Forced Vibration Tests Research Articles

Dynamic behavior of a square foundation in planar motion on a sand stratum

A comprehensive experimental study was conducted to explore the dynamic soil-structure interaction problem of square footings resting on a sand stratum by means of centrifuge modeling. To capture the horizontal, vertical and rocking response characteristics of the foundation, a program of forced hybrid-mode vibration tests in the form of eccentric vertical excitation on 4 foundation models was pursued. Corresponding to a variety of prototype foundation inertial and bearing pressure conditions, the response time histories and frequency response functions were obtained. With the aid of the experimental data, an evaluation of the performance of the traditional homogeneous and square-root-of-depth modulus theoretical soil models for the shallow sand stratum problem is also included.

Railway bridge using small post-tensioned concrete box girder

A girder depth is the critical parameter for rapid construction of bridge and clearance limitation in urban area such as high-density residential district. A standard post-tensioned I-shaped concrete girder usually demands relatively higher girder depth in order to retain sufficient moment arm between compressive and tensile fiber. To elaborate this issue, a small rectangular hollowed section can be used as a replacement of I-shaped standard girder. This small post-tensioned concrete box girder allows more flexible girder depth adjustment rather than standard I-shaped posttensioned girder plus additional torsion resistance benefits of closed section. A 30 m long, 1.7 m high and 3.63 m wide actual small post-tensioned concrete box girder is designed. A laboratory test was performed for its static behaviors by applying 6400 kN amount of load in the form of 4-point bending test. The load-deflection curve and crack patterns at different loading stage are recorded. In addition, to extract the dynamic characteristics, natural frequency and damping ratio of this girder, several forced vibration tests using oscillator are carried out with varying operational frequency. Nonlinear finite element analysis of this 4-point bending test under monotonic static load is discussed with the aid of concrete damaged plasticity using ABAQUS program. Finally, a series of modal dynamic analyses of different span length and girder depth of small post-tensioned girder bridge is performed in order to validate the applicability to railway bridges and compared to the UIC design criteria in the form of time dependent dynamic responses such as deflection, acceleration and end rotation.

THE NUMERICAL AND EMPIRICAL EVALUATION OF STRUCTURAL PERFORMANCE OF ELEVATED TANKS CONSIDERING SOIL–STRUCTURE INTERACTION EFFECTS

Water tanks are an essential lifeline whose continuing availability and serviceability immediately after earthquake events are crucial for providing undisrupted emergency services. Their seismic performance is, therefore, of paramount importance. The seismic response of an elevated liquid tank situated on a soft soil deposit was studied by means of field vibration tests and numerical simulations. The ambient and forced vibration tests were conducted to identify the soil–structure interaction (SSI) effects on the small strain dynamic behavior of the structure. A series of time domain numerical analyses were performed to evaluate the seismic performance of these structures from a performance based design point of view. The results showed that consideration of SSI increased the displacement demand significantly. Thus, the calculated maximum displacement demand for supporting frame components of the tank may be underestimated significantly when the SSI effects are neglected. In addition, the seismic induced shear forces considering SSI effects were much smaller than the seismic shear forces for the fixed based case. For some soil types, the effect of this reduction on the overall response may become more prominent than the structural ductility mechanism. This resulted in the failure mechanism being initiated by a coupled compression — bending moment effect, rather than shear failure. Finally, the sloshing response is significantly increased due to the SSI.

Soil–pile–structure interaction simulations in liquefiable soils via dynamic macroelements: Formulation and validation

We present a macroelement for soil–structure interaction analyses of piles in liquefiable soils, which captures efficiently the fundamental mechanisms of saturated granular soil behavior. The mechanical model comprises a nonlinear Winkler-type model that accounts for soil resistance acting along the circumference of the pile, and a coupled viscous damper that simulates changes in radiation damping with increasing material nonlinearity. The formulation for a gap element is also proposed to account for formation of gap at pile–soil interface. Validation of the macroelement is conducted using full-scale forced vibration test data from a blast-induced liquefaction test bed, and centrifuge data for seismic loading of piles with superstructure. The macroelement parameters are estimated as a function of the measured soil properties and the level of effective stress. Predictions of bending moments and acceleration time histories at the top of pile and the superstructure are found to be in good agreement with both the full and the model scale data. A comparison with predictions from alternative established methodologies is also presented.

Seismic and vibration tests for assessing the effectiveness of GFRP for retrofitting masonry structures

Full-scale shake table seismic experiments and low-amplitude vibration tests on a masonry building are carried out to assess its seismic performance as well as study the effectiveness of a new multifunctional textile material for retrofitting masonry structures against earthquakes. The un-reinforced and the retrofitted with glass fiber reinforced polymer (GFRP) strips masonry building was subjected to a series of earthquake excitations of increasing magnitude in order to progressively induce various small, moderate and severe levels of damage to the masonry walls. The performance of the original and retrofitted building states is evaluated. Changes in the dynamic characteristics (lowest four modal frequencies and damping ratios) of the building are used to assess and quantify the damage states of the masonry walls. For this, the dynamic modal characteristics of the structure states after each earthquake event were estimated by performing low-amplitude impulse hammer and sine-sweep forced vibration tests. Comparisons between the modal results calculated using traditional accelerometers and those using Fiber Bragg Grating (FBG) sensors embedded in the reinforcing textile were carried on to investigate the reliability and accuracy of FBG sensors in tracking the dynamic behaviour of the building. The retrofitting actions restored the stiffness characteristics of the reinforced masonry structure to the levels of the original undamaged un-reinforced structure. The results show that despite a similar dynamic behavior identified, corresponding to reduction of the modal frequencies, the un-reinforced masonry building was severely damaged, while the reinforced masonry building was able to withstand, without visual damage, the induced strong seismic excitations. The applied GFRP reinforcement architecture for one storey buildings was experimentally proven reliable for the most severe earthquake accelerations. It was easily placed in a short time and it is a cost effective solution (covering only 20% of the external wall surfaces) when compared to the cost for full wall coverage by GFRPs.

PZT 소자의 정압전 응답을 이용한 보 구조물의 모드 변형에너지기반 손상 모니터링

본 연구에서는 PZT 소자의 정압전 효과에 의한 동적 응답신호를 이용하는 보 구조물 손상 모니터링 기법을 제안하였다. 특히, 모드 변형에너지기반 보 구조물 손상 모니터링에 PZT 정압전 응답신호를 입력자료로 활용하는 방안에 대한 연구에 주안점이 있다. 먼저, PZT 소자의 정압전 효과 및 동적 변형률 응답의 이론적 배경을 요약하였다. 다음으로, 모드 변형에너지기반 보 구조물 손상위치 모니터링 기법을 제시하였다. 제시된 기법의 적합성을 검증하기 위해, 캔틸레버 보 모형을 대상으로 강제진동 실험을 수행하였으며, 세 종류의 센서(가속도계, PZT 센서, 변형률계)를 통해 동적 응답신호가 계측되었다. 손상 전후에 계측된 이들 진동신호들을 사용하여 모드 변형에너지기반의 손상위치 모니터링이 수행되었다. The main objective of this study is to examine the feasibility of using lead zirconate titanate (PZT)'s direct piezoelectric response as vibrational feature for damage monitoring in beam structures. For the purpose, modal strain energy (MSE)-based damage monitoring in beam structures using dynamic strain response based on the direct piezoelectric effect of PZT sensor is proposed in this paper. The following approaches are used to achieve the objective. First, the theoretical background of PZT's direct piezoelectric effect for dynamic strain response is presented. Next, the damage monitoring method that utilizes the change in MSE to locate of damage in beam structures is outlined. For validation, forced vibration tests are carried out on lab-scale cantilever beam. For several damage scenarios, dynamic responses are measured by three different sensor types (accelerometer, PZT sensor and electrical strain gage) and damage monitoring tasks are performed thereafter. The performance of PZT's direct piezoelectric response for MSE-based damage monitoring is evaluated by comparing the damage localization results from the three sensor types.

Full-scale dynamic testing and modal identification of a coupled floor slab system

This paper presents work on full-scale vibration testing of the 2nd and 3rd floor slabs of the Tin Shui Wai Indoor Recreation Center. The slabs are supported by one-way long span steel trusses, which are connected by diagonal members and vertical columns to form a mega-truss. On the 2nd floor are a large multi-function room and children play area, while the 3rd floor hosts two basketball courts. Based on their expected usage, significant cultural vibrations with possible rhythmic activities can be expected. To determine the dynamic characteristics of the constructed slab system, ambient and forced vibration tests were performed. Thirty-five setups were carried out in the ambient test to determine the mode shapes using six triaxial accelerometers. A recently developed Fast Bayesian FFT Method is used to identify the modal properties using the ambient data in individual setups. The mode shapes from the individual setups are assembled by a least square fitting procedure. Forced vibration tests were performed by loading the slabs at resonance with a long-stroke electromagnetic shaker, resulting in vibration amplitudes in the order of a few milli-g. A steady-state frequency sweep was carried out and the modal properties were identified by least square fitting of the measured steady-state amplitude spectra with a linear dynamic model. The dynamic properties identified from the ambient and forced vibration tests, as well as their posterior uncertainty and setup-to-setup variability, will be compared and discussed. The field tests provide an opportunity to apply the Fast Bayesian FFT Method in a practical context.

Prestress Force Identification Based on Forced Vibration Tests of Eccentrically Prestressed Concrete Beam

The measurement of residue prestress force is one main issue for condition and performance assessment of prestressed concrete beam bridge. This paper proposes a vibration based parameter estimation technique for this purpose. Under given form external excitation, beam velocity responses at multiple points are collected firstly. The prestress force of the beam is then identified based on the minimization of the least square difference between the measured response and the baseline response. A numerical study on a beam of variant length, subjected to a constant prestress force with variant eccentricity, is conducted to show the effect of prestress force and the effect of bending moment due to eccentricity on fundamental frequency of the beam. The results show that this vibration based method for prestress force identification is both theoretically feasible and practically workable.

Ambient and Forced Vibration Testing of a Reinforced Concrete Building before and after Its Seismic Retrofitting

AbstractThis paper investigates the effects of seismic retrofitting on the modal characteristics of a 6-story RC building located in Istanbul, Turkey. Ambient vibration surveys were carried out before, during, and after the retrofitting work, which took place between June and December 2010. The building was retrofitted via jacketing of columns, addition of structural walls, and construction of a mat foundation. These studies were complemented with data from forced vibration tests performed with an eccentric-mass shaker after the retrofitting work was completed. During retrofitting, partitions were demolished; as a result, the first modal frequency of the building decreased by 11%, based on the results of the ambient vibration survey. The ambient vibration survey also showed that the modal frequencies after the seismic retrofitting increased by almost 96%. During the forced vibration tests, the building was excited around its modal frequencies using an eccentric-mass shaker. It was found that the modal dam...

605 小型・軽量な試験片を対象とした強制加振による放射音測定のための加振機製作(機械力学・計測制御II)

605 小型・軽量な試験片を対象とした強制加振による放射音測定のための加振機製作(機械力学・計測制御II)

Modal identification of Shanghai World Financial Center both from free and ambient vibration response

The Shanghai World Financial Center (SWFC), a currently built super high-rise building, is located in Lujiazui area. The height of this 101-storey building is 492m above ground. A set of dynamic field tests were conducted on the building from April to May 2008. To identify the dynamic properties of the building, three output-only modal identification techniques are applied to the ambient and forced vibration measurements. These methods consist of: the Peak-Picking method (PP) combined with the half-power bandwidth method, the Random Decrement based method (RDT) combined with curve-fitting method, and the Hilbert–Huang transform method (HHT). The fundamental frequencies and damping ratios in two translational directions are identified from the free decays of forced vibration tests. The estimated eleven modal frequencies and damping ratios under microtremors from the Peaking-Picking method and the Hilbert–Huang transform method are compared to each other with favorable correlation. The modal frequencies from the finite element analysis and the shaking table test are further studied with the results of field test. The effect of the installed Active Tuned Mass Damper (ATMD) on the damping characteristics of the building is presented. Based on the identified results, accuracy and efficiency of these methods are investigated with the length of Fast-Fourier Transform and the effect of RDT. The modal properties of the SWFC presented in this paper can be used as baseline in future health monitoring.

MODAL ANALYSIS OF BRIDGE STRUCTURES BY MEANS OF FORCED VIBRATION TESTS / TILTO KONSTRUKCIJŲ MODALINĖ ANALIZĖ TAIKANT PRIVERSTINĖS VIBRACIJOS TESTUS

New modern bridges and older structures exposed to accelerated deterioration need permanent condition monitoring. The major tools used in bridge condition evaluation are based on the results of the experimental modal analysis of bridge structures. This paper focuses on the methodology of Forced Vibration Tests performed by means of mechanical exciters. A comprehensive testing system called MANABRIS, developed at Wroclaw University of Technology, is presented. The system is equipped with a rotational eccentric mass exciter and dedicated software for programming and controlling the tests and for acquiring and processing measurement data. The results of a Forced Vibration Test carried out on a typical railway bridge are compared with the results of a conventional modal test of the structure excited by trains. The precision of the considered testing techniques is evaluated and the range of their effective application is proposed. Santrauka Naujų šiuolaikinių tiltų ir senesnių konstrukcijų, kurias veikia greitesnio nusidėvėjimo procesai, būklę reikia nuolat stebėti. Pagrindinės priemonės, taikomos tilto būklei vertinti, pagrįstos tilto konstrukcijų eksperimentinės modalinės analizės rezultatais. Šiame straipsnyje pagrindinis dėmesys skiriamas priverstinės vibracijos testams (angl. Forced Vibration Tests), atliktiems naudojant mechaninio žadinimo generatorius. Pristatoma išsami bandymo sistema, pavadinta MANABRIS, kuri buvo sukurta Vroclavo technologijos universitete. Sistemoje yra sukamasis ekscentriškas masės sužadintuvas ir programinė įranga, skirta testams programuoti ir kontroliuoti, norint gauti ir apdoroti matavimų duomenis. Priverstinės vibracijos testo (angl. Forced Vibration Test), atlikto su įprastu geležinkelio tiltu, rezultatai lyginami su tradicinio modalinio testo rezultatais, gautais konstrukcijas veikiant traukiniais. Vertinamas testo tikslumas ir siūlomi jo efektyvaus pritaikymo būdai.

Damage detection of RC beams based on experiment and analysis of nonlinear dynamic characteristics

The experimental and analytical investigation on damage detection of reinforced concrete (RC) beams has been carried out using a testing RC beam. The correlations between the damage extent and the nonlinear dynamic characteristics (NDC) of the testing beam are presented in this paper. The NDC of the RC beam were obtained by means of time–frequency analysis (TFA) of displacement responses from the impulsive and harmonic vibration tests. The correlation between the displacement amplitude and the excitation frequency in the case of forced vibration was obtained. Combining the results from the TFA with the relationship of displacement amplitude and corresponding natural vibration frequency (DA-CNVF) from the harmonic vibration tests, a reasonable window width for TFA was suggested. It is observed that the window width of seven times of the fundamental period of the testing structure is a suitable option in the time–frequency analysis. Using the TFA with this appropriate moving window width, the relationship between DA-CNVF at different damage levels in the case of free vibration was used to depict and analyze the NDC of the damaged RC beam. The feasibility of using NDC of a damaged RC beam from either forced harmonic or free vibration tests to detect damage level is discussed. The NDC of the reinforced concrete beam can be used to detect its damage levels if the apparent damage is not significant. Therefore, this damage detection method may be suitable for structures with small damage invisible or hard to be found by naked eyes.

Effect of interlayer slip on the damping of layered and welded aluminium beams

Damping is a complex phenomenon, which acts in the form of absorption and dissipation of energy in the vibrating systems. Joints are inherently present in all most all assembled structures that contribute significantly to the slip damping. The joints fabricated by welding contribute to the damping mechanism in assembled structures. In this study, the relationship between the dynamic slip and damping capacity has been theoretically investigated considering the higher modes of vibration of the welded aluminium cantilever beams. The damping ratios of these structures are determined using forced vibration test method. It has been confirmed through the comparison of theoretical and experimental results that the damping capacity of a jointed cantilever beams can be well estimated using the proposed method.

Calculation methods of Structure–Soil–Structure Interaction (3SI) for embedded buildings: Application to NUPEC tests

This work aims at improving and validating methods coupling Finite Element (FE) and Boundary Element (BE) Methods in the context of Soil–Structure Interaction (SSI) and Structure–Soil–Structure Interaction (3SI) tests performed by NUPEC on mock-up structures built on an unmade ground. Several cases have been tested: single and juxtaposed buildings, shallow and embedded foundations, with various loading conditions: forced and natural seismic loadings. The numerical simulations of forced vibration tests are in good agreement with the results of the NUPEC experiments in the case of two embedded buildings either in terms of amplitude and resonance. The numerical simulation of seismic response tests by FEM and BEM allows for a proper choice of the “reference point” where the computed and the experimental displacements coincide. A parametric analysis of Structure–Soil–Structure Interaction carried out by the FEM has allowed to determine the influence of some parameters on SSI. Most of them like the position of the building in the excavation, the direction of the load, the quality of the contact between the sidewalls of the buildings and the soil for embedded foundations, do not show to have a strong influence on the dynamic system behaviour, which is mainly governed by the stiffness of the first soil layer. As far as 3SI is concerned, this paper shows that when the cross interaction has a small effect on the building response in the case of surface foundations, it has a strong influence in the case of embedded foundations with an important decrease of the response at the top of the buildings.

Extracting energy from Vortex-Induced Vibrations: A parametric study

Here, Vortex-Induced Vibrations (VIVs) of a circular cylinder are analyzed as a potential source for energy harvesting. To this end, VIV is described by a one-degree-of-freedom model where fluid forces are introduced from experimental data from forced vibration tests. The influence of some influencing parameters, like the mass ratio m∗ or the mechanical damping ζ in the energy conversion factor is investigated. The analysis reveals that: (i) the maximum efficiency ηM is principally influenced by the mass-damping parameter m∗ζ and there is an optimum value of m∗ζ where ηM presents a maximum; (ii) the range of reduced velocities with significant efficiency is mainly governed by m∗, and (iii) it seems that encouraging high efficiency values can be achieved for high Reynolds numbers.

Dynamic Assessment about Simple-Supported Girder Bridge

The objective of this study is to introduce a dynamic assessment method about simple-supported girder bridge .This paper introduces the dynamic test contents, test methods, test results and the assessment index. The dynamic tests included ambient vibration tests which were used to identify the bridge modal parameters and forced vibration tests based on controlled traffic loads which were used to obtain the and vibratory response. The analytical results showed that the dynamic performance was in good condition and the bridge had great potential of carrying capacity. This method provided effective means for the assessment of the existing simple-supported girder bridge.

Vibration damping characteristics of carbon fiber-reinforced composites containing multi-walled carbon nanotubes

Vibration damping characteristic of nanocomposites and carbon fiber reinforced polymer composites (CFRPs) containing multiwall carbon nanotubes (CNTs) have been studied using the free and forced vibration tests. Several vibration parameters are varied to characterize the damping behavior in different amplitudes, natural frequencies and vibration modes. The damping ratio of the hybrid composites is enhanced with the addition of CNTs, which is attributed to sliding at the CNT–matrix interfaces. The damping ratio is dependent on the amplitude as a result of the random orientation of CNTs in the epoxy matrix. The natural frequency shows negligible influence on the damping properties. The forced vibration test indicates that the damping ratios of the CFRP composites increase with increasing CNT content in both the 1st and 2nd vibration modes. The CNT–epoxy nanocomposites also show similar increasing trends of damping ratio with CNT content, indicating the enhanced damping property of CFRPs arising mainly from the improved damping property of the modified matrix. The dynamic mechanical analysis further confirms that the CNTs have a strong influence on the composites damping properties. Both the dynamic loss modulus and loss factor of the nanocomposites and the corresponding CFRPs show consistent increases with the addition of CNTs, an indication of enhanced damping performance.

Incertidumbre paramétrica en modelos dinámicos de estructuras civiles

El análisis dinámico es un procedimiento esencial para diseñar una estructura sujeta a cargas dinámicas. Su objetivo es determinar la respuesta estructural e interpretar los resultados teóricos para obtener un diseño adecuado para la estructura. Sin embargo, la existencia de incertidumbres asociadas a parámetros geométricos, propiedades de materiales y dinámica no modelada, que afectan la respuesta dinámica de la estructura, actualmente no son consideradas en los modelos estructurales. Aunque se han desarrollado diferentes herramientas para el tratamiento de incertidumbre en sistemas, la incertidumbre estructurada resulta idónea para ser incorporada en modelos estructurales debido a que permite modelar las variaciones tanto en parámetros físicos como en la dinámica no considerada en los sistemas dinámicos. Este trabajo presenta una descripción de incertidumbre paramétrica para un modelo de tribuna metálica construido en la Escuela de Ingeniería Civil y Geomática de la Universidad del Valle. Los resultados muestran que el modelo analítico con incertidumbre es capaz de representar un conjunto de modelos identificados a partir de ensayos de vibración forzada mediante la variación de los parámetros de masa y amortiguamiento.

Influence of System Uncertainties on Structural Damage Identification through Ambient Vibrations of Steel Structures

The practical difficulties presented by forced vibration testing of large steel structures, such as tall buildings, transmission lines or bridges, led to an increased interest in structural monitoring through ambient vibrations, which usually allows the proper identification of modal properties, natural frequencies, damping and modes of vibration. Changes in these modal properties constitute an indication of structural damage, which may then be assessed on the basis of experimental evidence. The authors proposed an approach to determine the so-called damage damping and stiffness matrices, which are essential to identify the location and intensity of damage. No restrictions were introduced on the damping matrix of the system. The approach requires ambient vibration data of all relevant coordinates used in the structural model, which are processed employing the SSI method. In practice, the identification method is seriously hampered by ambient factors such as temperature or humidity. In general those effects must be filtered out in other to obtain a reliable diagnosis of damage, approach that demands long term monitoring. In this paper, an alternative approach is explored, based on the introduction of error damping and stiffness matrices. Data on both matrices is generated on the basis of observed variations of structural member stiffness and damping caused by ambient factors. The influence of this uncertainty on the identified spectral properties is assessed by simulation.