Full-scale experimental modal analysis of an arch dam: The first experience in Iran

Forced vibration field tests and finite-element studies were conducted on the Shahid Rajaee concrete arch dam in Northern Iran to determine the dynamic properties of the dam–reservoir–foundation system. The first forced vibration tests on the dam were performed with two different types of exciter units, with a limited maximum force, bolted on the dam crest for alternative in-phase and out-of-phase sequencing. Because of an insufficient number of recording sensors, two arrangements of sensors were used to cover sufficient points on the dam crest and one gallery during tests. Two kinds of vibration tests, on–off and frequency sweeping, were carried out on the dam. The primary natural frequencies of the coupled system for both symmetric and anti-symmetric vibration modes were approximated during on–off tests in two types of sequencing of exciters, in phase and out-of-phase, with a maximum frequency of 14 Hz. The principal forced vibration tests were performed at precise resonant frequencies based on the results of the on–off tests in which sweeping around the approximated frequencies at 0.1 Hz increments was performed. Baseline correction and suitable bandpass filtering were applied to the test records and then signal processing was carried out to compute the auto power, cross power and coherence spectra. Nine middle modes of vibration of the coupled system and corresponding damping ratios were estimated. The empirical results are compared against the results from calibrated finite-element modeling of the system using former ambient vibration tests, considering the dam–reservoir–foundation interaction effects. Good agreement is obtained between experimental and numerical results for eight middle modes of the dam–reservoir–foundation system.     

CFRP composite retrofitting effect on the dynamic characteristics of arch dams

The purpose of this study is to investigate the effect of retrofitting dynamic characteristics of a damaged laboratory arch dam model, subsequently repaired with high-strength structural mortar and strengthened with composite carbon fiber reinforced polymer. This study constructed in laboratory conditions is a prototype arch dam–reservoir–foundation model. Five test cases of ambient vibration on the arch dam model illustrate the changes in dynamic characteristics: natural frequency, mode shape, and damping ratio, before and after retrofitting. The ambient vibration tests collected data from the dam body during vibrations by natural excitations which provided small impacts and response signals from sensitivity accelerometers placed at crest points. Enhanced Frequency Domain Decomposition Method in the frequency domain extracts the experimental dynamic characteristics. At the end of the study, experimentally identified dynamic characteristics obtained from all test cases have been compared with each other. Apparently, after the retrofitting, the natural frequencies of the dam body increased considerably, demonstrating that the retrofitting, including repairing and strengthening is very effective on the flashback of initial dynamic characteristics.     

An experimental investigation of water level effects on the dynamic behaviour of a large arch dam

The need for full‐scale dynamic tests, which are recognized as the most reliable method to evaluate a structure's vibration properties, is increasing as new analysis techniques are developed that take into account the complex interaction phenomenons that occur in dam–reservoir–foundation systems. They are extremely useful to obtain reliable data for the calibration of newly developed numerical methods. The Earthquake Engineering and Structural Dynamics Research Center (CRGP) at the University of Sherbrooke has been developing and applying dynamic testing methods for large structures in the past 10 years. This paper presents the experimental evaluation of the effects of the varying water level on the dynamic response of the 180 m Emosson arch dam in Switzerland. Repeated forced‐vibration tests were carried out on the dam during four different periods of the reservoir's filling cycle during a one‐year span. Acceleration and hydrodynamic pressure frequency responses were obtained at several locations while the dam was subjected to horizontal harmonic loading. The variation of the resonant frequencies as a function of the reservoir level is investigated. A summary of the ongoing numerical correlation phase with a three‐dimensional finite element model for the dam–reservoir–foundation system is also presented. Copyright © 2001 John Wiley & Sons, Ltd.     

Continuous ambient‐vibration monitoring of the arch dam of Mauvoisin

This paper presents the experimental programme and results of a continuous ambient vibrations recording programme carried out on the 250 m arch dam of Mauvoisin. This project follows a series of previous measurements completed for seven different water levels. An automated system was set up on the dam and the ambient vibrations were recorded twice daily for a period of 6 months. Frequency shifts were tracked throughout the testing period and the effects of the varying water level were identified. The results confirmed the behaviour observed in previous ambient‐ and forced‐vibration tests. The added‐mass effects are overcome by the stiffening of the dam due to increasing hydrostatic pressure for lower reservoir levels. This trend is then reversed for higher water levels. Any temperature‐related effects were not identified. The experimental techniques are briefly described and the frequency identification process and its limitations are discussed. Copyright © 2001 John Wiley & Sons, Ltd.     

Experimental and finite element studies of the forced vibration response of morrow point dam

Measured accelerations and water pressures obtained during a recent forced vibration test on a large thin arch dam at high water are compared to predictions from a finite element model for which water compressibility is both included and neglected. The numerical model is calibrated using the antisymmetric response data because they are only slightly affected by water compressibility; good agreement is obtained. In the effort to reproduce the symmetric response data, for which water compressibility plays a strong role, the calibrated model shows better correlation when water compressibility is included, but the agreement is still inadequate. A successful isolation of the fundamental water resonance from the experimental data shows significantly different features from those of the numerical water model, indicating possible inaccuracy in the assumed geometry and/or boundary conditions for the reservoir. Some other results at low water level are also included.     

Identification of Fei-Tsui arch dam from both ambient and seismic response data

This purpose of this paper is to study the dynamic characteristics of the Fei-Tsui arch dam using the seismic response data and the ambient vibration data. For the identification of dam properties from seismic response data, the multiple inputs from the abutment of the dam to represent the nonuniform excitations of seismic input motion are considered, and the ARX model is applied using the discrete-time linear filtering approach with least-squares approximation to identify the dynamic characteristics of the dam. The system modal dampings, natural frequencies and frequency response functions are identified. A comparison of the identified modal parameters is made among different seismic events. Post-earthquake safety evaluation of the dam can be made based on the identified model. Finally, the ambient vibration test of the dam is performed to identify the mode shapes along the dam crest.     

猴子岩高面板坝振动台模型试验——大坝结构动力特性研究

利用大型振动台模型试验,测得了猴子岩高面板堆石坝缩尺模型坝的动力特性参数,包括大坝结构的自振频率、阻尼比和振型系数等;分析了多种因素对坝体动力特性的影响规律;根据模型试验相似率推算得到原型坝的相应动力特性参数。研究表明:大坝结构有相对稳定的振型;坝体的动力特性参数值受激振白噪声强度和振动历史等因素影响;水库蓄水使得模型坝结构自振频率小幅提升。这些试验结果和研究结论,可以为该坝的动力分析提供基本资料和定性参考。     

多点激励下拱桥模型振动台试验及数值模拟研究

对多点激励下大跨度桥梁的抗震性能研究,目前主要集中在理论和数值模拟方面,缺乏振动台试验数据的验证。本文通过对一座拱桥模型的多点激励振动台试验和相应的数值模拟分析,在一定程度上揭示了拱桥在一致激励、考虑行波效应和衰减效应的多点激励下的反应特点。同时,本文的数值模拟结果与试验结果也得到了较好的吻合。     

Natural frequencies measured from ambient vibration response of the arch dam of Mauvoisin

The resonance frequencies of the 250‐m‐high arch dam of Mauvoisin are obtained by way of ambient vibration tests. It is observed that the resonance frequencies initially increase with rising water level and then decrease with a further rise. This is linked to the two competing features of increasing entrained mass of water (reduction of the resonance frequencies) and of dam stiffening due to closing of the vertical construction joints (augmentation of the resonance frequencies). The ambient vibration test results are complemented by those obtained during earthquakes at an array of 12 accelerographs. Copyright © 2000 John Wiley & Sons, Ltd.     

Effective ambient vibration testing for validating numerical models of concrete dams

Ambient vibration tests were conducted on a 56 metre high concrete gravity dam to measure its modal properties for validating a finite element model of the dam–reservoir–foundation system. Excitation was provided by wind, by reservoir water cascading down the spillweir, and by the force of water released through outlet-pipes. Vibrations of the dam were measured using accelerometers, and 3-hour data records were acquired from each location. Data were processed by testing for stationarity and rejecting non-stationary portions before Fourier analysis. Power spectra with low variance were generated from which natural frequencies of the dam were identified clearly and modal damping factors estimated. Modal analysis of the frequency response spectra yielded mode shapes for the six lowest lateral modes of vibration of the dam. The finite element model for the dam was analysed using EACD-3D, and the computed mode shapes and natural frequencies compared well with the measured results. The study demonstrates that ambient vibration testing can offer a viable alternative to forced vibration testing when only the modal properties of a dam are required. Copyright © 1999 John Wiley & Sons, Ltd.     

Arch dam system identification using vibration test data

In an effort to study the dynamic characteristics of an arch dam system from the vibration test results, a systematic method of frequency-domain system identification is developed. The governing equations for system identification are based on a non-classical modal superposition method. The non-classical model is shown to be derivable from a general matrix formulation of the dam system. The conventional classical modal formulation becomes a special case of the general non-classical formulation. The modal parameters of the non-classical and the classical formulation are to be identified. The system identification method includes a single-mode sweep procedure for initial parameter estimation and a progressive multiple-mode parameter identification scheme that contains an information criterion for the determination of the optimal number of modes to be included in the identification process. The method is applicable to data measured at more than one point on the dam and to data that include both the amplitude response and the phase response. The method is applied to the vibration test data of two dams. Based on the results of these applications, the adequacy of the classical model and the non-classical model is compared and the effect of the phase data on the parameter determination is discussed.     

Vibration tests and analysis of a model arch dam

Vibration tests were conducted on a 1/24-scale model of the North Fork Dam, a double-curvature arch dam, to determine natural frequencies, mode shapes and hydrodynamic pressures. The mode shapes and natural frequencies were determined from tests using two vibrators mounted on the crest of the dam. Hydrodynamic pressures at the dam/reservoir interface were determined from tests in which the vibrator was attached to the downstream foundation of the dam. The hydrodynamic pressures calculated using Westergaard's theory and a theory for arch dams developed by Perumalswami and Kar accurately predicted the measured pressure at frequencies below the first mode frequency of the dam. The differences in the two theories were insignificant. The Structural Analysis Program (SAP), a linear three-dimensional (3-D) finite element code, was used to compute mode shapes and frequencies for the dam with its base fixed and with a foundation. Numerical solution schemes used in the finite element analysis consisted of a Ritz analysis and a subspace iteration method. Calculations were conducted for both full and empty reservoir conditions. The accuracy of the Ritz analysis improved considerably as more nodes in flexible regions of the dam were loaded. However, the lowest eigenvalues were computed using the subspace iteration method. For the full reservoir, the natural frequencies decreased by 20-30 per cent when the foundation was included in the finite element model. The difference was less when the reservoir was empty. The calculations using the subspace iteration scheme and including the foundation agreed closely with experimental mode shapes and corresponding natural frequencies.     

Experimental study of dynamic damage of an arch dam

Seismic responses of a 292‐m high arch dam were studied by experiment on a shaking table. The model system included the arch dam with contraction joints, a part of a reservoir, and a partial foundation with a topographic feature near the dam. Potential rock wedges on the abutments and the mechanical properties including uplift on the kinematic planes were carefully simulated. A damping boundary consisting of a viscous liquid was introduced to simulate the effect of dynamic energy emission to far field, which made the dynamic interaction between the dam and the foundation be adequately represented in the model test of an arch dam system. Dynamic responses of the arch dam system under a sequence of seismic loadings in increasing strength were examined. Eleven cracks or overstresses on the model dam due to the earthquake excitations were observed, and consequently, its natural frequency dropped by about 14%, but the model dam was stable under the hydrostatic load of the impounded water after the test. Copyright © 2006 John Wiley & Sons, Ltd.     

Numerical simulation of reinforcement strengthening for high‐arch dams to resist strong earthquakes

This paper focuses on analyzing the nonlinear seismic response of high‐arch dams with cantilever reinforcement strengthening. A modified embedded‐steel model is presented to evaluate the effects of the strengthening measure on alleviating the extension and opening of cracks under strong earthquakes. By stiffening reinforced steel, this model can easily consider the steel–concrete interaction for lightly reinforced concrete (RC) members without the need of dividing them into RC and plain concrete zones. The new tensile constitutive relations of reinforced steel are derived from the load–deformation relationship of RC members in direct tension. This model has been implemented in the finite element code and its applicability is verified by two numerical simulations for RC tests. Subsequently, numerical analyses for a 210‐m high‐arch dam (Dagangshan arch dam) are conducted with and without the presence of cantilever reinforcement. Numerical results show that reinforcement strengthening can reduce the nonlinear response of the arch dam, e.g. joint opening and crest displacement, and limit the extension and opening width of concrete cracks. Copyright © 2008 John Wiley & Sons, Ltd.     

水库输水隧洞施工爆破振动对大坝安全影响监测及分析

阐述柏峰水库除险加固工程主坝输水隧洞施工爆破振动对大坝安全影响。选择有代表性的几个部位,采用原位质点振动速度测试的方法对整个施工期间爆破振动对大坝安全影响进行全过程监测与分析。对实测结果分析表明,爆破振动对大坝安全未产生明显的危害。监测成果可为工程爆破振动安全评价提供科学依据。     

Dynamic behaviour and seismic response of El Infiernillo dam

El Infiernillo, a 145 m high rockfill dam in Mexico built in a narrow V-shaped canyon, was subjected to eight major earthquakes since its construction. In this study, the dynamic dam response is analysed using (i) the recorded November 15, 1975 bedrock-crest acceleration histories and (ii) the results of a 1970 full-scale test conducted by UNAM, in which eight upstream–downstream and longitudinal resonant frequencies and configurations were observed and documented. These observed and seismically induced dynamic responses are compared herein to predictions of two different numerical models of El Infiernillo dam; a newly developed simplified three-dimensional (3D) model, and a 3D finite element model. The dynamic dam response characteristics are assessed, and performance of the employed numerical models is evaluated. It is found that (i) higher modes of vibration had participated significantly in the recorded seismic response and (ii) upstream–downstream response is well represented by the two numerical models employed. Using the simplified model, the September 19, 1985 earthquake non-linear response is computed and shown to compare satisfactorily with its recorded counterpart in the upstream-downstream and vertical directions. The largest computed dynamic accelerations, stresses and strains are found to occur within the upper third of the dam body.     

Numerical simulation of reservoir sediment and effects on hydro‐dynamic response of arch dams

Based on the dynamic theory for saturated porous media by Biot (Journal of the Acoustical Society of America 1956; 28 : 168–178), a numerical model is presented to analyse the reflection behaviours of reservoir sediment and compared with those from the visco‐elastic model. It is concluded that the two models give very similar results of reflection coefficient α within the frequency range of interest. Then, using the two models, the change of the reflection coefficients α with various sedimentation parameters and excitation frequencies are studied in detail. The results are further used in the analysis of response functions of hydro‐dynamic pressures on, and structural displacements of the Xiang Hong Dian arch dam, for which some results from a field vibration test are available. It appears that effects of water compressibility with sediment reflection on hydro‐dynamic pressures and structural response are not significant for this specific case. Copyright © 2001 John Wiley & Sons, Ltd.     

Experimental study of seismic overloading of large arch dam

A dynamic overloading model test has been carried out on a shaking table for an arch dam of 278 m in height to investigate its behaviours under strong earthquake. The model system included the arch dam with contraction joints, part of reservoir, partial foundation rock with topographic feature near the dam. A damping boundary consisting of viscous liquid has been used to simulate the effect of dynamic energy emission to far field, which made the dynamic interaction between dam and foundation in model arch dam system be represented properly. Three sets of different seismic waves of design level have been used as the input to investigate the difference in the responses of arch dam. Artificial waves of different levels have been used to verify the behaviours of arch dam under seismic overloading. Since the opening of joints during strong earthquake reduced the response acceleration and tensile arch stress, cantilever stress on downstream face exceeded the tensile strength first for the model dam. And the arch dam responded in a non‐linear way when input seismic load increased. Some cracks appeared near abutments, and the damage made the natural frequency of arch dam to drop obviously, but the static function did not seem to change for the model tested. Copyright © 2005 John Wiley & Sons, Ltd.     

Vibration tests on emosson arch dam,Switzerland

Measurements have been made of vibrations induced in the 180 m high Emosson arch dam in Switzerland³ by a system of mechanical eccentric-mass vibrators and by the natural wind. Details are given of the dam itself, the tests and the experimental results obtained. Finite element calculations made before the tests, which include reservoir and foundation effects, are compared with the measurements.     

中承式钢筋混凝土拱桥自振特性分析

利用有限元通用软件ANSYS,对一中承式钢筋混凝土拱桥进行了结构自振特性的数值模拟分析,求出了自振频率、振型等动力参数,对其模态特征进行了描述,并通过变换拱桥的主要结构参数对比分析了它们对结构自振特性的影响.结果表明:自振频率随矢跨比的减小略有提高;横撑有助于增强拱肋的横向刚度,减小拱肋面外振动,提高抗风稳定性;各振型的自振频率值随拱肋刚度的提高近似成线性增大,可以通过改变拱肋刚度来调整其自振特性.研究结果可以为同类桥梁的抗震设计提供参考.