purpose of this paper is to highlight that the need for improved system identification methods within the domain of modal analysis increases under the impulse of the broadening field of applications, e.g., damage detection and vibro-acoustics, and the increased complexity of today’s structures. Although significant research efforts during the last two decades have resulted in an extensive number of parametric identification algorithms, most of them are certainly not directly applicable for modal parameter extraction. So, based on this, the aim of the present work is to develop a technique for modal parameter extraction from the measured signal. Design/methodology/approach A survey and classification of the different modal analysis methods are made; however, the focus of this thesis is placed on modal parameter extraction from measured time signal. Some of the methods are examined in detail, including both single-degree-of-freedom and multi-degree-of-freedom approaches using single and global frequency-response analysis concepts. The theory behind each of these various analysis methods is presented in depth, together with the development of computer programs, theoretical and experimental examples and discussion, in order to evaluate the capabilities of those methods. The problem of identifying properties of structures that possess close modes is treated in particular detail, as this is a difficult situation to handle and yet a very common one in many structures. It is essential to obtain a good model for the behavior of the structure in order to pursue various applications of experimental modal analysis (EMA), namely: updating of finite element models, structural modification, subsystem-coupling and calculation of real modes from complex modes, to name a few. This last topic is particularly important for the validation of finite element models, and for this reason, a number of different methods to calculate real modes from complex modes are presented and discussed in this paper. Findings In this paper, Modal parameters like mode shapes and natural frequencies are extracted using an FFT analyzer and with the help of ARTeMiS, and subsequently, an algorithm has been developed based on frequency domain decomposition (FDD) technique to check the accuracy of the results as obtained from ARTeMiS. It is observed that the frequency domain-based algorithm shows good agreement with the extracted results. Hence the following conclusion may be drawn: among several frequency domain-based algorithms for modal parameter extraction, the FDD technique is more reliable and it shows a very good agreement with the experimental results. Research limitations/implications In the case of extraction techniques using measured data in the frequency domain, it is reported that the model using derivatives of modal parameters performed better in many situations. Lack of accurate and repeatable dynamic response measurements on complex structures in a real-life situation is a challenging problem to analyze exact modal parameters. Practical implications During the last two decades, there has been a growing interest in the domain of modal analysis. Evolved from a simple technique for troubleshooting, modal analysis has become an established technique to analyze the dynamical behavior of complex mechanical structures. Important examples are found in the automotive (cars, trucks, motorcycles), railway, maritime, aerospace (aircrafts, satellites, space shuttle), civil (bridges, buildings, offshore platforms) and heavy equipment industry. Social implications Presently structural health monitoring has become a significantly important issue in the area of structural engineering particularly in the context of safety and future usefulness of a structure. A lot of research is being carried out in this area incorporating the modern sophisticated instrumentations and efficient numerical techniques. The dynamic approach is mostly employed to detect structural damage, due to its inherent advantage of having global and location-independent responses. EMA has been attempted by many researchers in a controlled laboratory environment. However, measuring input excitation force(s) seems to be very expensive and difficult for the health assessment of an existing real-life structure. So Ambient Vibration Analysis is a good alternative to overcome those difficulties associated with the measurement of input excitation force. Originality/value Three single bay two storey frame structure has been chosen for the experiment. The frame has been divided into six small elements. An algorithm has been developed to determine the natural frequency of those frame structures of which one is undamaged and the rest two damages in single element and double element, respectively. The experimental results from ARTeMIS and from developed algorithm have been compared to verify the effectiveness of the developed algorithm. Modal parameters like mode shapes and natural frequencies are extracted using an FFT analyzer and with the help of ARTeMiS, and subsequently, an algorithm has been programmed in MATLAB based on the FDD technique to check the accuracy of the results as obtained from ARTeMiS. Using singular value decomposition, the power Spectral density function matrix is decomposed using the MATLAB program. It is observed that the frequency domain-based algorithm shows good consistency with the extracted results.

On the parameter sensitivity in structural parameter identification using Eigensystem Realization Algorithm for a MW-size wind turbine

Accurate estimation of structural parameters such as the natural frequencies, modal shapes and damping ratios is important for both numerical modelling and structural health monitoring, and ambient vibration analysis is usually used for the estimation. Although the Eigensystem Realization Algorithm (ERA) is widely used, the results are known to depend on the selection of the calculation parameters. In this study, first the sensitivity of the calculation parameters of the NExT-ERA method for the full-scale measurement data of a 2,4MW wind turbine is studied to provide reference for practical application of the method. Small initial model order resulted in unstable estimation of the damping ratios, while higher values gave robust results. The optimum value for the initial model order was 400 to 600, which was much larger than the recommendation in the literature as four times the model order. The optimal values for the correlation length parameter are 3500 to 4000 for the 1st modes, and 100 to 350 for the 2nd modes, which corresponds to the length of the free decay process seen in the cross- correlation function. Higher number of points used for FFT gave lower damping ratios for the 1st mode in both fore-aft and side-side directions. The effect of the number of the FFT points is smaller in 2nd mode, which may be due to difference of the length of data required to capture the characteristics of the system. The structural parameters estimated with NExT-ERA method agreed well with the results of the previously conducted forced vibration test except for the fore-aft 1st mode damping ratio, of which value was especially small compared to other modes.

Ambient Vibration Test of a 18-Story Reinforced Concrete Building

Ambient vibration analysis of a 18-story reinforced concrete building was carried out to investigate its dynamic properties. Ambient vibration test was conducted on the building, and modal parameters including natural frequencies and damping ratios were identified using the Frequency domain decomposition (FDD) method and the Enhanced frequency domain decomposition (EFDD) method. The results demonstrate that ambient vibration test is a feasible and efficient technique to excite structures, and that FDD and EFDD methods are reliable and efficient techniques for ambient vibration data.

Ambient seismic vibration analysis and ground characterization in the vicinity of Algiers seismic zone

The Dar El Beida area, situated in the Mitidja basin, in the north central Tellian Atlas is characterized by its Plio-Quaternary sediments. The site effect analysis in this zone surrounding Algiers city is very important. The soil is constituted from recent soft sediments capable of amplifying seismic motion. Indeed, the analysis thanks to the ratio H/V ambient vibration method confirms these assumptions. An electric resistivity tomography strengthened the results obtained from ambient vibration for site characterization. This demonstrates once again the more or less homogeneous character of formations in this area. However, strongly amplifying its sediments largely explains the seismic acceleration of 0.52 g recorded following the 2003 Boumerdes earthquake.

Building frequency fluctuations from continuous monitoring of ambient vibrations and their relationship to temperature variations

Building frequencies (fundamental and higher modes) are a critical parameter especially in the field of structural health monitoring mainly based on the stability of the structural dynamic parameters of individual building (frequencies, damping and modes shape). One of the most used methods to find out these parameters is based on the use on ambient vibration analysis. In this work, we study the fluctuations over a month period of the fundamental frequencies (transverse and longitudinal) of a 3.5-story RC-building made of 2 identical units connected by a structural joint. Time independent building frequencies is a strong assumption; as illustrated by our experiment showing that over an observation period of a month, building frequencies fluctuate of about 3.5 %. A clear correlation is found between the building frequency fluctuations and temperature variations, with a phase-shift interpreted as the characteristic time of heat diffusion within the walls. This allows: (1) determining the thermal diffusivity of the structure, (2) inferring its relative stiffness variations, and (3) showing that its Young modulus varies linearly with temperature.

Dynamic characteristics of a bridge estimated with new bolt-type sensor, ambient vibration measurements and finite element analysis

Applicability of new bolt-type piezoelectric bolt sensor to estimate dynamic properties of bridges is verified by comparing measurement results with those obtained from ambient vibration observations and from finite element method of analysis. For this study a bridge located at Yurihonjo city, Japan, was chosen as the target structure. The structure is a multiple-span type bridge with steel beams of variable sections and reinforced concrete slabs to support the asphalt carpet. The bridge consists of seven spans with a total length of 256 m. The experimental measurements using the new bolt-type sensor were performed only on the first span near the left abutment. However, ambient vibration measure ments and analysis were done for all spans. In general, responses were obtained appropriately using the new sensor and that results are comparable with those obtained from analysis and ambient vibration measurements.

Building monitoring in Bishkek and Dushanbe by the use of ambient vibration analysis

<p>Within the framework of the EMCA - Earthquake Model Central Asia - project, the cities of Bishkek (Kyrgyzstan) and Dushanbe (Tajikistan) were selected for building monitoring using measurements of seismic noise to obtain the dynamical properties of the buildings. Eight buildings of different construction types, date of construction and building height, both, Soviet-era and recently constructed buildings, were instrumented for a period of a few hours. In this study, an overview of the experiment is given, including a short description of each monitored building’s structure and the performed installation. Preliminary results for a representative Soviet-era residential building in Dushanbe are presented. Modal analysis is performed using the Frequency Domain Decomposition (FDD) method to estimate the natural frequencies and the mode shapes. The wave propagation velocities in the two directions along the building axes are determined by an interferometric approach. Although the experimental set-up was not the optimal, valuable information about the dynamical characteristics of the buildings were still obtained.</p>

Seismic response of an 8-story RC-building from ambient vibration analysis

In this study, we assess the dynamic characteristics of an 8-story RC-building composed by two units connected through a structural joint. This building, belonging to one of the largest hospitals in northern Greece, has been selected in the framework of an European funded project as test site for developing a structural health monitoring system and it is instrumented with a permanent strong motion network. The assessment of the dynamic characteristics is performed using ambient vibration recorded by a temporary seismic network installed inside the structure. Non-parametric identification methods, namely the peak picking and frequency domain decomposition, are applied to perform operational modal analysis and extract the natural frequencies and mode shapes of the structural system. Since the detection of changes in the shear wave velocity inside the building is relevant for health monitoring analysis, we use the ambient vibration recordings to perform a deconvolution interferometry. Moreover, a shear-beam model is considered to estimate the velocity in the first three floors, where the distribution of internal sources introduces complex patterns in the impulse response functions. The velocity for lowest part of the building is estimated by optimizing the match between the arrival times of the empirical and theoretical pulses. Finally, the velocities and quality factors estimated from ambient vibration analysis are consistent with preliminary results obtained analyzing earthquake data recorded in the same building.

Ambient vibration analysis of an unstable mountain slope

A field experiment with small aperture seismic arrays was performed on the unstable rock slope above the village of Randa in the southern Swiss Alps. The aim of this experiment was to constrain the seismic response of a potential future rockslide using ambient vibration recordings. Weak seismic events were identified on the recordings and site-to-reference spectral ratios were calculated using a reference site located on the stable part of the slope. Spectral ratios of up to 30 were observed at sites located within the unstable portion of the slope. A strong variation of spectral ratios with azimuth indicates a directional site effect. Neither amplification nor directionality were observed at sites located in the stable part of the slope. Furthermore, time-frequency polarization analysis of the ambient noise was performed to provide robust estimates of frequency dependent directions of the maximum polarization. It was found that the unstable part of the slope vibrates within a narrow range of directions (130 ± 10°) for the frequency range centred around 5 Hz. The polarization directions estimated from ambient seismic vibrations are in good agreement with the deformation directions obtained by geodetic and in situ measurements. No directionality of ambient vibrations was observed at sites within the stable part of the slope.

Ambient Vibration Analysis with Subspace Methods and Automated Mode Selection: Case Studies

This study presents an investigation of the performance of subspace techniques for modal identification using ambient vibration measurements. Several models and structures characterized by increasing degrees of complexity are investigated to assess the potential benefits of stochastic subspace identification algorithms, and the difficulties that might be experienced during a modal identification analysis. The case studies include a simple three-degree-of-freedom (3 DOF) mass-spring-dashpot model, the 120 DOF finite element model, as well as the physical laboratory model of a small scale steel frame, and a long span suspension bridge. Stabilization diagram and clustering analysis approaches are adopted for spurious mode rejection, and the latter is found to be promising for automating the mode selection process. The experiences with experimental data reveal that some preconditioning tools are quite helpful in order to properly focus on the structural modes of interest, and that preconditioning improves the performance of the subspace methods. On the whole, the approaches investigated in this study are found to perform quite satisfactorily for operational modal analysis of engineering structures.