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1.
Near-fault records of the 1971 San Fernando earthquake contain severe, long duration acceleration pulses which result in unusually large ground velocity increments. A review of these records along with the results of available theoretical studies of near-fault ground motions indicates that such acceleration pulses may be characteristic of near-fault sites in general. The results of an analytical study of a building severely damaged during the San Fernando earthquake indicate that such severe, long duration acceleration pulses were the cause of the main features of the observed structural damage. The implications of such pulses on current aseismic design methods, particularly those used to establish design earthquakes, are examined for buildings located near potential earthquake faults. Analytical studies of the non-linear dynamic response of single and multiple degree-of-freedom systems to several near-fault records, as well as to a more standard accelerogram, indicate that at near-fault sites: (a) very large displacement ductilities may result for current levels of code design forces; (b) smoothed elastic design response spectra should reflect the larger ground velocities that may occur; and (c) peak inelastic response cannot reliably be inferred from elastic response predictions. 相似文献
2.
Due to the damage suffered by many skewed highway bridges during the San Fernando earthquake, 1971, a study is made on the dynamic response of this type of bridge. Using a beam model capable of flexural and torsional deformations, the mode shapes and frequency equations are presented. Due to the skewness of the intermediate supports, coupled flexural and torsional vibrations are shown to be excited by the vertical component of ground motion An analysis is carried out for the Foothill Boulevard Undercrossing, S.E. Bridge in the San Fernando Valley, California which suffered damage to the intermediate supports. The results of the analysis correlate well with the observed damage pattern of the bridge. 相似文献
3.
The objectives of this paper are to present a comparison of the dynamic characteristics of a seven-storey reinforced concrete building (Van Nuys–Holiday Inn) identified from four recorded strong-motion response data (Whittier earthquake, Landers earthquake, Big Bear earthquake and Northridge earthquake). In the analysis, time-domain methods for estimating the system parameters and the modal properties of the building are studied. Both off-line and on-line identification algorithms are applied to these seismic response data. Under the assumption of a linear time-invariant system the ARX model and ARMAX model are used. Comparison of the identification results using different models are made. In addition, recursive procedures are adapted as on-line identification and the time-varying modal parameters are estimated. For structural systems under strong earthquake excitation, a recursive identification method, adaptive forgetting through multiple models (AFMM), is introduced to identify systems with rapidly changing parameters. Through the analysis of the seismic response data of the building subjected to four earthquakes the identification algorithm and the identification results are discussed. 相似文献
4.
The problem of determining linear models of structures from seismic response data is investigated using ideas from the theory of system identification. The approach is to determine the optimal estimates of the model parameters by minimizing a selected measure-of-fit between the responses of the structure and the model. Because earthquake records are normally available from only a small number of locations in a structure, and because of noise in the records, it is necessary in practice to estimate parameters of the dominant modes in the records, rather than the stiffness and damping matrices of the linear model. A new algorithm is developed to determine the optimal estimates of the modal parameters. After tests with simulated data, the method is applied to a multi-storey building using records from the 1971 San Fernando earthquake in California. New information is obtained concerning the properties of the lower modes of the building and the time-varying character of the equivalent linear parameters. 相似文献
5.
This paper addresses the issue of system identification for linear structural systems using earthquake induced time histories of the structural response. The proposed methodology is based on the Eigensystem Realization Algorithm (ERA) and on the Observer/Kalman filter IDentification (OKID) approach to perform identification of structural systems using general input–output data via Markov parameters. The efficiency of the proposed technique is shown by numerical examples for the case of eight-storey building finite element models subjected to earthquake excitation and by the analysis of the data from the dynamic response of the Vincent-Thomas cable suspension bridge (Long Beach, CA) recorded during the Whittier and the Northridge earthquakes. The effects of noise in the measurements and of inadequate instrumentation are investigated. It is shown that the identified models show excellent agreement with the real systems in predicting the structural response time histories when subjected to earthquake-induced ground motion. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
6.
Empirical scaling equations are presented which relate the average number of water pipe breaks per km2,
, with the peak strain in the soil or intensity of shaking at the site. These equations are based on contour maps of peak surface strain evaluated from strong motion recordings, and observations of intensity of ground shaking and damage following the Northridge, California, earthquake of 17 January 1994. Histograms for the number of pipe breaks per km2, n, are presented for several ranges of values of the horizontal peak strain and for several values of the site intensity. The observed distribution of pipe breaks is also used to speculate on possible more detailed geographical distribution of near surface strains in the San Fernando Valley and in central Los Angeles. The results can be used to predict number of pipe breaks in the San Fernando Valley and in Los Angeles, for a scenario earthquake or in probabilistic seismic hazard calculations, considering all possible scenarios that contribute to the hazard and the likelihood of their occurrence during specified exposure. Such predictions will be useful for emergency response planning (as the functioning of the water supply is critical for sanitation and in fighting fires caused by earthquakes), to estimate strains during future and past earthquakes in areas where no strong motion was recorded and in defining design guidelines for pipelines and other structures and structural systems sensitive to strains in the ground. 相似文献
7.
Thomas C. Hanks 《地震工程与结构动力学》1976,4(5):473-488
While the accurate estimation of ground-motion amplitudes across the entire frequency band of engineering interest is not possible at the present time, the excitation and propagation of long-period strong-ground motion can be understood with existing seismological methodology. In the Los Angeles Basin, the long-period strong ground motion excited by the San Fernando earthquake is dominated by the presence of surface waves, whose gross amplitude and frequency content are easily attributable to physical properties of the earthquake source and source-station propagation paths. Observed measures of the long-period strong ground motion of the Kern County earthquake relative to the San Fernando earthquake at two sites in the Los Angeles Basin which recorded both shocks can be predicted with considerable accuracy by a simple earthquake source model. This source model is extrapolated to represent the maximum credible earthquake likely to affect the Los Angeles area, taken to be a repeat of the Fort Tejon (1857) earthquake along the San Andreas fault. The measures of long-period strong ground motion in the Los Angeles Basin estimated for it agree well with the comparable measures of Earthquake A-2, intended to represent the same situation. For the purpose of aseismic design of long-period structures, Earthquake A-2 is a reasonable, if not all inclusive, estimate of the long-period strong ground motion in the Los Angeles Basin generated by a magnitude 8+ earthquake along the San Andreas fault north and east of Los Angeles. 相似文献
8.
Seepage driving effect on deformations of San Fernando dams 总被引：2，自引：0，他引：2
In the process of flow deformation of an earth dam, the seepage force inside the dam plays a role as a driving force. The seepage force acts just like the gravitational force in terms of pushing soils away from their original locations after liquefaction is triggered. This paper draws attention to this seepage driving effect by presenting a set of fully coupled finite element analyses on the well-known San Fernando dams, with the objective of evaluating the impact of this seepage effect. The results indicate that while this effect is always there, its practical significance depends on a number of factors. In the case of the upper San Fernando dam, which experienced a significant, but restricted, downstream movement during the 1971 earthquake, the seepage driving effect was indeed significant. On the contrary, for the lower dam, which failed and slid into the upstream reservoir during the same earthquake, this seepage effect was relatively less pronounced. The detailed results of the analyses reveal the likely mechanisms of failure and deformation of the two dams and the likely cause behind the difference between their responses during the earthquake. 相似文献
9.
In this paper, long- and short-period vibrations in sedimentary basins are studied. First, two-dimensional, long-period vibrations of deep semi-circular basins for excitation by earthquake faults, which can be inside or outside the basin, are analyzed. Second, recurring intermediate peak frequencies of Fourier-spectrum amplitudes of recorded accelerations along the east–west axis of the San Fernando Valley during the 1994 Northridge, California earthquake are reviewed. It is shown that these intermediate frequencies cannot be associated with vibrations of the entire San Fernando basin because the frequency range of typical strong-motion recordings (0.04 to 15.0 s) is too narrow to include the long-period vibration of the whole basin. These intermediate vibrations are consistent with Kanai׳s one-dimensional models consisting of parallel layers and excited by vertically incident shear waves. 相似文献
10.
Power spectral density which describes frequency content is considered one of the most significant properties to be taken into account when generating ground motions through the use of stochastic processes. Using a smoothed and normalized Fourier amplitude spectrum, frequency content for components of motion along a set of principal axes is estimated. Fourier amplitude spectra obtained by this moving-window technique are presented which show the time dependency of frequency content for motions produced by the San Fernando earthquake of 9 February 1971. A mathematical model to simulate ground motion processes is proposed for which both the intensity and frequency content are non-stationary. Using this mathematical model with parameter characteristics along principal axes similar to those of the motions recorded during the San Fernando earthquake, three-dimensional ground motions are synthetically generated. The properties of the simulated motions show general characteristics similar to the characteristics observed in real accelerograms. The suggested model is considered adequate for engineering purposes. 相似文献
11.
J. H. Wood 《地震工程与结构动力学》1976,4(4):349-377
A study has been made of the response, during the San Fernando earthquake 9 February 1971, of the nine-storey steel frame Building 180, located at the California Institute of Technology, Jet Propulsion Laboratory, Pasadena. The analysis throws light on the actual dynamical properties of the building during the earthquake, and also demonstrates that it is possible, when the ground motion is specified, to make accurate predictions of building motions during moderate earthquakes by using a linear viscously damped model. Methods of evaluating the lower mode periods and damping ratios from the earthquake records are described and the values obtained are compared with results from dynamic testing before and after the earthquake and with the periods computed from computer models of the building. Although no structural damage occurred and computed stresses in the steel frame were less than yield stresses, the periods measured by an ambient vibration test after the earthquake were of the order of 10 per cent higher than the pre-earthquake values. The maximum periods during the earthquake were found to be about 30 per cent higher than the post-earthquake periods. 相似文献
12.
Kiyoshi Muto 《地震工程与结构动力学》1972,1(1):33-43
The KII Building was subjected to the San Fernando earthquake on 9 February 1971 without suffering any damage. During the earthquake, strong motion accelerations were recorded at the base, middle and top of the building. The simulation analysis of the building was carried out taking into account the interaction between the building and the soil. The internal viscous damping theory considering the different damping coefficients for the building and the soil was adopted. Comparison of the calculated and observed acceleration time histories as well as the floor response spectra showed satisfactory agreement. On the basis of the response spectrum values, the fact that no damage was observed in this building is justified theoretically. 相似文献
13.
C. B. Crouse 《地震工程与结构动力学》1976,4(4):333-347
Accelerograms, Fourier amplitude spectra and response spectra from ground motion recorded in the San Fernando earthquake at selected sites in Los Angeles were examined and compared. Although differences exist in the data from the bases of a group of six tall buildings close together near the intersection of Wilshire Boulevard and Normandie Avenue, the differences are not significant from the standpoint of design. A single design spectrum is most appropriate for this area, even though there exists a variety of foundation conditions, building properties and basement configurations for the six sites. The degree to which soil-structure interaction and the local geology affected the response could not be precisely determined from the basement accelerograms, although the data from the six Wilshire-Normandie sites offered some evidence that soil-structure interaction and local site conditions did not contribute significantly to the character of the recorded motions. There was some correlation between the degree of similarity in the response at two sites and their distance apart. Thus, it is possible that some differences in the response of the closely spaced buildings could result from different superposition of the seismic waves. The shapes of response spectra from all of the sites studied, including the spectra from seven buildings 2–3 miles distant from the buildings near the intersection of Wilshire Boulevard and Normandie Avenue, indicate that the character of the ground motion was probably more dependent on the source mechanism and the travel paths of the earthquake waves, rather than on more local effects. 相似文献
14.
Mihailo D. Trifunac Maria I. Todorovska 《Soil Dynamics and Earthquake Engineering》2004,24(3):225-239
This paper compares the distribution of damage from the San Fernando, 1971, and Northridge, 1994, earthquakes. Both events had similar size, occurred on blind thrust faults beneath the densely populated San Fernando Valley of the Los Angeles metropolitan area, and hence offer a rare opportunity to compare the effects of the two earthquakes. In a previous study of the distribution of red-tagged (‘unsafe’) buildings and of breaks in the water distribution system caused by the Northridge earthquake, the authors discovered that buildings were damaged less where the soil response was not linear (as indicated by the breaks in the water pipes), except in localized areas of very severe shaking (peak ground velocity exceeding 150 cm/s). The study in this paper shows that the same applies to the damage caused by the San Fernando earthquake, and that the areas with severely damaged buildings (so called ‘gray zones’) for both earthquakes overlapped. This reoccurrence of damage within the same area is interpreted to result from some specific properties of local soil and geology. These properties are not fully understood at present, but should be explored to provide a basis for a new tool for forecasting microzonation maps, and reducing future seismic hazard. 相似文献
15.
Three studies of site amplification factors, based on the recorded aftershocks, and one study based on strong motion data, are compared one with another and with the observed distribution of damage from the Northridge, CA, earthquake of 17 January 1994 (ML=6.4). In the epicentral area, when the peak ground velocities are larger than vm≈15 cm/s, nonlinear response of soil begins to distort the amplification factors determined from small amplitude (linear) wave motion. Moving into the area of near-field and strong ground motion (vm>30 cm/s), the site response becomes progressively more affected by the nonlinear soil response. Based on the published results, it is concluded that site amplification factors determined from small amplitude waves (aftershocks, small earthquakes, coda waves) and their transfer-function representation may be useful for small and distant earthquake motions, where soils and structures respond to earthquake waves in a linear manner. However in San Fernando Valley, during the Northridge earthquake, the observed distribution of damage did not correlate with site amplification determined from spectra of recorded weak motions. Mapping geographical distribution of site amplification using other than very strong motion data, therefore appears to be of little use for seismic hazard analyses. 相似文献
16.
The model presented in the companion paper is validated in both the linear and nonlinear cases under steady-state single frequency harmonic and transient ground motions. The crest acceleration responses of the Santa Felicia earth dam subjected to the 1971 San Fernando earthquake and of the Long Valley earth dam subjected to the strongest of the 1980 Mammoth Lake earthquakes are computed and compared with the motions recorded at the site. Acceleration time histories for the solid and fluid phases in both horizontal and vertical directions, as well as stress-strain and pore water pressure-strain time histories for points along the height of the dam are presented. The ability of the model to simulate the occurrence of liquefaction in a dam is also demonstrated. 相似文献
17.
An orthogonal set of principal axes is defined for earthquake ground motions. These principal axes are obtained such that the corresponding variances of motion have maximum, minimum and intermediate values and the covariances equal zero. This indicates that the corresponding components of motion along the principal axes are uncorrelated with respect to each other. Since real earthquake accelerograms are assumed to be reasonably well represented by Gaussian random processes, the three components of motion along the principal axes are statistically independent of each other. Using these principal axes and applying the moving-window technique to the ground accelerograms recorded during the San Fernando earthquake of 9 February 1971, time-dependent characteristics of three-dimensional ground motions along principal axes are determined. Results of the analysis indicate significant correlation between directions of principal axes and directions from the recording stations to the fault slip zone. It is concluded that three components of ground motion can be generated stochastically with statistical independence being maintained, provided they are assumed to be directed along principal axes. 相似文献
18.
In this paper a generalization of the well known endochronic densification law for dry sand under dynamic loading is presented, taking into account the dependence of its parameters on some measured and calculated sand properties and the variation of these parameters with time. This new generalized law needs the calibration of only one numerical coefficient. For this purpose two sets of reported experimental data of dry sand under cyclic shear strain controlled tests have been used. For non-harmonic loading a new method to determine the equivalent number of cycles has been developed. Finally, the proposed law is implemented in a one-dimensional FEM code to simulate the behavior of a sand layer subjected to the San Fernando earthquake (February 9, 1971), in order to compare the computed results with previous researches and field observations. 相似文献
19.
This paper presents estimates of wavenumbers of propagating waves in a seven-story reinforced concrete building in Van Nuys, California, using recorded response to four earthquakes. The phase velocities inferred from these wavenumbers are consistent from one earthquake to another. They are also consistent, inside the building, with independent estimates of the shear wave velocities in the building (e.g. using ambient vibration tests), and along the base, with phase velocities of Love waves typical for San Fernando Valley. 相似文献
20.
Results from analytical studies conducted on an instrumented ten-storey reinforced concrete building which experienced ground accelerations in excess of 0⋅6g during the 1987 Whittier-Narrows California earthquake and suffered only minimal damage are presented. Using the dynamic characteristics inferred from accelerations recorded in the building during the earthquake, a mathematical model was calibrated to study the response of the building and to explain its good behaviour despite the apparent severity of the motions recorded in the basement of the building. Very good correlation was obtained between the computed and recorded response of the building. Non-linear analyses were conducted to evaluate the strength and deformation capacity of the building and to estimate its response in the event of more severe earthquake ground motions. Special emphasis is given to the evaluation of the overstrength of the building. Lateral overstrengths larger than 4⋅2 and larger than 5⋅7 were computed for the longitudinal and transverse directions of the building, respectively. It is concluded that these high levels overstrength in the building played an important role in limiting the damage during the Whittier-Narrow earthquake. Since the estimation of inelastic deformations during severe earthquake ground motions depends on the actual strength of the building, it is recommended to consider explicitly probable values of this overstrength in the strength reduction factors. 相似文献