Design Response Spectra for Large Dams and important structures in Iran

The present paper has been prepared in connection with “Implementation of a probabilistic approach to the assessment of risk, the role of data bases in reducing uncertainty”.To study the dynamic behavior of a Dam during strong earthquakes and then design it safe and economical, it is necessary to consider the effects of strong ground motion characteristics at the location where the structure is to be built. The peak ground acceleration alone cannot show the extent of the effect of vibrations of a structure. Other factors such as strong motion duration, site geology, frequency content of strong ground motion of the site and the corresponding structural behavior and, in all, the amount of the released energy govern the extent of detraction or damage suffered by a structure. Should the dominant frequency of the resulting spectrum from an earthquake at a certain location be close to the range of the vibration frequency of a structure, the vibration of the structure will be amplified and will result in more damages.Several methods can be used to generate response spectra; such as a Standard response spectrum, (SRS) Site specific response spectrum and Uniform confidence response spectrum. However, the best method is the one by which the relevant spectrum is generated on the basis of the recorded accelerograms of different earthquakes at the same location. Therefore, more suitable design spectrum can be prepared for a place if sufficient number of accelerograms be available for that place, but due to the lack of sufficient number of accelerograms, the available ones can be divided into different groups and then, design spectra are prepared. Housner [1] for the first time and later, other researchers used more data and, at each stage, prepared design spectrum for the design of structures. The most important factors that affect the shapes of response spectra (as design spectrum) are as follows: Seismic moment or magnitude of earthquake, physical properties and thickness of the site foundation material, distance from the site to source, the mechanisms of the earthquake and significant duration of strong ground motion. The significant duration of earthquake is affected by the distance from the source to the recording place of acceleration, site geology and the amount of the released energy. The effect of the waves propagation route (path effect) on the spectrum in high frequencies and the effect of seismic source on the response spectrum are clearly seen. Also local geological setting may demonstrate its effect at any frequency, but this effect is quite clear on the relevant spectra mostly at lower frequencies (higher periods). These factors can help us in our attempt to classify the accelerograms response spectra suitably. Response spectra characterizing the different design levels may be determined from peak ground acceleration, velocity and displacement consideration [2], or by using site-dependent or site-independent generalized spectra shapes.In this study, attempt has been made to consider the effect of the foregoing factors on the accelerograms of the earthquakes recorded at different stations in Iran and classify them suitably and, then, present design spectrum. These computations have been carried out for the two seismotectonic provinces of Zagros5 and Center Iran (Iran excluding Zagros).In this study we applied SRS method to prepare design spectrum. A standard response spectrum may be used when the contribution of multiple seismic sources may be represented by an envelop. The prescribed shape of the standard response spectra is to be obtained on the basis of the statistical analyses from many response spectra derived using earthquake records. This SRS is scaled to the relevant site specific value of ground acceleration, velocity and/or displacement [3]. Seed et al [4] and Mohraz [5&6] presented general spectral shapes for a median and median plus one standard deviation which are applicable to rock or other types of sites.For the more conservative analyses of important structures as large dam and atomic power plants, use is mostly made of a median spectrum plus one standard deviation. Five percentage (5%) damping is the most commonly applied value of damping for defining the characteristics of seismic movement and its main reason is the presence of spectral shapes extended to this damping [7]. Regardless of which approach is adopted for specifying the design response spectrum, account must be taken of the uncertainties with the spectral ordinates.