Pacoima Dam Research Articles

A framework for seismic response analysis of dams using numerical source‐to‐structure simulation

AbstractThe determination of spatially‐varying broadband ground motions is a prerequisite for the seismic analysis of dams. However, generating realistic ground motions at a specific dam site remains a challenge. Source‐to‐structure simulation, which considers source, propagation path, and site, is a promising way to reasonably synthesize site‐specific ground motions. This paper proposes a practical framework for the seismic analysis of dams based on a deterministic numerical source‐to‐structure simulation. The site‐specific broadband ground motions are generated by a physics‐based 3D numerical simulation using the spectral element method (SEM) in a coarse mesh, while the nonlinear dynamic response of the dam is simulated by the finite element method (FEM) in a fine mesh. The proposed framework bridges the gap between wavefield simulation and seismic analysis of dams. The seismic response analysis of Pacoima dam during the 1994 Northridge earthquake was conducted as a case study, making it the first demonstration of a complete source‐to‐structure simulation for realistic concrete dams using the purely numerical method. Results show that the calculated seismic response agrees with the observation, thus indicating the utility of the proposed framework for source‐to‐dam simulation.

A new solution to estimate the time delay on the topographic site using time domain 3D boundary element method

This study focuses on investigating spatial variation of ground motion that has great influence on the dynamic behavior of the large structures located on the surface topography. One of the most effective parameters on the spatial variation of ground motion is the difference between the arrival time of seismic waves in different points located on the abutments. In this research, a three-dimensional model of the Pacoima Dam site is prepared. The time domain 3D boundary element method is used to apply non-uniform excitation at the dam supports. This model is subjected to vertically propagating incident SH and P waves. The time delay can be characterized by calculating the value of the time delay for which the cross-correlation between two records is maximized. Finally, to obtain the time delay in a topographic site, a function considering effective parameters such as the height from the canyon base, wave velocity and predominant frequency, is presented. Furthermore, a code was developed for generating the spatially variation of seismic ground motions. The results show that the proposed functions have an acceptable accuracy in estimating the time delay to generate non-uniform ground motion.

A procedure to predict the precise seismic response of arch dams in time domain using boundary element formulation

In this context, a new boundary element algorithm based on the time-convoluted traction kernels is employed to evaluate the spatially varying earthquake ground motions of the Pacoima dam in the USA subjected to SH, SV and P incident waves. An accurate three-dimensional (3D) model of the dam canyon is implemented into the computer code BEMSA to investigate the seismic response of the dam. The analyses are performed in time domain with a linearly elastic constitutive model for the medium. This modeling procedure has been validated by the results reported in the existing literature. According to the results of this study, the response of the dam to earthquake waves is generally influenced by predominant frequency of the incident motion, surface topography, relative distance of observation points, and type of the incident seismic wave. For the cases considered, the incident SV wave has led to the maximum amplification of incident motions, especially at the left side of the dam. The results indicate that the proposed procedure can be employed for accurate prediction of a dam response during an earthquake.

Nonlinear Seismic Response Analysis of High Arch Dams to Spatially-Varying Ground motions

The failure of a large dam can be catastrophic to human life and property downstream. Therefore, the seismic safety is of particular concern for high dams in seismically active regions. This paper addresses the nonlinear seismic response analysis of high arch dams due to spatially-varying ground motions. Firstly, a comprehensive analysis model developed at Tsinghua University is presented, which takes into account radiation damping effect of semi-unbounded canyon, dynamic interaction of dam-water, opening of contraction joints, seismic damage cracking and strengthening of dam concrete, and nonlinearity of foundation rock. Subsequently, the seismic damage of Pacoima dam during the 1994 Northridge earthquake is qualitatively analyzed by the developed analysis model. The results agree with the actual damage observed after the earthquake. Most of the contraction joints opened and closed during the earthquake, and a larger residual opening occurred at the thrust block joint after the earthquake. The cracks continue from the bottom of the thrust block joint in three directions: diagonal, horizontal, and vertical. Finally, a large-scale numerical simulation of seismic ground motion from source rupture to dam canyon is introduced, which can simulate the characteristics of near-field ground motions at dam sites by considering the effect of source mechanism, propagation media, and local site.

Modal identification of arch dams using balanced stochastic subspace identification

Dynamic characteristics extracted from ambient and forced vibration tests are always associated with some level of uncertainties because of unknown nature of applied forces, existence of ambient noises as well as measurement errors. Stochastic Subspace Methods are among the most accurate and consistent methods within the domain of operational modal analysis. In this research, a new technique for Operational Modal Analysis is proposed using Stochastic Realization Theory and Canonical Correlation Analysis, which in comparison with previous methodologies, identification process are directly performed in the prediction space by extracting orthonormal vectors of data space. Considering its optimized nature, the proposed method is expected to have superior accuracy in terms of elimination of unstable poles as well as low time consumption analysis. To indicate the efficiency and accuracy of the proposed algorithm, it is applied to reanalyze the results of the forced vibration tests performed on the Shahid-Rajaee arch dam in northern Iran. These tests were conducted via steady-state sinusoidal stimulation method. More accurate natural frequencies are obtained compared to those of previously reported results, besides the fact that the first three modes of the structure were identified by the new approach, while they were not observed via the previous one. In order to examine the capabilities of the proposed method for processing of ambient vibration records, the dynamic characteristics of the Pacoima dam was identified using the recorded responses during 2001 earthquake in San Fernando, California. The results indicated good accuracy in the obtained frequencies and damping ratios compared to those obtained via data driven subspace method. Time consumption of identification process were reduced significantly (up to 50%) for both case studies indicating a faster convergence rate provided by the proposed method.

Extended FDD-WT method based on correcting the errors due to non-synchronous sensing of sensors

In this research, a combinational non-parametric method called frequency domain decomposition-wavelet transform (FDD-WT) that was recently presented by the authors, is extended for correction of the errors resulting from asynchronous sensing of sensors, in order to extend the application of the algorithm for different kinds of structures, especially for huge structures. Therefore, the analysis process is based on time-frequency domain decomposition and is performed with emphasis on correcting time delays between sensors.Time delay estimation (TDE) methods are investigated for their efficiency and accuracy for noisy environmental records and the Phase Transform – β (PHAT-β) technique was selected as an appropriate method to modify the operation of traditional FDD-WT in order to achieve the exact results. In this paper, a theoretical example (3DOF system) has been provided in order to indicate the non-synchronous sensing effects of the sensors on the modal parameters; moreover, the Pacoima dam subjected to 13 Jan 2001 earthquake excitation was selected as a case study. The modal parameters of the dam obtained from the extended FDD-WT method were compared with the output of the classical signal processing method, which is referred to as 4-Spectral method, as well as other literatures relating to the dynamic characteristics of Pacoima dam. The results comparison indicates that values are correct and reliable.

Response of an arch dam to non-uniform excitation generated by a seismic wave scattering model

Non-uniform ground motions are generated based on a single record available at a site and seismic wave scattering analysis. The Chino Hills 2008 earthquake records at the Pacoima Dam site are used to indicate the accuracy of the method. Dynamic analysis of the Pacoima dam-reservoir-foundation under uniform and non-uniform ground motions is carried out using the EACD-3D2008 software, and the results are compared to recorded responses at different locations on the dam. There is good agreement between computed and recorded displacements of the dam for non-uniform excitation. For uniform excitation, the displacements are underestimated in comparison with those obtained from recorded excitation. Significant intensification of stresses, especially near the foundation, and different patterns of stress distribution are observed for non-uniform excitation in comparison with uniform excitation. For uniform excitation maximum stresses occur in the crown cantilever near the crest, but for non-uniform excitation the maximum stresses occur along the sides and near the foundation.

Earthquake damage analysis of arch dams considering dam–water–foundation interaction

A comprehensive model is integrated in this paper to analyze the nonlinear earthquake response of arch dams. The following factors are taken into account: the semi-unbounded size of foundation rock and compressible water, the opening of contraction joints, the cracking of the dam body, and the spatial variation of ground motions. As a case study, the earthquake response of the Pacoima dam to the 1994 Northridge earthquake is investigated using the proposed model. The earthquake damage is simulated based on the actual conditions during the earthquake to verify the developed analysis model. The joint opening and concrete cracking are qualitatively reproduced, wherein the ground motion excitation is spatially defined based on the acceleration records at the dam–rock interface. Several influencing factors, such as the earthquake input mechanism, shape of the thrust block, and water level, are further considered to analyze their contribution to the actual damage. The result shows that the variation of ground motions along the dam–foundation interface may be the most important factor for the damage mode of the Pacoima dam.

Seismic responses of arch dams due to non-uniform ground motions

In the present paper, spatially variation input effects on seismic responses of arch dams have been studied. Recorded ground accelerations at the dam foundation interface of Pacoima dam during January 13, 2001 were used for the purpose of this investigation. A numerical finite element model was developed for dynamic analysis of the dam reservoir system. The modified version of NSAD-DRI finite element program was used for the analysis and the ground acceleration time histories were interpolated at all dam foundation interface nodal points. Total and pseudo static displacements as well as developed stresses due to uniform and non uniform excitations are obtained. The results reveal that multiple support excitations can have profound effects on the dam responses.

Comparison of uniform and spatially varying ground motion effects on the stochastic response of fluid-structure interaction systems

The effects of the uniform and spatially varying ground motions on the stochastic response of fluid-structure interaction system during an earthquake are investigated by using the displacement based fluid finite elements in this paper. For this purpose, variable-number-nodes two-dimensional fluid finite elements based on the Lagrangian approach is programmed in FORTRAN language and incorporated into a general-purpose computer program SVEM, which is used for stochastic dynamic analysis of solid systems under spatially varying earthquake ground motion. The spatially varying earthquake ground motion model includes wave-passage, incoherence and site-response effects. The effect of the wave-passage is considered by using various wave velocities. The incoherence effect is examined by considering the Harichandran-Vanmarcke and Luco-Wong coherency models. Homogeneous medium and firm soil types are selected for considering the site-response effect where the foundation supports are constructed. A concrete gravity dam is selected for numerical example. The S16E component recorded at Pacoima dam during the San Fernando Earthquake in 1971 is used as a ground motion. Three different analysis cases are considered for spatially varying ground motion. Displacements, stresses and hydrodynamic pressures occurring on the upstream face of the dam are calculated for each case and compare with those of uniform ground motion. It is concluded that spatially varying earthquake ground motions have important effects on the stochastic response of fluid-structure interaction systems.

System identification of a concrete arch dam and calibration of its finite element model

AbstractA magnitude 4.3 earthquake occurred near Pacoima Dam on 13 January 2001. An accelerometer array that had been upgraded after the Northridge earthquake recorded the motion with 17 channels on the dam and the dam–foundation interface. Using this data, properties of the first two modes are found from a system identification study. Modal properties are also determined from a forced vibration experiment performed in 2002 and indicate a significantly stiffer system than is estimated from the 2001 earthquake records. The 2001 earthquake, although small, must have induced temporary nonlinearity. This has implications for structural health monitoring. The source of the nonlinear behaviour is believed to be loss of stiffness in the foundation rock. A finite element model of Pacoima Dam is constructed and calibrated to match modal properties determined from the system identification study. A dynamic simulation of the 2001 earthquake response produces computed motions that agree fairly well with the recorded ones. Copyright © 2006 John Wiley & Sons, Ltd.

Influence of base-rock characteristics on the stochastic dynamic response of dam–reservoir–foundation systems

The effect of base-rock characteristics on the stochastic dynamic response of dam–reservoir–foundation systems subjected to different earthquake input mechanisms is investigated by using the Lagrangian approach in this paper. The effects of compressible wave propagation and surface sloshing motion are included in a finite element formulation of the fluid elements. A concrete gravity dam is chosen as a numerical example. The Pacoima Dam record of the San Fernando earthquake in 1971 is selected as a ground motion. Three different earthquake input mechanisms are used to consider the effect of base-rock characteristics in the analyses: the standard rigid-base input, the massless-foundation input and the deconvolved-base-rock input models. The means of the maximum values of the displacements, stresses and hydrodynamic pressures obtained from the three input model are compared with each other.

Comparison of stochastic and deterministic dynamic responses of gravity dam–reservoir systems using fluid finite elements

In this paper, stochastic and deterministic dynamic responses of gravity dam–reservoir systems are compared using the displacement fluid finite elements. For stochastic dynamic analysis of gravity dam–reservoir systems, variable-number-nodes two-dimensional fluid finite elements based on the Lagrangian approach is programmed in FORTRAN language and incorporated into a general-purpose computer program, which is used for stochastic dynamic analysis of solid systems, STOCAL. The Sarıyar concrete gravity dam, constructed in Turkey is selected for numerical example. The S16E component recorded at Pacoima dam during the San Fernando Earthquake in 1971 is used as a ground motion. Displacements and hydrodynamic pressures occurring on the upstream face of the dam and stresses occurring on the dam section are calculated by using stochastic and deterministic methods and compared with each other.

A method for coupled arch dam-foundation-reservoir seismic behaviour analysis

This paper presents a method for coupled arch dam-foundation-reservoir seismic behaviour analysis. The dam is discretized by finite elements (FE) and the foundation and reservoir are discretized by boundary elements (BE). The opening of contraction joints and the spatial variability of the seismic action is taken into account. The study of Pacoima dam by this method is also presented. The computed results show that no cracks were to be expected due to the vibrations induced during the Feb. 9, 1971 San Fernando earthquake.

Stochastic response to earthquake forces of a cable-stayed bridge

Stochastic methods of dynamic response are applied to the earthquake analysis of cable-stayed bridges. Certain types of bridge model with fan configuration and A-type towers are used as an example. The input ground acceleration used is that of the Pacoima dam record of the 1971 San Fernando earthquake. It has been shown that the stochastic method gives practically acceptable abstract information such as mean-of-maxima values for vertical displacements at the deck and cumulative probability functions for vertical displacements at mid-main span of cable-stayed bridges. Mean-of-maxima values for bending moments at the bridge deck have also been presented. All the results obtained by the stochastic method have been compared with absolute maximum values by the complete quadratic combination method and time-history analysis. Since the calculation of the spectral density of filtered white noise is relatively simple, the particular form proposed by Kanai for representing earthquake ground acceleration has also been used in this paper for comparison with the actual earthquake record. It gives satisfactory results for cable-stayed bridges.

Foam rubber modeling of topographic and dam interaction effects at Pacoima Dam : Anooshehpoor, A; Brune, J N Bull Seismol Soc Am V79, N5, Oct 1989, P1347–1360

Foam rubber modeling of topographic and dam interaction effects at Pacoima Dam : Anooshehpoor, A; Brune, J N Bull Seismol Soc Am V79, N5, Oct 1989, P1347–1360

Foam rubber modeling of topographic and dam interaction effects at Pacoima dam

A study of the topographic and dam interaction effects was made using a 3- D foam rubber model of the actual topography around the Pacoima Dam accelerograph which recorded over 1 g high-frequency horizontal ground accelerations during the 1971 San Fernando earthquake. Scaling of frequency from the model to the earth depends on the average value of shear-wave velocity in the upper few hundred meters. Assuming βe = 2 km/sec, for vertically incident SH waves, the spectral ratio of the ground acceleration on the ridge to the free field (flat surface) indicates an amplification of about 60 per cent around 6.5 Hz on the N76°W component. Topography has little effect upon the motion recorded on the S14°W component. Motion on the ridge is lower than the free-field motion on both horizontal components for frequencies above 9 Hz. Amplification peaks shift to higher or lower frequencies depending on the assumed shear-wave velocity in the upper few hundred meters. Results from nonvertically incident SH waves show that the topographic effect is dependent on the direction of approach of the seismic energy. The effect is either de-amplification (in part by shadowing) or amplification (relative to the case where no topography is present), depending on whether the canyon is on the ray path or not. The Fourier spectrum of the ground motion at the dam crest shows peak frequencies at about 5 Hz and 10 Hz (resonance), which correspond to the normal modes of the dam. A study of dynamic interaction between the Pacoima Dam and the ridge shows that the coupling is less than 2 per cent at about 10 Hz and less than 12 percent at about 5 Hz.

Nonlinear Seismic Analysis of Arch Dams

A nonlinear finite element procedure for arch dams is described in which the gradual opening and closing of vertical contraction joints and predetermined horizontal cracking planes are considered. A special joint element approximately represents the deformations due to plane sections not remaining plane at each open joint and allows a single shell element discretization in the thickness direction to be used for the dam. Compressive and sliding nonlinearities are not included. Finite element treatments are also used for the water, assumed incompressible, and for the foundation rock, assumed massless, with all degrees of freedom (dof) off the dam condensed out. For efficiency in the computations, the condensed water and foundation matrices are localized in a way which maintains good accuracy. The response of Pacoima Dam to the 1971 San Fernando ground motion recorded on a ridge over one abutment and scaled by two.‐thirds is computed first for water at the intermediate level that existed during the 1971 earthquake and then for full reservoir. In the first analysis, the dam exhibits pronounced opening and separation of the contraction joints, allowing violation of the no‐slip assumption. The presence of a full reservoir greatly increases the dam response, enough to bring some of the other assumptions of the analysis into question. Reducing the ground motion scale to 0.44 with full reservoir drops the response back to a reasonable level, but the contraction joint separations remain.

The faulting mechanism of the San Fernando Earthquake

The large displacement pulse occurring at Pacoima dam approximately 2.5 s after the triggering of the accelerogram is identified as the shear radiation emanating from the point of initial rupture, this interpretation suggesting that the San Fernando, California, earthquake was initiated with massive but localized rupture in the hypocentral region. On the basis of the resulting S-P time at Pacoima dam and teleseismic observations of the reflected phase pP, together with the estimated uncertainties in the local hypocentral determination of Allen et al. (1973), the point of initial rupture is located at 34°27.0′N, 118°24.0′W and h = 13 km. The breakout phases resulting from the rupture of the earth's surface are tentatively identified on the Pacoima dam accelerograms and teleseismic World-Wide Standard Seismograph Network seismograms. For the San Fernando earthquake the breakout phases do not appear to be particularly energetic, but this may only reflect the especially energetic nature of the initial rupture phases. The inferred length of faulting and the estimated origin times of the initial rupture and the breakout phases yield an average rupture velocity, of at least 2.8 km/s. Although they are subject to the uncertainties of the suggested effects of source propagation and of single-station observations, the source parameters of the initial rupture re estimated as follows: source dimension, 6–3 km; stress drop, 350–1400 bars; average slip, 4.6–9.2 m; and seismic moment, 1.7–0.85 × 1026 dynes cm. A 0.8-s time interval after the arrival of S1 contributes 40% of the radiated energy flux at Pacoima dam; the initial rupture event may have generated as much as 80% of the total energy radiated by the San Fernando earthquake. The stress difference accompanying the initial rupture is comparable to an estimate of the north-south compressive stress that is assumed to maintain the continued uplift of the San Gabriel Mountains; the stress drop obtained for the entire faulting process, however, is smaller than either of these by 1–2 orders of magnitude. That the San Fernando earthquake was apparently initiated with massive but localized failure not representative statically or dynamically of the full faulting process most likely reflects a highly nonuniform distribution of strain energy density in the incipient source region. This possibility adds a new dimension to laboratory studies, numerical models, and conventional seismic investigations of the earthquake mechanism.

Analysis of the Pacoima dam accelerogram—San Fernando, California, earthquake of 1971

Integrated ground velocities and displacements calculated from the accelerogram recorded at the Pacoima dam site indicate that the strong ground motion was predominately in the vertical and NS direction, in general agreement with the mechanism of faulting as inferred from aftershock studies, and with fault displacements observed in the field. High-frequency peak accelerations of 1.25 g were recorded in two horizontal directions, these being the highest ground accelerations so far recorded for earthquakes. Response spectrum curves calculated from the accelerograms do not show unusual features, and the numerical values are consistent with past experience. The high-frequency, high-amplitude impulsive ground motion associated with the highest peak accelerations did not contribute significantly to the over-all response spectrum values. The presence of the high-frequency motions in the recorded accelerograms is presumably the consequence of the proximity of the recording site to the fault dislocation.