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1.
Two-dimensional analytical elastic models are developed for evaluating dynamic characteristics, namely natural frequencies and modes of vibration of a wide class of earth dams in a direction parallel to the dam axis. In these models the non-homogeneity of the dam materials is taken into account by assuming a specific variation of the stiffness properties along the depth (due to the continuous increase in confining pressure). In addition, both shear and normal (axial) deformations are considered. Cases having constant elastic moduli, linear and trapezoidal variations of elastic moduli, and elastic moduli increasing as the one-half, one-third, two fifths, and a general (l/m)th powers of the depth are studied. Dynamic properties of three real earth dams in a seismically active area (Southern California) estimated from their earthquake records (input ground motion and crest response in the longitudinal direction) as well as results from full-scale dynamic tests on one of these dams (including ambient and forced vibration tests) are compared with those from the suggested models. It was found that the models in which the shear modulus and the modulus of elasticity of the dam material vary along the depth are the most appropriate representations for predicting the dynamic characteristics. The agreement between the experimental and earthquake data and the theoretical results from some of the models is reasonably good.  相似文献   

2.
The aim of this study is to show the effect of geological factors in predicting the level of blast-induced ground vibrations. The site-specific character of ground must be involved in the prediction models especially if the ground conditions have a variable character like in this case. But in a blasting environment, this is only possible by using an empirical way. Towards this aim, an in-situ experimental study in a highly jointed sandstone quarry was carried out to incorporate the variable conditions into the prediction models. Therefore, 60 shots were organized and their ground vibrations monitored in two different directions to compare the results. These shots were normal production shots involving the true technological properties as well as geological properties into the prediction model. Based on these, the empirical relations between particle velocity, the amount of explosive and distance have been developed. The results show that the performances of these estimations depend on the site-specific character of these empirical relations. The best prediction was obtained with only 2.08% error level provided that the true technological and geological properties are involved.  相似文献   

3.
The classical continuum model for the linear vertical vibrations of a suspension bridge (Bleich et al., 1950 [1]) is re-examined. The primary objective of the study is to extend the definitive analytical and numerical results of Irvine and Caughey (1974) [2], Irvine and Griffin (1976) [3] and Irvine (1980, 1981) [4], [5] for the natural frequencies, mode shapes, and modal participation factors for an extensible suspension cable, which depend on one dimensionless parameter related to the elasticity of the cable, to the case of a stiffened suspension bridge in which the response depends also on a second dimensionless parameter related to the stiffness of the girder. The continuum suspension bridge model is also used to understand the pattern of variation of mode shapes as a function of cable elasticity and girder stiffness, which has been shown by West et al. (1984) [6] to be considerably more complex than that for a suspension cable. Finally, the threshold amplitudes of free vibrations that would result in the incipient slackening of the hangers are determined.  相似文献   

4.
The strong ground motion observation network began to operate in the Taipei Basin area as part of the Taiwan Strong Motion Instrumentation Program (TSMIP). It is operated by the Seismological Observation Centre of the Central Weather Bureau. Forty-three free-field accelerometers have been installed in the Taipei Basin, and several earthquakes have been recorded by this network since its installation. Three events with magnitudes of 5–7, 5–4 and 6–2 that triggered more than ten stations are used here to study the characteristics of the basin effects on ground motions both in the time and frequency domains. From this study, it is clear that vibrations of different frequencies have different amplifications in the Taipei Basin. In fact, the frequency responses show the variation can even be up to about 7 times. The horizontal peak ground acceleration (PGA) and spectral ratio contours in the low-frequency band are closely correlated with the geological structure in the Taipei Basin. The analysis of the response spectra also shows this correlation phenomenon and the relation between the damage from an earthquake and the basin effects.  相似文献   

5.
The physical bases and empirical equations for modelling the duration of strong earthquake ground motion in terms of the earthquake magnitude, the epicentral distance and the geological and local soil site conditions are investigated. At 12 narrow frequency bands, the duration of a function of motion f(t), where f(t) is acceleration, velocity or displacement, is defined as the sum of time intervals during which the integral S f2 (τ) dτ gains a significant portion of its final value. All the records are band-pass filtered through 12 narrow filters and the duration of strong ground motion is studied separately in these frequency bands. It is shown that the duration of strong motion can be modelled as a sum of the source duration, the prolongation due to propagation effects and the prolongation due to the presence of the sediments and local soils. It is shown how the influence of the magnitude on the duration of strong ground motion becomes progressively stronger, in going from low to moderate frequencies, and that the duration is longer for ‘soft’ than for ‘hard’ propagation paths, at low and at moderate frequencies. At high frequencies, the nature of the broadening of the strong motion portion of the record with increasing distance is different, and is most likely related to the diffraction and scattering of the short waves by the velocity inhomogeneities along the wave path. It is also shown that the geological and local soil conditions should both be included in the model. The duration can be prolonged by 3–5 sec at a site on a deep sedimentary layer at frequencies near 0.5 Hz, and by as much as 5–6 sec by the presence of soft soil underneath the station, at a frequency of about 1 Hz. An empirical equation for a probabilistic estimate of the discrepancies of the predictions by our models relative to the observed data (distribution function of the residuals) is presented.  相似文献   

6.
The continuous connection method of analysis is extended to deal with the free vibrations of a coupled shear wall structure. The natural modes and frequencies are determined from the Galerkin technique, and the dynamic response following an imposed lateral displacement is evaluated. A comparison is made between theoretical predictions of natural frequencies and the results from tests on model structures.  相似文献   

7.
The seismic response of the Mexico City Cathedral built of very soft soil deposits is evaluated by using motions recorded in various parts of the structure during several moderate earthquakes. This unique set of records provides significant insight into the seismic response of this and other similar historic stone masonry structures. Free‐field ground motions are carefully compared in time and frequency domains with motions recorded at building basement. The dynamic characteristics of the structure are inferred from the earthquake records by using system identification techniques. Variation of seismic response for different seismic intensities is discussed. It is shown that, due to the soil–structure interaction, due to large differences between dominant frequencies of earthquake ground motions at the site and modal frequencies of vibration of the structure, and due to a particularly high viscous damping, seismic amplifications of ground motion in this and similar historic buildings erected on soft soil deposits are much smaller than that induced in most modern constructions. Nevertheless, earthquake records and analytical results show that several components of the structure such as its central dome and the bell towers may be subjected to local vibrations that significantly amplify ground motions. Overall, results indicate that in its present state the structure has an acceptable level of seismic safety. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

8.
This paper is concerned with the free vibrations of a restricted class of multi-storey shear buildings in which inertial coupling exists between the torsional and the two sway vibrations. The restrictions imposed are that (a) the shear centres of all storeys lie on a vertical straight line, (b) the principal axes of shear are in the same directions in all storeys, (c) the centres of mass of all floors lie on another vertical straight line, (d) the radius of gyration about the shear centre of every floor mass is the same and (e) the ratios of the two shear stiffnesses to the torsional stiffness do not vary from storey to storey. In consequence of the last restriction it is proved that the 3n natural frequencies, normal modes and generalized masses, where n is the number of storeys, are expressible very simply in terms of products of the three natural frequencies, normal modes and generalized masses of the single-storey, three-dimensional building formed by removing everything above the first floor, with the n natural frequencies, normal modes and generalized masses of a certain n-storey, two-dimensional shear frame. In the special case of a uniform building, a simple closed form solution, valid for any number of storeys, is given.  相似文献   

9.
We present some numerical methods which simplify the analysis of equipment-structure systems composed of an equipment component which is light with respect to the structure component. We consider completely general forms of equipment-structure interaction. In particular, we consider that an arbitrary number of natural frequencies of the equipment and the structure are nearly equal (or equal) to each other. We show that this coincidence of natural frequencies implies that the equipment-structure system will itself have several closely spaced natural frequencies. The essence of equipment-structure interaction is the formation of beats, with slowly varying amplitude in each degree of freedom, between these natural frequencies-at least in response to a δ-function ground motion. We derive some differential equations, the solutions of which are a slowly varying envelope function (a vector) which yields the above slowly varying amplitudes. It is obtained by numerical integration using some conventional, but modified, methods and using a step size appropriate to capture its slowly varying nature. This envelope function defines the response to a δ-function ground motion. We show how it may be combined with the properties of an arbitrary ground motion (acceleration record or response spectrum) to yield the response to such a ground motion.  相似文献   

10.
Seismic characterization and monitoring of Fucino Basin (Central Italy)   总被引：1，自引：1，他引：0  
The Fucino basin (Central Italy) is one of the largest intramountain alluvial plain in the Apennines range. It has a tectonic origin related to the presence of important systems of faults located in its northern and eastern edges. Some of these faults are still active and capable of generating strong seismic events. Site effects related to the soft soils filling the basin can be very important. In this paper we show the preliminary results of a seismic network installed in the Fucino area in order to collect information about site amplification effects and geometry of the basin. We analyze ambient seismic vibrations and recordings of about 150 local earthquakes mainly related to the seismic sequence of the April 6th 2009 Mw 6.3 L’Aquila event. Moreover the strongest events of L’Aquila sequence were analyzed at the three permanent strong-motion stations operating in the area. Using standard spectral techniques we investigate the variation of resonance frequencies within the basin. The ground motion recorded in the Fucino plain is mainly characterized by strong energy at low-frequencies (f < 1 Hz) affecting both horizontal and vertical components. This is particularly evident for stations deployed in correspondence of very thick deposits of sedimentary filling, where a significant increase of ground-motion amplitude and duration is likely caused by locally generated surface waves. The amplification at low-frequencies (<1 Hz) on the horizontal components can reach up a factor of 10 in comparison to nearby stiff sites. However, we found evidences of seismic amplification phenomena also for stiff sites surrounding the basin, including stations of the Italian strong motion network. The independent geological information and the shallow shear-velocity profiles available for the basin can be combined with resonance frequencies for deriving representative geological sections to be used as base for future numerical 2D–3D modeling of the basin.  相似文献   

11.
Although railway-generated ground vibrations usually have greater energy levels at lower frequencies, vibrations in the audible range above 20 Hz can nevertheless be relevant for secondary noise problems in buildings. One countermeasure is soil stabilization under the track embankment. While effective at low frequencies, a potential side effect is amplification in some audible bands. Presented here are both experimental and theoretical assessments of the countermeasure in the audible bands. The main innovation is the treatment of an infinite periodic track–ground system, using a transfer matrix approach with a repeating element including the rail, pad, sleeper, and an underlying half-space (ballast and soil). Excitation in this band is attributed to rail and wheel roughness. The model makes successful predictions when the half-space properties are allowed to be frequency-dependent such that the dispersion of the surface wave matches that in the actual layered earth (including ballast and underlying soil layers). The field measurements are also unique in that both before and after evaluation of the countermeasure was possible.  相似文献   

12.
The transmission and reduction of vibrations in the far-field of the surface of the ground due to a surface load is investigated theoretically and validated with given field measurement data. The performance of a given stabilization column, located directly underneath the load, at a number of receiver positions is studied and measured in terms of insertion loss. A numerical model is presented, which enables the wave-field in the region of the column to be determined, based on an integral equation formulation of the problem which is solved using a boundary element approach. It is shown that the column has a beneficial effect at low frequencies especially in certain frequency bandwidths and is validated with field data. However, when the Rayleigh wavelength becomes short compared with the depth and width of the column adverse effects occur at some frequencies which are also observed in the far-field. Various depths of columns and material properties of the surrounding soil medium are studied and results presented so that some preliminary physical conclusions may be derived.  相似文献   

13.
High-frequency filtering of strong-motion records   总被引：5，自引：3，他引：2  
The influence of noise in strong-motion records is most problematic at low and high frequencies where the signal to noise ratio is commonly low compared to that in the mid-spectrum. The impact of low-frequency noise (<1 Hz) on strong-motion intensity parameters such as ground velocities, displacements and response spectral ordinates can be dramatic and consequentially it has become standard practice to low-cut (high-pass) filter strong-motion data with corner frequencies often chosen based on the shape of Fourier amplitude spectra and the signal-to-noise ratio. It has been shown that response spectral ordinates should not be used beyond some fraction of the corner period (reciprocal of the corner frequency) of the low-cut filter. This article examines the effect of high-frequency noise (>5 Hz) on computed pseudo-absolute response spectral accelerations (PSAs). In contrast to the case of low-frequency noise our analysis shows that filtering to remove high-frequency noise is only necessary in certain situations and that PSAs can often be used up to 100 Hz even if much lower high-cut corner frequencies are required to remove the noise. This apparent contradiction can be explained by the fact that PSAs are often controlled by ground accelerations associated with much lower frequencies than the natural frequency of the oscillator because path and site attenuation (often modelled by Q and κ, respectively) have removed the highest frequencies. We demonstrate that if high-cut filters are to be used, then their corner frequencies should be selected on an individual basis, as has been done in a few recent studies.  相似文献   

14.
In order to effectively control vibration related problems, the development of a reliable vibration monitoring system and the proper assessment of attenuation characteristics of various vibrations are essential. Various ground vibrations caused by train loading, blasting, friction pile driving and hydraulic hammer compaction were measured using 3D geophones inside of the borehole as well as on the ground surface, and the propagation and attenuation characteristics of various source generated vibrations were investigated by analyzing particle motions. For the geometric modeling of various vibrations, the types of various sources and their induced waves were characterized and the geometric damping coefficients were determined. The measured attenuation data matched well with the predicted data when using the suggested geometric damping coefficient, and the estimated soil damping ratios were quite reasonable taking soil type of the site and experiencing strain level into consideration.  相似文献   

15.
The continuous connection technique is used to analyse the free vibrations of a system of coupled shear walls. The dynamic equation is expressed in integro-differential form, and the natural modes and frequencies are determined by the Galerkin method. Theoretical results are compared with published experimental data.  相似文献   

16.
Modelling and modal analysis of seismic vibrator baseplate   总被引：1，自引：0，他引：1  
The vibroseis method must be extended to its limits as the search for oil and gas continues on land. To successfully improve vibroseis data quality, it is crucial to evaluate each element in the vibroseis data acquisition system and ensure that the contribution from each element is successful. Vibroseis systems depend greatly upon the ability of vibrators to generate synchronous, repeatable ground-force sweeps over a broad frequency range. This requires that the reaction mass and the baseplate of the vibrator move as rigid bodies. However, rigid-body motion is not completely true for high- frequency vibrations, especially for the vibrator baseplate. In order to accurately understand the motion of the vibrator baseplate, a finite element analysis model of the vibrator baseplate and the coupled ground has been developed. This model is useful for simulating the vibrator baseplate dynamics, evaluating the impact of the baseplate on the coupled ground and vibrator baseplate design. Model data demonstrate that the vibrator baseplate and its stilt structure are subject to six significant resonant frequencies in the range of 10–80 Hz. Due to the low rigidity of the baseplate, the baseplate stilt structure experiences severe rocking motions at lower frequencies and the baseplate pad experiences severe flexing motions at higher frequencies. Flexing motions cause partial decoupling, which gives rise to increased levels of harmonic distortion and less useable signal energy. In general, the baseplate pad suffers more bending and flexing motions at high frequencies than low frequencies, leading less efficiency in transmitting the useable energy into the ground.  相似文献   

17.
Permanent displacements of a gas turbine founded on a fine, poorly graded, and medium density sand are studied. The amplitudes and modes of vibration are computed using Barkan´s formulation, and the “High-Cycle Accumulation” (HCA) model is employed to account for accumulated deformations due to the high number of cycles. The methodology is simple: it can be easily incorporated into standard mathematical software, and HCA model parameters can be estimated based on granulometry and index properties. Special attention is devoted to ‘transient’ situations at equipment´s start-up, during which a range of frequencies – including frequencies that could be similar to the natural frequencies of the ground – is traversed. Results show that such transient situations could be more restrictive than stationary situations corresponding to normal operation. Therefore, checking the stationary situation only might not be enough, and studying the influence of transient situations on computed permanent displacements is needed to produce a proper foundation design.  相似文献   

18.
The results of a numerical study on the influence of a number of structural design parameters on the fundamental frequency of reinforced-soil retaining wall models are presented and discussed. The design parameters in the study include the wall height, backfill width, reinforcement stiffness, reinforcement length, backfill friction angle and toe restraint condition. The intensity of ground motion, characterized by peak ground acceleration, is also included in the study as an additional parameter. The study shows that the fundamental frequency of reinforced-soil wall models with sufficiently wide backfill subjected to moderately strong vibrations can be estimated with reasonable accuracy from a few available formulae based on linear elastic wave theory using the shear wave speed in the backfill and the wall height. Numerical analyses showed no significant influence of the reinforcement stiffness, reinforcement length or toe restraint condition on the fundamental frequency of wall models. The strength of the granular backfill, characterized by its friction angle, also did not show any observable effect on the fundamental frequency of the reinforced-soil retaining wall. However, the resonance frequencies of wall models were dependent on the ground motion intensity and to a lesser extent, on the width to height ratio of the backfill.  相似文献   

19.

In nonlinear dynamic structural analysis, a suite of pulse-like ground motions is required for the performance-based design of structures near active faults. The dissimilarity in the amplitude and frequency content of the earthquake time series referred to nonstationary properties in temporal and spectral, respectively. An approach is proposed based on the nonstationary properties of the far-field records and the seismological information in an event for simulating pulse-like records. The pulse-like earthquake time history is estimated via the superposition of the residual part of the earthquake with the estimated pulse. The wavelet-based Hilbert transform is utilized to characterize the nonstationary properties, the instantaneous amplitude, and frequencies of far-field records to model residual part. The effects of near-fault and pulse are estimated based on the seismological properties of the region. The validation of the procedure is indicated by comparing simulated time-series, response spectra, and Arias intensity with recorded pulse-like records in two different earthquakes in California; the Mw 6.7 1994 Northridge and the Mw 6.5 1979 Imperial valley.

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20.
This paper presents a comparison between measured train-induced ground vibrations in the free-field before and after countermeasures had been taken at Kåhög near Gothenburg in Sweden. A wave barrier of lime–cement columns was constructed parallel to the railway in order to reduce the ground-borne vibrations inside nearby buildings. On top of the barrier an embankment was built to reduce air-borne vibrations. Due to the wave barrier design, part of the energy content of the waves was expected to be reflected by the screen and transmitted energy was expected to be partly scattered. Contribution from the noise-embankment was not thought likely but could not be ruled out due to its fairly large mass and its close proximity to the railway. The effect of the mitigating measures resulted in a 67% reduction of the maximum particle velocity at 30 m and 41% at 60 m from the railway. A simple two-dimensional finite element model has been used to study the relative importance of the wave barrier and the noise-embankment as contributors to the mitigation recorded of the ground vibrations in the field. It is concluded with respect to ground vibrations that both the barrier and the embankment had a mitigating effect but that the contribution from the barrier dominated. Furthermore, it is seen from the field results as well as the simulation that the effect of the mitigating action is reduced with increasing distance from the railway.  相似文献   

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