Ambient Vibration Measurements Research Articles

Scour depth estimation in a balanced cantilever bridge with deteriorated central hinges based on natural frequencies: field measurements, methodology for estimation and verification

A scheme based on field measurements of vibrations is proposed for estimating the scour depths of a multi-span balanced cantilever bridge resting on foundations embedded in soft soil. Field measurements of ambient vibrations using velocity sensors indicated elongated periods of vibration in the transverse direction at the pier tops due to scour underneath. The modal properties were identified using the Eigensystem Realization Algorithm. The unknown spring constants of the central hinges were determined to represent the deteriorated performance of the bridge superstructure under no-scour benchmark conditions from a finite element model incorporating soft soil conditions. A single set of spring constants compatible with the measured natural frequencies was thus identified for the transverse movement of the central hinges. The strong dependence of the transverse bending mode of the pier and weak dependence of the longitudinal bending mode of the pier on scour phenomena were clearly observed. The observed relationships between the scour depth and natural frequency were sensitive to soft soil conditions in both modes. The estimated spring constants representing soft soil were used to assess the unknown scour depths in the pier foundations suffering from scour. Independent bathymetric surveys verified the general applicability of the proposed scheme for estimating the scour depths around bridges of this type within reasonable limits.

Site Effect Assessment of the Gros-Morne Hill Area in Port-au-Prince, Haiti, Part B: Mapping and Modelling Results

This paper presents the general results in terms of maps, as well as geological and numerical models of a site effect study, that aimed at a better understanding of the ground motion amplification on the Gros-Morne hill, in the southeastern part of Port-au-Prince, Haiti, which might have influenced the 2010 event damage pattern in that area. These maps and models are based on multiple geophysical–seismological survey outputs that are presented, in detail, in Part A of this publication. Those outputs include electrical resistivity tomography sections, P-wave velocity profiles, S-wave logs, estimates of the fundamental resonance frequency for many locations, as well as earthquake recordings at three sites and associated site amplification assessment for the top of the hill. Related results are discussed in Part A with respect to outputs and interpretations that had been published earlier by other research teams for the same site. Our results only partly confirm the strong seismic amplification effects highlighted by some of the previous studies for this hill site, which had been attributed to the influence of local topographic and soil characteristics on seismic ground motion. Here, we focus on the imaging of different site effect components over the entire survey area; we present maps of shear wave velocity variations, of changing fundamental resonance frequencies, and of related estimates of soft soil/rock thickness, of peak spectral amplitudes, and of ambient ground motion polarization. Results have also been compiled within a 3D surface–subsurface model of the hill, which helps visualize the geological characteristics of the area, which are relevant for site effect analyses. From the 3D geomodel, we extracted one 2D geological section along the short-axis of the hill, crossing it near the location of Hotel Montana on top of the hill, which had been destroyed during the earthquake, and has now been rebuilt. This cross-section was used for dynamic numerical modelling of seismic ground motion, and for related site amplification calculation. The numerical results are compared with the site amplification characteristics that had been estimated from the ambient vibration measurements and the earthquake recordings.

Correlation between earthquake damage and seismic site effects: The study case of Lentini and Carlentini, Italy

On December 13th 1990 a magnitude 5.6 seismic event, felt throughout Sicily, and having maximum seismic intensity VII-VIII, occurred. Although, it is classified as moderate magnitude, it caused causalities and the collapse of a few buildings. Two of the most struck cities were Carlentini and Lentini, located in south-eastern Sicily at about 20 km from the instrumental epicenter. In order to evaluate the influence of ground features on the damage distribution, both vulnerability and damage maps were drawn using information collected in about 6000 forms coming from official ascertainments. Significant damage observed in some portions of the investigated areas, appears related to the modest quality of the buildings that were classified as medium to high vulnerability. To evaluate the role played by the geologic setting, two sites were instrumented to measure shear-wave velocity of the main lithotypes. Sixty-three ambient vibrations measurements were also performed both in free field and inside some buildings. Moreover, using information coming from geological and geophysical surveys, 1D numerical modelling were done to achieve information about the main ground motion parameters. The results highlight the occurrence of significant site effects and the potential presence of resonance phenomena between site and structure during the occurrence of a seismic event. Seismic site amplifications together with the buildings' vulnerability, observed through damage surveys, contributed to enhance the inherent seismic risk of the investigated area.

Ambient Vibration Measurements in Representative Buildings in Kathmandu Valley Following the Gorkha Earthquake

The horizontal-to-vertical spectral ratio is a widely used technique to identify fundamental frequency and amplification by both structural and geotechnical engineers. Following the 2015 Gorkha earthquake, ambient vibration measurement was conducted in central Kathmandu Valley considering representative building types. Well-designed high-rise reinforced concrete (RC), residential RC (poorly designed), brick masonry (cement mortar), medium-rise RC, and medium-rise precode RC buildings were chosen for ambient vibration measurement. As the first attempt to estimate the natural frequencies of buildings in Kathmandu Valley, ambient vibration records were taken at various heights of the buildings. Another aim of this study was to identify the possibility of vibration resonance in the existing building types; thus, free-field measurements were also taken during the building measurements.

Natural Frequencies Evaluation on Partially Damaged Building using Ambient Vibration Technique

Severe damages observed on the school blocks, roads, retaining walls and drainage within the compound of SMK Kundasang Sabah possibly due to the ground movements triggered by the Ranau earthquake in 1991. Ambient vibration measurements were carried on the remaining demolished 3-storey building which partially damaged in order to measure the predominant building frequencies using tri-axial 1 Hz seismometer sensors. Popular methods of Horizontal-to-vertical spectral ratios (HVSR) and Fourier amplitude spectra (FAS) were used to compute the ambient vibration wave fields of each building axes (Transverse or North-South (NS), Longitudinal or East-West (EW) and vertical) into Fourier spectra. Two main modes of translation and torsion were observed from the peaks frequencies obtained at 2.99 to 3.10 Hz (1st mode), 4.85 Hz (2nd mode) and 5.63 to 5.85 Hz (3rd mode). The building experiencing translation modes of bending and shear in the NS and EW directions. It could be seen when the amplitudes tends to increase when the floor are increased. Meanwhile, the torsional bending mode is expected to occur when the deformation amplitudes are found to be increasing horizontally, when moving into partially structural damaged section located on the East wing of building.

S‐wave velocity structure of Chlef City, Algeria, by inversion of Rayleigh wave ellipticity

ABSTRACTChlef City, Algeria, which is located in the Lower Cheliff Basin, is vulnerable to seismic hazards. Since there is no constrained velocity model for the Lower Cheliff Basin, particularly at greater depths, we conducted an ambient vibration investigation to map the shear‐wave velocity structure beneath the city, with the primary goal of supplementing the existing microzonation studies. Here, we inverted the Rayleigh wave ellipticity measurement curve of ambient vibrations measurements from 120 sites in Chlef City to estimate the shear‐wave velocity structure. The study area was subdivided into six zones based on similarities between the observed horizontal‐to‐vertical spectral ratio of the ambient vibrations curves. Our resultant shear‐wave velocity models show that the observed fundamental frequencies (0.3–1.6 Hz) are related to a thick layer (~800 m) of upper Miocene deposits, where on average. Generally, the Mesozoic basement shows . Moreover, the clear peaks observed at higher frequencies and only in the northwest part of the city are related to a thin layer of Quaternary deposits ().

Site characterisation in Barreiro urban area (Portugal) using H/V and ReMi techniques

ABSTRACTBarreiro is an urban county located in the Lisbon metropolitan area (Portugal) with several hazardous industrial facilities. This area shows a moderate seismicity, but due to its geotectonic location, it has been subject in the past to earthquakes responsible for high social and economic losses. Earthquakes that occurred in 1531 (Mw ≈ 6.6), 1755 (M ≥ 8) and 1909 (Mw ≈ 6) are examples of destructive events that hit Barreiro. In this study, we present an analysis of ambient vibration measurements together with local geology and geotechnical properties of the shallower units. Horizontal‐to‐vertical spectral ratio (H/V) computed from ambient vibrations, shear wave velocity profiles, and value distributions were compared with the geological and geotechnical properties of the shallower units, looking for potential site effects.Geotechnical characterisation was based on the analysis of 289 logs and 1109 Standard Penetration Tests. A total of 214 refraction microtremor measurements were performed, obtaining profiles and values for 34 different sites. The results from ambient vibration records carried out at 136 sites (Vicêncio, Teves‐Costa and Sá Caetano ) were also used. Spatial distributions of profiles, values, the thickness of the shallow formations, the number of blows from Standard Penetration Tests (NSPT values), and H/V peak frequencies are presented and discussed. Correlations between these parameters were established. Three areas prone to seismic amplification, for frequencies between 2.5 Hz to 8 Hz, were identified. Ground classification was performed according to Eurocode 8 using the estimated values. The results will be made available to the Barreiro municipality for support of land and emergency planning.

Deeper Vs profile constraining the dispersion curve with the ellipticity curve: A case study in Lower Tagus Valley, Portugal

Shear wave velocity profile and bedrock depth are key parameters for seismic site response estimation and a reliable tool to evaluate liquefaction potential in soil deposits. They can be determined using in-situ geotechnical tests such as the seismic Cross-Hole (CH), seismic Cone Penetration Test (SCPT), seismic Dilatometer Test (SDMT), or through geophysical surface wave methods. The main advantages of surface wave methods are their non-invasive nature and the ability to characterize the shear wave velocity of the soil at a larger scale. However, the investigation depth in general is less than 20 m. Using the Rayleigh ellipticity curve to constrain the dispersion curve from active and/or passive measurements, deeper Vs-profile is obtained.In this study, the Vs profile of the soil at a site located over Lower Tagus alluvial Valley was obtained using different surface wave methods. For this purpose, ambient vibration measurements using a single three-component seismic station were made, to complement active and passive linear measurements. The Rayleigh wave ellipticity curve was computed from the single station recordings using the RayDec method and dispersion curves were estimated with the array recordings processed using f-k based methods: MASW, ReMi and conventional f-k method for non-linear array data. A joint inversion procedure was applied to the data and the results were compared with Vs profiles obtained from direct measurements with Cross-Hole and SDMT tests. The results show that considering the passive ellipticity curve in the joint inversion process with the dispersion curve, it is possible to obtain deeper and less scattered Vs profiles.

Ambient Vibrations Measurements and 1D Site Response Modelling as a Tool for Soil and Building Properties Investigation

The safety of historic buildings heritage is an important task that becomes more substantial when the buildings are directed to educational purposes. The present study aims at evaluating the dynamic features of the Benedettini complex, an historic monastery located in downtown Catania, which is at present the headquarters of the humanistic studies department of the University of Catania. Both the building’s complex response to a seismic input and the soil-to-structure interaction were investigated using ambient noise recordings. The results point out a multiple dynamic behaviour of the monastery structure that shows several oscillation modes, whereas the identification of a single natural frequency can be observed in some sites where the structure can more freely oscillate. This observation is also confirmed by the variability of computed damping values that appear linked to the different rigidity of the structure, as a function of the either the longitudinal or transversal orientation of the investigated structural elements. Moreover, the comparison between the building’s fundamental period and spectral ratios frequencies, which were obtained from free field ambient noise measurements located outside the monastery, outline the presence of potential resonance effects between the site and structure during a seismic event. Numerical modelling of the local seismic response confirms the obtained experimental site frequencies, setting into evidence that higher amplification factors are reached in the same frequency range characterizing the building.

Study on the application of ambient vibration tests to evaluate the effectiveness of seismic retrofitting

In recent years, earthquakes have occurred frequently, and the seismic performance of existing school buildings has become particularly important. The main method for improving the seismic resistance of existing buildings is reinforcement. However, there are few effective methods to evaluate the effect of reinforcement. Ambient vibration measurement experiments were conducted before and after seismic retrofitting using wireless measurement system and the changes of vibration characteristics were compared. The changes of acceleration response spectrum, natural periods and vibration modes indicate that the wireless vibration measurement system can be effectively applied to evaluate the effect of seismic retrofitting. The method can evaluate the effect of seismic retrofitting qualitatively, it is difficult to evaluate the effect of seismic retrofitting quantitatively at this stage.

Uncertainty quantification and propagation in dynamic models using ambient vibration measurements, application to a 10-story building

Abstract This paper investigates the application of Hierarchical Bayesian model updating for uncertainty quantification and response prediction of civil structures. In this updating framework, structural parameters of an initial finite element (FE) model (e.g., stiffness or mass) are calibrated by minimizing error functions between the identified modal parameters and the corresponding parameters of the model. These error functions are assumed to have Gaussian probability distributions with unknown parameters to be determined. The estimated parameters of error functions represent the uncertainty of the calibrated model in predicting building’s response (modal parameters here). The focus of this paper is to answer whether the quantified model uncertainties using dynamic measurement at building’s reference/calibration state can be used to improve the model prediction accuracies at a different structural state, e.g., damaged structure. Also, the effects of prediction error bias on the uncertainty of the predicted values is studied. The test structure considered here is a ten-story concrete building located in Utica, NY. The modal parameters of the building at its reference state are identified from ambient vibration data and used to calibrate parameters of the initial FE model as well as the error functions. Before demolishing the building, six of its exterior walls were removed and ambient vibration measurements were also collected from the structure after the wall removal. These data are not used to calibrate the model; they are only used to assess the predicted results. The model updating framework proposed in this paper is applied to estimate the modal parameters of the building at its reference state as well as two damaged states: moderate damage (removal of four walls) and severe damage (removal of six walls). Good agreement is observed between the model-predicted modal parameters and those identified from vibration tests. Moreover, it is shown that including prediction error bias in the updating process instead of commonly-used zero-mean error function can significantly reduce the prediction uncertainties.

Dynamic characterisation of a historic bell-tower using a sensitivity-based technique for model tuning

The most relevant results of the vibration-based investigations performed on a historic masonry tower in Italy namely the Santa Maria a Vico bell-tower is here presented. The first part of the study involves preliminary full-scale ambient vibration measurements in operational conditions and dynamics-based finite element (FE) modelling. At first, a manual tuning of the uncertain parameters of the model was carried out to adjust material properties, soil-structure interaction and constraining effect of the neighbouring structures. Then, based on the sensitivity analysis, only the most sensitive parameters were chosen as updating parameters. Finally, a model updating technique based on a sensitivity-based method was used to minimise the error between experimental vibration data and numerical response values. To this aim, a residual vector defined as the weighted difference between the measured quantities and calculated quantities was used. The uncertain structural parameters of the FE model were identified by minimising a robust penalty function. The calibrated model was used as an important tool for the seismic assessment of the structure using pushover analysis. Since the assumed value of the masonry compressive strength is the most sensitive parameter of non-linear behaviour, a sensitivity analysis was performed considering reference values in the range of interest. The seismic safety corresponding to increasing levels of the seismic hazard was finally investigated.

Structural modal parameter identification using local mean decomposition

Modal parameter identification is the first step in structural health monitoring of existing structures. Already, many powerful methods have been proposed for this concept and each method has some benefits and shortcomings. In this study, a new method based on local mean decomposition is proposed for modal identification of civil structures from free or ambient vibration measurements. The ability of the proposed method was investigated using some numerical studies and the results compared with those obtained from the Hilbert–Huang transform (HHT). As a major advantage, the proposed method can extract natural frequencies and damping ratios of all active modes from only one measurement. The accuracy of the identified modes depends on their participation in the measured responses. Nevertheless, the identified natural frequencies have reasonable accuracy in both cases of free and ambient vibration measurements, even in the presence of noise. The instantaneous phase angle and the natural logarithm of instantaneous amplitude curves obtained from the proposed method have more linearity rather than those from the HHT algorithm. Also, the end effect is more restricted for the proposed method.

Effectiveness of seismic repairing stages with CFRPs on the seismic performance of damaged RC frames

This study aims at evaluating the performance of repairing technique with CFRPs in recovering cyclic performance of damaged columns in flexure in terms of structural response parameters such as strength, dissipated energy, stiffness degradation. A 2/3 scaled substandard reinforced concrete frame was constructed to represent the substandard RC buildings especially in developing countries. These substandard buildings have several structural deficiencies such as strong beam-weak column phenomenon, improper reinforcement detailing and poor material properties. Flexural plastic hinges occurred at the columns ends after testing the substandard specimen under both constant axial load and reversed cyclic lateral loading. Afterwards, the damaged columns were externally wrapped with CFRP sheets both in transverse and longitudinal directions and then retested under the same loading protocol. In addition, ambient vibration measurements were taken from the undamaged, damaged and the repaired specimens at each structural repair steps to identify the effectiveness of each repairing step by monitoring the change in the natural frequencies of the tested specimen. The ambient vibration test results showed that the applied repairing technique with external CFRP wrapping was proved to recover stiffness of the pre-damaged specimen. Moreover, the lateral load capacity of the pre-damaged substandard RC frame was restored with externally bonded CFRP sheets.

Determining Time Variation of Cable Tension Forces in Suspended Bridges Using Time‐Frequency Analysis

A feasibility study was conducted to develop a novel method to determine the temporal changes of tensile forces in bridge suspender cables using time‐frequency analysis of ambient vibration measurements. An analytical model of the suspender cables was developed to evaluate the power spectral density (PSD) function of a cable with consideration of cable flexural stiffness. Discrete‐time, short‐time Fourier transform (STFT) was utilized to analyze the recorded acceleration histories in both time and frequency domains. A mathematical convolution of the analytical PSD function and time‐frequency data was completed to evaluate changes in cable tension force over time. The method was implemented using acceleration measurements collected from an in‐service steel arch bridge with a suspended deck to calculate the temporal variation in cable forces from the vibration measurements. The observations served as proof of concept that the proposed method may be used for cable fatigue life calculations and bridge weigh‐in‐motion studies.

Evaluation of the seismic retrofitting of an unreinforced masonry building using numerical modeling and ambient vibration measurements

Ambient vibration measurements and 3-D nonlinear time-history numerical modeling are used to assess the retrofitting measures conducted in a 6-story unreinforced masonry building (URM) built in the end of the 19th century in Switzerland. Retrofitting measures were taken in order to improve the soundproofing and possibly the seismic performance of the building. Reinforced concrete (RC) footings were added under the walls and horizontal steel beams were added to link the walls together with a RC slab at each floor, though the wooden beams were left in place. Several ambient vibration recordings were performed before, during and after the retrofitting work in order to monitor the evolution of the dynamic behavior of the structure. Moreover, numerical models representing the state of the building before and after the retrofit work have been developed to perform nonlinear dynamic analyses using various ground motion records. The change in the modal vibration frequencies, mode shapes, and failure mechanism are presented and discussed in further details. According to ambient vibration measurements, the performed retrofitting resulted in an increase of about 25% of the fundamental frequency. From the results of both the numerical modeling and the ambient vibration measurements, it is confirmed that the in-plane behavior of the slabs evolved from non-rigid floors with in-plane deformation to rigid floors with diaphragm effects. The ambient vibration measurements show that the new stiff slabs could lead to torsion behavior in the building as the result of the diaphragm effect and to higher seismic demand. However, the numerical models show that the displacement capacity of the building increases as a result of those new stiff slabs. Consequently, higher deformation capacity, indicated by the inter-story drift values, on average, are observed for all the damage grades in the post-retrofit state of the building. Finally, the overall seismic safety was only slightly improved.

Efficiency investigation of structural interventions based on in situ ambient vibration measurements on Acheiropoietos Early Byzantine basilica, Thessaloniki, Greece

Acheiropoietos in its current state is a typical Early Byzantine three-aisled timber-roofed basilica with narthex and galleries. During the destructive earthquake of June 20, 1978, the basilica suffered significant damage. In the following years, ambient vibration measurements were performed at the damaged structure before any intervention and main eigenperiods of vibration were calculated. In 1990s, an extensive structural restoration project was undertaken aiming at the provision of diaphragms of controlled shear strength and rigidity at the level of the timber floors and roofs of the galleries. Horizontal steel frames were placed at the perimeter of these diaphragms providing the required diaphragmatic response. The interventions were intended to support the long north and south walls and galleries of the basilica by its stiffer east and west walls against horizontal actions. Within the framework of the present study, the efficiency of the interventions to achieve the desired diaphragmatic response was investigated by measuring the structure’s response to ambient vibration. The in situ measurements and the postprocessing data document that the timber diaphragms are actually behaving as rigid levels. Therefore, it is concluded that the implemented interventions on the basilika assure its proper diaphragmatic function, improving thus successfully its dynamic behavior in future earthquake excitations, according to the redesigned structural approach.

Automated Model Updating of Historical Masonry Structures Based on Ambient Vibration Measurements

AbstractFinite-element (FE) model updating is a very effective procedure for determining the uncertainty parameters in a structural model and minimize differences between experimentally and analyti...

Stability of beam positioning in ShenGuangIII

This paper describes how major beam positioning stability challenges faced by ShenGuangIII (SGIII) facility were met and solved by support system designs. The facility must provide vibrational and thermal stability conditions for alignment-sensitive components. The positioning stability error budget is developed for the alignment-sensitive components to provide connections between mechanical designs and performances of the support systems when they are subjected to multiple input sources that can cause structural vibration and drift. The largest allocation for vibration sources is broadband ambient vibration that is specified as acceleration power spectral density (PSD) at top-of-foundation. The components are supported in the laser bay primarily by relatively large and massive hybrid support structures of reinforced concrete (RC) pedestals and steel frames or vessels and in the target bay/switchyard primarily by building and large steel space frames. The support systems are required to afford the fundamental frequencies as best as they could. Finite element models of key modules in the laser bay and the global target bay/switchyard structures have been used to guide the designs of the combined conventional materials and optical system structures. Finite element analyses of broadband ambient vibration have demonstrated that the designs satisfy the positioning error budget. SGIII construction was completed in 2015. Ambient vibration measurements were performed and demonstrated that the acceleration PSDs remaining within guidelines. The dedicated shots to flat gold-plated targets showed that SGIII met its design requirement of ±30μm rms beam pointing to target.

System Identification of a Base-Isolated Bridge by Ambient and Forced Vibration Tests

Abstract Sciarapotamo Bridge, located in Reggio Calabria Province, Italy, has been recently retrofitted by replacing its deteriorated concrete bridge deck with a composite steel- concrete deck, and isolating the new deck with eight high damping rubber, and four multi-directional sliding bearings. Full-scale ambient and forced vibration dynamic commissioning tests were performed on the bridge just before the bridge has become operational. Three different output-only system identification methods with ambient vibration measurements were used for modal parameter identification, namely: (1) multiple reference natural excitation technique in conjunction with eigen-system realization algorithm, (2) enhanced frequency domain decomposition, and (3) data driven stochastic subspace identification methods. Modal parameter estimation results using different output-only identification methods were presented and compared within the paper. In addition, relatively higher level forced vibration test data was used for estimating frequency response functions, identifying isolated natural frequencies, and damping ratios of the retrofitted bridge in as-built condition. Estimated damping coefficients under different excitation levels are of great value, showing the total damping in the system at low level ambient and relatively higher level forced vibration tests. Experimentally obtained modal parameters of the bridge set the undamaged benchmark state of the bridge for future potential condition assessment studies.