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- Academic researcher in Earthquake Engineering, Engineering Seismology and Engineering Mechanics.Topics: Structural He... moreAcademic researcher in Earthquake Engineering, Engineering Seismology and Engineering Mechanics.Topics: Structural Health Monitoring, Wave Propagation in Structures and Soils, Soil/Structure Interction, Probabilistic Seismic Hazard Assessment, Strong Ground Motionedit
We present a simple nonlinear model of a shear-beam building that experiences large nonlinear deformations and collapse when excited by large pulses of strong earthquake ground motion. In this paper, we introduce the model and show that... more
We present a simple nonlinear model of a shear-beam building that experiences large nonlinear deformations and collapse when excited by large pulses of strong earthquake ground motion. In this paper, we introduce the model and show that its properties can be selected to be consistent with the damage observed in a seven-story hotel in San Fernando Valley of the Los Angeles metropolitan area during the 1994 Northridge earthquake. We also show an example of excitation that leads to the collapse of the model. We illustrate the response only for a sequence of horizontal pulses. We will describe the response of the same model to horizontal, vertical, and rocking motions at its base, as well as for more general excitation by strong earthquake ground motion, in future papers.
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Research Interests: Geology and Ground Motion
This report explores the advantages of expansion in orthonormal wavelet bases -- as a preprocessing tool -- in analysis of large sets of seismic vibration monitoring data, of ground or structural response, for possible application in data... more
This report explores the advantages of expansion in orthonormal wavelet bases -- as a preprocessing tool -- in analysis of large sets of seismic vibration monitoring data, of ground or structural response, for possible application in data mining. The focus is on the insight that can be gained from the wavelet domain representation, convenience in estimation of energy and correlation, estimation of derivatives, efficiency of representation (data compression property), and dimensionality reduction. Local and global aggregates and distributions of energy and related quantities (e.g. power, power spectrum density, Fourier amplitude, cross-energy, crosspower, cross power spectrum density, etc.), computed directly in the wavelet domain, are introduced and interpreted as information granules, representative of a frequency interval, of a partition of the phase plane, or of the entire record. The concepts explored are illustrated on a mini database of strong motion records from the 1994 Nort...
Variability of the fixed-base and soil–structure system
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Wave travel times of vertically propagating waves are measured in the Van Nuys 7-storey hotel, located in the Los Angeles metropolitan area, from impulse response functions computed by deconvolution of the recorded earthquake response.... more
Wave travel times of vertically propagating waves are measured in the Van Nuys 7-storey hotel, located in the Los Angeles metropolitan area, from impulse response functions computed by deconvolution of the recorded earthquake response. East-West response data from 11 earthquakes over a period of 24 years are analysed. Changes in wave travel times are used to infer about local (between sensors) and global changes of structural stiffness, from one event to another, and with time during the earthquakes that damaged the building (San Fernando and Northridge). Also, wave travel times are used to estimate the fundamental fixed-base frequency of the building, f1; which is compared with independent estimates of the soil–structure system frequency fsys during the same earthquakes and during five ambient vibration tests. The analysis shows that monitoring only the changes of fsys can be misleading for structural health monitoring and can lead to erroneous alarms, while monitoring changes of f...
The former Imperial County Services Building⎯a 6-story RC building severely damaged by the 1979 Imperial Valley earthquake, is a rare example of a well instrumented building that has been damaged by an earthquake. Analysis is presented of... more
The former Imperial County Services Building⎯a 6-story RC building severely damaged by the 1979 Imperial Valley earthquake, is a rare example of a well instrumented building that has been damaged by an earthquake. Analysis is presented of damage detection by a method based on detection of abrupt changes in the response using wavelet basis expansion. The changes in system frequency, determined from the ridge of the Gabor transform, are also analyzed. It is shown that most of the detected abrupt changes are consistent with the degree and spatial distribution of damage. Some smaller abrupt changes can be explained as high frequency pulses of the input motion that have propagated through the building. However, there are also prominent abrupt changes (usually in the roof records) that cannot be explained by the reported damage, or by the input motion.
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Environmental effects on measured structural frequencies— model prediction of short-term shift during heavy rainfall and comparison with full-scale observations
This paper introduces a new and unique full‐scale testbed site for structural health monitoring and soil–structure system identification studies being developed in southwest China. The site is a 48‐story skyscraper with an extended... more
This paper introduces a new and unique full‐scale testbed site for structural health monitoring and soil–structure system identification studies being developed in southwest China. The site is a 48‐story skyscraper with an extended four‐level basement, supported by piles, located in Kunming, the capital of Yunnan Province. Located in the diffused zone of collision of the Indian and Eurasian tectonic plates, Yunnan is one of the most active seismic areas in mainland China. The final sensor deployment will consist of 43 triaxial accelerometers (129 channels) and one weather station. The accelerometer array comprises (a) a structural array of 25 accelerometers installed at 10 levels aboveground, (b) a basement array of 14 accelerometers distributed in the first and fourth basements, and (c) two borehole arrays installed close to the basement perimeter wall, each with one accelerometers at the surface and another one at 50‐m depth, which is the depth reached by the piles. With such dense instrumentation of structure, basement, and pile foundation, this site will be the first permanently instrumented full‐scale testbed to enable identification of a soil‐foundation–basement‐structure system and validation of many assumptions commonly made in the prediction of the soil–structure interaction effects. A high‐performance wired local area network has been installed in the building, featuring a Precision Time Protocol‐enabled time synchronization and real‐time remote access over the Internet. The site will be fully operational in late spring of 2020. Results of preliminary system identification of the structure from ambient vibration test data are presented.
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Earthquake response records and ambient and forced vibration tests data have been used for structural system identification and health monitor of full-scale structures. These three types of excitation differ in amplitude and nature.... more
Earthquake response records and ambient and forced vibration tests data have been used for structural system identification and health monitor of full-scale structures. These three types of excitation differ in amplitude and nature. Rarely there have been opportunities to compare directly their effectiveness for health monitoring. The experiment of a full-scale slice of a 7-story reinforced concrete (RC) building carried out on the UCSD-NEES shake table presents such an opportunity. Data is available on NEES website (Panagiotou et al. 2013 on https://nees.org/). The specimen is a slice of a RC building, consisting of two perpendicular shear walls. It was shaken by four earthquakes of increasing intensity which progressively damaged the structure. Ambient vibration and white noise excitation tests of two intensities were carried out before and after each earthquake test.We compare the changes in two damage sensitive parameters, the fundamental frequency of vibration (f1) and the comp...
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This collection of eleven papers has been brought out to recognize the scientific contributions of Professor Mihailo D. Trifunac, fondly called as Misha by his friends and colleagues, on the occasion of his 60th birthday. Professor... more
This collection of eleven papers has been brought out to recognize the scientific contributions of Professor Mihailo D. Trifunac, fondly called as Misha by his friends and colleagues, on the occasion of his 60th birthday. Professor Trifunac is singularly credited for emphasizing the relevance of strong motion seismology in earthquake engineering, thus enabling it to become more realistic in ground motion characterization. He is also credited for his vision in emphasizing several issues in earthquake engineering, which were seriously considered by the professional community much later. The papers in this collection are the review papers on those topics that have formed the core of his vast research contributions to earthquake engineering and engineering seismology during a period of about 35 years. These papers have been written by his past students (I.D. Gupta, Ljupco Jordanovski, Vinay Gupta, Tzong-Ying Hao), and by his colleagues and friends (Francisco Sánchez-Sesma, Ivanka Paskal...
Foundation-soil and building-foundation-soil interaction has been studied using a two-dimensional analytical model, including both the dynamic and the kinematic interaction effects. The building has been represented by an infinitely long... more
Foundation-soil and building-foundation-soil interaction has been studied using a two-dimensional analytical model, including both the dynamic and the kinematic interaction effects. The building has been represented by an infinitely long shear wall resting on a rigid circular foundation, embedded into an elastic homogeneous half-space. Deep and shallow foundations have been considered (with depth-to-half-width ratio of 1 and 0.5) and excitation consisting of plane P- and SV- and surface Rayleigh waves. The wave function expansion method has been used to represent the motion in the half-space and a substructure approach to solve the interaction problem. Special attention has been paid to the wave passage effects, and to the influence of the depth of the embedment.
One dimensional wave propagation is studied in a cantilever conical structure deforming in shear. The structure has rectangular cross‐section both dimensions of which decrease linearly along the polar axis (hence, doubly tapered). The... more
One dimensional wave propagation is studied in a cantilever conical structure deforming in shear. The structure has rectangular cross‐section both dimensions of which decrease linearly along the polar axis (hence, doubly tapered). The equation motion is derived and solved in terms of spherical Bessel functions, which can be expressed exactly as finite algebraic expressions in terms of powers and sine and cosine functions of the polar coordinate. The transfer‐matrix for a truncated pyramid element is then derived and generalized to a chain of elements. The model is used to represent a 48‐story pyramid‐shaped steel‐frame skyscraper, the Transamerica Tower in San Francisco, California, in which the Loma Prieta, 1989 earthquake ( Mw=6.9${M}_{\mathrm{w}}=6.9$ , epicentral distance, R≈90$R \approx 90$ km) was recorded by an array of accelerometers. The building is modeled by homogeneous and layered truncated pyramids. Wave propagation through the building is studied by analysis of impulse response functions computed from the observed earthquake accelerations. In addition, its equivalent homogeneous beam shear‐wave velocity is identified solely from its geometry and observed fundamental frequency of vibration. Further, the variation of wave velocity along is height is identified by least squares fit of a layered pyramid model in the observed impulse response functions. The results reveal equivalent homogeneous pyramid wave velocity of about 150–160 m/s in both directions, which is similar to other steel‐frame structures. The identified wave velocity profiles are consistent with the structural design. The identified parameters can be used as reference in future structural health monitoring of this structure.
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Nonparametric techniques for estimation of wave dispersion in buildings by seismic interferometry are applied to a simple model of a soil-structure interaction (SSI) system with coupled horizontal and rocking response. The system consists... more
Nonparametric techniques for estimation of wave dispersion in buildings by seismic interferometry are applied to a simple model of a soil-structure interaction (SSI) system with coupled horizontal and rocking response. The system consists of a viscously damped shearbeam, representing a building, on a rigid foundation embedded in a half-space. The analysis shows that (1) wave propagation through the system is dispersive. The dispersion is characterized by lower phase velocity (softening) in the band containing the fundamental system mode of vibration, and little change in the higher frequency bands, relative to the building shear wave velocity. This mirrors its well known effect on the frequencies of vibration, i.e. reduction for the fundamental mode (softening) and no significant change for the higher modes of vibration, in agreement with the duality of the wave and vibrational nature of structural response. Nevertheless, the phase velocity identified from broader band IRFs is very ...
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AbstractAn algorithm of the wave method for structural health monitoring (SHM) is tested and calibrated using shake table experiment data of a full-scale, seven-story, reinforced-concrete building ...
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Abstract Coherency has been used to describe the similarity of strong earthquake ground motion at nearby sites, and for seismic response analyses of structures that are spatially extended, on multiple supports and on large foundations.... more
Abstract Coherency has been used to describe the similarity of strong earthquake ground motion at nearby sites, and for seismic response analyses of structures that are spatially extended, on multiple supports and on large foundations. For that purpose, functions describing its decay with increasing separation distance and frequency are used that have been derived from recorded data by a small number of dense arrays worldwide. Although it has been long recognized that the coherency depends on the site conditions, as demonstrated from comparison of coherencies recorded by arrays with different geology, the recorded data has not been sufficient to study systematically the effects of the site conditions. In this paper, we study the effects of the site conditions using synthetic earthquake ground motion at the ground surface, generated by the SYNACC method. Nine sites are considered, each characterized by a combination of the local soil condition parameter, s L , and the geologic site parameter, s , and by a corresponding set of parallel layers. Three variations of s L were considered representing: rock soil, stiff soil, and deep soil, and three variations of s , representing sediments, geological basement rock and intermediate geology. The results show that, for small separation distances (100 m in this case), the incoherence of the ground motion depends on the properties of the layers of soil, sediments and underlying geology. It is shown that the coherency decreases with decreasing stiffness of the soil and rock layers near the ground surface, and with progressively deeper soil deposits. The additional incoherence caused by irregular layer geometry, surface topography and from three-dimensional inhomogeneities beneath the site is not considered in this study.
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Abstract It is shown that the synthetic earthquake displacements over a large area (100×100 m 2 ) on ground surface can be described well by a plane surface undergoing translations and rotations, with only minor departures of... more
Abstract It is shown that the synthetic earthquake displacements over a large area (100×100 m 2 ) on ground surface can be described well by a plane surface undergoing translations and rotations, with only minor departures of displacements from those of a plane flat surface. Rotational motions are largest on deep soil sites over sediments and smallest for motions on the geological basement rocks.
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... research activities: (1) software development for routine and specialized processing of analogue and digital strong motion accelerograms, (2) routine processing of large accelerogram data sets ... “Routine computer processing of... more
... research activities: (1) software development for routine and specialized processing of analogue and digital strong motion accelerograms, (2) routine processing of large accelerogram data sets ... “Routine computer processing of strong motion accelerograms,” Report EERL 73 ...
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We present examples of a controlled numerical experiment that contribute towards understanding of the physical phenomena that lead to the reduction of coherency of strong earthquake ground motion. We show examples for separation distance... more
We present examples of a controlled numerical experiment that contribute towards understanding of the physical phenomena that lead to the reduction of coherency of strong earthquake ground motion. We show examples for separation distance of 100 m between the two points on the ground surface, which is in the range of engineering interest. Our examples illustrate the consequences of: (a) standing waves that result from interference of the incident and reflected waves from a near vertical contrast in material properties, (b) standing waves within a concave inhomogeneity (a semi-circular valley in our examples), and (c) smaller motions in the diffraction zone, behind the inhomogeneity. We show that it is possible to reduce coherency, to the extent observed for recorded strong earthquake ground motion, even by a single inclusion in a half space, for incident ground motion that is coherent. We also illustrate the combined effects of geometric spreading and finite fault width, superimposed on the otherwise dominating effects caused by interference. Our examples show reduction of coherence for specific angles of incident waves, while, for other angles of incidence, the coherence remains essentially equal to one.
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The variations of identified wave velocities of vertically propagating waves through the structure are investigated for a 54-story steel-frame building in downtown Los Angeles, California, over a period of 19 years since construction... more
The variations of identified wave velocities of vertically propagating waves through the structure are investigated for a 54-story steel-frame building in downtown Los Angeles, California, over a period of 19 years since construction (1992–2010), using records of six earthquakes. The set includes all significant earthquakes that shook this building, which produced maximum transient drift ~0.3% and caused no reported damage. Wave velocity profiles β( z) are identified for the NS, EW, and torsional responses by fitting layered shear beam/torsional shaft models in the recorded responses, by waveform inversion of pulses in impulse response functions. The results suggest variations larger than the estimation error, with a coefficient of variation about 2–4.4%. About 10% permanent reduction of the building stiffness is detected, caused mainly by the Landers and Big Bear earthquake sequence of 28 June 1992, and the Northridge earthquake of 17 January 1994. Permanent changes of comparable m...
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A summary is presented of analyses of variations of the system frequency of 21 instrumented buildings in the Los Angeles area, which recorded the Northridge earthquake (MS = 6.7) of January 17, 1994, and some of its aftershocks. Some of... more
A summary is presented of analyses of variations of the system frequency of 21 instrumented buildings in the Los Angeles area, which recorded the Northridge earthquake (MS = 6.7) of January 17, 1994, and some of its aftershocks. Some of these buildings also recorded other
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The motion of a point is specified completely by its six components: three translations and three rotations. Traditionally, only the translational components of the earthquake ground shaking and structural response have been recorded. In... more
The motion of a point is specified completely by its six components: three translations and three rotations. Traditionally, only the translational components of the earthquake ground shaking and structural response have been recorded. In part, the absence of direct observations of rotational motions resulted from lack of inexpensive rotational sensors with sufficient sensitivity to measure small rotations caused by earthquakes. Recently, however, rotations from teleseismic and small local earthquakes were successfully recorded (ring laser gyros, fiber optic gyros,
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A signal processing method for structural health monitoring is applied to detect damage in the former Imperial County Services (ICS) Building, caused by the 1979 Imperial Valley earthquake in southern California. The building response was... more
A signal processing method for structural health monitoring is applied to detect damage in the former Imperial County Services (ICS) Building, caused by the 1979 Imperial Valley earthquake in southern California. The building response was recorded by a 13-chanel array of accelerometers, and a description of the distribution of the damage throughout the structure is available. The method is based on detecting abrupt changes in the seismic vibration response by analysis of the finest detail coefficients of a wavelet basis expansion of the recorded response. This method has been previously proven to work for numerically simulated response of simple models with postulated damage, but not for real earthquake data. The analysis in this paper critically examines the capabilities of this method to detect damage in real data. The analysis shows that most of the detected prominent abrupt changes are consistent with the spatial distribution and severity of the reported damage. Other less promi...
The ability to monitor the health of an instrumented structure, detect damage as it occurs, and issue an early warning during or soon after the earthquake (or some other natural or man made disaster), and before physical inspection is... more
The ability to monitor the health of an instrumented structure, detect damage as it occurs, and issue an early warning during or soon after the earthquake (or some other natural or man made disaster), and before physical inspection is possible, has significant potential benefits in reducing loss of life and injuries, in emergency response, and in recovery following the disaster. The timeliness of such information, even when the damage is obvious or there is no structural damage, is very useful to a building owner, of an important business or a critical facility. To be practically useful, the structural health monitoring systems must be robust when applied to real data, reliable, and sufficiently sensitive and accurate. This paper reviews briefly the current methods, trends and outstanding issues in practical implementation of such systems, with emphasis on a new method based on detecting changes in wave travel times using impulse response functions. This method can be viewed as an i...
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Most seismometers and strong motion transducers deployed in seismic monitoring networks are (or are equivalent to) penduli, and are therefore sensitive to rotation of the support, besides the component of translation that is to be... more
Most seismometers and strong motion transducers deployed in seismic monitoring networks are (or are equivalent to) penduli, and are therefore sensitive to rotation of the support, besides the component of translation that is to be recorded. Consequently, pendulum transducers record a mixture of translations and rotations, which cannot be separated unless rotations are also recorded at the same point. Traditionally, the rotational components of earthquake ground motion and of the response of structures have not being recorded by seismic arrays. Direct measurements of rotation during earthquakes are rare but do exist, and there is a growing interest in the seismological and earthquake engineering communities to deploy more rotational sensors in the field, and record all six degrees-of-freedom (6 DOF) of seismic motion at a point. The separation of “pure translations” from the recorded mixture of motions using data from 6 DOF sensors also requires a special instrument correction algori...
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ABSTRACT The SYNACC method for synthesizing earthquake ground motion time histories is reviewed, with emphasis on the recent extension to generating motions on an array of points in space. This method is a combination of empirical and... more
ABSTRACT The SYNACC method for synthesizing earthquake ground motion time histories is reviewed, with emphasis on the recent extension to generating motions on an array of points in space. This method is a combination of empirical and physical model-based approach, and involves unfolding in time a site-specific Fourier amplitude spectrum of ground acceleration, obtained for a scenario earthquake by an empirical scaling model, or for an ensemble of earthquakes by probabilistic seismic hazard analysis. The unfolding consists of representing the ground motion as a superposition of traveling surface Love and Rayleigh waves and of body P and S waves, which propagate with phase and group velocities consistent with the dispersion characteristic of the site geology, approximated by parallel layers. The synthesized motions are site specific and consistent statistically with observations within the recording range of typical accelerographs (0.02-25 Hz). The output consists of synthetic accelerations, velocities and displacements, and also of point strains, rotations and curvatures, at a point or at an array of points. Such time histories are useful for the design of spatially extended structures, like pipelines, bridges and tunnels.
This paper presents a brief review of the SYNACC method for synthesizing earthquake ground motion time histories on an array of points in space. SYNACC is a combination of an empirical and physical model-based method, and involves... more
This paper presents a brief review of the SYNACC method for synthesizing earthquake ground motion time histories on an array of points in space. SYNACC is a combination of an empirical and physical model-based method, and involves unfolding in time a site-specific Fourier amplitude spectrum of ground acceleration obtained for a scenario earthquake by an empirical scaling model. The unfolding consists of representing the ground motion as a superposition of traveling surface Love and Rayleigh waves and of body P and S waves, which propagate with phase and group velocities consistent with the dispersion characteristic of the site geology, approximated by parallel layers. Because the coefficients of expansion are scaled so that the Fourier spectrum of the synthetic motion matches a site specific empirical spectrum, the synthetic motions are consistent statistically with observations within the recording range of typical accelerographs (0.02-25 Hz). A Uniform Hazard Fourier spectrum or a...
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ABSTRACT Nonparametric techniques for estimation of wave dispersion in buildings by seismic interferometry are applied to a simple model of a soil–structure interaction (SSI) system with coupled horizontal and rocking response. The system... more
ABSTRACT Nonparametric techniques for estimation of wave dispersion in buildings by seismic interferometry are applied to a simple model of a soil–structure interaction (SSI) system with coupled horizontal and rocking response. The system consists of a viscously damped shear beam, representing a building, on a rigid foundation embedded in a half-space. The analysis shows that (i) wave propagation through the system is dispersive. The dispersion is characterized by lower phase velocity (softening) in the band containing the fundamental system mode of vibration, and little change in the higher frequency bands, relative to the building shear wave velocity. This mirrors its well-known effect on the frequencies of vibration, i.e. reduction for the fundamental mode and no significant change for the higher modes of vibration, in agreement with the duality of the wave and vibrational nature of structural response. Nevertheless, the phase velocity identified from broader band impulse response functions is very close to the superstructure shear wave velocity, as found by an earlier study of the same model. The analysis reveals that (ii) the reason for this apparent paradox is that the latter estimates are biased towards the higher values, representative of the higher frequencies in the band, where the response is less affected by SSI. It is also discussed that (iii) bending flexibility and soil flexibility produce similar effects on the phase velocities and frequencies of vibration of a building.
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ABSTRACT It is shown that the observed loss of coherency of synthetic strong motion on ground surface for separation distances less than about 100 m can be described in terms of the dispersion of strong motion waves. Additional... more
ABSTRACT It is shown that the observed loss of coherency of synthetic strong motion on ground surface for separation distances less than about 100 m can be described in terms of the dispersion of strong motion waves. Additional contributions to the loss of coherency from variations of material properties in the soil and from geometrical departures from perfectly flat ground surface and irregular layer geometries are not considered in this paper. It is also shown that the synthetic surface displacements over a large rectangular area (100×100 m) on ground surface can be described well by a flat surface undergoing translations and rotations only, and with only minor departures from the plane flat surface.