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<br />extending or compressing the time scale or by adding portions <br />of other records that contain the desired frequencies. The <br />predominant period can be changed by adjusting the time scale <br />by the ratio of the predominant period desired to the <br />predominant period of the unmodified record. The duration of <br />the record can be extended by repeating significant portions or <br />it can be reduced by truncating. The modified time-histories <br />should be critically reviewed to ensure that the desired <br />characteristics are incorporated and reasonably represent an <br />expec ted earthquake. Modified ground motion records are the <br />most commonly used sources of ground motion time-histories for <br />dynamic ana lyses. <br /> <br />5. Uncertainties in Ground Motion Design Values <br /> <br />Definition of the uncertainty in ground-motion design values is a <br />complex task depending upon the seismotectonic province where the <br />earthquake occurs and the physical parameters controlling the <br />source, path, and local ground-response effects. The complexity <br />of the problem arises from the unknown or poorly known statistical <br />distribution of many of the physical parameters and not knowing <br />the precise way to combine the uncertainties of the individual <br />physical parameters of the system, even if the statistical <br />distribution for each parameter is well known. The difficulty is <br />a consequence of the short and incomplete world seismicity record <br />and the lack of adequate geologic data to define earthquake <br />potential at a site. The quality of the data base is one of the <br />most important factors leading to the capability for precise <br />specifications of earthquake ground motions. Empirical procedures <br />currently used will be refined and extended in the future. In the <br />meantime, parametric studies can provide the bounds within which <br />the evaluation of structural adequacy can be made. <br /> <br />G. Earthquake Analyses <br /> <br />1. Need for and Extent of Analyses <br /> <br />The extent and type of analysis required for the seismic design of <br />a dam depends on the following factors: the seismicity of the <br />region, foundation conditions at the site of the dam and impounded <br />reservoir, type and height of dam, construction methods and as- <br />buil t ma teria1 propertie s, and sound engineering judgment based on <br />past experiences. <br /> <br />Experience has shown that well-engineered dams which are designed <br />to resist the static forces with generally accepted factors of <br />safety and which are well constructed have the ability to resist <br />strong earthquake ground motions. Consequently, all dams or their <br />appurtenant structures will not necessarily require additional <br />earthquake design studies; however, some analyses to document that <br />additional studies are not needed should be performed. <br /> <br />-17- <br />