An attempt is made to conduct a preliminary statistical analysis based on the calculated variations in peak accelerations at ground surface and predominant soil periods. In this way it is believed that the effect of the
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differences in the input earthquake characteristics can be taken into account with respect to the safety level required for structures located on these layers. It appears realistic to assume that the selected strong motion records represent the range of possible earthquakes that may take place in the near vicinity of Istanbul and the variation of peak ground acceleration and predominant periods can be modelled statistically by a normal distribution. Under these circumstances, it is relatively simple to calculate the probability of exceedence in terms of peak ground accelerations and predominant soil periods as shown on Figure 9. At this stage it is justifiable to consider the variations in the probability separately with respect to the selected base rock peak acceleration levels since they represent approximately the seismicity of the region. The magnitude of a possible earthquake in a region should be estimated based on available seismological data with relation to adopted risk levels or return periods. After these analyses have been conducted then the above mentioned probabilities concerning the characteristics of possible earthquakes can be taken into account. However, in order to be consistent in the final evaluation of the earthquake characteristics at the investigated site, the probability level selected for the peak ground acceleration and predominant soil period should match the risk level adopted in the seismicity study.
Figure 9. Probability of exceedence for the calculated predominant soil periods and peak ground accelerations for the soil profile S4
CONCLUSIONS
An attempt is made by conducting an analytical study to evaluate the effects of input earthquake characteristics on response of soil layers. Although the dominant factor controlling the response of structures located on the ground surface is local soil conditions, it is evident from this study that earthquake characteristics also may play an important role. From an engineering perspective, since it appears rather difficult, if not impossible, to make a deterministic evaluation
concerning the earthquake characteristics, a statistical approach can be utilized to estimate the range of input earthquake effects and to calculate the corresponding probabilities.
REFERENCES
1. Faccioli, E., Battistella, C., Alemani, P., Lo Presti, D. and Tibaldi, A. Seismic Microzoning and Soil Dynamics Studies in San Salvador, Infuence of Local Soil Conditions on Seismic Response, 12th ISMFE Conf., Rio de Jenerio, Brazil, pp. 21–36, 1984.
2. Idriss, I.M. and Seed, H.B. An Analysis of Ground Motion During the 1957 San Francisco Earthquake, Bull. Seismological Soc. America, Vol. 58(6), pp. 2013–2032, 1969.
3. Katayama, I., Iwasaki, T., and Seaiki, M. Statistical analysis of earthquake acceleration response spectra, Trans. Japanese Soc.Civ.Eng., Vol. 10, pp. 311–313, 1978.
4. Kiremidjian, A. and Shah, H.C. Probability Site-Dependent Response Spectra, ASCE, J. Struc.Div., Vol. 106(ST1), pp. 69–86, 1980.
5. Schnabel, P.B., Lysmer, J., and Seed, H.B.Shake—A Computer Program for Earthquake Analysis of Horizontally Layered Sites, EERC Report No. 72–12, Uni.of California, Berkeley, 1972.
6. Valera, J.E. and Donovan, N.C. Incorporation of uncertainties in seismic response of soils, Proc.5th World Conf. Earthquake Engng., Rome, Vol. 1, pp370–379, 1973.
7. Vinale, F. Microzonazione Sismica di Un’Area Campione di Napoli, Rivista Italiani di Geotechnica, Organo della Associazione Geotecnica Italiana, Vol. XXII (3), pp.141–162, 1989.
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The Artificial Wave in Earthquake Safety Analysis for Nuclear Plant Shield
X.Shen, J.Yu
Institute of Structural Theory, Tongji University, Siping Road 1239, Shanghai, 200092 China
ABSTRACT
Usually, the artificial earthquake waves used for the earthquake safety analysis of Nuclear Plant Shield are determined by fit technique based upon the frequency spectrum of NRC RG—1.60 design criteria of USA . In this paper the authors’ basic idea about the design of the artificial earthquake wave is introduced. Finally, 10 groups of time history curves of the artificial earthquake wave are proposed and used in practice. Comparing the 10 groups of curves with the 3 curves from USA, the results are satisfactory for the earthquake reliability analysis of the Nuclear Plant Shield.
INTRODUCTION
The nuclear plant shield is an important protective structure which protects the environment from leakage of nuclear materials in case of an accident. In the earthquake resistance design for the nuclear plant shield, it is necessary to calculate the dynamic response of the structure with acceleration time history inputted in its base. An earthquake safety analysis is also needed for the structure. Affected by various factors, such as source mechanism, wave propagation and local site effect, the earthquake ground motion may involve some uncertainties. For that reason, it is appropriate to treat earthquake signals as a random process. With such an assumption , a certain amount of ground motion waves is needed. The best input signal to the structure is the acceleration history measured at the site where the structure is situated. A simple way to obtain these input signals is to find a real acceleration history whose parameters are similar to that of the real
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site, and make some modifications of the magnitude, time duration and frequency components according to the local conditions. Unfortunately, it is very difficult to have these real acceleration histories. In 1960s, the artificial earthquake wave method was first adopted [1], [2], [3]. Recently, the artificial wave method has developed rapidly due to the development of computer technology and the advantages of this artificial method, which can easily meet the requirements of the target spectrum in magnitude, time duration and frequency characteristic [4], [5], [6], [7].
Artificial earthquake waves produced by using the computer are a number of acceleration time histories , which will satisfy some requirements (such as ground peak acceleration, frequency characteristic and duration) and in the meantime, other condition are not controlled. These uncontrolled factors are just the right random characteristics of the earthquake motion. In the artificial earthquake wave method the earthquake is usually regarded as the product of a certain time function and a stable Gauss process. In this way, the non-stable characteristics of earthquake are reflected, and the theory for stable process can be used.